Misting of Newtonian Liquids in Forward Roll Coating

Abstract

Misting of Newtonian liquids, in the film-split region of two counter rotating rigid and deformable cylinders was visualized with standard and high-speed cameras. Flow instabilities begin with ribbing and eventually lead to generation of air-borne droplets called mist. As speed was raised, a uniform film thickness evolved into one with ribs, which evolved into continuous sheets of liquid extending downstream of the gap between the cylinders. The edge of each sheet formed a rim whose two ends were attached to the ribs on the cylinders. Still images via high-speed photography revealed a new mechanism for mist generation: the sheets extended downstream, became unstable and ruptured to produce air-borne droplets. A droplet time-of-flight measurement technique quantified the effect of process settings (speed and speed ratio) and material properties (viscosity and surface tension) on droplet size, count, and mass concentration of mist.