By combining a previously developed model of droplet spreading with simple equations to describe liquid penetration, we develop a versatile new model that successfully describes the spreading and imbibition of liquid droplets on porous surfaces. The model is experimentally verified for a range of porous membranes and should be of particular relevance to the interaction of inkjet droplets with porous receivers such as paper.
We apply the molecular kinetic theory of wetting to the relaxation of the dynamic contact angle during the spreading of liquid droplets on solid surfaces. Experimental data have been obtained for several solid−liquid combinations and are successfully fitted using the theory. Physical parameters are extracted and interpreted in terms of the microscopic characteristics of the liquid and the solid. We also show that droplet spreading and forced wetting can be described by the same equations. A comparison is made between the results obtained within the molecular-kinetic framework and those obtained with conventional hydrodynamic theory.
The macroscopic behavior of the contact line during the spontaneous spreading of a droplet can be described in terms of microscopic quantities, specifically the substrate-liquid and liquid-liquid interactions. Here, for the first time, the results of molecular dynamics simulations of very large sessile drop systems are compared with the predictions of the molecular-kinetic theory of wetting. Good agreement is obtained, with both approaches yielding a consistent set of molecular parameters and macroscopic behavior which is consistent with experiments.
Electrostatic assist is widely used in liquid coating processes to promote high coating speeds by postponing the onset of air entrainment. However, very little research has been published on the interaction of electrostatic forces with coating flows. The present work seeks to address this deficiency and to show the way in which an electrostatic field can promote dynamic wetting. A simple theory is developed in which the electrostatic field and the resultant forces are treated independently of the liquid flow. Electrostatic equations are derived and incorporated within the molecular kinetic theory of dynamic wetting. Experimental evidence is presented which demonstrates the utility of this approach.
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...
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