Wetting transition on patterned surfaces

10 Feb 2016 NUS mathematicians investigate liquid-vapour transition on patterned surfaces in shear flow using the minimum action method (MAM) developed based on large deviation theory.

The hydrophobicity of a solid surface can be greatly enhanced when patterned with nano- or microscale structures. Such patterned surfaces have many applications in industry, such as drag reduction, self-cleaning, defrosting, and anti-icing. Liquid placed on the patterned surface can exhibit two different wetting states: the Cassie-Baxter state and the Wenzel state.

The transitions between the two states and the effects of surface topography and surface chemistry on the transitions have been extensively studied in earlier work. However, most of the earlier work focused on the study of the free energy landscape and energy barriers. A team led by Prof REN Weiqing from the Department of Mathematics in NUS considered the transitions in the presence of a shear flow, and investigated the influence of hydrodynamics on the wetting transition. They computed the minimum action path using an efficient numerical method, MAM. The MAM was developed by one of the authors in his earlier work based on principles provided in large deviation theory.

Numerical results were obtained for transitions on a surface patterned with straight pillars. It was found that the shear flow facilitates the transition from the Wenzel state to the Cassie state, while inhibitingthe transition backwards. The Wenzel state becomes unstable when the shear rate reaches a certain critical value. Two different scenarios for the Wenzel-Cassie transition are observed. At low shear rate, the transition happens via nucleation of the vapour phase at the bottom of the groove followed by its growth. At high shear rate, in contrary, the nucleation of the vapour phase occurs at the top corner of a pillar (see Figure). These findings provide a quantitative basis for optimal design of patterned surfaces.

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The critical points along the minimum action path for transition from the Wenzel state (the first panel) to the Cassie-Baxter state (last panel) on patterned surfaces under a shear flow. The middle panel is an intermediate metastable state. The second and fourth panels are the transition states between the neighbouring (meta-) stable states. [Image credit: Ren WQ]

 

Reference

Yao W, Ren W. “Liquid-vapor transition on patterned solid surfaces in a shear flow.” J. Chem. Phys. 143 (2015) 244701.