Study on the evaporative flow and deposition pattern depending on a confined geometry and droplet shape

Abstract

Surface-coating technologies are important for various applications, e.g., ink-jet printing, micro-electronic engineering and biological arrays. In this study, we introduce a novel idea to obtain uniform patterns using the evaporation of multi-component spherical and non-spherical asymmetric droplets in a confined geometry. When the multi-component liquid droplet evaporates in the confined area, the evaporated vapors are stagnant around the droplet inside the confined chamber because the evaporated vapor molecules are much heavier then the ambient air. These vapors induce long-continuous flows driven by Marangoni effects opposite to coffee-ring flows during evaporation, which help to obtain uniform deposition. Finally, we show that the coffee-ring is totally suppressed and a uniformly dried pattern is achieved. From PIV experiments, we also find that the confinement effect still works for not only spherical but also non-spherical asymmetric droplets although the non-spherical droplet has non-uniform evaporation rates along the contact line due to the vertex where the singularities have a relatively lower contact angle and higher evaporation rate. The newly proposed coating method is expected to be widely used in quantum dots (QDs) color filter in QD-LED industry field where diverse shapes of QDs array, e.g., triangular, square, hexagonal geometries, can be considered depending on streamlining manufacturing processes, lowering a production cost and applying to flexible displays such as curved, bendable and rollable ones.