Soap Bubbles

A soap or detergent molecule consists of a long slender nonpolar hydrocarbon chain with a highly polar oxygen-rich group attached to one end.  When such molecules are added to water, they tend to migrate to the surface and orient themselves so that the nonpolar ends are sticking out.

    The surface of the water is covered with a non-polar layer which drastically reduces the surface tension and adds stabilizing elastic properties to the liquid surface along with an increase in surface area.  When a wire or plastic frame is placed into the solution and then withdrawn, the water tends to drain from the inside of the raised surface, making the surface begin to collapse on itself forming a multilayered film.

    The soap limits the minimum thickness of the soap film to the length of two soap molecules stacked end to end.  Moderately thin films, such as these, are self-healing with respect to small punctures.

    In the case of a bubble, the water drains to the bottom of the bubble producing a small bulb on the bottom.  One can observe water dripping from the bottom of a large bubble.  When the top of the bubble becomes too thin to support the total mass of the bubble, it breaks.  The addition of glycerin, or other viscous material, adds strength to a bubble since this material does not drain out of the soap film readily.  The swirling colors observed in the bubble are a result of thin film interference and the changing thickness of the film due to the draining liquid.

Extension:

Water is considered a polar molecule.  Oxygen has a negative charge.  Hydrogen has a positive charge.  The hydrogen of one water molecule is attracted to the oxygen of another molecule.  Because of this attraction, water molecules will bond with other water molecules to take up the least amount of space in a given area.

Cohesion is a force of attraction between molecules within a liquid.  Adhesion is an attractive force between liquids and solids.  One example of adhesion is a meniscus, or the curved upper surface of water that adheres to the glass when the water is placed in a small diameter cylinder.  Another example occurs when droplets of water remain on a glass rod that is lifted out of the water.  In this case, adhesion of water molecules to the glass rod is greater than the cohesion of the water molecules to each other.  If the glass rod was dipped into mercury, no mercury droplets would adhere to the rod because mercury has a greater cohesion to itself than to the glass.

Surface tension is the tendency of the surface of a liquid to contract to its smallest area.  It is the result of cohesive forces in which molecules, such as water, exert electromagnetic attraction on each other.  Examples of this surface tension are seen in soap bubbles or in the way a water strider bug can skim across the surface of water.  Surface tension keeps it afloat.  The strider is supported by an upward force that is exerted by the surface water molecules.  Once again, the water molecules always arrange themselves in the closest orientation -- in this case, the surface molecules of the water act like a stretched skin.  Properties of surface tension are also illustrated when a rock is thrown into a pool.  When a force (the rock) acts upon the surface film and distorts it, the water's cohesive property exerts an opposite force, restoring the surface to its calm again.  Gravity can act at the surface to pull the surface molecules together.

Surface tension also applies to water droplets.  The cohesive force of the surface water molecules pulls the water in around itself, into the smallest possible area -- in this case, a spherical bead or droplet.  A sphere has the smallest surface area for a given volume.

In liquid films, surface tension creates contraction forces as well.  The soap bubble demonstration done in class demonstrates the surface tension properties of water -- cohesion as demonstrated by the water contracting into the smallest possible area when air is introduced into the film, and adhesion, demonstrated as the string is pulled along by water molecules.

Information by Science Kit and Boreal Laboratories

Page Last Updated: Friday March 02, 2007           Webmaster: Larry Jones                 Pickens County School District