Malleable patterns from the evaporation of colloidal liquid bridge: coffee ring to the scallop shell

TL;DR

This study explores how varying the confinement distance in drying colloidal droplets creates diverse patterns, from coffee rings to scallop shell-like structures, driven by evaporation flux and thin film dynamics.

Contribution

It introduces a novel method to control particle deposition patterns by adjusting confinement length, linking it to evaporation flux variations and thin film behavior during drying.

Findings

Smaller confinement lengths produce spoke-like patterns.

Larger confinement lengths lead to pinned contact lines.

Particle concentration significantly influences pattern formation.

Abstract

The present article highlights an approach to generate contrasting patterns from drying droplets in a liquid bridge configuration, different from well-known coffee rings. Reduction of the confinement distance (the gap between the solid surfaces) leads to systematized nano-particle agglomeration yielding to spokes-like patterns similar to those found on scallop shells instead of circumferential edge deposition. Alteration of the confinement length modulates the curvature that entails variations in the evaporation flux across the liquid-vapor interface. Consequently, flow inside different liquid bridges (LBs) varies significantly for different confinement lengths. Small confinement lengths result in the stick-slip motion of squeezed liquid bridges. On the contrary, the stretched LBs exhibit pinned contact lines. We decipher a proposition that a drying liquid thin film present during dewetting near the three-phase contact line is responsible for the aligned deposition of particles. The confinement distance determines the height of this thin film, and its theoretical estimations are validated against the experimental observations using reflection interferometry, further exhibiting good agreement (in order of magnitude). Modulating the particle size does not significantly influence the precipitate patterns; however, particle concentration can substantially affect the deposition patterns. The differences in deposition patterns are attributed to the complex interplay of the gradient of evaporation flux induced motion of contact line in combination with the drying of thin liquid film during dewetting.