Abstract
Superhydrophobic coatings have extraordinary properties like self-cleaning and staying dry, and have recently appeared on industrial and consumer markets. The stochastic nature of the coating components and coating processes (e.g., spraying, painting) affects the uniformity of the water repellency across the coated substrate. The wetting properties of those coatings are typically quantified on macroscale using contact angle goniometry (CAG). Here, highly sensitive force-based methods, scanning droplet adhesion microscopy (SDAM), and micropipette force sensor (MFS), are used, to quantify the microscale heterogeneity in the wetting properties of stochastic superhydrophobic coatings with irregular surface topography that cannot be investigated by CAG. By mapping the wetting adhesion forces with SDAM and friction forces with MFS, it is demonstrated that even the best coatings on the market are prone to heterogeneities that induce stick-slip motion of droplets. Thus, owing to their high spatial and force resolution, the advantages of these techniques over CAG are demonstrated.
Highlights
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Micro- or nanopillar surfaces, fabricated via micropatterning followed by surfacehighly sensitive force-based methods, scanning droplet adhesion microschemical modification[13,14] are frequently copy (SDAM), and micropipette force sensor (MFS), are used, to quantify the used to systematically explore superhydromicroscale heterogeneity in the wetting properties of stochastic superhydrophobic coatings with irregular surface topography that cannot be investigated by contact angle goniometry (CAG)
To map droplet adhesion on the coating surfaces, SDAM measurements on a 1 mm × 1 mm area were done with 100 μm intervals
Summary
We investigated the wettability of the following stochastic superhydrophobic coatings: Glaco, Hydrobead, UltraEverDry, Supraliq Z, and Supraliq T (see Table S1, Supporting Information, for details). Aside from superhydrophobic surfaces, high droplet mobility can be achieved on a variety of slippery surfaces with comparatively low contact angles but very low contact angle hysteresis.[30] On the other hand, the normal adhesion force, quantified here as the pull-off force (FA), relates to the droplet–surface contact area upon separation of the droplet from the surface, and depends on the receding contact angle θrec exclusively.[23,34] FA only becomes small for very high θrec, that is, for surfaces that are superhydrophobic In our measurements, this explains how coatings such as UltraEverDy and Hydrobead can show the lowest FA in SDAM, but comparably high Fp /L in MFS. In SDAM, normal forces are measured in spatially localized manner: Aside from the local receding contact angle θrec, the pull-off force is affected only by the size and features of the droplet–surface contact area, as demonstrated by earlier measurements carried out on micropillars of varying diameters.[23]
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