Droplet impact on vibrating superhydrophobic surfaces

Many unanswered questions remain pertaining to droplet dynamics during impact on vibrating surfaces. Using optical high-speed imaging, we investigate the impact dynamics of macroscopic water droplets (≈2.5mm) on rigid and elastic superhydrophobic surfaces vibrating at 60-320 Hz and amplitudes of 0.2-2.7 mm. Specifically, we study the influence of the frequency, amplitude, rigidity, and substrate phase at the moment of impact on the contact time of impacting droplets. We show that a critical impact phase exists at which the contact time transitions from a minimum to a maximum greater than the theoretical contact time on a rigid, nonvibrating superhydrophobic surface. For impact at phases higher than the critical phase, contact times decrease until reaching a minimum of half the theoretical contact time just before the critical phase. The frequency of oscillation determines the phase-dependent variability of droplet contact times at different impact phases: higher frequencies (> 120 Hz) show less contact time variability and have overall shorter contact times compared to lower frequencies (60-120 Hz). The amplitude of vibration has little direct effect on the contact time. Through semiempirical modeling and comparison to experiments, we show that phase-averaged contact times can increase or decrease relative to a nonvibrating substrate for low (<80Hz) or high (>100Hz) vibration frequencies, respectively. This study not only provides new insights into droplet impact physics on vibrating surfaces, but also develops guidelines for the rational design of surfaces to achieve controllable droplet wetting in applications utilizing vibration.