Mass transfer during drying of colloidal film beneath a patterned mask that contains a hexagonal array of holes

TL;DR

This study models the drying process of a colloidal droplet beneath a patterned mask, revealing particle accumulation patterns influenced by evaporation, advection, diffusion, and sedimentation.

Contribution

It introduces a detailed simulation of colloidal drying under a patterned mask considering multiple transport mechanisms, which was not previously modeled in this context.

Findings

Particles accumulate beneath mask holes during drying

Diffusion reduces particle accumulation

Simulation highlights the interplay of advection, diffusion, and sedimentation

Abstract

We simulated an experiment in which a thin colloidal sessile droplet is allowed to dry out on a horizontal hydrophilic surface when a mask just above the droplet predominantly allows evaporation from the droplet free surface directly beneath the holes in the mask [Harris D J, Hu H, Conrad J C and Lewis J A 2007 \textit{Phys. Rev. Lett.} \textbf{98} 148301]. We considered one particular case when centre-to-centre spacing between the holes is much less than the drop diameter. In our model, advection, diffusion, and sedimentation were taken into account. FlexPDE was utilized to solve an advection-diffusion equation using the finite element method. The simulation demonstrated that the colloidal particles accumulate below the holes as the solvent evaporates. Diffusion can reduce this accumulation.