Beads of water hopping over hurdles

On lotus leaves and raincoat sleeves, water droplets roll off as beads. It is a mesmerizing and common sight whose physics is relevant to a range of industrial applications, such as those involving weather-protective coating.

Using a superhydrophobic surface, a few pipettes and a high-speed camera, de Maleprade et al. studied what little hurdles would do to water droplets rolling down an incline. Using droplets with volumes ranging from 0.06 to 0.2 mL and hurdles 1 mm tall and 8 mm apart, they sent the beads down the ridged hydrophobic surface.

Compared to a smooth track, the hurdled track cut the droplets’ velocity down to a tenth of its original value. The authors attribute this loss in kinetic energy to the jiggling motion of the droplets as they hit and hop over the hurdles.

Depending on the droplet size, the track needed to reach a critical tilt before a droplet could overcome the first hurdle, after which, it had enough momentum to progress down the rest of the hurdled track.

The authors established an equation for describing the physics, as well as predicting the critical tilt by treating the condition provided by the hurdles as a special form of “friction.”

They also tested water-glycerol mixtures to study how viscosity affects the droplet behavior.

“This study offers an efficient solution to control the velocity of highly mobile liquids. The technique is robust for a wide range of viscosities, and it could be extended to liquids with various properties,” said author David Quéré.

Source: “Droplet hurdles race,” by Hélène de Maleprade, Rafid Bendimerad, Christophe Clanet, and David Quéré, Applied Physics Letters (2021). The article can be accessed at https://doi.org/10.1063/5.0043908 .