Effects of gravity-driven drainage on particle filtration during dip coating

When a solid substrate is withdrawn from a liquid bath at velocity V, a coating of thickness h is deposited according to the classical Landau Levich Derjaguin model. For a particle-laden suspension, it has been recently demonstrated by Sauret et al. (2019, 2020) that particles whose size is smaller than the resulting film thickness can be filtered. This mechanism  is both tunable and relevant to numerous industrial and medical applications.  Here, we perform an experimental study of particle filtration during dip coating using a large range of particle sizes and fluid viscosities, such that our flow is in the gravity-driven drainage regime. Low volume fraction suspensions are utilized in order to minimize changes to the rheology of the fluid and clumping of particles. The filter is tuned by altering the withdraw velocity and data is collected on the thickness of the coating using gravimetric methods while particle entrainment is measured though imaging. Our extensive data set allows us to understand this soft particle filtering process over a wide range of flow conditions.