Angling the wetting line retards air entrainment in premetered coating flows

The dynamic wetting line not perpendicular to the substrates motions was studied as an effective way of delaying air entrainment in coating flows. Slide coating was considered because flow does not give a significant hydrodynamic assist of the wetting line and can show the effect of angling the wetting line was kinematic. Four mineral oils were used as Newtonian liquids and flow rates were measured. When speed was increased until a low-flow limit of coatability was achieved, bead instability led to the appearance of dry patches on the substrate. When the phenomenon was observed, flow rate was increased until dynamic wetting line was attained corresponding to the onset of air entrainment.; Air entrainment is a major limitation to many coating processes where one or several thin layers of liquid are deposited onto a continuous substrate. At low substrate speeds, a uniform film is formed however, as the speed is increased above a certain threshold, air is entrained between the coating and the substrate and spoils the quality of the coating. The study of air entrainment in coating flows has largely been based on dip coating experiments where a smooth flat substrate is plunged into a large pool of stagnant liquid (for a review, see Burley, 1992). The investigations pointed out for the major role of the fluid properties in determining the air entrainment velocity. Recently, experiments were carried out by the present authors (Cohu and Benkreira, 1998) in an original dip coating arrangment (labeled 'angled dip coating') where the wetting line was not perpendicular to the substrate speed. It was established experimentally that air entrainment in dip coating begins only when the component of the susbtrate speed normal to the wetting line exceeds a critical velocity which depends only on the properties of the liquid used. In other words, having the angle formed by the wetting line and the substrate velocity vector differ from 90° by an angle β increased the air entrainment velocity by a factor 1/cosβ. This result had been anticipated earlier by Blake and Ruschak (1979) who, however, did not show conclusive experimental data. In this article, the application of the same principle to another class of coating flows, referred to as premetered coating procsses, is investigated for the first time.