Droplet impact on hydrophobic surfaces with hierarchical roughness

We investigate the dynamic properties of microliter droplets impacting with velocities up to $0.4\:{\rm{m}}\:{{\rm{s}}^{ - 1}}$ on hydrophobic surfaces with hierarchical roughness. The substrates consist of multiple layers of silica microspheres, which are decorated with gold nanoparticles; the superstructures are hydrophobized by chemical modification. The initial impact event is analysed, primarily focusing on the bouncing of the droplets. The number of bounces increases exponentially with substrate hydrophobicity as expressed by the contact angle. The subsequent relaxation regime is analysed in terms of the frequency and damping rate of the droplet oscillations. Both quantities exhibit a substantial decrease for large contact angles. Results are discussed in relation to reports in literature; damping is most likely due to viscous dissipation.