Tailoring the Coffee Ring Effect by Chemically Active Janus Colloids

Abstract

The coffee ring effect (CRE), which results from the evaporation of sessile droplets containing colloids, is critical to processes such as inkjet printing, microfabrication, and biomedical diagnostics. This study investigates the CRE dynamics of droplets containing a mixture of passive and chemically active Janus particles that exhibit self-propelled activity via self-diffusiophoresis/self-electrophoresis. Our experiments demonstrate that depending on the fraction of active colloids, their activity influences the capillary flow and interfacial interactions, leading to significant alterations in both the peripheral growth and the central accumulation of colloids. Droplets containing solely active particles produce loose peripheral networks and a “coffee-eye” pattern in the center. The scaling of growth fluctuations reveals a shift from diffusion-like aggregation to the Kardar–Parisi–Zhang (KPZ) universality class. This work enhances our understanding of the CRE phenomenon and opens avenues for advancing fundamental knowledge in colloidal science and expanding the applications of particle deposition techniques.