School Science Lessons
(UNPh19)
2024-07-25
Capillary action, Surface tension
Contents
19.3.0 Capillary action
19.4.0 Surface tension
19.3.0 Capillary action
19.3.1 Damp course, damp proof course, in a building
19.3.2 Teapot effect
Experiments
19.3.3 Capillarity, capillary action, capillary rise in wicks
Rise of liquid
SI unit
Mercury
Capillary length
Poiseuille
19.3.4 Bursting water bubble
19.3.5 Capillary action in soil and deposition by groundwater
19.3.6 Capillarity between glass slides, water rise by capillary action
19.3.7 Capillary action, capillary tubes
19.3.8 Capillary phenomena
19.3.9 Castor oil drop
19.3.10 Catenoid soap film
19.3.11 Changing drop size
19.3.12 Clumping plastic
19.3.13 Cohesion plates
19.3.14 Cohesion tube
19.3.15 Drop liquid soap on water surface covered with powder
19.3.16 Electrostatic dispersion of water drop
19.3.17 Infiltration and capillary action by groundwater
19.3.18 Connect two unequal size balloons
19.3.19 Connect two unequal size soap bubbles
19.3.20 Leaky boats
19.3.21 Pressure in a soap bubble, soap bubble snuffer
19.3.22 Soap film and sliding wire, soap film minimal surfaces
19.3.23 Sponge action
19.3.24 String siphon empties dish of water, twisting tea bag string
19.3.25 Submerged float
19.3.26 "Tears of wine"
19.3.27 Thread ring in soap film, minimum energy thread
19.3.28 Water drops in tapered tubes
19.3.29 Water droplets
19.3.30 Water on different surfaces
19.4.0 Surface tension
19.4.1 Surface tension
19.4.2 Drops and films, wetting
19.4.3 Bubbles in the air
19.0.3 Surfactants
Experiments
19.4.5 Blow soap bubbles
19.4.6 Bubbles in water and aqueous solutions
19.4.7 Burn butane bubbles
19.4.8 Coalescing oil drops
19.4.9 Drive a boat with surface tension, camphor boat, spinning dancers
19.4.10 Float razor blades
19.4.11 Float a needle, paper clip, razor blade, cotton loop on water
19.4.12 Heap up water up in a glass, liquid surface is higher than the brim
19.4.13 Heap up water up in a glass
19.4.14 Hold water in a sieve
19.4.15 Lift the water surface
19.4.16 Measure surface tension, surface tension balance
19.4.17 Oscillating mercury
19.4.18 Move a boat with surface tension, detergent, soap
19.4.19 Pinch together water streams
19.4.20 Soap and surface tension
19.4.21 Soap bubble support
19.4.22 Surface tension with electric field
19.4.23 Wet mop
19.4.24 Air takes up space
19.3.1 Damp course, damp proof course, in a building
Rising damp
Capillarity may occur in soil, bricks and other building materials, so moisture may rise up the walls called "rising damp".
To prevent this rising moisture a damp proof course of lead, bitumen or plastic may be including in the walls.
A damp course is a layer of impervious material, e.g. plastic or bitumen sheet, built into a wall about 20 cm above ground level.
It should prevent water from the foundations rising in brick walls by capillarity to cause dampness in the home or storage area.
Also, vertical damp courses may be fitted at the sides and door and window openings.
Damp courses in chimneys are used to prevent downward passage of water by capillarity.
19.3.2 Teapot effect
The teapot effect refers to the problem of the dribbling teapot when tea is poured from a teapot runs along the under side of the spout instead of falling into the teacup.
The effect is greater at low pouring speeds and is not caused by surface tension nor by force of adhesion between the tea and the inside of the spout.
The explanation is based on solutions of hydrodynamic equations described as a "hydro-capillary effect".
A common solution to the problem is to use a teapot with a spout made of thin metal.
Another solution is to coat the inside of the spout with " super hydrophobic materials that repel water, e.g. butter.
19.3.3 Capillarity, capillary action, capillary rise in wicks
"Crystal Tree", a salt solution moves up the paper tree due to capillary forces, toy product
See diagram 19.19.1: Contact angle.
1. For liquid in contact with solid in air,
S1 = surface tension between air and liquid
S2 = surface tension between liquid and solid
S3 = surface tension between air and solid
Total potential energy and total gravitational factors, mgh, tend towards
a minimum.
S1 + S2 cause liquid to form a drop on the surface.
S3 + weight of liquid cause liquid to form a film.
2. Wet the glass
A large drop of water on clean glass spreads over the glass is said to "wet the glass", because S3 + weight of liquid > S1 + S2
However, a very small drop of water remains as a drop, because the gravitational factor is small, so S1 + S2 > S3 + weight of liquid
Similarly the very dense element mercury remains as a drop, because S3 + weight of liquid > S1 + S2.
3. Capillarity
Capillarity refers to whether a liquid tends to rise above or fall below the hydrostatic level in a capillary tube, caused by surface tension.
The liquid rises in the tube if it "wets" the solid, i.e. the angle of contact between the liquid and the solid is less than 90oC.
It will fall if the angle of contact is greater than 90oC.
Capillarity occurs in behaviour of liquids in wicks, paper handkerchiefs, paper serviettes, paper towels, baby's nappy (diaper), blotting paper, soil.
Capillarity describes the behaviour of water in vertical thin tubes due to surface tension.
(Capillaries are the narrowest blood vessels, 0.008-0.02 mm, barely wider than a red blood cell.)
Rise of liquid
See diagram 19.19.2: Capillarity.
The rise of a liquid in a thin tube depends on the following:
* the surface tension of the liquid,
* the angle of contact of the surface of the liquid with the capillary, i.e. where the meniscus meets the capillary,
* the density of the liquid and,
* the radius of the tube.
Height of meniscus, h = 2 γ cosθ / ρ g r, where γ = liquid-air surface tension, θ = angle of contact, ρ = density of liquid, g = acceleration due to gravity, r = radius of tube.
SI unit for surface tension.
The SI unit for surface tension is mN / m, millinewton per meter.
The surface tension of water = 0oC 75.64, 25oC 71.97,
100oC 75.64.
So water will rise higher in a thin tube compared to a thick tube.
For water, the attractive force between a glass and a water molecule, i.e. adhesion, > the attractive force between water molecules, i.e. cohesion.
So water rises to a height depending on the tube diameter.
For example, in a 0.4 mm diameter tube, radius 0.2 mm, water rises 70 mm.
Water can rise from the subsoil by capillarity when water in the soil is lost by evaporation or transpiration by plants.
4. Mercury
The level in the capillary tube is depressed below the level in the reservoir, because the forces of cohesion within the mercury > the forces of adhesion between the mercury and the glass wall.
With mercury and glass, the intermolecular forces within the liquid exceed the intermolecular forces between the solid and the liquid, convex meniscus forms and capillary action works in reverse.
5. Capillary length
The characteristic capillarity potential of different liquids can be rated by their capillary length.
Capillary length, λc= √ (γ / ρg), where g = gravitational acceleration, ρ = density of the fluid, γ = surface tension of the fluid-fluid interface.
Poiseuille
6. Jean Léonard Marie Poiseuille, 1797-1869, France, discovered an equation to determine flow rate in capillary tubes.
Volume flow rate, Q = (pressure difference × radius4) / [8/ π]× length of the tube.
So flow rate, Q, is inversely proportional to the length of the tube.
Experiments
1. Put 1 cm depth of ink or coloured water in a beaker.
Dip a stick of white chalk in the beaker.
The ink rises in the chalk by capillarity.
2. Dip different absorbent materials, e.g. jute string in the ink and observe the rate of movement of ink up the material.
3. Dip the end of a glass tube with narrow bore, capillary tube, in the ink and observe the rise of the ink.
The narrower the bore the greater the rise.
The forces of adhesion between the water and the glass are greater than the forces of cohesion between the water molecules.
The meniscus curves up and the water rises in the capillary tube.
19.3.4 Bursting water bubble
A jet of water directed upward against the apex of a cone will cause the water to flow around and form a bubble.
A drop of ether will decrease the surface tension and the bubble will collapse.
19.3.5 Capillary action in soil
and deposition by groundwater
See diagram 6.56: Solution and deposition.
1. Place a mixture of table salt and fine, dry sand in the bottom of a small aquarium to a depth of 2 to 5 cm.
Cover this layer with about 5 cm of clean sand, no salt.
Insert a glass tube with a funnel, supported by a clamp stand, into the sand at one corner of the aquarium, see overleaf.
Make sure that the tube reaches the salt layer.
Clamp a heat lamp on a stand so that it can shine down on the other side of the aquarium.
Pour water into the funnel.
The tube may have to be shaken slightly to get the water to move down the tube.
Observe the side of the aquarium.
The water can be seen to move through the sand.
Put in enough water to wet a layer about 2 cm deep along the bottom of the aquarium.
Light the lamp and let it burn for several hours.
In the vicinity of the lamp, the water will rise through the sand by capillary action, bringing the salt in solution up with it.
The heat will cause the water to evaporate, and the salt will be deposited near and at the surface.
Taste some of the sand near the heat lamp to see if it is salty.
In nature, the sun has the same effect as the heat lamp in this experiment.
2. Put a 2 cm depth of a mixture of sodium chloride and fine dry sand in a big plastic container.
Cover this layer with 5 cm of clean sand with no salt in it.
At one end of the container insert a long stem funnel vertically into the sand so that the end of the stem reaches the sand and salt layer.
At the other end of the container fix a heat lamp or put only that end in direct sunlight.
Pour enough water into the funnel to wet a layer about 2 cm deep along the bottom of the container.
Agitate the stem of the funnel to help the water to move down.
Turn on the heat lamp or leave that end of the container in the sunlight for 2 hours.
Observe through the side of the container so that you can see water moving through the sand.
Near the lamp or sunlight, water rises through the sand by capillary action, bringing the salt in solution up with it.
The heat causes the water to evaporate, and the salt forms deposits near and at the surface of the sand.
19.3.6 Capillarity between glass slides, water rise by capillary action
See diagram 19.3.2: Capillarity between glass slides.
1. Half fill a drinking glass with water.
Add a few drops of ink to colour the water and stir it.
Immerse several absorbent papers such as face tissues, toilet paper and napkin paper that can absorb water into water.
Observe and compare water rises along the different paper.
2. Use a glass tube, its inner diameter is smaller than 1.2 mm, and put it into coloured water.
Observe what happens in the tube.
Use other two glass tubes, one with a smaller inner diameter, another with a larger diameter.
Repeat the experiment above, observe in which tube water rises to highest height.
3. Use two glass slides with rubber bands to hold them together.
Insert a match stick from top of the glass slides to separate them.
Place the assembly in a pan and pour some coloured water into the bottom of the pan.
Observe the coloured water rises between two glass plates.
Note in which position water rises most and rises least.
19.3.7 Capillary action, capillary tubes
Sets of capillary tubes of various diameters show capillary rise with water.
Touch the end of a small glass surface with a small glass tube and the water is drawn into the tube.
19.3.8 Capillary phenomena
* Dip your finger in water covered with sawdust or Lycopodium powder.
* Dip a wet paintbrush in and out of water.
* Pour water down a wet string.
* Pour water in a flexible paper box.
19.3.9 Castor oil drop
Draw a large drop of castor oil under water where it forms a spherical drop.
19.3.10 Catenoid soap film
Dip two concentric circles of wire in soap and separate them to form a catenoid soap film.
19.3.11 Changing drop size
As the amount of sodium hydroxide is varied in a dilute solution, the size of drops formed by an olive oil jet changes with the variation of surface tension.
Olive oil sprayed on hot water forms droplets, but on cold water forms an oil slick.
19.3.12 Clumping plastic
Add water to 2 polystyrene cups as follows:
* to 2 cm from top,
* to almost overflowing.
Put 1 cm pieces of polystyrene foam on the surfaces.
In polystyrene cup 1., pieces move to the edge to reduce plastic foam surface touching the water.
In polystyrene cup 2., pieces move away, because would need to float downhill, but clump to reduce total amount of hydrophobic plastic surface touching water.
19.3.13 Cohesion plates
Two heavy glass plates stick together when a film of water is between them.
Note the difference in cohesion of dry and wet plate glass.
However, if they show cohesion, why do they fall apart when placed in a bell jar that is evacuated?
Atmospheric pressure holds two plate glass panes together.
19.3.14 Cohesion tube
A 2 m tube full of water and sealed at the top will support the water column against gravity.
19.3.15 Drop liquid soap on water surface covered with powder
Drop soap on a water surface covered with sawdust or Lycopodium powder.
19.3.16 Electrostatic dispersion of water drop
Water drops from a pipette at high potential are dispersed into droplets.
19.3.17 Infiltration and capillary action by groundwater
See diagram 6.57: Capillary action through soil.
1. Fill two glass tubes, 2 cm in diameter and 30 cm long about half full of dry, fine sand.
Support the tubes vertically by clamp stands, with their bottoms resting in some type of flat dish or aquarium.
Pour water into one tube.
The water will infiltrate down through the pore spaces of the sand, move into the dish, and partially move up the other tube by capillary action.
2. Half fill two glass tubes, 2 cm in diameter and 30 cm long with dry, fine sand.
Support the tubes vertically with clamp stands so that the lower end rest in a dish.
Pour water into one glass tube.
Observe the water moving down through the sand pore spaces, then moving into the dish, then starting to move up the other tube by capillary action.
19.3.18 Connect two unequal size balloons
Inflate two identical balloons and then deflate them to ensure that they inflate easily.
Inflate one balloon so it is almost fully inflated.
Attach it to a rubber tube with a clothes peg (cloths pin) valve and keep the valve shut and the balloon inflated.
Half inflate the second balloon and attach it to the other end of the rubber tube with the clothes-peg valve.
Open the clothes peg valve.
Air passes from the smaller balloon into the larger balloon.
The smaller balloon shrinks and the larger balloon expands.
Air flows from the balloon at higher pressure to the balloon at lower pressure.
The air flow ceases when the two balloons have equal pressure.
Although this experiment is similar to the following experiment to connect two unequal size bubbles, they may differ.
This is because the rubber in the balloons has different qualities to the wall of the soap bubble.
For example, rubber exhibits a high degree of elastic hysteresis, so at a given pressure when the balloon is being inflated,
its diameter is greater than at the same pressure when the balloon is being deflated.
19.3.19 Connect two unequal size soap bubbles
1. Blow a small bubble and a larger bubble on two pipes with the communicating tap between them closed.
See diagram 19.3.11: Large and small soap bubbles.
Open the communication tap.
The small bubble gets smaller and blows out the larger bubble.
So the pressure was larger in the smaller bubble.
In a soap bubble, the excess pressure, i.e. the difference between the interior pressure pi and the exterior pressure pe = 4γ / R, where R =
the radius of curvature of the bubble surface, and γ = the surface tension.
So the smaller the drop, the greater its internal pressure.
Foam occurs when the surface tension of water, i.e. the attraction of surface molecules toward the centre to gives a water drop of water its round shape, is reduced, and air is mixed in to cause bubble formation.
2. Use a T-tube to blow two soap bubbles of different diameters then interconnect them.
See diagram 19.3.11.1: Connected soap bubbles.
A smaller bubble blows up a larger one when connected by a tube.
Connect 3 rubber tubes and 3 tube clamps to the ends of a T-tube, A, B and C.
Connect two L-tubes to the rubber tubes at the ends of the cross arm of the T-tube, B and C.
Mix 1 cc of detergent, 2.5 cc of glycerine and 3 cc of water.
Dip separately the ends of the L-tubes, B and C, in the solution.
Press open the clamps on the rubber tubes A and B (close clamp C) and blow into A to make a small bubble at B.
Press open the clamps on the rubber tubes A and C (close clamp B) and blow into A to make a larger bubble at C.
Connect the two bubbles by pressing open clamps B and C (close clamp A).
Observe any change in size of the bubbles.
The smaller bubble blows up the larger bubble, because the surface tension in a soap bubble is inversely proportional to the radius of the bubble.
The larger the bubble, the smaller the surface tension and the lower the pressure inside it.
3. Dip two inverted glass funnels into a soap solution, so a soap film forms across their openings.
Blow air into the stems of the funnels so the soap films becomes a spherical bubbles still attached to the funnels.
Connect the funnels by Tygon tubing and a three-way valve.
By using the valve three-way valve, two bubbles, a large bubble and a small bubble can be blown up separately, then connected.
The smaller diameter bubble will shrink and collapse to blow up the larger diameter bubble.
This action demonstrates the Laplace's law for the phenomenon of minimizing the surface area of a soap film.
Using Laplace's law, the gauge pressure of a spherical membrane is 2γ/r, where γ is the surface tension and r is the radius of the sphere.
For soap bubbles, which have an inside and outside surface, the gauge pressure is twice this, i.e. 4γ/r.
Pressure is inversely proportional to the radius, so a small bubble is capable of blowing up a larger one.
4. If the effect of large bubbles absorbing smaller ones happened in the human lungs, the smaller alveoli would all collapse and the larger alveoli would all expand.
However, this does not happen, because the surfactant in the lungs is a phospholipid that changes the surface tension as the radius of the alveoli changes.
This action stabilizes the pressure between different-sized alveoli.
19.3.20 Leaky boats
Float 30 cm long flat bottom boats made of different screen material.
A small metal boat with a large hole floats on water.
19.3.21 Pressure in a soap bubble, soap bubble snuffer
1. Connect a slanted water manometer to a tube supporting a bubble.
Vary the size of the bubble and note the change of pressure.
2. Dip the open end of a funnel into detergent solution or soap solution.
Blow into the funnel until a big bubble forms, e.g. 25 cm
diameter, then close the end of the funnel with the tip of a finger.
Bring the tip of the funnel near a lighted candle.
Remove you finger to blow out the candle flame.
The surface tension of the soap bubble is great enough to force air escaping from the bubble to snuff the candle.
3. If the force of escaping air is not enough to blow out the candle flame,
fold a small piece of paper diagonally into four the balance the centre of the piece of paper on a point.
The air escaping from a bursting bubble should cause some movement in the balancing piece of paper.
19.3.22 Soap film and sliding wire, soap film minimal surfaces
See diagram 19.300: Soap films and sliding wire.
1. Force exerted by a soap film.
Liquid surfaces are in a state of tension.
Dip a rectangular wire frame in a soap bubble solution then place it on an overhead projector.
Attach a small weight to the connecting cotton to show that a force is necessary to increase the area of the soap bubble.
2. Wire frames dipped in soap film form minimal surfaces.
Twist wire to make a rectangle with one side missing.
Twist each end of another piece of wire to make a slider.
The slider forms the fourth side of the rectangle.
Dip the wire rectangle in the soap solution.
Pull the slider out slightly and watch the film stretch.
Release the slider.
The contraction of the film pulls back the slider.
In sliding wire experiments, the soap film provides the force to pull a light wire on a U-shape frame.
A sliding wire frame film with a spring on one end and a string pull on the other shows that tension does not increase with length.
3. Make a wire loop with a handle.
Place a piece of wire across the loop and dip the loop in soap solution.
Lift the loop and use a finger to break the film on one side, away from the handle.
The remaining film will draw closer to the handle.
4. Make a T-shape from coat hanger wire with cross arm 10 cm.
Make a soap solution with 1 cc detergent, 2.5 cc glycerine, and 3 cc water.
Attach a 10 cm piece of wire to the cross arm of the T-shape by connecting them with equal length threads at their ends.
Dip the wires into the soap solution.
Pull them out by holding the leg of the T-shape wire.
Observe the threads joining the pieces of wire.
The threads are curved.
Puncture the soap film between the wires.
The threads are now hanging down straight.
The surface tension in the soap film between the wires and thread consists of,
forces of adhesion between the liquid molecules and the solids,
and forces of cohesion between the molecules of the soap solution.
5. Light a candle and attach it to the table.
Hold a funnel by its long end and dip it into the soap solution.
Blow through the funnel and to form a bubble that sticks to the funnel.
Point the long thin end of the funnel towards the candle flame and observe the candle flame.
Adhesive and cohesive forces exists between the bubble and the funnel and within the soap solution in the wall of the bubble.
These forces push the air in the funnel out of the opening to blow on the candle flame.
19.3.23 Sponge action
Water picked up by a wet sponge is greater than that picked up by a dry sponge.
19.3.24 String siphon empties dish of water, twisting tea bag string
1. Wash a length of string in hot water and soap.
Put a dish filled with water on the edge of the table.
Put one end of the string be in the water and let the other end hang down over the edge of the table.
Water in the dish rises into the string by capillarity then falls down the string and forms a siphon.
Water drops slowly from the end of the string to empty the dish.
Using wet string to do the experiment needs less time, but makes observing the phenomenon of capillarity more difficult.
2. Repeat the experiment with different lengths of string to test whether length of string affect the speed of the water lost from the dish.
3. Thread a long thin knitting needle through a plastic drinking straw and bend them both into a U-shape.
Hang the U-shape over the edge of a jar of water.
Water climbs up the drinking straw by capillary action then trickles down the other side to empty the jar.
4. Examine the twisted cotton fibres in tea bag string.
Dip the tea bag in a cup of hot water.
Remove the tea bag with a steady vertical pull and observe the twisted cotton threads unwind to spin the tea bag.
The cotton threads absorb the tea solution so the threads swell lengthen and unwind.
19.3.25 Submerged float
When submerged, a wire hoop keeps afloat beneath the surface of water due to surface tension.
A cork and lead device floats with a wire ring above the surface.
Push the ring down below the surface and it remains until soap is added to reduce the surface tension.
19.3.26 "Tears of wine"
As 50 proof alcohol evaporates in a watch glass the remaining liquid forms drops that run down the sides like tears!
19.3.27 Thread ring in soap film, minimum energy thread
See diagram 19.3.20: Minimum area of soap film.
1. Dip wire frame A with attached cotton loop into a soap solution.
A soap film forms and the loop keeps its irregular shape.
Use a heated needle to break the film inside the loop.
The irregular loop becomes circular.
Dip wire frame B with a cotton side into a soap solution.
Apply a force at F to alter the curvature of the cotton.
Different frames are used to show the minimum surfaces formed when dipped
into the soap solution.
2. A loop of thread in the middle of a soap film forms a circle when the centre is popped.
Make a soap solution with 1 cc detergent, 2.5 cc glycerine, and 3 cc water,
and put it in a shallow container.
Make a soap film by dipping a wire frame in the soap solution.
Make a 4 cm diameter loop of thread.
Wet the loop in the soap solution then put it in the soap film.
The thread is probably wrinkled and has no regular shape.
Pierce the centre of the loop with the point of a dry pencil.
Move the wire frame and observe the shape of the loop of thread.
It forms a circle and you can shake the frame and move the thread circle.
The thread loop lying in the soap film forms a circle after you pierce the inside of the loop.
This happens because the surface tension forces inside the loop stop and equal forces on the outside of the loop pull on the thread.
The forces of cohesion are of equal magnitude throughout the soap film and this allows you to tilt the wire frame so that the thread loop moves without changing shape.
19.3.28 Water drops in tapered tubes
19.3.28 Water drops in tapered tubes
A drop of water in a tapered tube moves to the narrow end.
19.3.29 Water droplets
Small water droplets form on a surface not wet by water.
Droplets bounce off when sprayed on with an atomizer.
Water droplets will roll across the surface of an over full glass of water when projected out of a pipette at a small angle.
19.3.30 Water on different surfaces
Clean the glass with washing powder or soap, rinse with clean water and dry.
Divide the glass into three parts as follows:
* Wipe butter or lard on the glass.
* Wipe a thin even layer of wax.
* Keep clean and dry glass as a control.
Put equal size drops of water on each part.
Put equal size drops of water on the surfaces of newspaper, paint, different types of glass, perspex, tile, your skin and a leaf of a floating water plant, e.g. Lotus.
Observe and compare the shape of the drops.
Put a drop of water on some dirty greasy clothes.
Observe the drop.
Add a very small amount of detergent.
Describe the drop again.
19.4.1 Surface tension
See diagram 19.19.1: Contact angle.
See diagram 19.19.2: Capillarity.
A molecule in the interior of a liquid experience attractive forces from all the other molecules in all directions, so there is a net force of zero on that molecule.
However, a molecule at the surface of a drop of liquid experiences a net attractive force towards the centre of the drop of liquid.
So the surface of the drop of liquid tends to be pulled inward leading to a surface of minimum area.
The surface of water acts like an elastic film that resists any deformation when a small weight is placed on it.
For example, a water strider insect leaves dimples in the water surface as it strides across the surface of the water.
The stretched membrane effect is caused by molecules at the surface that have only sideways and downwards hydrogen bonding forces on them.
Surface tension values of liquids in contact with air at 20oC,
Water = 72.8 × 10-3 N / m, i.e. 72.8 mN / m (millinewton per metre, mN.m-1),
ethanol = 22.3 mN /m, glycerol = 63.1 mN/m,
mercury = 465 mN /m, olive oil = 32.0 mN /m,
soap solution = 25.0 mN /m.
Water at 0oC = 75.6 mN / m, Water at 100oC = 58.9 mN / m
Sometimes surface tension values are quoted in the CGS (cgs) unit. the dyne (dyn).
However, 1 dyn / cm, dyn.cm-1 = 1 mN /m, mN.m-1, millinewton per metre.
Liquid surfaces behave as if they are enclosed in a stretched elastic membrane that tends to contract until the surface area is a minimum.
Small quantities of liquids assume the shape of a spherical drop, because a sphere is the solid with the smallest surface area for a given volume.
Surface tension is caused by the forces of molecular attraction, forces of cohesion.
A molecule in the liquid is attracted by all the molecules around it within a sphere of molecular attraction.
For a molecule within the liquid the forces of molecular attraction act equally in all directions, so there is no resultant force in any direction.
However, a molecule near the surface has more molecules below it than above it within the radius of attraction.
So there is a resultant force acting inwards and work must be done to transfer molecules from the interior to the surface of a liquid.
Surface tension is the work that must be done to increase a liquid surface by unit area.
The work done in increasing a liquid surface by an area A = S × A, where S = the surface tension, measured in Nm-1.
Surface tension may also be regarded as a force per unit length.
If a line length l is drawn in a liquid surface there is a force equal to S × l acting in the surface of the liquid and at right angles to the line.
If a line length l is drawn along the line of contact of the liquid and another medium, e.g. air, there is an unbalanced force due to surface tension.
It tends to make the liquid contract, and a force equal to S1 that be applied to prevent the surface contracting.
The surfaces of a liquid act as if a thin elastic membrane covers them.
The surface molecules attract each other by cohesion, force of cohesion, measured in Nm-1.
As the surface molecules are under tension the liquid contracts to minimize the surface area, so a falling water drop is nearly spherical.
Surface tension is the property that causes the surface of a liquid to behave as if covered with a weak elastic skin.
It is caused by the exposed surface's tendency to contract to the smallest possible area.
This occurs because of unequal cohesive forces between molecules at the surface, unbalanced molecular cohesive forces.
Surface tension is measured in N m-1.
Similar phenomena include:
the formation of droplets, the concave profile of a meniscus,
and the capillary action, capillarity, by which water soaks into a sponge or porous material,
or "wets" the surface of a material.
The surface tension of water is the highest of all liquids, 73 mN m-1.
The high surface tension controls the shape of a meniscus, raindrops and sea spray.
Organic impurities lower the surface tension of water.
Increase in temperature lowers the surface tension of a liquid.
For water, St = So - 0.14t, where St and So are the surface tensions at
toC and 0oC respectively.
19.4.2 Drops and films, wetting
See diagram 19.0.1: Wetting water drops (See Figs. 1. to 4.).
Fig. 1.The pendant drop test, which is used to measure the surface tension of a liquid, γ, (gamma), using the formula mg = π d γ.
Fig.2. A drop of liquid on a wettable surface
The attractive forces to the surface exceed the surface tension of the glass attach to water molecules by hydrogen bonds.
Fig. 3. A drop of liquid on a non-wettable surface, e.g. Teflon or a plastic bowl
The the surface tension forces within the water drop increase the curvature of the surface and are greater than any attracting forces to the surface.
The angle of contact, θ < 90o.
Fig. 4. The drop increased in size and angle of contact advanced by adding drops to it from a dropper.
Similarly sucking out some water from a drop would recede the contact angle.
Work must be done to extend a liquid surface, so when the liquid surface is increased the potential energy increases.
Potential energy tends to decrease, so a liquid surface tends to decrease, making the air-liquid, liquid-solid, and air-solid surfaces as small as possible.
The air, liquid and solid contacts of the three surface tensions are as follows:
S1 between air and liquid,
S2 between liquid and solid, and, S3 between air
S1 and S2 cause a liquid to form a drop.
S3 and the weight cause a liquid to form a film.
A small volume of mercury on glass forms a drop, but the same small volume of water on glass forms a film, because S3 is greater than S1 and S2.
So we say that water "wets " glass.
For a very small drop, ignoring gravitation, the form assumed by the liquid is S3 = S2 + S1 cos θ,
where θ is the "angle of contact", between the liquid and the solid surface.
A substance becomes wet if a small volume of a liquid spreads evenly over it, instead of forming separate droplets over it.
19.4.3 Bubbles in the air
A bubble of water floating in air will tend to contract, increasing the pressure air inside the bubble.
This action continues until the tendency to contract due to surface tension equals the tendency to expand due to the excess pressure.
The excess pressure inside the bubble of radius r = 4S / r.
The pressure inside a drop = 2S / r.
The bubble has an inside and an outside skin to contribute to the pressure due to surface tension.
Organic impurities and increase in temperature lowers the surface tension of water.
If St = the surface temperature of water at toC, and S0 = surface tension of eater at 0oC, St = S0 - 0.14 t.
19.4.4 Surfactants
Surfactants in detergents
Surfactants, surface active agents, are organic molecules with a lipophilic end and a polar end that emulsify and disperse oil and grease.
Surfactants also lower the surface tension to improve the wetting of clothes so that dirt may be more easily removed.
Surfactants do not precipitate in hard water.
Cationic surfactants may act as fabric softeners, e.g. aminoethylethanolamine, AEEA, C4H12N2O, surfactant, fabric softener, fuel additive, chelate.
Detergents or soaps contain surfactants, surface active agents, which reduce the surface tension of solvents used for washing.
When the distance between solid and liquid is less than 10-8 m, molecular forces between them will apply.
The attraction between molecules of solid and liquid forms a liquid layer attached to the surface of the solid.
When the action of the solid molecules on the liquid molecules of the liquid layer is stronger than the interaction of the liquid molecules, or force of cohesion
,
the interface spreads and molecules stick to each other between liquid and solid.
It is called adhesion, force of adhesion.
Surface energy, surface tension - the force of attraction for itself that gives a liquid, e.g. water, an apparent skin.
The "skin" contracts to form drops rather than sheets on surfaces it does not wet.
On some surfaces, e.g. clean glass, the attraction of the water is greater for the glass than for itself, so the water "wets" the glass.
Water spreads out smoothly on clean glass, but remains in droplets on dirty glass.
So wetting refers to the covering of a solid by a liquid with a thin film.
The contact angle the liquid makes on the solid is small.
19.4.5 Blow soap bubbles
The spherical film of liquid surrounding a soap bubble exists, because the gas pressure inside the bubble is greater than the air pressure outside the bubble.
The gas inside the bubble pushes out and this outward push is opposed by the surface tension of the liquid molecules in the wall of the bubble.
In time, enough gas leaks out of the bubble through the bubble walls and to decrease the pressure inside the bubble until the bubble bursts.
Bubbles in aerated water, fizzy drinks, burst when they rise to the surface, because the atmospheric pressure is much lower than the pressure inside the liquid drink.
Experiments
1. Make a soap bubble solution by putting three level tablespoonfuls of
soap powder or soap flakes into four cups of hot water.
Let the solution stand for three days before using.
Try blowing bubbles with a bubble blower or a drinking straw by slitting the end of the straw with a razor blade into four parts, extending about 1 cm from the end.
Bend these pieces outwards.
2. Make the soap solution the day before the lesson.
Show how to make soapy water by putting the pieces of soap or some detergent
into the water in the jar and shake.
Put some soapy water in the palms of your hands.
Press your hands together
so that a small hole forms.
Blow through this hole.
Dip one end of the stem or straw into the soapy water and blow gently
through the stem in the air.
Dip a loop of wire into the soap solution.
Is there a thin film of soap across the loop?
Blow through the loop slowly.
Blow a big bubble.
To make small bubbles, blow quickly.
To make big bubbles, blow slowly.
A bubble has a skin of soap.
Inside is air.
Describe the shape and colour of a soap bubble.
A bubble breaks if the skin is too thin, if it hits something the skin
breaks, if the air inside gets bigger the skin breaks.
3. Rinse a milk carton and fill it with water.
Add 1 / 3 cup household soap and one tablespoon glycerine.
Close the carton and turn it over a couple times to mix, without shaking
to avoid suds.
Leave to stand for 24 hours.
Use the plastic ring to blow bubbles.
Dip it in the bubble solution..
19.4.6 Bubbles in water and aqueous solutions
1. Shake pure water and leave to stand.
A few bubbles form, but they soon disappear.
2. Shake solutions of substances in water and leave to stand, e.g. soap and water.
Many bubbles appear to form a froth that remain for some time after the solution becomes motionless.
The dissolved substances decrease the surface tension, depending on their concentration.
Also the concentration of substance is greater at the surface of a film.
3. Make big bubbles with glycerine.
Use a 7 : 3: 1 solution of a water : dish-washing detergent : glycerine solution or 100 mL soap solution, 900 mL water, 50 mL glycerine (glycerol).
Leave the solution to stand for half an hour or, better still, overnight, and remove any foam on the surface of the bubble mix before using it.
4. Make big bubbles with light corn syrup.
Use a 6 : 2: 1 solution of a water : dish-washing detergent : corn syrup solution.
5. Make big bubble wands and bubble stands
* To make big bubble blowers,
use a wire coat hanger pulled outwards in a circle and twist the crook of the coat hanger to make a handle,
or cut off the rim of large plastic containers,
or use a plastic funnel,
or use a paper cup with a hole in the bottom,
or use a plastic drink bottle with the bottom cut off.
* Thread a string through two drinking straws, tie the ends of the string together, but hide the knot in one straw.
Grab one straw in each hand, dip the straws into a strong bubble solution then pull the straws apart.
* To make bubble stands,
use an inverted plastic container,
or use a plastic funnel in the rim of a bottle,
or use a piece of wire with a loop at one end then wind the other end around a pencil fixed in a cotton reel spool.
6. Make bubbles by hand.
Dip you hand in a soap solution with the tips of the first finger and thumb touching to make a ring.
Take your hand out of the soap solution and blow through the thumb and finger ring.
7. Make two bubbles with one drinking straw.
Cut a slit across the middle of a drinking straw then bend the straw at the slit to make two connected half straws.
Dip the end of one half straw in a bubble solution then blow at the slit to make a bubble at the end.
Dip the end of the other half straw into the bubble solution, then blow at the slit to make a second bubble.
When you blow the second bubble the first bubble gets larger.
Make the drinking straw straight again and close the slit with your finger.
The first bubble gets even larger and the second bubble get even smaller.
This occursbecause the air pressure exerted on the smaller bubble with smaller surface area is greater than the air pressure on the larger bubble.
8. Make long lasting colourful bubbles.
Add sugar to a bubble mix and cool it in the refrigerator.
Make a big bubble from the cooled bubble mix and put it on a bubble stand.
At first, the bubble has no colour as white light passes through it, but later some air in the bubble evaporates and it becomes smaller, so colours appear,
Uneven changes in the width of the soap solution in the bubble causes reflection of some light from the outer or inner walls of the bubble.
The colours formed are caused by interference.
9. Make a bubble inside a bubble.
Make a big bubble with a drinking straw dipped in soap solution and place it on a bubble stand made from a plastic cup with both ends missing.
Invert the bubble stand, suck more soap solution into the drinking straw and push its end through the big bubble.
Blow a smaller bubble inside the big bubble.
19.4.7 Burn butane bubbles
Do this experiment outside, but NOT in the laboratory.
1. Dissolve liquid soap or detergent in water.
Hold a cigarette lighter under the water then push the button to release butane, C4H10.
Butane bubbles appear on the surface of the water.
Stand well away then use a long match to light the bubbles.
2C4H10 + 13O2 --> 8CO2 + 10H2O
C4H10 + 13/2 O2 --> 4CO2 + 5H2O
2. Butane may be sold as "purified butane lighter gas" cartridges.
From the equation, 2 moles of butane reacts with 13 moles of oxygen.
2/13= 0.15 and air contains 21% oxygen, so the volume of butane to air for complete combustion of the butane is: 0.21 X 0.15 = 0.0315, a ratio of 1 : 31.5.
19.4.8 Coalescing oil drops.
Let drops of oil fall into a beaker of water.
Stir the water and observe drops of oil hitting each other and coalescing.
Potential energy tends to decrease.
The surface tension potential energy of the drops depends on their total surface area.
When two drops coalesce, the total surface area is reduced and potential energy is reduced.
19.4.9 Drive a boat with surface tension, camphor boat, spinning dancers
See diagram 4.219: Surface tension boats: A Middle
notch, B Left notch, C Right notch.
See diagram 19.297: Spinning dancers.
This phenomenon is called the Marangoni effect (C.G. M. Marangoni (1840-1925), where surface tension along an interface causes tangential forces (shear forces).
Experiments
1. If a piece of camphor is scraped into a dish full of water it will dart over the surface in a seemingly random manner.
Camphor is only slightly soluble in water.
On an uneven piece of camphor, solution occurs more rapidly at a point P, than where the surface is smooth.
So the strength of the camphor-water solution near P is greater than at any other point and hence the surface tension is least at P.
Then the greater tensions on the other side pull the camphor away from P.
Drop small particles of camphor onto the surface of water.
The particles move quickly over the surface of the water.
This occurs, because some camphor dissolves in water especially at sharp points reducing the surface tension of the water where the concentration of camphor is greatest.
The greater surface tension on the opposite side of the camphor causes it to be pulled in that direction.
Touch the surface of the water with a rod smeared with castor oil or add soap solution.
The oil or soap spreads over the surface of the water forming a thin layer and reducing the surface tension all around the particle of camphor, so it stops moving.
2. Make a camphor boat.
Cut out a triangular piece of aluminium foil.
Fold up the side to make a triangular boat.
Float the boat on water.
Make a pin hole in one of the corners of the boat and put a piece of camphor in the boat.
Put the boat in water.
The dissolving camphor will very slowly propel the boat as it spreads along the surface of the water.
Some soaps can be used to replace camphor.
3. Cut out the shape of a 4.5 cm boat from stiff paper.
Cut a notch in the middle of the stern large enough to hold a small lump of gum camphor or naphthalene mothball in contact with the water without letting it fall out.
Float the boat in a large round dish of water.
Make other boats with the notch in the stern on the right or on the left of the middle.
Achieve the same effect with a drop of dishwashing detergent or vegetable oil that will slowly spread across the surface of the water in a thin film.
This flow is strong enough to act as a jet to propel the boat across the water until the surface tension of all the water in the round dish is reduced.
4. Cut out a picture of two dancers.
Use glue to attach a small piece of camphor to the smaller end of four small corks.
Use pliers to force the blunt ends of four needles into a large flat cork to form a cross.
Attach the small corks to the central large flat cork by sticking the sharp ends of the needles into the sides of the small corks.
Place the apparatus on the table and turn the small corks to that the small ends with the camphor attached are all facing in the same direction, e.g. clockwise.
Attach the picture of the dancers to the large flat cork.
Place the apparatus on water to observe the perpetually spinning dancers.
5. Attach camphor to the edges of a light aluminium propeller cause it to spin on the surface of water.
6. A drop of Duco cement will dart around on the surface of water and two drops will play tag.
19.4.10 Float razor blades
Use a razor blade of the double edge type.
Place the razor blade across the prongs of a fork and lower it into the water.
The razor blade floats on the surface of water, as if occupying a depression in the surface of the water, so this experiment is not an example of buoyancy.
The surface of the water behaves like a stretched, elastic membrane.
When the razor blade pushes down on the surface, because of its weight, the stretched membrane exerts an upwards force on the razor blade.
The razor blade sinks if a drop on lubricating oil is added to the water, because the oil interferes with the cohesive forces between the water particles.
19.4.11 Float a needle, paper clip, razor blade, cotton loop, on water
See diagram 4.213: Float a needle.
See diagram 9.8.0: Mosquito larva.
1. Place the needle on a bit of tissue and place on the surface of fresh water.
Sink the tissue with a stick, leaving the needle floating.
Add a little soap to sink the needle.
2. Use a steel needle and dry it thoroughly.
Place it on the tines of a dinner fork and gently break the surface of some clean water in a dish with the fork.
If you are careful, the needle will float as you take the fork away.
Look at the water surface closely.
See how the surface film seems to bend under the weight of the needle.
The weight of the needle is greater than the weight of the water it displaces.
It does not sink, because it is supported by the elastic "skin" of water molecules in contact with air.
Add a drop of detergent solution.
The needle sinks, because the water molecules in the "skin" are now dispersed.
3. Put a needle on a small area of absorbent paper tissue, tissue paper.
Float the tissue and needle on a water surface.
Use a small stick to sink the absorbent tissue or leave it to sink when
it becomes soaked with water.
The needle remains floating on the water.
Add soap solution to the water.
The needle sinks.
4. Float a paper clip on water.
Hold a paper clip in a sling made of a paper towel.
Dip the paper clip on to the water surface.
Take away the paper towel.
See the water surface bending under the paper clip.
Add a drop of detergent solution.
5. Float needles, paper clips, rings of wire, and
a razor blade on water.
Add a drop of detergent solution.
6. Float an aluminium sheet on the surface of deionized
water and add weights until the metal sinks.
7. Use a razor blade of the double edge type.
Try floating it on the surface of water.
Note the surface and see the surface film.
8. Tie together two ends of a small piece of cotton
to make a cotton loop.
Drop the cotton loop onto clean water so that it makes an irregular shape
on the surface of the water.
Dip the end of a needle inside the floating cotton loop to break the surface
film of water.
The cotton loop forms a circle that minimizes the surface area of cotton
in contact with the water.
Add a drop of detergent solution.
19.4.12 Heap up water up in a glass, liquid surface is higher than the brim
1. Place a drinking glass in a shallow pan or on a saucer.
Rub the top edge of the glass with a dry cloth.
Pour water into the glass until it is full to the brim.
You will note that you can fill the glass several millimetres above the top.
Now drop coins or thin metal washers into the water edgewise.
By dropping these in, see how far you can heap the water up before it runs over.
2. The volume of an object with an irregular shape can be measured with an overflow can.
The volume of the object is equal to the volume of water that overflows.
However, small thin objects, e.g. coins, can be added to a small container of water, e.g. a drinking glass
Iinstead of an overflow, the surface of the water, the elastic "skin", curves outwards.
Fill a drinking glass until no more water can be added.
Draw the curvature of the surface of the water.
Add coins and draw the changes in curvature until the water spills over the rim of the drinking glass.
3. To observe that water higher than the rim of a cup does not overflow due to the surface tension of water,
dry a glass cup by wiping it with a clean dry cloth then place it on a dish with flat bottom.
Fill the cup with water.
Put coins or small stones in the glass to make the water heap up.
Shake some salt in the heaped up water.
The salt dissolves, but he water does not overflow.
You can put in more water without it overflowing, because the convex meniscus
keeps the water in place.
Gently place a needle on the water surface; it floats on water; estimate
and record the height of the water above the cup rim.
Add a needle on the water surface; the two needles still float on water;
the water surface rises slightly.
Again add a needle on the water surface; then they still float on water;
the water surface rises again.
Try to add needles on the water surface until water overflows.
Record the final height of the water and the amount of the needles.
4. Add nails to a full glass of water until it overflows.
Objects floating in a vessel cling to the edge until it is over full when
they go to the middle.
5. Make a surface tension hyperbola (See: 3.8.0
Conic sections, hyperbola)
A large meniscus forms between two sheets of glass held at an angle in
a pan of water.
Two glass plates are clamped on one edge and separated by a wire on the
other edge.
6. Place two very clean dry glasses on the table and fill one just to the brim, i.e. no meniscus.
Put coins in the glass of water, edge down.
Count the coins are put in before the meniscus breaks.
Repeat the experiment with the second glass in which you have put one drop of detergent.
Count the coins are put in before the meniscus breaks.
The water forms a convex meniscus due to the surface tension caused by the cohesion of the surface molecules.
In the second glass, the surface tension was broken by the detergent so the cohesion between the surface water molecules less and the water overflows much sooner.
19.4.13 Heap up water up in a glass
1. Place a drinking glass in a shallow pan or on a saucer.
Rub the top edge of the glass with a dry cloth.
Pour water into the glass until it is full to the brim.
You will note that you can fill the glass several millimetres above the top.
Now drop coins or thin metal washers into the water edgewise.
By dropping these in see how far you can heap the water up before it runs over.
19.4.14 Hold water in a sieve
1. Pour some oil over the wire mesh of a kitchen sieve and shake out the excess so that the holes are open.
Use a pitcher of water and carefully pour it into the sieve by letting it run down the side of the sieve.
When the sieve is about half full, hold it over a sink or bucket and observe the bottom.
You will see water pushing through the openings, but the surface tension keeps it from running through.
Touch the bottom of the sieve with your finger and the water should run through.
2. To observe the surface tension on a liquid surface, pour some oil into a sieve with dense net then shake it to get rid of unwanted oil.
Add water along the side of the sieve.
Slowly move the sieve above a bucket when the water reaches half way.
Water may come out from the sieve holes, but does not flow down.
The water will flow down when you touch the sieve bottom gently.
3. Make watertight sieves.
A mesh boat floats until a drop of water is placed inside it.
Dry cheesecloth holds water in an inverted beaker.
19.4.15 Lift the water surface
1. Bend the pointed end of a pin or use a piece of fine wire to make a hook.
File the point of the hook until it is very sharp.
Put your eye on a level with the surface of the water in a drinking glass.
Put the hook under the surface of the water and gently raise the point to the surface.
If you are careful, the point will not penetrate the surface film, but will lift it slightly upwards.
2. To observe surface tension on a liquid surface, bend the sharp end of a pin into a hook or make a hook with a piece of thin wire.
Polish the hook to make it sharp.
Fill a glass cup with water.
Immerse the end of the hook then gently lift it up.
The hook does not pierce the water surface, but lifts it slightly.
Be careful to keep your line of sight at the same horizontal plane with the water surface.
3. If you dive into water with your arms stretched out and hands clasped together, you do not feel much force on the ends of your fingers.
However, if you cannot dive properly and do a "belly flop", you feel the force on your stomach as you break the surface tension of the water.
19.4.16 Measure surface tension, surface tension balance
See diagram 19.2.7: Measure surface tension.
The surface tension of water in contact with air at 20oC = 7.28 × 10-2 N / M
1. If you can measure the force along the edge of a water surface, you can express surface tension as a force per unit length.
Clean a microscope slide thoroughly with caustic soda solution then deionized water.
Attach the slide to a balance with a light thread as in the diagram.
Add masses to counterpoise the slide so it does not move up or down.
Put a clean beaker of deionized water underneath the slide and to allow the water surface to attach itself to the slide.
Add very small masses, m, to the balance to cause the slide to just break away from the water surface.
Measure the length, l cm, and the width, d cm of the glass slide.
Perimeter of the bottom edge of the slide = 2(l + d) cm = 2(l + d) / 100 m.
Mass added to cause break = m / 1, 000 k
Force to balance surface tension = m / 1 000 newton
Surface tension = (m × 9.8 × 100) / 1 000 × 2 (1 + d) newton per metre.
Repeat the experiment with different solutions and measure their surface tension.
2. Adhesion balance: Measure surface tension by the direct pull on a frame touching water surface.
A glass plate on one end of a balance beam is in contact with water.
Pull a large ring away from the surface of a liquid with a spring sale.
A flat glass slide on a soft spring is lowered onto the surface of deionized water and the extension upon pulling the off the water is noted.
19.4.17 Oscillating mercury
See diagram 4.222a: Touch the mercury. Oscillating mercury.
Mercury is not permitted in schools except in some school systems where
it is allowed in barometers and thermometers.
This experiment is difficult to do well and get the mercury oscillating.
Experiments
1. Prepare a solution of 5 ml of dilute sulfuric
acid in 500 mL of water.
Add a few crystals of potassium dichromate.
Add a drop of mercury.
Dip the point of a clean iron nail into the solution.
The mercury starts to oscillate.
The solution acts as a chemical battery
with electrode mercury and iron nail.
Electrical charges concentrate at surfaces where their repulsive forces
on each other subtracts from the surface tension.
When the mercury and iron touch, the battery is short circuited and the
charge density on the mercury surface is reduced.
The mercury drop pulls up due to increased surface tension.
In a certain position of the nail, the mercury oscillates when it repeatedly short circuits the electrical charges building up when the mercury is not touching the nail.
2. Moving mercury
See diagram 4.222c: Moving mercury.
Place a drop of mercury in dilute nitric acid.
Add a few crystals of potassium dichromate added.
The mercury is moved towards the potassium dichromate crystals by motions in the solutions caused of electrical and chemical changes at the surfaces.
19.4.18 Move a boat with surface tension, detergent, soap
See diagram 4.219: Surface tension boats: A Middle notch, B Left notch, C Right notch.
1. Cut out the shape of a 2.5 cm boat from stiff paper.
Cut a notch in the middle of the stern large enough to hold a small lump of gum camphor or naphthalene mothball in contact with the water, without letting it fall out.
Float the boat in a large round dish of water.
Make other boats with the notch in the stern on the right or on the left of the middle.
You can achieve the same effect with a drop of dishwashing detergent, or piece of soap.
Surface tension pulls the boat in every direction.
The substances in the notch at the back of the boat reduces the surface tension behind the boat so it moves forward pulled by the surface tension around it.
As the substances mix with the water in the dish, the boat moves more slowly, until it stops.
19.4.19 Pinch together water streams
See diagram 4.218: Pinch together water streams.
1. Use a metal can.
Punch five holes in the sides about 5 mm apart.
Fill the can with water.
Observe that the water comes from the can in five streams.
Pinch the jets of water together with your thumb and forefinger to make one stream.
Brush your hand across the holes in the can and the water again flows in five separate streams.
2. Use an empty tall tin can or plastic drink bottle and a nail to drill five holes, very close to the bottom 5 mm apart.
Fill with water.
Water flows from the five holes.
Move your fingers over the holes.
The five streams of water gather into one stream.
If you rub on any hole on the can, the water changes into five streams again.
19.4.20 Soap and surface tension
1. Place a stocking in a beaker of clean water.
The stocking floats in the water.
Add drops of soap solution.
The surface tension is reduced and the stocking sinks.
2. Place a large drop of water on a wire gauze mat.
The drop has an almost spherical shape, because of the surface tension.
Add a small drop of soap solution.
The surface tension is reduced and the drop spreads over the gauze mat.
3. Fill the large clean plate with cold water and
let it stand for a time on the table until the water is still.
Sprinkle some talcum powder lightly over the surface of the water.
Wet a piece of soap in water and touch it to the water near the edge of the plate.
The talcum powder will be drawn to the opposite side of the plate at once.
The soap reduced the surface tension at one point.
The increased surface tension on the other side contracts the surface and pulls the talcum with it.
Try a similar experiment, but substitute flowers of sulfur for the powder and synthetic liquid detergent instead of the soap.
If you use a transparent dish, you can place it on an overhead projector and display the results on a screen.
4. Select a large plate and rinse it until you are sure that it is very clean.
Fill the plate with cold water and let it stand for a time on the table until the water is still.
Sprinkle some talcum powder lightly over the surface of the water.
Wet a piece of soap in water and touch it to the water near the edge of the plate.
The talcum powder will be drawn to the opposite side of the plate at once.
The soap reduced the surface tension at one point.
The increased surface tension on the other side contracts the surface and pulls the talcum with it.
Repeat the experiment by substituting flowers of sulfur for the powder and synthetic liquid detergent instead of the soap.
If a transparent dish is used, it can be placed on an overhead projector and the results displayed on a screen.
5. A short thread difficult to catch.
To observe that different liquids have different surface tensions, pour clear water into a clean dish.
Immerse a 1 cm thread in hot wax to get a thin coating then dry it.
Place it on a water surface.
Use a toothpick with soap or washing power to touch the thread or the water near the thread.
The thread may run away like a frightened rabbit.
Improve the demonstration by using a transparent dish and display the process with a projector.
Surface tension coefficient of soap liquid is smaller than that of water so the surface tension of water pulls the thread.
19.4.21 Soap bubble support
See diagram 4.221: Soap bubble support.
Make a soap bubble support with a wire loop about 10 cm in diameter.
Dip the loop in soap solution.
Blow a large soap bubble and put it in the loop.
Now wet a drinking straw in the soap solution and put it through the large bubble.
Blow a smaller bubble inside the large bubble.
19.4.22 Surface tension with electric field
Droplets from an orifice become a steady stream when connected to a Wimshurst generator surface tension with electric field.
19.4.23 Wet mop
Surface tension pulls the strands of a small fluffy mop together when wet.
19.4.24 Air takes up space
See diagram 4.225.1: Air takes up space.
See diagram 4.225.2: Transfer air under water.
1. Place the funnel in the neck of the bottle.
Seal the space between the funnel and the neck of the bottle with heavy grease or Plasticine (modelling clay).
Pour water slowly into the funnel.
The water stops running, because the air takes up space.
Repeat the experiment and pour in water until it comes nearly to the top of the funnel.
Use a nail to punch a hole through the seal.
All the water drops into the bottle.
The water replaces the air that comes out through the punched hole.
2. Pour water into a large glass container until it is half full.
Float a cork on the water and lower a drinking glass, mouth downward, over the cork.
Repeat the experiment with a piece of paper wedged tightly into the bottom of the glass.
The paper does not get wet.
3. Almost fill a fish tank with water.
Lower a drinking glass, mouth downward, into the fish tank.
With your other hand lower another glass into the fish tank.
Let this second glass fill with water by tilting its mouth upwards.
Now hold this glass above the first one mouth downwards.
Carefully tilt the first glass to let the air escape slowly.
Fill the second glass with air from the first glass to transfer the air under water.
Air replaces some water in the second glass.