Drop dynamics on chemically patterned surfaces

– We compare numerical and experimental results exploring the behaviour of liquid drops moving across a surface patterned with hydrophobic and hydrophilic stripes. A lattice Boltzmann algorithm is used to solve the hydrodynamic equations of motion of the drops allowing us to investigate their behaviour as the stripe widths and the wettability contrast are altered. We explain how the motion of the drop is determined by the interplay between the driving force and the variation in surface force as the drop moves between regions of different contact angle and we find that the shape of the drops can undergo large periodic deviations from spherical. When compared, the numerical results agree well with experiments on micron–scale drops moving across substrates patterned by microcontact printing. Introduction. – The question of how liquid drops wet and move across a solid surface has long caught the interests of academic and industrial communities alike, with applications ranging from microfluidic devices to ink-jet printing and surface coating. Though much progress has been made since the first pioneering work by Young and Laplace, many interesting, unanswered questions remain. One which has recently come to the fore because of experimental advances allowing the fabrication of surfaces with mesoscopic hydrophobic and hydrophilic regions is the behaviour of drops on chemically patterned substrates. Several authors [1–6] have shown that the wetting behaviour on these substrates can be very rich, with the drop shapes depending sensitively on parameters such as the dimensions and contact angles of the patterning. In this letter we build on this work to address the dynamics of drops moving across an array of alternating hydrophobic and hydrophilic stripes, focussing on the centre of mass motion as well as the morphological transitions induced by the imposed external flow. The drop is pushed by a constant gravity-like acceleration as opposed to [7] where a thermal gradient is applied to generate the drop motion. (∗) E-mail: [email protected] (∗∗) Present address: Laboratoire de Physico-Chimie des Polymères, Université de Mons-Hainaut, 20, Place du Parc B-7000 Mons, Belgium (∗∗∗) Present address: Institute of computational science, ETH Zurich, 8092 Zurich, Switzerland