INFLUENCE OF STRUCTURAL AND CHEMICAL ASYMMETRY OF NANOSTRUCTURES ON THE KINETICS OF WETTING

Abstract

The kinetics of wetting of a liquid droplet deposited onto a surface consisting of ordered arrays of nanostructures with either structural or chemical asymmetry was studied. Structurally anisotropic Si nanostructures were obtained by fabricating elliptical nanofins using interference lithography and metal assisted chemical etching. Chemically anisotropic nanostructures, on the other hand, were obtained by the oblique angle deposition of a metal onto an array of polystyrene nanostructures fabricated by interference lithography and O2/CF4 plasma etching. It was found that when there is chemical asymmetry, that is, a difference between the surface energy of the two faces of a nanostructure, an uneven pinning strength on the triple phase contact line causes preferential wetting to occur on the more hydrophilic face. Depending on the shape of the nanostructure, which can be controlled by the fabrication process, wetting can be made uni-, bi- or tri-directional. For the case of chemically homogeneous nanostructures, it was found that when the nanostructures are sufficiently rough, a form of wetting different from Wenzel and Cassie-Baxter states will arise. This form of wetting is commonly known as hemiwicking or 2D wicking, and involves a film of liquid wicking from the base of the droplet into the space between the nanostructures. The rate of imbibition of the wicking film is determined by the balance between capillary energy gained from wetting the nanostructures and energy losses in the form of skin drag and form drag. It was found that skin drag tends to be stronger along the length of the nanofins while the converse is true for form drag. Therefore, depending on the exact geometry of the nanofins, the wicking film may adopt an isotropic or anisotropic shape on nanofin arrays. In contrast, droplets spreading on 2D wicking surfaces made of nanofins are always isotropic in shape. This can be attributed to the elimination of contact line pinning by the wicking film which, by advancing ahead of the droplet edge, causes the droplet to effectively spread on a flat, composite surface made up of solid and liquid phases.