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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]

3D printing plays an important role in manufacturing of parts. Recent developments in 3D printing technology enable to print various shapes, sizes, and functional surfaces. The printed surfaces can have texture topology, which enables changing wetting state from hydrophilic to hydrophobic. This requires proper setting of printing parameters. In the present study, 3D printing of samples from titanium alloy (Ti-6Al-4V) is carried out. The resulting surface topology is examined using the analytical tools. The wetting state of 3D printed sample surfaces is assessed incorporating the contact angle method. The surface free energy of the printed samples is determined adopting the liquid contact angle technique. It is found that printed surfaces possess hydrophobic wetting state with droplet contact angle of 135° ± 5°. The droplet contact angle hysteresis is of the order of 36°, which causes pinning of the droplet on the surface. The surface free energy is determined as 54.2 mJ/m2. [ABSTRACT FROM AUTHOR]

Impacting droplets and droplet ejection from hydrophobic mesh surfaces have interest in biomedicine, heat transfer engineering, and self-cleaning of surfaces. The rate and the size of newborn droplets can vary depending on the droplet fluid properties, Weber number, mesh geometry, and surface wetting states. In this study, impacting water droplets onto hydrophobic mesh surface is investigated and impact properties including, spreading, rebounding, and droplet fluid penetration and ejection rates are examined. Droplet behavior is assessed using high recording facilities and predicted in line with the experiments. The findings reveal that the critical Weber number for droplet fluid penetrating/ejecting from mesh screen mainly depends on the droplet fluid capillary length, and hydrophobic mesh size. The contact time of impacting droplet over mesh surface reduces with increasing droplet Weber number, which opposes the case observed for impacting droplets over flat hydrophobic surfaces. The restitution coefficient attains lower values for impacting droplets over mesh surfaces than that of flat surfaces. The rate and diameter of the ejected droplet from the mesh increases as droplet Weber increases. At the onset of impact, streamline curvature is formed inside droplet fluid, which creates a stagnation zone with radially varying pressure at the droplet fluid mesh interface. This reduces the ejected droplet diameter from mesh cells as mesh cells are located away from the impacting vertical axis. [ABSTRACT FROM AUTHOR]

Carbonated water drops impact on a hydrophobic surface is examined. The influence of CO2 gas bubbles in droplet fluid on impacting droplet characteristics, such as spreading rates and restitution coefficient, are explored. The predictions of droplet wetting diameter and spreading rates are validated through the experimental data obtained from high-speed recording. The findings reveal that predictions agree well with the experimental data. CO2 gas bubbles in the droplet are compressed by the total impact pressure of the droplet liquid while slightly reducing the gas bubble sizes. The small size of close by bubbles at high pressure can merge forming large size bubbles, which occur toward the end of droplet spreading and retraction periods. The pressure increase in the fluid gives rise to the increased vertical height of the droplet and slightly reduces the droplet contact diameter on the impacted surface. The work done during the compression of CO2 gas in bubbles lowers the restitution coefficient of the droplet after the retraction period. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Impacting droplet on a hydrophobic surface is investigated and droplet size effect on impacting properties is examined. Liquid pressure variation inside droplet is numerically simulated in the impacting and rebounding periods. Droplet motion on impacted hydrophobic surface is monitored using a high-speed recording system. We showed that predictions and high-speed data for droplet shape and geometric features appear to be almost identical in the spreading and retraction of the droplet on sample surface. Increased volume of droplet gives rise to the peak pressure enhancement in droplet liquid during impact. The maximum droplet height remains larger for large volume droplets in both spreading and retraction cycles. Increasing size of droplet enlarges the wetting diameter on the impacted surface during droplet deformation on sample surfaces. The rate of peak velocity of the spreading surface of the droplet is faster for larger droplets as compared to that corresponding to small droplets. The ratio of spreading period over the retraction period of the droplet becomes small for droplets with small size. [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]

Summary: In order to minimize dust effects on optical surfaces related to the solar energy harvesting, adhesion between the active surface of energy harvesting device and the dust particles needs to be lowered. Consequently, the pinning force for the dust particles reduces, and the dust particles can be removed via creating the self‐cleaning effect. The wetting state of the active surface becomes critical towards reducing the pinning force. In this case, hydrophobic surfaces remain favorable reducing the dust adhesion on the surfaces. In the present perspective, characteristics of the environmental dust particles and their effects in humid air ambient are presented. The methods for dust removal from the surfaces and the optical transmittance reduction by the dust particles are discussed. The challenges and future perspectives of surface texturing towards achieving hydrophobicity and optical transmittance are also introduced. [ABSTRACT FROM AUTHOR]

Highlights • Flow and temperature fields inside droplet are significantly influenced by hydrophobic plates spacing. • Almost stagnation region is developed in central region of droplet when plates spacing is reduced. • Nusselt number remains almost same for further reduction of the plates spacing. • Maximum Bond number remains less than unity in droplet indicating importance of Marangoni current. Abstract Heat transfer and fluid flow inside the droplet located in between two parallel hydrophobic surfaces are considered. The functionalized silica nano-particles are deposited on the glass plates to create hydrophobic wetting state. The influence of hydrophobic glass plate spacing (plate heights) on the heat transfer characteristics and internal fluidity of droplet is examined. Temperature disturbance is introduced across the droplet via altering temperature settings on the top and the bottom plates. An experiment is carried out to monitor the droplet geometric features via high speed camera during squeezing action of the plates. The flow and temperature fields in the droplet fluid are simulated incorporating the experimental conditions. The direction of heat transfer from the plate surface to the droplet fluid is changed and the effect of heat transfer direction on internal fluidity of the droplet is investigated. The velocity predictions are validated with the Particle image velocimetry (PIV) data. It is found that velocity predictions agree well with the PIV data. The flow and temperature fields are influenced significantly by changing the plates height. In this case, reducing the plates height alters the size and orientation of the circulation cells inside the droplet fluid. Varying the plates temperature changes the Nusselt number; in which case, droplet heating from the bottom plate results in higher values of the Nusselt number than that of the top plate heating. The Bond number remains less than unity while indicating the importance of the Marangoni current on the flow field and heat transfer. [ABSTRACT FROM AUTHOR]

Water droplet mobility on the inclined hydrophobic surface coated with n-octadecane phase change material is examined. Polycarbonate surface is initially crystallized and later nano-size functionalized silica particles are deposited on the crystallized surface to increase the water droplet contact angle and reduce the contact angle hysteresis. Reversible exchange of wetting state is achieved via introducing a thin film of n-octadecane phase change material with 1.5 μm thickness on the functionalized silica particles deposited surface. The resulting surface remains hydrophobic if n-octadecane is in the solid phase while it becomes hydrophilic if thin film remains in the liquid phase. The surface wetting state reversibly exchanges during melting and re-solidification of n-octadecane film. Water droplet undergoes rolling and sliding on the inclined hydrophobic surface if n-octadecane is in solid phase. In this case, the solid n-octadecane flakes are formed on the surface and the functionalized silica particles deposited surface exposes to the free surface from the close regions of the flakes. This gives rise to a hydrophobic characteristic on the surface with droplet contact angle 140° and contact angle hysteresis 8°. Droplet rolling and sliding are influenced by the solid flake distribution on the n-octadecane coated surface. The closely spaced solid flakes results in hydrophilic surface while contributing to the droplet sliding on the inclined surface, which is more pronounced for large volume droplets. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Polycarbonate sheets are optically transparent and have the potential to be used as one of the cover materials for PV applications. Solution treatment of polycarbonate surfaces enables to create surface texture topology giving rise to a hydrophobic state, which is favorable for self-cleaning applications. In the present study, hydrophobization of polycarbonate surface is investigated via crystallization of surface by a one-step process. The influence of texture topology, which is created via crystallization, on water droplet mobility and optical transmittance is examined. Findings revealed that solution treatment, using acetone, results in crystallized polycarbonate surfaces with a hydrophobic state. Depending on the treatment duration, the texture characteristics of crystallized surface change while influencing the water contact angle hysteresis. This in turn affects the droplet mobility over the inclined crystallized surface and alters the UV visible transmittance. Moreover, the droplet mobility improves and dust mitigation rates from the treated surface increase as the solution treatment duration are reduced to 2 min. Oil impregnated samples result in improved UV visible transmittance; however, droplet motion changes from rolling to sliding over the surface. A sliding water droplet enables the removal of the dust particles from the oil-impregnated sample surface. [ABSTRACT FROM AUTHOR]

Self-cleaning of surfaces becomes challenging for energy harvesting devices because of the requirements of high optical transmittance of device surfaces. Surface texturing towards hydrophobizing can improve the self-cleaning ability of surfaces, yet lowers the optical transmittance. Introducing optical matching fluid, such as silicon oil, over the hydrophobized surface improves the optical transmittance. However, self-cleaning ability, such as dust mitigation, of the oil-impregnated hydrophobic surfaces needs to be investigated. Hence, solution crystallization of the polycarbonate surface towards creating hydrophobic texture is considered and silicon oil impregnation of the crystallized surface is explored for improved optical transmittance and self-cleaning ability. The condition for silicon oil spreading over the solution treated surface is assessed and silicon oil and water infusions on the dust particles are evaluated. The movement of the water droplet over the silicon oil-impregnated sample is examined utilizing the high-speed facility and the tracker program. The effect of oil film thickness and the tilting angle of the surface on the sliding droplet velocity is estimated for two droplet volumes. The mechanism for the dust particle mitigation from the oil film surface by the sliding water droplet is analyzed. The findings reveal that silicon oil impregnation of the crystallized sample surface improves the optical transmittance significantly. The sliding velocity of the water droplet over the thick film (~700 µm) remains higher than that of the small thickness oil film (~50 µm), which is attributed to the large interfacial resistance created between the moving droplet and the oil on the crystallized surface. The environmental dust particles can be mitigated from the oil film surface by the sliding water droplet. The droplet fluid infusion over the dust particle enables to reorient the particle inside the droplet fluid. As the dust particle settles at the trailing edge of the droplet, the sliding velocity decays on the oil-impregnated sample. [ABSTRACT FROM AUTHOR]

Dynamics of dust mitigation from inclined hydrophilic/hydrophobic glass surfaces are examined after introducing the electrostatic repulsion. Glass surfaces are hydrophobized via coating by functionalized nano-silica particles. An electronic circuit and electrostatic plate were constructed to create electrostatic force mitigating the dust particles from the surface. A high speed recording system is used to track and record the motion of the inflight particles repelled from the surfaces. Adhesion of dust on the hydrophilic/hydrophobic glasses is assessed using the atomic force microscopy technique. The surface of the samples is tilted to monitor the influence of surface tilting on: i) dynamics of the inflight particles, ii) dust mitigation rate, iii) and optical characteristics of surfaces prior and after particle repelling. The findings reveal that hydrophobizing the sample surfaces reduces the particle adhesion on the sample surfaces and the force of adhesion remains larger for small particle (0.8 μm) than that of the large size particle (10 μm), which is because of the large interfacial force created between the low dimensional particles and the sample surface. The dust particle displacement alongside x, y, and x-axes increases for the hydrophobic surface onset of electrostatic excitation, which becomes more apparent for the tilted sample surfaces. Optical transmittance improves for the particle repelled from hydrophobic surfaces and it further improves as the surface tilting angle increases to 50°. • Dust particles with small sizes (≤0.8 μm) strongly adhere to the glass surface. • Adhesion force reduces as the surface changes from hydrophilic to hydrophobic. • Maximum particle displacement occurs along x and y-axes under electrostatic influence. • Coverage area of dust repelled from inclined hydrophobic surface glass surface is large. • Optical transmittance improves after electrostatic dust repelling from hydrophobic surface. [ABSTRACT FROM AUTHOR]

Dust repelling from transparent polyvinyl chloride film surface via sonic excitation is examined and dynamics of repelled (inflight) dust particles are analyzed. An experimental rig is designed and built to assess the vibrational characteristics of the polyvinyl chloride film at different frequencies of sonic excitation. A high speed recording system and tracking program are utilized monitoring and evaluating the dynamics of the inflight particles. The dynamics of inflight particles are also simulated numerically and the predictions are compared with those of the experimental data. In order to examine the influence of dust particle adhesion on the dynamics of the inflight particles, the polyvinyl chloride film surface is hydrophobized through dip coating by functionalized nano-silica particles. Improvement of the optical transmittance of the dust mitigated film is determined via outdoor tests. The findings demonstrate that sonic excitation repels the particles from the film surface and it is more pronounced at 64 Hz excitation frequency while demonstrating that sonic excitation can be used for dust removal from transparent surfaces. The mitigation via sonic excitation improves the optical transmittance of the dusty surface by 77%, which becomes more apparent for hydrophobic surfaces. [ABSTRACT FROM AUTHOR]

Mitigation of environmental dust from surfaces becomes one of the challenges for maintaining the optical characteristics of surfaces. Dust repelling from hydrophobic and hydrophilic surfaces under vibrational excitation is investigated and the percentage of dust repelled from surfaces is evaluated. The characteristics of the dust particles are examined and dust adhesion on surfaces under molecular forces (van der Walls) is explored. High speed recording system is utilized to monitor dust repelling from the surfaces. The dust residues, which are not repelled from the sample surfaces, are analyzed and the percentage of area coverage of the dust repelled from the surfaces is assessed. The repelling height of the dust is predicted analytically, and the findings are compared with the experimental data. Findings revealed that the analytical predictions of dust repelling height are in good agreement with the experimental data. Due to none-stoichiometric elemental compositions in the dust compounds, ionic forces are created while forming the cluster-like structures because of particle adhesion. The vibrational excitation repels dust from sample surfaces in the form of cluster-like structures. Dust repelled from hydrophobic surface results in a larger clean area on the hydrophobic surface (80% of total surface area) than that of the hydrophilic surface (20% of total surface area). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Thermal analysis of a locally heated droplet located in two parallel plates with hydrophobic surfaces is considered. The spacing between the parallel plates is altered to modify the three-phase contact line of the droplet on both hydrophobic surfaces. The thermal state of the walls is altered via changing temperature difference between the parallel plates while initiating the droplet heat transfer. In this case, the bottom plate temperature is increased, while the top plate temperature is kept constant. The thermal field developed inside the droplet is obtained numerically incorporating the conditions used in the experiments. The influence of the droplet volume on the Nusselt and the Bond numbers is examined for the cases of droplets formed (i) on a hydrophobic plate and (ii) in between two parallel plates with hydrophobic wetting state. It is found that the droplet wetted by the parallelly located hydrophobic plates alters the circulation cell structure formed inside the droplet. Temperature field extends further inside the droplet as the wetted diameter is extended on the hydrophobic surfaces. The Nusselt number attains larger values for the two-plate case than the free droplet on a single plate case. The Bond number remains less than unity for the free and two-plate wetted droplets. [ABSTRACT FROM AUTHOR]