Enhanced Coffee-Ring Effect via Substrate Roughness in Evaporation of Colloidal Droplets

Abstract

The analysis of dried drop patterns has various applications in research fields like archeology, medical practice, printing, and so on. In this paper, we studied the evaporation and pattern formation of polytetrafluoroethylene (PTFE) colloid droplets on smooth substrate and rough substrates with different roughness. We found that the evaporation of droplets shows remarkable coffee-ring effect on smooth substrate and that the cross-section of the ring is wedge-shaped with its thickness decreasing from the edge to the center. However, with increasing roughness, the effect strengthened, with the section of the coffee-ring changing from wedge- to hill-shaped. The contact angle decreased with increasing roughness, leading to an increase in evaporation rate. Moreover, wicking led to additional evaporation, which also enhanced capillary flow, moving more particles to the edge. In addition, the rough structure of the substrate inhibited the back-flow of the capillary flow, preventing the particles’ move to the center. The formation of radial wrinkles on the edge also led to particle retention, preventing them from moving to the center. All these factors contribute to the decreased width and increased height of the coffee-ring pattern after evaporation on rough surfaces. It is an effective method to regulate the deposition pattern of evaporating droplet by changing the substrate roughness.