Slip - controlled thin film dynamics

– In this study, we present a novel method to assess the slip length and the viscosity of thin films of highly viscous Newtonian liquids. We quantitatively analyse dewetting fronts of low molecular weight polystyrene melts on Octadecyl-(OTS) and Dodecyltrichlorosilane (DTS) polymer brushes. Using a thin film (lubrication) model derived in the limit of large slip lengths, we can extract slip length and viscosity. We study polymer films with thicknesses between 50 nm and 230 nm and various temperatures above the glass transition. We find slip lengths from 100 nm up to 1 µm on OTS and between 300 nm and 10 µm on DTS covered silicon wafers. The slip length decreases with temperature. The obtained values for the viscosity are consistent with independent measurements. Introduction. – Miniaturization of chemical appliances into so-called microfluidic devices allows to handle smaller and smaller amounts of liquid. However, in narrow channels, the effect of the hydrodynamic boundary conditions becomes extremely important. In particular, small amounts of slip on the channel walls can improve throughput and decrease the dispersion of chemical signals [1]. In electronics industry, downscaling photolitographic processes requires extremely thin films of photoresist, a polymeric liquid. The dynamics of these thin films is also significantly affected by the boundary condition at the solid/liquid interface. Hence, there is a strong need to quantify slippage of different liquids on various substrates, and to char-acterise the influence of system parameters on the effective slip length. Common techniques to measure the velocity at the substrate involve tracer particles [2–4] or fluorescence recovery after photobleaching [5–7]. In addition, there are various indirect methods to determine the amount of slippage, mostly drainage experiments, e.g., in a surface forces apparatus [8–10] or between a colloidal probe particle and a wall [11–13]. For recent reviews see refs. [14, 15]. As none of these techniques can be applied to all liquid/substrate-combinations, we present