Colloidal Shape Effects in Evaporating Drops

TL;DR

This paper investigates how particle shape, confinement, and surfactants influence the deposition patterns of evaporating drops, revealing new mechanisms for uniform and heterogeneous particle deposition.

Contribution

It introduces novel insights into the effects of particle anisotropy, confinement, and surfactant-induced flows on evaporation-driven deposition patterns.

Findings

Ellipsoids deposit uniformly, spheres form rings.

Increased particle anisotropy increases CMM bending rigidity.

Surfactants induce Marangoni flows affecting particle deposition.

Abstract

We explore the influence of particle shape on the behavior of evaporating drops. A first set of experiments discovered that particle shape modifies particle deposition after drying. For sessile drops, spheres are deposited in a ring-like stain, while ellipsoids are deposited uniformly. Experiments elucidate the kinetics of ellipsoids and spheres at the drop's edge. A second set of experiments examined evaporating drops confined between glass plates. In this case, colloidal particles coat the ribbon-like air-water interface, forming colloidal monolayer membranes (CMMs). As particle anisotropy increases, CMM bending rigidity was found to increase, which in turn introduces a new mechanism that produces a uniform deposition of ellipsoids and a heterogeneous deposition of spheres after drying. A final set of experiments investigates the effect of surfactants in evaporating drops. The radially outward flow that pushes particles to the drop's edge also pushes surfactants to the drop's edge, which leads to a radially inward flow on the drop surface. The presence of radially outward flows in the bulk fluid and radially inward flows at the drop surface creates a Marangoni eddy, among other effects, which also modifies deposition after drying.