IntroductionDynamic wetting is central to many physical processes. An ambient fluid, often air, in contact with a solid is displaced by a liquid. At sufficiently high displacement speeds, wetting failure occurs, and the ambient fluid is entrained between the liquid and the solid. Usually dynamic wetting failure has undesirable consequences, and in the case of coating operations, for example, air entrainment limits processing speed.Dynamic wetting has been most frequently studied by plunging a solid into a relatively large, stagnant pool of liquid (for example, Perry, 1967;Inverarity, 1969;Burley and Kennedy, 1976; Gutot'f and Kendrick, 1982;Burley and Jolly, 1984;Blake, 1988; Brache et al., 1989;Seebergh and Berg, 1992). The angle at which the interface intersects the solid, as viewed through a low-power microscope or inferred from measurement of meniscus shape or force exerted on the solid, is determined as a function of speed. At low magnification, the interface appears to intersect the solid as a line, and the angle measured through the liquid is termed the apparent or macroscopic dynamic contact angle.At the Ion est speeds, the contact angle approaches the static advancing contact angle. The contact angle increases as speed Correspondma corlcerning this article should be addressed to K. J. Ruschak.is increased, with the wetting line remaining straight and horizontal (that is, normal to the velocity of the solid). However, when the angle reaches a nominal value of 180", wetting failure occurs, and a thin film of air, on the order of a micron in thickness (Perry, 1967), forms between the liquid and solid. The wetting line moves downward, becomes unsteady, and breaks up into straight-line segments that are inclined from the horizontal. The angle of inclination increases steadily with speed. At any instant, the wetting line has the appearance of sawteeth (Burley and Kennedy, 1976;Blake and Ruschak, 1979;Burley, 1992), and air bubbles may be formed at the downstream vertices of the sawteeth and carried into the liquid. The speed at which wetting failure begins varies inversely with viscosity raised to a power of about 0.7. Speed can also vary about a factor of 5 due to the chemical and physical properties of thesolid (Buonopaneet al., 1984;Blake and Ruschak, 1994).The case where only one sawtooth, having one vertex, forms is rare but particularly instructive. The wetting line then consists of two slanted and steady straight-line segments in the shape of a "V." Blake and Ruschak (1979) showed that the component of the speed of the solid normal to the segments is a constant that they termed the maximum speed of wetting. Initially, no air bubbles are produced, but as speed and the
AIChE JournalFebruary 1994 Vol. 40, No. 2 229 inclination angle increase, air bubbles begin to form at the vertex. At still higher speed, an air tube may form and persist for a significant distance before breaking up. The vertex may be several centimeters downstream of the level of the pool, and over this distance the liquid is separated fr...