Superhydrophobicity is a remarkable surface property found in nature and mimicked in many engineering applications, including anti-wetting, anti-fogging, and anti-fouling coatings. As synthetic superhydrophobic coatings approach the extreme non-wetting limit, quantification of their slipperiness becomes increasingly challenging: although contact angle goniometry remains widely used as the gold standard method, it has proven insufficient. Here, micropipette force sensors are used to directly measure the friction force of water droplets moving on super-slippery superhydrophobic surfaces that cannot be quantified with contact angle goniometry. Superhydrophobic etched silicon surfaces with tunable slipperiness are investigated as model samples. Micropipette force sensors render up to three orders of magnitude better force sensitivity than using the indirect contact angle goniometry approach. We directly measure a friction force as low as 7 ± 4 nN for a millimetric water droplet moving on the most slippery surface. Finally, we combine micropipette force sensors with particle image velocimetry and reveal purely rolling water droplets on superhydrophobic surfaces.
Accurate wetting characterization is crucial for the development of next-generation superhydrophobic surfaces. Traditionally, wetting properties are measured with a contact angle goniometer (CAG) suitable for a broad range of surfaces. However, due to optical errors and challenges in baseline positioning, the CAG method suffers from inaccuracies on superhydrophobic surfaces. Here we present an improved version of the oscillating droplet tribometer (ODT), which can reliably assess wetting properties on superhydrophobic surfaces by measuring the frictional forces of a water-based ferrofluid droplet oscillating in a magnetic field. We demonstrate that ODT has superior accuracy compared to CAG by measuring the wetting properties of four different superhydrophobic surfaces (commercial Glaco and Hydrobead coatings, black silicon coated with fluoropolymer, and nanostructured copper modified with lauric acid). We show that ODT can detect the small but significant changes in wetting properties caused by the thermal restructuring of surfaces that are undetectable by CAG. Even more, unlike any other wetting characterization technique, ODT features an inverse sensitivity: the more repellent the surface, the lower the error of measurement, which was demonstrated by experiments and simulations.
Lubrication is one of the most important ways to reduce the effect of friction, which is the single largest cause for energy losses in society. Typically, friction reduction is done by lubrication with petroleum‐based oils, while technology focus is shifting toward environmentally‐friendly green lubrication. Lowest friction coefficients with water‐based lubrication have previously been achieved with smooth surfaces such as silicon carbide and silicon nitride or polyzwitterionic polymer brushes with typical coefficients of friction in the order of 0.002. Here, a novel concept for green lubrication using a bilayer of water and ambient air acting as the lubricant between a hydrophilic and superhydrophobic surface is shown. This method achieves superlubricity with friction coefficients down to 0.002 as measured with oscillating tribometer and tilting stage. In addition, possible applications for superhydrophobic lubrication such as tunable lubrication and a 2D mouse treadmill, are shown.
Inside Front Cover: The cover image is based on the Research Article Oscillating droplet tribometer for sensitive and reliable wetting characterization of superhydrophobic surfaces by Junaid et al.
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