Visualization study on capillary-spreading behavior of liquid droplet in vertically aligned carbon nanotube array

Vertically aligned carbon nanotube (CNT) arrays exhibit distinct wetting characteristics. Thus, they are a promising solution for many applications such as heat-pipe wicks. In this study, the dynamic behavior of liquid-droplet spreading via capillary action in a vertically aligned CNT array has been investigated experimentally and theoretically. Two groups of CNT array with heights of 150 μm and 250 μm were tested. Two organic solvents, namely ethanol and acetone, were selected as the wetting liquids because of their good wettability with the as-grown CNTs. A visualization system equipped with a high-speed camera was used to record the capillary-spreading process. The results of the CNT array with different array heights showed two distinct modes of spreading: a steady uniform mode and an unsteady random mode. In the unsteady mode, the liquid predominantly spreads above the array. In the steady mode, the spreading process can be divided into three phases based on the different dominating effects. After the first flooding phase, the spreading in the second phase was found to follow the Lucas-Washburn model, whereas the third phase can be described using a constant evaporation-rate model. The self-assembly behavior was observed using an SEM after the wetting experiments. Honeycomb-shaped microstructures were formed spontaneously in the precursor-film region, whereas striped lines were observed on the front end of the contact line.