Dynamic characteristics of vibrated-droplets on a microstructured hydrophobic surface

Abstract

The square-shaped pillars rough surface was prepared with the polydimethylsiloxane(PDMS) material by using the photolithographic techniques. The Wenzel-Cassie wetting transition of different size of droplets, gently placed on the surface, had been observed under the vertical vibration at different amplitudes achieved by adjusting, at a given frequency, the amplitude of the vertical displacement. When the forced vibration frequency is in accordance with the nature frequency of the droplet, that is to say, at the resonance frequency, the forced amplitude for Wenzel-Cassie wetting transition arrives to the lowest value. Although the Wenzel-Cassie wetting transition of other droplets were observed at this frequency, the energy for wetting transition is not the minimum as the imposed vibration frequency deviates from their natural frequencies. The farther the deviation between the vibration frequency and the natural frequency is, the more energy it requires. Meanwhile, the phenomenon is analyzed theoretically with the surface physical chemistry and mechanics of vibration.