Order-to-disorder transition in ring-shaped colloidal stains

TL;DR

This study investigates the transition from ordered to disordered structures in ring-shaped colloidal stains formed by evaporating droplets, revealing that flow dynamics at evaporation end dictate particle arrangement.

Contribution

It uncovers the mechanism behind the order-to-disorder transition in colloidal stains, linking it to flow velocity singularities during evaporation.

Findings

Ordered crystals form at low deposition speeds.

Disordered packings occur at high flow velocities.

Transition is driven by flow velocity singularity at evaporation end.

Abstract

A colloidal dispersion droplet evaporating from a surface, such as a drying coffee drop, leaves a distinct ring-shaped stain. Although this mechanism is frequently used for particle self-assembly, the conditions for crystallization have remained unclear. Our experiments with monodisperse colloidal particles reveal a structural transition in the stain, from ordered crystals to disordered packings. We show that this sharp transition originates from a temporal singularity of the flow velocity inside the evaporating droplet at the end of its life. When the deposition speed is low, particles have time to arrange by Brownian motion, while at the end, high-speed particles are jammed into a disordered phase.