Capillary filtering of particles during dip coating

TL;DR

This paper investigates how capillary forces during dip coating can prevent particles from contaminating substrates, revealing regimes of particle entrainment and demonstrating a method for contamination control.

Contribution

It introduces a model system showing how capillary forces can filter particles during dip coating, preventing substrate contamination in polluted baths.

Findings

Particles can be trapped in the meniscus, preventing contamination.

Different entrainment regimes depend on withdrawal velocity.

Capillary filtration effectively blocks particle transfer at low velocities.

Abstract

An object withdrawn from a liquid bath is coated with a thin layer of liquid. Along with the liquid, impurities such as particles present in the bath can be transferred to the withdrawn substrate. Entrained particles locally modify the thickness of the film, hence altering the quality and properties of the coating. In this study, we show that it is possible to entrain the liquid alone and avoid contamination of the substrate, at sufficiently low withdrawal velocity in diluted suspensions. Using a model system consisting of a plate exiting a liquid bath, we observe that particles can remain trapped in the meniscus which exerts a resistive capillary force to the entrainment. We characterize different entrainment regimes as the withdrawal velocity increases: from a pure liquid film, to a liquid film containing clusters of particles, and eventually individual particles. This capillary filtration is an effective barrier against the contamination of substrates withdrawn from a polluted bath and finds application against biocontamination.