Nanowall Textured Hydrophobic Surfaces and Liquid Droplet Impact.

Water droplet impact on nanowires/nanowalls' textured hydrophobic silicon surfaces was examined by assessing the influence of texture on the droplet impact dynamics. Silicon wafer surfaces were treated, resulting in closely packed nanowire/nanowall textures with an average spacing and height of 130 nm and 10.45 µm, respectively. The top surfaces of the nanowires/nanowalls were hydrophobized through the deposition of functionalized silica nanoparticles, resulting in a droplet contact angle of 158° ± 2° with a hysteresis of 4° ± 1°. A high-speed camera was utilized to monitor the impacting droplets on hydrophobized nanowires/nanowalls' textured surfaces. The nanowires/nanowalls texturing of the surface enhances the pinning of the droplet on the impacted surface and lowers the droplet spreading. The maximum spreading diameter of the impacting droplet on the hydrophobized nanowires/nanowalls surfaces becomes smaller than that of the hydrophobized as-received silicon, hydrophobized graphite, micro-grooved, and nano-springs surfaces. Penetration of the impacted droplet fluid into the nanowall-cell structures increases trapped air pressure in the cells, acting as an air cushion at the interface of the droplet fluid and nanowalls' top surface. This lowers the droplet pinning and reduces the work of droplet volume deformation while enhancing the droplet rebound height.

On the mechanism of droplet rolling and spinning in inclined hydrophobic plates in wedge with different wetting states.

A water droplet rolling and spinning in an inclined hydrophobic wedge with different wetting states of wedge plates is examined pertinent to self-cleaning applications. The droplet motion in the hydrophobic wedge is simulated in 3D space incorporating the experimental data. A high-speed recording system is used to store the motion of droplets in 3D space and a tracker program is utilized to quantify the recorded data in terms of droplet translational, rotational, spinning, and slipping velocities. The predictions of flow velocity in the droplet fluid are compared with those of experimental results. The findings revealed that velocity predictions agree with those of the experimental results. Tangential momentum generated, via droplet adhesion along the three-phase contact line on the hydrophobic plate surfaces, creates the spinning motion on the rolling droplet in the wedge. The flow field generated in the droplet fluid is considerably influenced by the shear rate created at the interface between the droplet fluid and hydrophobic plate surfaces. Besides, droplet wobbling under the influence of gravity contributes to the flow inside the rolling and spinning droplet. The parallel-sided droplet path is resulted for droplet emerging from the wedge over the dusty surface.

Avalanche effect for chemically modified dust mitigation from surfaces.

Cost effective dust mitigation from surfaces is one of the challenges in various sectors. The reduction of dust adhesion on surfaces plays a vital role for dust mitigation from surfaces under the gravitational influence. Creating an avalanche effect on dusty surfaces improves the dust mitigation rate and provides an effective cleaning process. Hence, solution treatment of dust by low concentration hydrofluoric acid is considered towards reducing dust adhesion on glass surfaces. To increase the rate of dust mitigation, the avalanche influence is created by the higher density and larger size particles (5300 kg/m3 and ~ 50 µm) than the average size dust particles (2800 kg/m3 and 1.2 µm) via locating them in the top region of the dusty glass surfaces. Mitigation velocity of the dust particles is evaluated using a high-speed recording system and the tracker program. Findings revealed that solution treatment (curing) of the dust particles results in the formation of fluorine compounds, such as CaF2 and MgF2, on dust surfaces, which suppress dust adhesion on surfaces. OSHA Globally Harmonized System lists the fluorine compounds formed as environmentally non-harmful. Avalanche's influence results in dust mitigation at a smaller tilt angle of the glass surface (~ 52°) than that of the case with none-avalanche influence (63°). Area cleaned on the glass surface, via dust mitigation, is larger as the avalanche is introduced, which becomes more apparent for the solution treated dust particles. Dust mitigation under avalanche influence improves optical transmittance of the dusty glass samples by a factor of 98%.

Carbonated water droplets on a dusty hydrophobic surface.

Dust mitigation from surfaces remains essential, particularly for the efficient operation of energy harnessing devices. Although various dust removal methods have been introduced, the self-cleaning method is favorable because of the cost-effective cleaning process. Dust mitigation from surfaces by water droplets, mimicking nature, is fruitful because it involves low-cost operations. The dust removal rate from surfaces by rolling water droplets can be increased by creating bubbles inside the rolling droplets through which dust pinning on surfaces can be lowered and the droplet liquid infusion on dust surfaces can be enhanced. This study provides insight into bubble formation and dust mitigation in carbonated and distilled water droplets located on hydrophobic surfaces by examining bubble formation and dust distribution inside the water droplets. The behavior of bubbles inside the carbonated water droplet and emanating from the hydrophobic surface was recorded and analyzed by incorporating high-speed camera data. The influence of environmental dust particles on bubble formation was also assessed. Bubble velocity was formulated analytically and the findings are compared with those of the experimental values. Findings revealed that the bubble formation inside the carbonated droplet fluid had a significant effect on the transition of dust particles from the hydrophobic surface towards the droplet fluid. The volume concentration of dust particles in the carbonated water droplet was almost 1.5 to 2.5 times larger than that of the distilled water droplet. The dissolution of alkaline and alkaline earth metal compounds in the carbonated droplet fluid acted like nucleation centers for bubble formation; hence, the number of bubbles formed on the dusty hydrophobic surface was greater than that of the clean hydrophobic surface. Some bubbles attached at the dust particle surface contributed to dust mobility in the droplet fluid, which occurred particularly in the droplet bottom region. This enhanced the velocity of the dust particles transiting from the dusty hydrophobic surface to the droplet fluid interior by almost 1.5 times in the early period.

Droplet Rolling and Spinning in V-Shaped Hydrophobic Surfaces for Environmental Dust Mitigation.

The motion of a water droplet in a hydrophobic wedge fixture was examined to assess droplet rolling and spinning for improved dust mitigation from surfaces. A wedge fixture composed of two inclined hydrophobic plates had different wetting states on surfaces. Droplet rolling and spinning velocities were analyzed and findings were compared with the experiments. A wedge fixture was designed and realized using a 3D printing facility and a high speed recording system was adopted to evaluate droplet motion in the wedge fixture. Polycarbonate sheets were used as plates in the fixture, and solution crystallization and functionalized silica particles coating were adopted separately on plate surfaces, which provided different wetting states on each plate surface while generating different droplet pinning forces on each hydrophobic plate surface. This arrangement also gave rise to the spinning of rolling droplets in the wedge fixture. Experiments were extended to include dust mitigation from inclined hydrophobic surfaces while incorporating spinning- and rolling droplet and rolling droplet-only cases. The findings revealed the wedge fixture arrangement resulted in spinning and rolling droplets and spinning velocity became almost 25% of the droplet rolling velocity, which agrees well with both predictions and experiments. Rolling and spinning droplet gave rise to parallel edges droplet paths on dusty hydrophobic surfaces while striations in droplet paths were observed for rolling droplet-only cases. Spinning and rolling droplets mitigated a relatively larger area of dust on inclined hydrophobic surfaces as compared to their counterparts corresponding to rolling droplet-only cases.