Minimum Energy Paths of Wetting Transitions on Grooved Surfaces

Abstract

A method that computes minimum energy paths (MEPs) of wetting transitions is developed. The method couples the Cahn-Hilliard formulation of a modified phase-field method with the simplified string method. Its main computational kernel is the fast Fourier transform that is efficiently performed on graphics processing units. The effectiveness of the proposed method is demonstrated on two types of transitions of droplets on grooved surfaces. The first is the transition from the Cassie-Baxter wetting state to the Wenzel state, where it is shown that it progresses in a sequential manner with the droplet wetting each groove successively. The second transition type is a lateral displacement of the droplet against the grooves, where the droplet successively detaches/attaches from/to the rear/front protrusion of the surface (a transition in the reverse order is also possible). The energy barriers of both the transitions are extracted from the MEP; they are useful for the evaluation of the robustness of superhydrophobic surfaces (resistance to the Cassie-Baxter to Wenzel transition) and the droplet mobility on those surfaces (high mobility/small resistance to lateral displacements). The relation of the MEP with the potential transition paths coming from the solution space mapping is discussed.

Wetting and recovery of nano-patterned surfaces beyond the classical picture

“…In addition, our results support the interpretation that the asymmetric wetting observed from time to time in experiments is due to the deposition of impurities on the textured walls. Finally, in contrast with the quasi-static picture, 28,[31][32][33]36,37,48,50,51 the present results show that dynamic effects are strongly dependent on the liquid pressure.…”

“…Thus, intense experimental and theoretical research has been devoted to the investigation of the wetting and recovery mechanisms. 20,[22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] In their seminal work, Lafuma and Quéré 22 have investigated the variation of the contact angle hysteresis as a function of the pressure of a liquid droplet deposited between two textured hydrophobic surfaces. A large and sudden increase of the contact angle hysteresis is a measure of the transition from the Cassie-Baxter to the Wenzel state.…”

“…Apart from notable exceptions, 46,47 the wetting and recovery of textured surfaces has been studied via quasi-static methods, such as umbrella sampling, 31 restrained molecular dynamics, 27,37,48 string and nudged elastic band methods. 32,33,36,[48][49][50][51] These methods assume that the process is slow and the system is at the local equilibrium all along wetting or recovery. In other words, these methods neglect dynamic effects, such as inertia.…”

The springtail cuticle as a blueprint for omniphobic surfaces

“…103,105 In determining the wetting transition barrier, two complementary concepts have been developed. [178][179][180] The concept of critical pressure is based on Laplace's law that takes the pressure difference, Dp, across a curved fluid interface into account: 11).…”

Wetting transition energy curves for a droplet on a square-post patterned surface

“…Ren [28] computed the transition states, the energy barriers and the minimum energy paths for Cassie-to-Wenzel transition using the string method. Pashos et al [29,30] developed a numerical method to investigate the minimum energy paths and the free energy changes were presented in their work. Prakash et al [31] studied the spontaneous recovery of superhydrophobicity on nanotextured surfaces using molecular simulations.…”