Water Droplet Dynamics on a Hydrophobic Surface in Relation to the Self-Cleaning of Environmental Dust

Abstract

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.

Figure 1

SEM micrographs of the dust particles: (a) particles with various shapes and sizes, (b) small dust particles attached to the surface of large particles, XPS peaks for binding energy of sodium, potassium, and chlorine.

Figure 2

X-ray diffractogram of the dust particles.

Figure 3

AFM data of the dust particles: (a) friction data for the dust particles and (b) retention force of the dust particle. The frictional force is also shown for the clean surface.

Figure 4

SEM micrographs of the solution-crystallized polycarbonate surface before and after functionalized particles deposition: (a) crystallized polycarbonate surface showing hierarchically distributed spherules and (b) crystallized polycarbonate surface depicting fibril-like structures formed on the spherules, and (c) micrograph of the surface after functionalized silica particles deposition.

Figure 5

AFM image and line scan of the solution-crystallized surface and that deposited with functionalized silica particles: (a) 3D image of the surface and (b) line scan on the surface.

Figure 6

Sessile water droplet images and contact angles on the surfaces of as received polycarbonate, crystallized polycarbonate, and functionalized silica particles deposited crystallized polycarbonate samples.

Figure 7

Advancing and receding angles of the water droplet on the hydrophobic surface: (a) clean surface and (b) dusty surface.

Figure 8

Rotational speed (a) and translational velocity (b) of a water droplet predicted analytically (S1 and S2) and obtained from a high-speed camera for the clean hydrophobic surface. Droplet volume is 40 μL, and inclination angle is 3°.

Figure 9

Droplet height along the clean and dusty hydrophobic surface.

Figure 10

Forces acting on the hydrophobic surface during droplet motion on clean and dusty surfaces: (a) retention force and (b) total retention force. The forces were calculated analytically (S1) after determining the droplet advancing and receding angles and the droplet diameter at each location on the hydrophobic surface.

Figure 11

Variation of Froude and Webber numbers along distance: (a) Froude Number variation. (b) Webber Number variation.

Figure 12

Translational velocity of the droplet on clean and dusty surfaces for various droplet locations. Droplet location is the distance between droplet initiation and the start of dusty region on the hydrophobic surface.

Figure 13

Side and top images of the droplet obtained from a high-speed camera recording for clean and hydrophobic surfaces at various durations.

Figure 14

3D optical image of the droplet path and SEM micrographs of dust particle residue left on the droplet path. The red circle depicts the dust particle residue along the droplet path.

Figure 15

Optical images of a droplet on the hydrophobic surface with functionalized dust particles: (a) droplet on the hydrophobic surface with functionalized dust particles at different time duration and (b) top image of the droplet on the surface with functionalized and normal dust particles.

Figure 16

Water cloaking of normal and functionalized dust particles: (a) cloaking velocity and cloaking images of normal dust particles at different duration and (b) non-cloaking images of functionalized dust particles at different durations. Water does not cloak the functionalized dust particles for extended periods.

Figure 17

Slip velocity (a) and rotational speed (b) of the droplet on clean and dusty hydrophobic surfaces for different droplet locations. The droplet location represents the distance between the droplet initiation and the start of the dusty region.

Figure 18

UV-visible transmittance of the hydrophobic surface before and after dust removal by water droplets. The inset figure depicts the improvement ratio of the transmittance. The ratio was determined from the difference between the optical transmittance before and after dust removal by water droplets over the difference between the optical transmittance of the hydrophobic surface before dust deposition and after removal of dust from surface by water droplets.