Thickness of Nano-Scale Poly(Dimethylsiloxane) Layers Determines the Motion of Sliding Water Drops.

Layers of nanometer thick polydimethylsiloxane (PDMS) are applied as hydrophobic coatings because of their environmentally-friendly and chemically-inert properties. In applications such as heat exchangers or fog harvesting, low water drop friction on surfaces is required. While the onset of motion (static friction) has been studied, our knowledge of dynamic friction needs to be improved. Which processes lead to energy dissipation and cause dynamic friction? This knowledge is important to minimize friction. Here, we measured the dynamic friction of drops on PDMS brushes of different thicknesses, covering the whole available velocity regime. The brush thickness L turned out to be a predictor for drop friction. 4-5 nm thick PDMS brush showed the lowest dynamic friction. A certain minimal thickness is necessary to form homogeneous surfaces and reduce the attractive Van der Waals interaction between water and the substrate. The increase in dynamic friction above L = 5 nm is also attributed to the increasing viscoelastic dissipation of the capillary ridge formed at the contact line. The height of the ridge is related to the brush thickness. Fluorescence correlation spectroscopy and atomic force measurements support this interpretation. Sum-frequency generation further indicates a maximum order at the PDMS-water interface at intermediate thickness. This article is protected by copyright. All rights reserved.