Confinement Effect on Dip-Coating of Yield-Stress Fluids*

Dip-coating is a widely-used industrial process in which surfaces of solid objects are covered by a thin layer of liquid. The thickness of the coating generally depends on both fluid and geometrical properties and its mastery is of major interest. Albeit the problem is well-studied for simple liquids, it remains largely elusive for yield-stress liquids—in spite of the industrial relevance. We study the effect of geometry by withdrawing rods from cylindrically-shaped baths filled with a Carbopol gel (yield stress 4—130 Pa). The coating thickness is observed to strongly depend on the geometrical confinement. The thickness decreases with decreasing separation between the rod and the sides of the bath. This confinement effect is stronger for high yield stress. A smaller rod is observed to result in thinner coatings. We will discuss and interpret these results.