Alternative mechanism for coffee-ring deposition based on active role of free surface

TL;DR

This paper proposes an alternative mechanism for the coffee-ring effect, emphasizing the active role of the free surface in transporting and depositing particles, supported by a new theoretical model that matches observed patterns.

Contribution

It introduces a novel model highlighting the free surface's role in particle deposition, contrasting with traditional flow-driven explanations.

Findings

The model successfully reproduces ring-shaped depositions.

Deposition patterns vary with surface wettability and evaporation mode.

The free surface actively captures and transports particles during drying.

Abstract

When a colloidal sessile droplet dries on a substrate, the particles suspended in it usually deposit in a ring-like pattern. This phenomenon is commonly referred to as the "coffee-ring" effect. One paradigm for why this occurs is as a consequence of the solutes being transported towards the pinned contact line by the flow inside the drop, which is induced by surface evaporation. From this perspective, the role of the liquid-gas interface in shaping the deposition pattern is somewhat minimized. Here, we propose an alternative mechanism for the coffee-ring deposition. It is based on the bulk flow within the drop transporting particles to the interface where they are captured by the receding free surface and subsequently transported along the interface until they are deposited near the contact line. That the interface captures the solutes as the evaporation proceeds is supported by a Lagrangian tracing of particles advected by the flow field within the droplet. We model the interfacial adsorption and transport of particles as a one-dimensional advection-generation process in toroidal coordinates and show that the theory reproduces ring-shaped depositions. Using this model, deposition patterns on both hydrophilic and hydrophobic surfaces are examined in which the evaporation is modeled as being either diffusive or uniform over the surface.