Fluid properties have large effects on droplet dynamics under air shear flow

In applications ranging from spray coating to icing on power lines, wind turbine blades and aircraft, changes in temperature can significantly affect the viscosity and surface tension of liquid droplets. These changes in droplet properties can significantly impact their dynamics.

Moghtadernejad et al. studied the deformation of four types of liquid droplets with different surface tensions and viscosities on a superhydrophobic surface under various wind speeds.

“Because the physical properties of liquids change with temperature and pressure, we wanted to study their dynamics when their viscosity or surface tension change,” said author Sara Moghtadernejad.

Using a high-speed camera, the researchers captured the behavior of these droplets over time at wind speeds ranging from 5 to 60 meters per second. To simulate water at different temperatures, they used distilled water, ethylene glycol, propylene glycol and glycerol.

In their experiments, they found three distinct behavior regimes. When both the surface tension and viscosity are low, droplets deform and move or rotate slightly without breaking up, but may detach from the surface as the wind speed increases. At high surface tension but low viscosity, the droplet deforms and detaches, and may break up at higher wind speeds. In the high surface tension and high viscosity case, the droplet breaks up or forms rivulets without detaching from the surface.

“This is the beginning of a large study to understand droplet dynamics on superhydrophobic surfaces,” said Moghtadernejad. Next, the authors will incorporate temperature effects directly in order to study reality closer.

Source: “Experimental study of droplet shedding on laser-patterned surfaces,” by Sara Moghtadernejad, Mehdi Jadidi, K. M. Tanvir Ahmmed, Christian Lee, Ali Dolatabadi, and Anne-Marie Kietzig, Physics of Fluids (2019). The article can be accessed at https://doi.org/10.1063/1.5126048 .