The dynamic motion of a water droplet on an inclined hydrophobic surface is analyzed with and without environmental dust particles on the surface. Solution crystallization of a polycarbonate surface is carried out to generate a hydrophobic surface with hierarchical texture composed of micro/nanosize spheroids and fibrils. Functionalized nanosize silica particles are deposited on the textured surface to reduce contact angle hysteresis. Environmental dust particles are collected and characterized using analytical tools prior to the experiments. The droplet motion on the hydrophobic surface is assessed using high-speed camera data, and then, the motion characteristics are compared with the corresponding analytical results. The influence of dust particles on the water droplet motion and the amount of dust particles picked up from the hydrophobic surface by the moving droplet is evaluated experimentally. A 40 μL droplet was observed to roll on the hydrophobic surface with and without dust particles, and the droplet slip velocity was lower than the rotational velocity. The rolling droplet removes almost all dust particles from the surface, and the mechanism for the removal of dust particles from the surface was determined to be water cloaking of the dust particles.
An understanding of the dynamic motion of a water droplet is critical to reduce the effort required to remove dust particles from such surfaces. In line with self-cleaning applications, the wobbling and geometric variations of a rolling droplet were experimentally assessed for various droplet sizes.Furthermore, the internal fluidity of a rolling droplet was numerically predicted. The findings revealed that the rotational Bond number influenced the droplet wobbling due to adhesion force variations during rolling. Small-sized droplets, which were comparable to the capillary length, resulted in higher rotational speeds than those of larger-sized droplets. The ability to alter the rolling characteristics of droplets on inclined hydrophobic surfaces could address the limitations of self-cleaning surfaces and has implications for efficiency enhancements in solar energy devices.
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