Life and death of a fakir droplet: Impalement transitions on superhydrophobic surfaces

Abstract

We show that the equilibrium state of a water drop deposited on a superhydrophobic surface cannot be solely determined by its macroscopic contact angle but also depends on the drop size. Following the evolution of the interface of evaporating droplets, we demonstrate that the liquid can explore a succession of equilibrium conformations which are neither of the usual fakir nor Wenzel types. A comprehensive description of the transition between these wetting states is provided. To do so, we have taken advantage of microfabrication techniques and interference microscopy which allows for the "3D" imaging of the liquid interface. In addition, we propose a simple theoretical description of the interface geometry which goes beyond the standard two-state picture for superhydrophobicity. This model accounts correctly for all our experimental observations. Finally, guided by potential microfluidic applications we propose an efficient design strategy to build robust liquid repellant surfaces.

The collapse transition on superhydrophobic surfaces

“…This was seen in simulations for drops of both cylindrical and spherical symmetry, and has recently been reported experimentally [19]. The depinning occurs to reduce the distortion of the interface from spherical.…”

Wetting Transition Characteristics on Microstructured Hydrophobic Surfaces

“…[16][17][18][19] Moulinet and Bartolo studied the droplet equilibrium state deposited on hydrophobic rough surfaces by using the real-time 3D imaging technique for different droplet sizes. 20) Erbil and Cansoy estimated the droplet penetration between the pillars by suggesting the modified Cassie-Baxter equation based on the geometry of square and cylindrical pillar patterns. 9) Although a number of studies have reported some important physics behind droplet wetting behavior on hydrophobic surfaces on the basis of experiments and modeling, clear understanding of transition wetting phenomena showing penetration of liquid between pillars is required for designing and controlling hydrophobic surfaces effectively.…”

Wetting state on hydrophilic and hydrophobic micro-textured surfaces: Thermodynamic analysis and X-ray visualization

“…Although wetting state has been extensively explored in previous reports, 13,14,16,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] the majority of these previous reports have focused on hydrophobic textured surfaces. Besides the wetting state was inferred from visible-wavelength images, making it difficult to identify the physical behavior at the vicinity of the micro-pillars on the test surfaces.…”