A first look reveals the surprising behavior of droplet reaction forces on a soft superhydrophobic surface

Superhydrophobic surfaces feature many industry-relevant benefits like anti-icing and anti-fouling properties, as well as the ability to self-clean and reduce drag. A team of researchers describes in their Physics of Fluids publication the reaction force of a water droplet impacting a flexible superhydrophobic surface during its recoil. Understanding these dynamics can help describe flexible superhydrophobic surfaces in nature, like insect wings and leaves, and in industry, like water-repellent textiles and medical devices.

In the experiment, the authors spray coated a smooth but flexible PDMS (polymer) surface with superhydrophobic paint. Then, they used a position-sensing system and high-speed imaging capable of resolving displacements of less than 2 micrometers to discover how a water droplet’s reaction force deflects the flexible superhydrophobic surfaces. Tests with various drop impact velocities revealed that the flexible surface was deflected downward while the droplet recoiled, since the droplet exerted a force on the flexible surface.

The team was surprised to find that the droplet’s reaction force occurred during recoil. Jeong-Hyun Kim, an author on the paper, explained that the droplet’s reaction and impact forces were comparable and found to displace the flexible superhydrophobic surface. Their experiment showed that a droplet’s reaction force can manipulate the flexible material’s surface dynamics by changing the surface tension and natural frequency of the surface.

Kim believes that their research could inspire future technological innovations, such as, for example, piezoelectric energy harvesting systems that use rain drops. He also emphasized that their novel experimental technique could “broaden our perspectives of fluid-structure interaction problems that involve the regimes of high flexural stiffness and small displacements.”

Source: “Dynamics of a flexible superhydrophobic surface during a drop impact,” by Jeong-Hyun Kim, Jonathan P. Rothstein, and Jessica K. Shang, Physics of Fluids (2018). The article can be accessed at https://doi.org/10.1063/1.5028127 .