Simulation of wetting behavior of liquid droplets on different types of micropatterned surfaces

Abstract

self-cleaning and anti-icing. Superhydrophobic surfaces have extreme water repellent properties associated with very high static contact angles and low resistance to liquid motion on surfaces. These properties result from the combination of the chemical hydrophobicity of the topmost layers of the substrate and its roughness, the latter being the dominant factor. The superhydrophobic behavior arises on rough surfaces when drops are suspended on microstructures, so that their contact with the substrate is minute. This suspended state (Cassie-Baxter state) is however tends to collapse to wetted state (Wenzel state) if the liquid-air interface is perturbed. Also, multiple metastable Cassie-wetting states may exist separated by an energy barrier from Wenzel state. In this study, numerical method is applied to investigate the wetting properties of micro-patterned rough surfaces with a particular focus on the stability of the Cassie-Baxter state as well as to find the optimal texture to support the superhydrophobic configuration. Using open source software, Surface Evolver, 3D drop-shape models are developed to numerically investigate the shapes and energies of Cassie drops on substrates with micro-posts for a wide range of parametric space. The wettability of the pillar-patterned surfaces is numerically quantified with an analysis of apparent static contact angle of droplets. The effect of pillar spacing on contact angles is explored and the results are found to be in good agreement with the classical wetting theory as well as with experimental data. A normalized form of interfacial energy is used to compare the stabilities of droplet on substrates with square pillars. The sequence of stable drop configurations with increasing droplet volume on substrate is analyzed and an explanation of superhydrophobic drop spreading has been provided for both isotropic and anisotropic wetting behavior. Numerical analysis shows that droplet spreading from one isotropic wetting configuration to one anisotropic configuration is not favorable unless the spreading of the droplet is restricted to be anisotropic. Applying dimensional variation in texture characterized by different pillar spacing and pillar width, the key parameter which plays dominant role in the stability of droplet is investigated. From numerical analysis it reveals that the solid fraction that avails at the drop-base is vital for the stable size of the droplet. If droplet of different volumes are deposited confirming same dropbase area on two distinct surfaces with difference in pillar spacing or width, the surface that allows less solid fraction at drop-base will give stability to larger droplets compared to that attains stability on the other surface. Finally, to investigate the effect of pillar structure on superhydrophobicity, drop shapes on cylindrical pillars and square pillars are explored under exactly same wetting configuration and it has been found that the pillar structure does not cause notable effect in wetting if the droplet size is too much large compared to the roughness structures. In general, the developed simulation techniques provide reliable prediction of the wetting behavior which will significantly aid the design of superhydrophobic surfaces through optimizing the geometric parameters such as pillar size, shape and spacing as well as the nature of surface coatings required.