Air Force

Controlling the behavior of liquids in microchannels

Modifying the wettability of a surface by changing its geometry

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Mercury drop on nonplanar acetate having an interstice with a 110° included angle: shows complete filling of the crevice. This surface could be designed such that no filling of the crevice would occur.

The contact angle of a liquid on a surface may be used to define to what extent, if any, a liquid will wet or contact a surface. Whenever a liquid contacts a solid surface, several different types of behavior can be exhibited. At one extreme, a drop of liquid contacting a solid surface will spread out until it forms a thin film on the surface, which is called total wetting and in this case, the liquid has a contact angle of zero with the surface. At the other extreme, a drop of liquid will sit on the surface with minimal contact like a marble. This behavior, known as total non-wetting, is defined by a contact angle of 180° with the surface.

Air Force researchers have developed methods to control the degree of solid-liquid contact by selection of the material, angles of contact, and surface characteristics of each. Through proper selection of the contact angles, the surface tension of a liquid can be designed to increase or decrease flow rates, wicking, precipitation, and other characteristics. The degree of wetting of the surface can be further controlled by parameters, such as temperature, radiation, and electromagnetic fields.

This type of surface modification has application in lubrication of sliding surfaces, fuel and catalyst interactions, adherence of coatings, heat transfer, and any other solid-liquid combination with the desired wettability. Other technologies that may be affected by this concept include metallurgy and fabrication of metal-matrix composites because surface tension and contact angles are both high for liquid metals.

Zero gravity and microgravity situations will similarly be influenced because surface tension and wettability become controlling parameters when viscous flow ceases in these environments. Finally, mercury porosimetry is closely associated with this behavior and may benefit from its application. If another liquid having a smaller contact angle, 120°, for example, is employed secondarily after mercury, any difference in data plots of intrusion volume versus pressure should be capable of interpretation by this phenomenon.

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