&Bullet; physics 14, p49
Biological and chemical assays are increasingly being automated with the help of microfluidic chips that control the movements of tiny droplets with microscopic pillars, bumps, and other features. Now Katrina Smith-Mannschott from the Swiss Federal Institute of Technology (ETH) in Zurich and colleagues are showing an alternative way of manipulating such droplets – by stretching the substrate on which they are sitting  . The approach could lead to simpler techniques for manufacturing microfluidic devices.
The researchers studied how glycerine droplets with a diameter of 2 mm slide on a soft, millimeter-thick layer of silicone gel. When they stretched the gel layer 23% of its original length, the droplets slid significantly faster in the direction parallel to the applied stretch than in the perpendicular direction. When the droplets stopped moving, they appeared to be elongated along the direction of elongation, with smaller droplets showing larger anisotropies.
According to existing wetting theories, static droplets should contract evenly on homogeneous surfaces and wet a circular area of the substrate. In order to understand the different dynamics on a stretched surface, the researchers examined the boundaries of the droplets microscopically. They found that the wetting comb – a normally even “curl” of the substrate around the perimeter of each droplet – was asymmetrical and that the degree of asymmetry depended on how much the substrate had been stretched. Based on this observation, they suggested that a droplet elongates because the variation in the comb shape around its circumference creates an imbalance in intermolecular forces between the substrate and the droplet. These forces, which are responsible for a number of elastocapillary effects in other systems (see synopsis: Hairstyling with physics), also explain the voltage-dependent increase in the droplet speed. The researchers hope that additional experiments will reveal the limits of asymmetric wetting behavior that can be achieved with this technique.
Sophia Chen is a freelance science writer based in Columbus, Ohio.
- K. Smith-Mannschott et al.“Droplets sit and slide anisotropically on soft, stretched substrates” Phys. Rev. Lett.126158004 (2021).