Wetting on surfaces with tailored nano-scale defects

Surface heterogeneity is acknowledged as a cause of contact angle hysteresis. More recently, it has also been recognized as having an important effect on contact line dynamics [1]. However, it has proven difficult to design quantitative experiments [2]. There are two fundamental difficulties: (i) to control the heterogeneity and (ii) to fabricate a true reference surface. In many methods, additional well-controlled defects may be added to a surface, however, the untreated substrate itself already presents some chemical or topographical disorder (typically nano-scale). This is illustrated by the fact that even on a purportedly defect-free surface the hysteresis is non-negligible (\textgreater a few degrees). We report on the use of adsorbed short-chain polymer surfaces as nearly ideal reference surfaces (H\textless 0.01$^{\circ}$). Topographical defects of controlled size, shape and density are then added using nanosphere lithography with dilute colloidal suspensions, and the dependence of the hysteresis and low-velocity dynamics on the defect parameters are determined.\\[4pt] [1] K. Davitt \textit{et al.}, \textit{Langmuir }\textbf{29}, 6884 (2013).\\[0pt] [2] S.M.M. Ramos \textit{et al.}, \textit{Phys. Rev. E }\textbf{67}, 031604 (2003).