Capillary Sorting of Particles by Dip Coating

TL;DR

This paper introduces a novel method for particle separation based on capillary forces during dip coating, enabling efficient size-based sorting of particles in suspensions.

Contribution

The study demonstrates how capillary forces during dip coating can be exploited for size-selective particle sorting, providing a scalable and robust separation technique.

Findings

Three distinct filtration regimes identified based on capillary number.

Small particles can be selectively entrained while larger ones are retained.

A range of capillary numbers for effective particle separation is estimated.

Abstract

In this letter, we describe the capillary sorting of particles by size based on dip coating. A substrate withdrawn from a liquid bath entrains a coating whose thickness depends on the withdrawal speed and the liquid properties. If the coating material contains particles, they will only be entrained when the viscous force pulling them with the substrate overcomes the opposing capillary force at the deformable meniscus. This force threshold occurs at different liquid thicknesses for particles of different sizes. Here, we show that this difference can be used to separate small particles from a mixed suspension through capillary filtration. In a bidisperse suspension, we observe three distinct filtration regimes. At low capillary numbers, Ca, no particles are entrained in the liquid coating. At high Ca, all particle sizes are entrained. For a range of capillary numbers between these two extremes, only the smallest particles are entrained while the larger ones remain in the reservoir. We explain how this technique can be applied to polydisperse suspension. We also provide an estimate of the range of capillary number to separate particles of given sizes. The combination of this technique with the scalability and robustness of dip coating makes it a promising candidate for high-throughput separation or purification of industrial and biomedical suspensions.