Mechanism for Excessive Air Entrainment in Coating Systems, 1993 Coating Conference Proceedings

One of the objects of our study is to determine the sequence of events leading to air entrainment. It is well known that, prior to air entrainment, the dynamic contact line destabilizes and forms sawtooth structures at a critical speed. Subsequently, two fundamentally distinct mechanisms can be foreseen to result in air entrainment. Further increase in speed could either force a stream of air through the tip of the V-shaped structures into the fluid, forming air bubbles in a periodic manner much like the air bubbles in a fish tank, or, as proposed by Scriven (1982 AIChE National Meeting), the air entrainment mechanism could involve the collapse of the thin air pockets and formation of bubbles.

Visualization experiments with a relatively dry and smooth surface plunging into a pool of liquid provide further insight into the mechanism. We have shown that with relatively smooth surfaces, the air pockets do indeed collapse and form air bubbles in a highly dynamic manner. As the air pockets, or V-shaped structures, form and elongate, the liquid surface destabilizes forming waves at the sides, and subsequently, nucleation sites form and the fluid contacts the solid surface. The surface tension forces rapidly expand the wetting region until a patch of air in the pocket becomes isolated and enters the liquid. Once surrounded by liquid, the isolated air patch rapidly forms a bubble. The sequence of events leading to air entrainment and the physics of the process are presented with the aid of long-wave stability analysis.