There is a huge interest in developing super-repellent surfaces for anti-fouling and heat-transfer applications. To characterize the wetting properties of such surfaces, the most common approach is to place a millimetric-sized droplet and measure its contact angles. The adhesion and friction forces can then be inferred indirectly using the Furmidge's relation. While easy to implement, contact angle measurements are semi-quantitative and cannot resolve wetting variations on a surface. Here, we attach a micrometric-sized droplet to an Atomic Force Microscope cantilever to directly measure adhesion and friction forces with nanonewton force resolutions. We spatially map the micron-scale wetting properties of superhydrophobic surfaces and observe the time-resolved pinning-depinning dynamics as a droplet detaches from or moves across the surface.
The functional properties of a surface, such as its antifogging or anti-fouling performance, are influenced by its wettability. To quantify surface wettability, the most common approach is to measure the contact angles of a liquid droplet on the surface. While well established and relatively easy to perform, contact angle measurements were developed to describe macroscopic wetting properties and are difficult to perform for submillimetric droplets. Moreover, they cannot spatially resolve surface heterogeneities that can contribute to surface fouling. To address these shortcomings, we report on using an atomic force microscopy technique to quantitatively measure the interaction forces between a microdroplet and a surface with piconewton force resolution. We show how our technique can be used to spatially map topographical and chemical heterogeneities with micron resolution.
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