Effects of Induced Acoustic Vibrations on Droplet Shedding on Hybrid Micro-structured Surfaces



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Artificial hydrophobic and superhydrophobic surfaces have been studied in the last ten years in an effort to understand the effects of structured micro- and nano- scale features on droplet motion and self-cleaning mechanisms. Among these structured surfaces, hybrid surfaces consisting of a combination of hydrophilic and hydrophobic materials have been designed, fabricated and characterized to understand how surface properties and morphology affect enhanced droplet growth rates and droplet shedding during condensation. However, use of hybrid surfaces in condensation leads to a strong pinning effect that takes place between the condensing droplets and the hydrophobic-hydrophillic edge, leading to a significant contact angle hysteresis effect. In an effort to circumvent the pinning effect, a vibration-induced droplet shedding method has been explored to overcome contact angle hysteresis and facilitate droplet shedding at lower rolling angles.

To understand the effects of hybrid surface morphology and vibration modes on droplet removal from surfaces used for condensation, this research study focuses on the effects of acoustic-induced vibrations on droplet sliding at different inclined angles on hybrid surfaces. A hydrophilic surface (silicon surface) has been used as the baseline in the study to be able to uncover the effects of vibration on pinned droplets. Firstly, the relationship between sliding angles and droplet volume was investigated experimentally for hybrid surfaces with different spacings. Then, the effects of natural resonance frequencies of droplets with different volumes on different surfaces were also studied using a resonance model and a customized experimental setup. Acoustic-induced vibrations were then applied to the surfaces to understand the effects of single or multiple resonance frequencies on droplet sliding angles. Droplet vibration and roll-off processes were experimentally characterized using a high speed imaging system. An acoustic sensor was also used to measure the induced frequencies and amplitudes.

Experimental results to date show that hybrid surfaces with larger spacing leads to lower droplet sliding angles. Furthermore, droplets under the influence of acoustic waves depict different contour morphologies when vibrating at different resonance frequencies. Moreover, droplet sliding angles can be reduced through vibration when the proper combination of droplet size and surface morphology is prescribed. Future studies will consider the use of acoustic waves in actual condensers.