Particulate suspension coating of capillary tubes

TL;DR

This study investigates how particle-laden suspensions deposit coatings inside capillary tubes, revealing different regimes and factors influencing film thickness, including particle size, suspension concentration, and shear-induced migration effects.

Contribution

It introduces a comprehensive analysis of coating regimes and predicts film thickness using effective viscosity, while highlighting shear migration effects in concentrated suspensions.

Findings

Three coating regimes identified: liquid only, heterogeneous, thick films.

Film thickness can be predicted by suspension viscosity for larger particles.

Shear-induced migration causes local variations in film composition.

Abstract

The displacement of a suspension of particles by an immiscible fluid in a capillary tube or in a porous media is a canonical configuration that finds application in a large number of natural and industrial applications, including water purification, dispersion of colloids and microplastics, coating and functionalization of tubings. The influence of particles dispersed in the fluid on the interfacial dynamics and on the properties of the liquid film left behind remain poorly understood. Here, we study the deposition of a coating film on the walls of a capillary tube induced by the translation of a suspension plug pushed by air. We identify the different deposition regimes as a function of the translation speed of the plug, the particle size, and the volume fraction of the suspension. The thickness of the coating film is characterized, and we show that similarly to dip coating, three coating regimes, liquid only, heterogeneous, and thick films, are observed. We also show that, at first order, the thickness of films thicker than the particle diameter can be predicted using the effective viscosity of the suspension. Nevertheless, we also report that for large particles and concentrated suspensions, a shear-induced migration mechanism leads to local variations in volume fraction and modifies the deposited film thickness and composition.