Harnessing Reversible Wetting Transition to Sweep Contaminated Superhydrophobic Surfaces

Abstract

Sweeping deposited particles is absolutely essential in order to maintain the excellent functionality of superhydrophobic surfaces. Many methods have been proposed to sweep microparticles deposited on tips of micro/nanostructures. However, how to sweep nanoparticles trapped in cavities of superhydrophobic surfaces has remained an outstanding issue. Here, we show that harnessing the reversible wetting transition provides a feasible way to sweep such nanoparticles. Using molecular dynamics simulations, we demonstrate that the electrically induced CB–W wetting transition makes liquid intrude into a groove and wet a trapped hydrophilic nanoparticle; however, once the electric field is removed, a spontaneous W–CB dewetting transition happens, and the extruded liquid transports the hydrophilic nanoparticle to the groove top, successfully picking up the trapped hydrophilic nanoparticle. We further find that the adhesion between the nanoparticle and groove bottom wall hinders the successful pickup, and picking up such a nanoparticle requires a stronger particle hydrophilicity. With the introduction of amphiphilic Janus particles into a liquid, we exhibit that the electrically induced reversible wetting transition can also successfully pick up a trapped hydrophobic nanoparticle. By means of calculations of the potential of mean force (PMF), we reveal pathways of both the CB–W wetting transition and the W–CB dewetting transition and hence answer why and how a hydrophilic or a hydrophobic nanoparticle is picked up successfully.

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

“…This result demonstrates that the Cassieto-Wenzel wetting transition of the large nanodroplet follows a classical energy pathway, similar to previous reports. 25,26,42 Two energy barriers are observed, and the dewetting energy barrier, βΔE i,2 , is much larger than the wetting energy barrier, βΔE i,1 , and hence, the nanodroplet would remain in the Wenzel state even if the electric field is removed. In other words, the spontaneous dewetting transition cannot take place.…”

“…These authors observed a sharp reduction in the apparent contact angle when the electric voltage exceeded 22 V, converting a rolling droplet into an immobile one, and hence, they judged that the Cassie-to-Wenzel wetting transition was triggered. After that, the electric-field-induced wetting transition has been extensively explored using MD simulations. − Through MD simulations, the critical electric field, E c , for triggering the Cassie-to-Wenzel wetting transition was found to depend on topology structures and intrinsic wettability of textured surfaces. Yuan and Zhao presented that the critical electric voltage increases with an increase in the roughness factor, r , or the solid surface fraction, φ s .…”

“…When an electric field is imposed on a droplet, the Cassie droplet temporarily collapses into the impaled Wenzel state; the droplet can return to the Cassie state when removing the electric field, without any additional energy input to the droplet. Using the potential of mean force (PMF) method, Zhang et al 26 revealed that the Wenzel-to-Cassie dewetting transition takes place only when the Wenzel state has higher free energy and the transition follows a nonclassical barrier-free pathway; in other words, the dewetting energy pathway differs from the wetting energy pathway.…”

A Comprehensive Review of Wetting Transition Mechanism on the Surfaces of Microstructures from Theory and Testing Methods

“…There are numerous methods that can be used to simulate the droplet states on superhydrophobic surface. From the microscale state, molecular dynamics (MD) study the flow behavior of statistical fluid from the perspective of molecular atoms to explore the wetting state transition of droplets at molecular scale [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ]. The simulation dynamics are sometimes given by the Monte Carlo method.…”

Molecular Dynamics Simulation on the Wettability of Nanoscale Wrinkles: High Water Adhesion of Rose Petals

“…The piston, 0.7134 nm in height, comprised four layers of carbon atoms. A water film (5937 molecules) with a thickness of 3.5 nm, which serves as the presentation of macroscopic droplets at the nanoscale, − was placed over the surface (Figure ). A periodic boundary condition was used to describe all directions of the simulation box.…”