Dip coating of viscous granular suspensions

Abstract

A solid substrate withdrawn from a liquid bath is coated with a uniform thickness given by the Landau–Levich–Derjaguin (LLD) law. If the bath is a suspension of neutrally buoyant particles, the liquid coating can entrain particles in a process known as capillary filtration that only depends on the capillary number Ca. Experiments were performed using a custom-built dip coating apparatus and gravimetry techniques. The working liquids were monodisperse and bidisperse suspensions with larger particle diameters and higher liquid viscosity than previously reported. The single particle entrainment point for monodisperse suspensions agrees well with the critical capillary number predicted in prior literature. Bidisperse suspensions exhibit active filtration regions where only the smallest particles are entrained with the range of Ca for active filtration consistent with reported models. These experimental results both validate and enhance our understanding of capillary filtration by exploring larger particle-sized granular suspensions of higher viscosity solutions, as relevant to applications in construction, medical care, personal care products, and food science.