05. Micro- and Nano-Scale Transport, MEMS

Surface patterning is frequently utilized to modulate surface characteristics due to the proven influence of surface structures on wetting phenomena and associated physics. In this study, chemical heterogeneity was introduced through stripe-patterned surfaces modified by self-assembled monolayers, each of which is composed of two types of organic molecules with different affinities to water. By this means, three distinct heterogeneous stripe-patterned surfaces were generated. Experiments have shown that the contact angle of a macroscopic droplet on a heterogeneous surface can be successfully described by the Cassie model. However, there is intense debate regarding the application of the Cassie equation to microscopic droplets, as it has been reported that the Cassie equation may be invalid if the droplet size is comparable to the heterogeneity size, which usually occurs at the nanoscale. Furthermore, when wetting such a heterogeneous surface, surface heterogeneity can induce contact line pinning, resulting in metastable contact angles as the droplet size changes. In this study, molecular dynamics simulations were employed to explore the wetting behavior of water droplets on heterogeneous surfaces at the nanoscale. Simulations were conducted for over a dozen systems, encompassing diverse surface heterogeneity sizes and droplet sizes, to comprehensively explore wetting behavior. The results demonstrated that a more pronounced difference in wettability between the two components leads to a more significant expansion in contact area with the same increase in droplet size.