Caution slippery when wet! Amazing stuff!
"The slipperiness of a surface can be tuned by tweaking its molecular-scale roughness, researchers in Finland have shown. This helps to explain why water droplets can easily slide off hydrophilic surfaces if they are chemically homogeneous, and also opens up new possibilities for the design of low-friction surfaces – allowing the researchers to produce what they believe is the most slippery surface ever created. Such surfaces could be used in self-cleaning coatings. ...
In the highly hydrophobic case, a water droplet touches the surface at a very steep angle, so only a relatively small area of the droplet made contact and slides off easily. This principle of texturing a surface to reduce a droplet’s contact area is well known and routinely exploited in superhydrophobic surfaces. The other cases surprised the researchers more. In the low-coverage, hydrophilic case, water spread out to form a thin film that covered the surface. Other water droplets formed large interfacial contacts with this layer, but as it was highly mobile it lubricated their passage across the surface. In the intermediate case, however, this hydrophilic surface effect was broken up by the hydrophobic islands. Instead of skating across the surface of a contiguous layer of water or skimming on a hydrophobic surface, water droplets were repeatedly puddling. ...
The researchers extended their work to make a super-slippery superhydrophobic surface. They grew their hydrophobic monolayer on top of a layer of aluminium oxide-covered black silicon, which was textured at the micrometre scale to minimise the droplet contact area and create an exceptionally superhydrophobic surface. ... Here, however, the researchers combined both physical and chemical hydrophobicity. ‘The contact area is very small, and where it does contact the surface you have this lubricating effect,’ ... The result, they believe, is the most slippery surface ever created. ... believes the approach is scalable ..."
From the abstract:
"Friction determines whether liquid droplets slide off a solid surface or stick to it. Surface heterogeneity is generally acknowledged as the major cause of increased contact angle hysteresis and contact line friction of droplets. Here we challenge this long-standing premise for chemical heterogeneity at the molecular length scale. By tuning the coverage of self-assembled monolayers (SAMs), water contact angles change gradually from about 10° to 110° yet contact angle hysteresis and contact line friction are low for the low-coverage hydrophilic SAMs as well as high-coverage hydrophobic SAMs. Their slipperiness is not expected based on the substantial chemical heterogeneity of the SAMs featuring uncoated areas of the substrate well beyond the size of a water molecule as probed by metal reactants. According to molecular dynamics simulations, the low friction of both low- and high-coverage SAMs originates from the mobility of interfacial water molecules. These findings reveal a yet unknown and counterintuitive mechanism for slipperiness, opening new avenues for enhancing the mobility of droplets."
Fig. 1: Controlling CLF [contact line friction ] by tuning surface hydrophobicity with OTS SAM.
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