How droplets move on laser-structured surfaces: Determination of droplet adhesion forces on nano- and microstructured surfaces

Abstract

Hypothesis: Lateral adhesion forces are a fundamental property of liquid-solid interactions and a key aspect of dynamic droplet mobility. But, commonly applied conventional wetting analysis is limited to static and quasi-static methods and cannot resolve dynamic and spatial liquid-solid interactions. However, droplet mobility is assumed to be affected by chemical and topographic surface inhomogeneities introduced by femtosecond laser treatment.

Experiments: In this study, we used a customized droplet adhesion force instrument to determine lateral adhesion forces on various femtosecond laser-structured surface designs to obtain a deeper understanding of the dynamic droplet motion with regard to chemical and topographic surface features.

Findings: We show that the droplet motion was highly affected by the chemical and topographical surface design and local inhomogeneities. The droplet mobility on femtosecond laser-structured surfaces could be classified into a static, a transfer, and a kinetic regime, which is essential for designing surfaces with extreme wetting characteristics and a wide range of scientific and industrial processes. Furthermore, with proper tailoring of surface structures and chemical modification, we were able to provoke adhesion forces on self-organized laser microstructures similar to those found on the natural lotus leaves.

Keywords: Droplet adhesion force analysis; Droplet mobility; Femtosecond laser; Laser texturing; Lateral adhesion force; Microstructures; Self-organized; Silanization; Superhydrophobic; Wetting.