Two liquids in contact cannot usually slide by each other at all, because their molecules are in intimate contact. But in Physical Review Letters, a French team suggests that this motion can be lubricated by placing tiny beads at the liquids’ interface, to act as ball bearings. If the concept can be achieved in experiments, it could provide new flexibility in manipulating fluid flow.
In recent years, researchers have created “superhydrophobic” surfaces that use arrays of tiny raised pillars to allow water to flow with very low friction. First, the surface of each pillar is hydrophobic, like wax, so water “beads up” on it. But in addition, the collection of pillars is much more hydrophobic than a flat hydrophobic surface would be, because the liquid bridges across the tops of the closely-spaced pillars and slides as if on an air cushion.
Olivier Pierre-Louis and Laurent Joly of the University of Lyon, France, wondered if a similar technique could modify the friction between two liquids. For example, in a microfluidics device that does some chemical processing, one might want two liquids to flow past one another easily and only mix at a specified location. To replace the pillars, they imagined placing an array of tiny spheres at the interface. The fluids would remain separated because the spheres would be hydrophobic, and surface tension would maintain an air gap in the regions between the spheres. “These kinds of interfaces have already been made experimentally,” Pierre-Louis says, but no one has measured their frictional properties. Using theoretical calculations and simulations, the researchers, working with student Quentin Ehlinger, now find that beads indeed dramatically reduce the friction.
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