Capillary Sorting of Particles by Dip Coating

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 A substrate withdrawn from a liquid bath entrains a coating whose thickness depends on the withdrawal speed and the liquid properties.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.

Entrainment of particles during the withdrawal of a fibre from a dilute suspension

“…We can, therefore, predict whether particles will be entrained in the coating film depending on the Goucher number Go = R/ c and the capillary number Ca. Unlike the 2-D situation with a plate, in which only the withdrawal rate affects the coating thickness and particle entrainment, the fibre radius provides a new level of control and an additional design criterion for dip-coating applications, in particular for size-based particle sorting (Dincau et al 2019).…”

“…Unlike the 2-D situation with a plate, in which only the withdrawal rate affects the coating thickness and particle entrainment, the fibre radius provides a new level of control and an additional design criterion for dip-coating applications, in particular for size-based particle sorting (Dincau et al. 2019).…”

“…Besides, dip-coating can also be used for the filtration and sorting of particles (Dincau et al. 2019). In some situations, the particle size becomes comparable to the thickness of the liquid film, and interfacial effects occur.…”

“…The existence of the different regimes have led to a new method of capillary filtration (Dincau et al. 2019). Yet, this filtration is limited by the threshold for entrainment, which can only be controlled through the capillary length and the capillary number for planar substrate, thus limiting the range of particle sizes that can be filtered.…”

Capillary filtering of particles during dip coating

“…The capillary filtering mechanism described here should apply to a wide range of quasispherical particles, including biological microorganisms. Besides, the threshold dependence on the particle size could allow the sorting of particle by size [52].…”