From Coffee Rings to Self-Driven Assembly: Active Matter Enabled Design of Drying Droplets

TL;DR

This paper reviews how active particles within evaporating droplets influence flow dynamics and particle assembly, advancing understanding beyond passive models and enabling novel interface design strategies.

Contribution

It synthesizes recent findings on active matter in droplets, emphasizing mechanisms like bubble-mediated flows and proposing strategies for controlled deposition.

Findings

Active particles alter flow and assembly in evaporating droplets.

Bubble-mediated flows significantly influence particle distribution.

Strategies for controlled deposition are outlined.

Abstract

Evaporating colloidal droplets have long been used as model systems to understand capillarity, interfacial transport, and particle assembly, most prominently through the coffee ring effect. In classical descriptions, suspended particles are treated as passive tracers carried by evaporation-driven capillary flow, with additional influence from Marangoni stresses, wettability, and contact line pinning. More recent studies, however, show that this picture changes significantly when the particles themselves are active. Systems containing motile microorganisms, chemically active colloids, or externally driven particles can continuously inject energy or generate gradients within the droplet, leading to self-driven flows, modified interfacial stresses, and dynamic contact line behavior. In this Perspective, we bring together these developments, identify the key mechanisms governing active droplets, highlight the role of bubble-mediated flows, and outline strategies for controlled deposition and functional interface design.