# Cross-interaction drives stratification in drying film of binary colloidal mixtures

## TL;DR

This paper investigates the counterintuitive stratification in drying colloidal films of different-sized particles, revealing that inter-particle interactions drive the formation of layered structures, contrary to expectations based solely on diffusion constants.

## Contribution

It introduces a diffusion model incorporating colloid interactions to explain and predict stratification in drying binary colloidal films, supported by numerical and analytical analysis.

## Key findings

- Interaction-driven stratification explained
- Derived conditions for stratified structure formation
- Validated model with numerical and analytical results

## Abstract

When a liquid film of colloidal solution consisting of particles of different sizes is dried on a substrate, the colloids often stratify, where smaller colloids are laid upon larger colloids. This phenomenon is counter intuitive because larger colloids which have smaller diffusion constant are expected to remain near the surface during the drying process, leaving the layer of larger colloids on top of smaller colloids. Here we show that the phenomenon is caused by the interaction between the colloids, and can be explained by the diffusion model which accounts for the interaction between the colloids. By studying the evolution equation both numerically and analytically, we derive the condition at which the stratified structures are obtained.