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The sonic excitation of the liquid droplet on a hydrophobic mesh surface gives rise to a different oscillation behavior than that of the flat hydrophobic surface having the same contact angle. To assess the droplet oscillatory behavior over the hydrophobic mesh, the droplet motion is examined under the external sonic excitations for various mesh screen aperture ratios. An experiment is carried out and the droplet motion is recorded by a high-speed facility. The findings revealed that increasing sonic excitation frequencies enhance the droplet maximum displacement in vertical and horizontal planes; however, the vertical displacements remain larger than those of the horizontal displacements. The resonance frequency measured agrees well with the predictions and the excitation frequency at 105 Hz results in a droplet oscillation mode (n) of 4. The maximum displacement of the droplet surface remains larger for the flat hydrophobic surface than that of the mesh surface with the same contact angle. In addition, the damping factor is considerably influenced by the sonic excitation frequencies; hence, increasing sonic frequency enhances the damping factor, which becomes more apparent for the large mesh screen aperture ratios. The small-amplitude surface tension waves create ripples on the droplet surface. [ABSTRACT FROM AUTHOR]

Impacting water droplet on a hydrophobic soft surface is investigated and impacting droplet behavior during the spreading and retraction phases is examined. Polyisoprene elastomer surface is hydrophobized via deposition with functionalized nanosilica particles using the dip coating method. Hydrophobized surface results in the contact angle of 135±3deg with a hysteresis of 2±1deg, and stretching and stretch relaxing of the coated samples do not alter the wetting state of the surfaces. Pressure variation, spreading, and retraction rates of droplet are simulated and findings are validated through the experimental data obtained from high-speed video system. The findings are also compared to those obtained for the impacting droplet on the hydrophobized glass surfaces. It is observed that predictions of droplet height and droplet shape agree well with the experimental data. Spreading period of the impacting droplet on the hydrophobized elastomer surface is longer than the hydrophobic glass surface; contrary, the retraction period of the impacting droplet is shorter on the elastomer surface than the glass surface. Impacting droplet generates large amplitude oscillatory surface waves on the elastomer surface and as the time progresses the wave amplitude reduces considerably, particularly along the surface (x-axis). [ABSTRACT FROM AUTHOR]

In this paper, the impact mechanisms of a water droplet on hydrophobized micro-post array surfaces are examined and the influence of micro-post arrays spacing on the droplet behavior in terms of spreading, retraction, and rebounding is investigated. Impacting droplet behavior was recorded using a high-speed facility and flow generated in the droplet fluid was simulated in 3D geometry accommodating conditions of the experiments. Micro-post arrays were initially formed lithographically on silicon wafer surfaces and, later, replicated by polydimethylsiloxane (PDMS). The replicated micro-post arrays surfaces were hydrophobized through coating by functionalized nano-silica particles. Hydrophobized surfaces result in a contact angle of 153° ± 3° with a hysteresis of 3° ± 1°. The predictions of the temporal behavior of droplet wetting diameter during spreading agree with the experimental data. Increasing micro-post arrays spacing reduces the maximum spreading diameter on the surface; in this case, droplet fluid penetrated micro-posts spacing creates a pinning effect while lowering droplet kinetic energy during the spreading cycle. Flow circulation results inside the droplet fluid in the edge region of the droplet during the spreading period; however, opposing flow occurs from the outer region towards the droplet center during the retraction cycle. This creates a stagnation zone in the central region of the droplet, which extends towards the droplet surface onset of droplet rebounding. Impacting droplet mitigates dust from hydrophobized micro-post array surfaces, and increasing droplet Weber number increases the area of dust mitigated from micro-post arrays surfaces. [ABSTRACT FROM AUTHOR]