Kevin J. Farrell, Principal Engineer, Computational Simulation & Validation
Water strider image courtesy of TimVickers [Public domain], from Wikimedia Commons
If you walk by a still body of water like a pond or a marsh, you might see little insects walking across the water’s surface. These water striders or water skippers seem to move easily over the water. The reason? Surface tension.
The term itself refers to the thermodynamic work needed to create a unit of interfacial area against the mutual attraction of liquid molecules or of liquid molecules and an adjacent surface.
We know that surface tension is an important contributor to several dimensionless parameters that affect two-phase flow and heat transfer: the Bond or Eötvös number, the Capillary number, the Marangoni number, the Morton number, the Ohnesorge number, and the Weber number. The magnitude of surface tension depends on the particular pair of substances (liquid and gas or two immiscible liquids) in contact, the temperature, and the presence of any active agents (surfactants) or impurities on a surface, even at very low concentrations.
Different fluids have inherent surface tensions. For example, water in contact with air has a surface tension of 73 mN/m at 20 °C. Most organic liquids have surface tensions ranging from 20 to 30 mN/m, but mercury has the highest at over 480 mN/m. Surface tension increases with temperature but can be reduced by adding organic solutes like soap and detergents to the liquid. These surfactants are used in catalysis, aerosols, wetting agents, emulsifiers, foaming agents, and dispersants.
Forces of cohesion bind the molecules of a liquid to one another, while forces of adhesion exist between the molecules of a liquid and a solid boundary surface. If the adhesion forces exceed the cohesion forces, the liquid is said to wet the surface. Wetting is an interesting attribute of many nanomaterials like graphene. The relative magnitude of these forces affects the contact angle, as shown in Table 1.
| Contact angle | |||||
| Wetting | Perfect nonwetting | Low | High | Perfect wetting | |
| Adhesion | Weak | Weak | Strong | Weak | Strong |
| Cohesion | Strong | Strong | Weak | ||
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The contact angle subtended in the liquid between a liquid and a solid depends on the surface tension, the surface condition, and the purity of the fluid. For water, a wettable surface is termed hydrophilic and a nonwettable surface hydrophobic. Hydrophobic surfaces can facilitate dropwise condensation, with extremely high values of the heat transfer coefficient.
Surface tension causes the interfacial area to behave as a membrane stretched over the fluid mass. Small droplets of mercury form spheres when placed on a smooth surface, and discrete gas bubbles form spheres in a liquid. Surface tension is what allows a cup to be filled slightly above its rim. In multicomponent liquids, surface tension effects may even produce irregular surfaces, like wine tears in which droplets of alcohol and water are drawn up the side of a glass and fall back to the liquid surface.
Surface tension is responsible for a lot of environmental “magic”. Try floating a metal needle in a bowl of water. I wonder how that works?