In an attempt to understand the effects of micro and nano-scale structures on droplet shedding as well as self-cleaning mechanism, several researchers have focused on artificial hydrophobic surfaces in the last decade. Artificial hydrophobic surfaces are characterized by high contact angles and low contact angle hysteresis, which result in easy liquid shedding during condensation. Recent studies have indicated that improved dropwise condensation happens after nucleation. Unfortunately, hydrophobic surfaces have been found to have lower nucleation rates owing to higher contact angles as well as a high nucleation energy barrier.
Therefore, micro-textured surfaces made of hydrophobic and hydrophilic materials have been designed, manufactured and characterized in a move to improve droplet shedding and droplet nucleation. However, droplet shedding on micro-textured surfaces is limited reference to the pinning effect occurring between the droplets and the hydrophobic-hydrophilic edge, which results in a considerable contact angle hysteresis effect.
There is needed an external force to mitigate or eliminate entirely the hysteresis effect. A number of previous studies have indicated that a surface vibration would be an effective means of inducing droplet shedding by forcing wetting transition. Droplet can shed from the surface once the energy barrier related to the pinning effect is eliminated through the induced surface vibration.
The vibration-induced wetting transition have been analyzed in recent studies. However, the mechanism of vibration-induced wetting transition is not yet well understood. Therefore, Chun-Wei Yao at Lamar University in collaboration with Jorge L. Alvarado at Texas A&M University focused on the effects of surface vibration on droplet sliding angle under varying vibration resonance conditions. This provided a possible method of shedding droplets off hydrophobic-hydrophilic micro-textured surfaces with high contact angle hysteresis. Their research work is published in journal, Applied Surface Science.
The authors first investigated through experimentation the droplet sliding angles and droplet volumes on the micro-textured surfaces with varying structured spacings. Using a resonance model and a customized experimental arrangement, the authors studied and analyzed the effects of natural resonance frequencies of droplet with varying volumes on various surfaces. They then applied vibrations to the surfaces in order to understand the effects of single- and multi-resonance frequencies on droplet sliding angles.
The research team observed from experimental droplet vibration results that the sliding angles decreased with first or the second resonance frequencies at varying droplet volumes on the micro-textured surfaces with various spacings. Droplets vibrating at the first resonance frequency posted lower sliding angles as compared to droplets vibrated at the second resonance frequency. This was referenced to the first resonance frequency’s over-active resonance motions as well as lower threshold amplitude needed for the dewetting transition.
Chun-Wei Yao and colleagues also observed that the first resonance frequencies could be used to overcome the droplet contact angle hysteresis effect easily. The results of the study indicated that resonance-based surface vibrations are efficient in reducing sliding angles on micro-textured surfaces with hydrophilic and hydrophobic materials.
Chun-Wei Yao, Chen-Ling Lai, Jorge L. Alvarado, Jiang Zhou, Kendrick T. Aung, Jose E. Mejia. Experimental study on effect of surface vibration on micro textured surfaces with hydrophobic and hydrophilic materials. Applied Surface Science, volume 412 (2017), pages 45–51.
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