Control of Stratification in Drying Particle Suspensions via Temperature Gradients

TL;DR

This study uses molecular dynamics simulations to explore how temperature gradients influence particle stratification during drying, revealing that thermal control can switch stratification from small-on-top to large-on-top.

Contribution

It demonstrates how thermal gradients, created by selective thermalization, can control particle stratification in drying suspensions, offering new strategies for material design.

Findings

Constant temperature drying yields small-on-top stratification.

Negative temperature gradients promote large-on-top stratification.

Positive thermal gradients can produce strong small-on-top stratification.

Abstract

A potential strategy for controlling stratification in a drying suspension of bidisperse particles is studied using molecular dynamics simulations. When the suspension is maintained at a constant temperature during fast drying, it can exhibit "small-on-top" stratification with an accumulation (depletion) of smaller (larger) particles in the top region of the drying film, consistent with the prediction of current theories based on diffusiophoresis. However, when only the region near the substrate is thermalized at a constant temperature, a negative temperature gradient develops in the suspension because of evaporative cooling at the liquid-vapor interface. Since the associated thermophoresis is stronger for larger nanoparticles, a higher fraction of larger nanoparticles migrate to the top of the drying film at fast evaporation rates. As a result, stratification is converted to "large-on-top". Very strong "small-on-top" stratification can be produced with a positive thermal gradient in the drying suspension. Here we explore a way to produce a positive thermal gradient by thermalizing the vapor at a temperature higher than that of the solvent. Possible experimental approaches to realize various thermal gradients in a suspension undergoing solvent evaporation, and thus to produce different stratification states in the drying film, are suggested.