lecdem.physics.umd.edu - Surface Tension

Surface Tension

F3-01: SURFACE TENSION - JOLLY BALANCE

F3-01

Demonstrate surface tension and to determine the value of the surface tension of water.

This is the classic experiment for determination of the surface tension of water. Weights can be attached to the bottom to determine the spring constant. Measure the equilibrium position of the rectangular frame when not immersed, and when the bottom wire is completely immersed. The average should be taken with the wire just making surface contact. Slowly and smoothly lower the platform until separation occurs, and measure the displacement. The surface tension T is given by T=F/2L, where F is the force of the spring and L is the length of the wire (which is 8 cm for this apparatus). The spring constant k is 7.27 g/cm, so the force is F=kgx, where x is the extension of the spring and g is the acceleration of gravity. The photographs at the center and rightabove show the effect of the surface tension pulling down the spring. Be careful. This apparatus is sensitive.
F3-02: SURFACE TENSION - BALLOONS

F3-02

demonstrate surface tension in a counterintuitive way.

Use two identical balloons. Blow up one balloon on the tube and clamp it. Then blow up the other balloon to a different size and slip it onto the other end of the tube.
Q: When you remove the clamp, what will happen?: (a) the small balloon will get smaller and the large one larger, (b) the two balloons will become equal, or (c) they will stay the way they are.

A: The small balloon will blow up the larger one, and get smaller, due to surface tension effects. The rubber is thicker in a smaller balloon, and thus produces greater surface tension.
F3-03: SURFACE TENSION - SOAP BUBBLES

F3-03

Demonstrate surface tension in a perhaps counterintuitive way using soap bubbles.

Pour some soap solution into the small glass lid of the soap solution container. Turn the valve so that only one side is connected to the hose, dip that end into the solution and blow a bubble. Rotate the nozzle so that the other side is connected and blow a bubble of a different size. Rotate the valve so that air can flow from one to the other.
Q: What will happen to the two bubbles when they are connected?

A: The smaller bubble will get even smaller and the bigger bubble will get even bigger, as seen in the photograph at the right.
F3-04: SURFACE TENSION - THREAD ON WATER

F3-04

Demonstrate how detergent decreases the surface tension of water.

Fill a container with water and place it on the overhead projector. Place a loop of thread on top of the water such that it forms an irregular shape. A drop of detergent solution gently placed inside the loop with the eyedropper reduces the surface tension of the water inside the loop. The greater surface tension outside the loop then pulls the thread outward into a uniform circle. Keep the thread near the center of the dish; use only a small drop of soap solution.
F3-05: SURFACE TENSION - THREAD IN FRAME

F3-05

Demonstrate surface tension.

A thread with a loop at its center is connected across a diameter of a wire loop, as shown in the photograph at the left. Dip the wire loop into soap solution, then puncture the surface inside the thread loop. The loop expands, as shown in the photograph at the right, due to surface tension.

F3
F3-06: SURFACE TENSION - NEEDLE ON WATER

F3-06

Illustrate surface tension by floating a needle on water.

The water bath is allowed to become quiescent and the needle is very carefully placed on the surface of the water. The surface tension of the water supports the needle. Practice! This demonstration requires a delicate touch.
F3-11: SURFACE TENSION - CAPILLARY TUBES

F3-11

Demonstrate surface tension in capillary tubes

Four glass tubes of different diameter are connected at the base. The height of the water in each tube is determined by the effects of surface tension. The water is spiked with green food coloring to enhance its visibility. For large groups, a camera may be provided.
F3-21: SURFACE TENSION - ALCOHOL AND WATER IN SAND

F3-21

Illustrate the difference between the surface tension in water and in alcohol.

One jar contains sand with water. The bottle has been tamped so that the sand grains have become aligned with very small cracks between them. The surface tension of the water will not allow the water to flow into these very small volumes. When the jar is squeezed, the cracks open up sufficiently to allow the water to flow into them, thus causing the water in the tube to fall.
A second jar contains sand with alcohol. Because alcohol has a much smaller surface tension than water, the alcohol will flow into the small cracks between sand grains, filling the space. When the alcohol bottle is squeezed, the volume is already filled with sand and liquid, so the alcohol level in the tube must rise.
F3-22: SURFACE TENSION - AT THE BEACH

F3-22

Illustrate the surface tension of water.

The bowl photographed above contains wet sand. When tapped on its side, the grains of sand settle with their sides so close that water cannot flow into the cracks due to its surface tension. The surface of the sand looks wet. When you push down on the sand with a foot, as though walking on the beach, the spacing between sand grains opens up sufficiently that water can flow down into the space. The sand in the vicinity of the foot appears dry. This effect causes the sand to appear lighter in color by your footprints at the beach. This effect is known as "dilatancy."
F3-31: WATER BELL

F3-3

Demonstrate surface tension in an artistic manner.

A stream of water hits a circular horizontal surface and projects out radially. Surface tension in the water pulls the water together, creating a bell or heart-shaped water surface as shown in the photograph.

F3
G4-11: SOAP FILM OSCILLATIONS

G4-11

Demonstrate standing waves in a two-dimensional medium.

A large rectangular frame or a circular frame (from M4) are provided for producing soap films using a specially-formulated soap solution. Careful movement allows production of various standing waves.

Please specify in comments when ordering if you want the circular form, square form, or both.

OS2, M4
G4-12: STANDING WAVES ON A SOAP FILM

G4-12

Demonstrate standing waves in a circular membrane.

A circular wire loop (from M4) holds a soap film which is positioned in front of a loudspeaker (the rectangular box to the left of the soap film). It is illuminated by a bright point source which casts a reflected pattern on the screen as shown in the photograph. A sine-wave oscillator attached to the speaker can be tuned to obtain various standing wave patterns, which are defined by the bright lines in the reflected light. Some standing wave patterns are shown above.

M4, ME3, LS1