&Bullet; physics 14, s86
Droplets from a mixture of two liquids with different volatilities take on the shape of a pancake.
Researchers are trying to control the movement and behavior of small droplets for a variety of uses, from automating chemical and biological experiments to making devices like liquid lenses. Typically, models assume that the droplets remain spherical even after they evaporate. Amir Pahlavan from Princeton University and colleagues have now shown that some droplets can take on pancake-like shapes under certain conditions. The team says they want to use their results to develop droplets that can switch between shapes.
The researchers conducted experiments with picoliter-volume droplets containing a mixture of two different alcohols, isopropanol and butanol. They placed each droplet on a glass surface and then observed it with a microscope.
Within a tenth of a second after deposition, Pahlavan and colleagues observed that droplets containing more than 40% isopropanol changed from initially spherical to flat to spherical again. They explained the behavior as follows: While the liquids in each droplet were initially well mixed, the isopropanol evaporated much faster than the butanol, creating heterogeneities in the flows within the liquid that deformed the droplet.
More specifically, simulations carried out by the team show that the shape changes were due to two competing flows: capillary flow, which arises from pressure fluctuations within the droplet, and Marangoni flow, which arises from a surface tension gradient along the droplet interface. The Marangoni flow caused liquid within the droplet to flow from the center of the droplet to its outer edge; the capillary flow counteracted this movement. When the Marangoni River dominated, the pancake shape was created. Over time, the capillary flow overtook the Marangoni flow and the droplets became spherical again. The results suggest that adding a more volatile liquid to a droplet could provide a way to tune the shape of a droplet.
Sophia Chen is a freelance science writer based in Columbus, Ohio.
- AA Pahlavan et al., “Evaporation of liquid droplets with a binary mixture: The formation of picolitre pancake-like shapes”, Phys. Rev. Lett.127, 024501 (2021).