With the support of renowned scientist Lord Rayleigh, her 1891 paper showed that surface contamination significantly reduces surface tension, and also that changing the characteristics of the surface (compressing or expanding it) also affects surface tension. Using homemade materials, she developed an instrument known as a trough for measuring surface contaminants and their effects. In the late 1800s, Agnes Pockels, who was initially blocked from pursuing a scientific career but studied on her own, began investigating the impact and characteristics of soapy and greasy films in water. Surface tension is affected by a variety of variables, including the introduction of additional substances on the surface. Some insects, like the one shown in Figure 10.17, even though they are denser than water, move on its surface because they are supported by the surface tension.įigure 10.17 Surface tension (right) prevents this insect, a “water strider,” from sinking into the water. A steel needle carefully placed on water will float. As a result of this high surface tension, the surface of water represents a relatively “tough skin” that can withstand considerable force without breaking.
Among common liquids, water exhibits a distinctly high surface tension due to strong hydrogen bonding between its molecules. Surface tensions of several liquids are presented in Table 10.3. This property results from the cohesive forces between molecules at the surface of a liquid, and it causes the surface of a liquid to behave like a stretched rubber membrane. Surface tension is defined as the energy required to increase the surface area of a liquid, or the force required to increase the length of a liquid surface by a given amount. (credit photo: modification of work by “OliBac”/Flickr) We can measure viscosity by measuring the rate at which a metal ball falls through a liquid (the ball falls more slowly through a more viscous liquid) or by measuring the rate at which a liquid flows through a narrow tube (more viscous liquids flow more slowly).įigure 10.16 Attractive forces result in a spherical water drop that minimizes surface area cohesive forces hold the sphere together adhesive forces keep the drop attached to the web. Honey, syrup, motor oil, and other liquids that do not flow freely, like those shown in Figure 10.15, have higher viscosities. Water, gasoline, and other liquids that flow freely have a low viscosity. The viscosity of a liquid is a measure of its resistance to flow. But when you pour syrup on pancakes or add oil to a car engine, you note that syrup and motor oil do not flow as readily. When you pour a glass of water, or fill a car with gasoline, you observe that water and gasoline flow freely.
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