Effects of Nanodroplet Sizes on Wettability, Electrowetting Transition, and Spontaneous Dewetting Transition on Nanopillar-Arrayed Surfaces

Abstract

In this study, the wetting and dewetting behaviors of water nanodroplets containing various molecule numbers on nanopillar-arrayed surfaces in the presence or absence of an external electric field are investigated via molecular dynamics (MD) simulations, aiming to examine whether there is a scale effect. The results show that, in the absence of an electric field, nanodroplets on coexisting Cassie/Wenzel surfaces may be in the Cassie or the Wenzel state depending on their initial states, and apparent contact angles of the Cassie or Wenzel nanodroplets increase monotonously with increasing the droplet size. Energy analysis shows that on the same coexisting Cassie/Wenzel surface, when an electric field is imposed, a small nanodroplet possesses a lower energy barrier separating the Cassie state from the Wenzel state. Therefore, the small nanodroplet is easier to collapse into the Wenzel state. Moreover, the spontaneous Wenzel-to-Cassie dewetting transition is not observed for the nanodroplets after the removal of the electric field because the Wenzel state is a globally stable energetic state. With the same pillar geometry, both the wetting transition and the dewetting transition are significantly modified for liquids with higher intrinsic contact angles. The energy barrier of the wetting transition increases for both the large and small nanodroplets, meaning that the Cassie state becomes more robust. The energy curve shows that the Wenzel state of the large nanodroplet has higher energy so that the droplet can return to the Cassie state when removing the electric field. Intriguingly, although the small Wenzel nanodroplet has lower energy in the presence of the electric field, the dewetting transition still occurs. The increased solid−liquid interfacial tension when removing the electric field is responsible for this abnormal result. The wetting and dewetting transitions follow different energy pathways, leading to a hysteresis energy loop. There exists a critical water molecule number separating the unstable/stable Wenzel configurations, above which the Cassie state is energetically favorable and the dewetting transition can occur spontaneously after removing the electric field.

Coalescence of multiple droplets induced by a constant DC electric field

“…The simple point charge/extension (SPC/E) model [28][29][30] is employed to investigate the dynamic coalescence process induced by an electric field, owing to its suitable capture of various liquid water properties in MD simulations. The water-water and ion-ion interactions are computed through pairwise potentials as a sum of Lennard-Jones (LJ) and electrostatic…”

Oblique Impacts of Nanodroplets upon Surfaces

“…For analysis, the position and velocity of each particle are extracted every 1 ps. The codes in this work have been validated by previous works. ,,,− …”

Freezing–Melting Mediated Dewetting Transition for Droplets on Superhydrophobic Surfaces with Condensation

“…In general, the change of the SN superhydrophobic surface results in an unstable state for the melting droplet on this changed surface and can make the droplet overcome the barrier to achieve the dewetting transition with the help of surface tension (or the released free energy). This kind of dewetting transition is due to the change of the superhydrophobic surface, which is different from the use of external activations to achieve the dewetting transition, such as surface vibration, 15,16 electric field, 53,54 magnetic field, 23,55 and droplet growth or coalescence. 33,43 All of these studies aim to change the contact state by directly acting on droplets, while our study alters the relationship between droplets and surfaces by acting on superhydrophobic surfaces through a freezing− melting process, which provides a new perspective for achieving the transition of the contact state.…”