Abstract
For a liquid droplet to slide down a solid planar surface, the surface usually has to be tilted above a critical angle of approximately 10°. By contrast, droplets of nearly any liquid "slip" on lubricant-infused textured surfaces - so termed slippery surfaces - when tilted by only a few degrees. The mechanism of how the lubricant alters the static and dynamic properties of the drop remains elusive because the drop-lubricant interface is hidden. Here, we image the shape of drops on lubricant-infused surfaces by laser scanning confocal microscopy. The contact angle of the drop-lubricant interface with the substrate exceeds 140°, although macroscopic contour images suggest angles as low as 60°. Confocal microscopy of moving drops reveals fundamentally different processes at the front and rear. Drops recede via discrete depinning events from surface protrusions at a defined receding contact angle, whereas the advancing contact angle is 180°. Drops slide easily, as the apparent contact angles with the substrate are high and the drop-lubricant interfacial tension is typically lower than the drop-air interfacial tension. Slippery surfaces resemble superhydrophobic surfaces with two main differences: drops on a slippery surface are surrounded by a wetting ridge of adjustable height and the air underneath the drop in the case of a superhydrophobic surface is replaced by lubricant in the case of a slippery surface.
Highlights
The SU-8 2025 photoresist was first mixed with the hydrophobic perylenemonoimide dye (PMI) at a concentration of 0.05 mg mLÀ1.37 the mixture was spincoated onto the glass slides
We investigated micropillar arrays (Fig. 2A, B, F and G) and inverse opals (Fig. 2C–E, H and I) to distinguish between the general and substrate-dependent properties of drops on slippery surfaces
The inverse opals were prepared by first depositing a layer of polystyrene and silica particles onto a glass substrate, subsequent calcination of the polystyrene template,[39] followed by hydrophobization of the inverse opals to lower its surface energy
Summary
The surfaces were hydrophobised with (1H,1H,2H,2H)-perfluorooctyltrichlorosilane via chemical vapor deposition during 3 h, after activation by O2 plasma for 0.6 minutes at 150 W This process increases the density of –OH groups which are anchoring points for the fluorosilane. Depending on the interplay of the interfacial tensions, a slippery surface can exhibit zero (Fig. 1B), one (Fig. 1A and D) or both (Fig. 1C) three-phase contact lines This theoretical prediction does not provide quantitative information on the height and the width of the wetting ridge, the solid-drop contact region, or the dynamics of the liquid- or solid-TPCL. The equilibrium and dynamical behaviour of drops on slippery surfaces, we used laser scanning confocal microscopy
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