Surface Tension and Bubbles

The surface tension of water provides the necessary wall tension for the formation of bubbles with water. The tendency to minimize that wall tension pulls the bubbles into spherical shapes (LaPlace's law).

The interference colors indicate that the thickness of the soap film is on the order of a few wavelengths of visible light. Even though the soap film has less surface tension than pure water, which would pull itself into tiny droplets, it is nevertheless strong to be able to maintain the bubble with such a small thickness.

The pressure difference between the inside and outside of a bubble depends upon the surface tension and the radius of the bubble. The relationship can be obtained by visualizing the bubble as two hemispheres and noting that the internal pressure which tends to push the hemispheres apart is counteracted by the surface tension acting around the cirumference of the circle.

For a bubble with two surfaces providing tension, the pressure relationship is:

Derive the relationshipSoap bubbles
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Bubble Pressure

The net upward force on the top hemisphere of the bubble is just the pressure difference times the area of the equatorial circle:

The force of the surface tension downward on the entire circumference of the circle is twice the surface tension times the circumference, since two surfaces contribute to the force:

This gives

This latter case also applies to the case of a bubble surrounded by a liquid, such as the case of the alveoli of the lungs.

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Surface Tension and Droplets

Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. The spherical shape minimizes then necessary "wall tension" of the surface layer according to LaPlace's law. At left is a single early morning dewdrop in an emerging dogwood blossom.

Surface tension and adhesion determine the shape of this drop on a twig. It dropped a short time later, and took a more nearly spherical shape as it fell. Falling drops take a variety of shapes due to oscillation and the effects of air friction.

A water droplet can act as lens and form an image as a simple magnifier.

The relatively high surface tension of water accounts for the ease with which it can be nebulized, or placed into aerosol form. Low surface tension liquids tend to evaporate quickly and are difficult to keep in an aerosol form. All liquids display surface tension to some degree. The surface tension of liquid lead is utilized to advantage in the manufacture of various sizes of lead shot. Molten lead is poured through a screen of the desired mesh size at the top of a tower. The surface tension pulls the lead into spherical balls, and it solidifies in that form before it reaches the bottom of the tower.

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Capillary Action

Capillary action is the result of adhesion and surface tension. Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward. The surface tension acts to hold the surface intact, so instead of just the edges moving upward, the whole liquid surface is dragged upward.


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Capillary Action

Capillary action occurs when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules. The height to which capillary action will take water in a uniform circular tube is limited by surface tension. Acting around the circumference, the upward force is

The height h to which capillary action will lift water depends upon the weight of water which the surface tension will lift:

The height to which the liquid can be lifted is given by

Show calculation
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Capillary Action Calculation

The height h to which Capillary action will lift water depends upon the weight of water which the surface tension will lift:

The height to which the liquid can be lifted is given by

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