Influence of nano-scaled roughness on evaporation patterns of colloidal droplets

Abstract

Evaporation of colloidal droplets often leads to various deposited patterns which are not only interesting but also provide a very simple and useful method to fabricate functional materials. The patterns induced by the evaporation can be tuned via several factors, among which the roughness of the substrate is an important one. However, the effect of nano-scaled roughness is scarcely studied and far from being fully understood. In this work, the evaporation and pattern formation of SiO2 colloid droplets are studied on smooth substrate and nano-rough substrate, respectively. The aim of this work is to clarify how the evaporation dynamics and patterns are influenced by nano-scaled roughness. The roughness of the substrate is analyzed by using a scanning electron microscope and an atomic force microscope, the evaporation process and pattern formation are monitored via an in-situ microscope observation. The obtained deposited patterns are analyzed by using stylus profiling. It is found that the evaporation of droplets is accompanied by an obvious coffee ring effect on smooth substrate and the deposition patterns are bowl-shaped. However, uniform thickness evaporation patterns are obtained through evaporation on rough substrate, moreover, the crack density increases obviously. The analysis shows that nano-roughness is able to inhibit the circumfluence of droplets along the substrate, which greatly weakens the compensation for capillary flow, leading to particles gathering at air-droplet interface and formulating a particle layer. This prevents the coffee ring effect, and eventually results in the formation of evaporation patterns with uniform thickness.