ABSTRACT

On macroscopic length scales observable by naked eye, or even on the scale of a few microns accessible with optical microscopes, the most prominent feature of dynamic wetting is the wetting line where the advancing fluid/fluid interface seems to intersect the solid surface at a measurable dynamic contact angle 9/). Figures la-c illustrate the appearance of the dynamic wetting region in forced liquid/air displacement for a liquid/ solid combination with a static contact angle well below 90°. At low speeds, the advancing angle 6/5 remains near its static value (Fig. la). Typically, though, small increments in speed suffice to rapidly increase B£> (Fig. Ib; 0£> is measured through the liquid). At high speeds of the sort encountered in industrial coating processes, contact angles often become less sensitive to speed and typically fall in the range 120° < QD < 170°. Eventually, at a critical speed, the dynamic contact angle approaches 180°. Beyond that limit, visible amounts of air are entrained between the liquid and solid (Fig. Ic). The wetting line may become serrated and unsteady before the onset of visible air entrainment.