Measurement of Contact-Angle Hysteresis for Droplets on Nanopillared Surface and in the Cassie and Wenzel States: A Molecular Dynamics Simulation Study

Abstract

We perform large-scale molecular dynamics simulations to measure the contact-angle hysteresis for a nanodroplet of water placed on a nanopillared surface. The water droplet can be in either the Cassie state (droplet being on top of the nanopillared surface) or the Wenzel state (droplet being in contact with the bottom of nanopillar grooves). To measure the contact-angle hysteresis in a quantitative fashion, the molecular dynamics simulation is designed such that the number of water molecules in the droplets can be systematically varied, but the number of base nanopillars that are in direct contact with the droplets is fixed. We find that the contact-angle hysteresis for the droplet in the Cassie state is weaker than that in the Wenzel state. This conclusion is consistent with the experimental observation. We also test a different definition of the contact-angle hysteresis, which can be extended to estimate hysteresis between the Cassie and Wenzel state. The idea is motivated from the appearance of the hysteresis loop typically seen in computer simulation of the first-order phase transition, which stems from the metastability of a system in different thermodynamic states. Since the initial shape of the droplet can be controlled arbitrarily in the computer simulation, the number of base nanopillars that are in contact with the droplet can be controlled as well. We show that the measured contact-angle hysteresis according to the second definition is indeed very sensitive to the initial shape of the droplet. Nevertheless, the contact-angle hystereses measured based on the conventional and new definition seem converging in the large droplet limit.

Contact angle hysteresis at the nanoscale: a molecular dynamics simulation study

“…It can be observed that the variation of the CA for different droplet sizes is <3°. These results are consistent with MD results of water droplet containing 12,000-100,000 molecules placed on nanopillared surfaces [27]. Thus, we concluded that the line tension effect was limited for the droplet we studied.…”

“…1 Sources (a-e) and some influencing factors (f-g) of the CAH significant. As far as we are aware, there are only a few studies which have reported the investigation of CAH of nanodroplets using molecular dynamics (MD) simulations [26,27]. Hong and coworkers studied the static and dynamic CAs of water droplets on solid surfaces of different surface energies [26].…”

“…Hong and coworkers studied the static and dynamic CAs of water droplets on solid surfaces of different surface energies [26]. Zeng and coworkers carried out MD simulations of CAH for droplets on nanopillared surfaces [27]. However, these MD studies mainly focus on the rigid substrates.…”

Self-cleaning of superhydrophobic surfaces by self-propelled jumping condensate

“…Note that the nanostructures (particularly the conical protuberances in Fig. 1B) also minimize the contact area and therefore adhesive forces between the particle and the superhydrophobic surface (11,13), which is corroborated by the small contact angle hysteresis associated with fine textures (34)(35)(36). As a proof of concept, the jumping-condensate selfcleaning was achieved on nanostructured copper surfaces prepared following ref.…”

Hybrid atomistic-continuum simulation of nucleate boiling with a domain re-decomposition method

“…Koishi et al [11] simulated a large scale transition between Wenzel State and Cassie State on hydrophobic surface. They also investigated the contact angle hysteresis for droplets on nanopillared surface [12] . Vrancken et al…”