Dynamics of droplet motion over hydrophobic surfaces with functionalized and non-functionalized ferro particles.

Dynamically manipulating droplet motion on hydrophobic surfaces is crucial in various fields, including biomedical, sensing, actuation, and oil-water separation applications. Ferrofluid droplets can be manipulated and controlled using external magnetic forces. The creation of ferrofluids involves multiple procedures that can affect the functionality and stability of droplet manipulation, limiting their use in sustainable applications. This study investigates the dynamics of droplet motion over functionalized and non-functionalized ferroparticles, considering different droplet volumes, ferroparticle layer widths, and wt% concentrations. The translational and sliding velocities of the droplets are measured using high-speed camera recording with a tracker application. The finding revealed the transformation of a droplet sliding motion into a rolling motion with propulsion under the magnetic influence. The sliding velocity increases for the droplets moving over the ordinary ferroparticles on the hydrophobic surface. However, the droplet motion is dominated by rolling in the case of hydrophobic ferro particles. The droplet sliding velocity rises sharply at high concentrations (or layer width) of ferroparticle as the magnetic bond number rises sharply to 3. A newborn droplet adheres to the magnet surface during droplet rolling and sliding motion.

Dust mitigation by a water droplet in between movable and modified wetting states surfaces.

A novel approach for mitigating environmental dust from hydrophobic surfaces using a water droplet is presented. A sessile droplet is sandwiched between two parallel plates, one of which is moveable and hydrophilic while the other is stationary and hydrophobic. Investigations are conducted into how plate spacing affects the dust mitigation rate and the droplet's level motion. The high-speed camera analyzes the droplet motion for various plate spacing, dusty regions, and droplet sizes. In a controlled laboratory setting, the movement of fluid and dust particles inside a droplet is simulated. The results showed that when a droplet is still, it effectively reduces dust. The droplet meniscus expands by decreasing the gap between the droplet and the surface, increasing the dust removal rate. While the Magdeburg force and surface tension influence the droplet's adhesion to a hydrophobic surface, surface tension remains the primary factor affecting droplet pinning on a hydrophilic plate, more so than pinning on a dusty hydrophobic surface. When compressing, a current is created in the droplet fluid, greatly accelerating the rate at which dust is removed from the hydrophobic surface. We also move a dangling droplet over a dirty surface to evaluate its cleaning effectiveness and find that a 60 µL droplet has a 97% cleaning effectiveness and can remove dust from up to 450 mm2 of surface area. Our study provides insight into the unique method of removing dust from active surfaces and sheds light on droplet pinning forces generated by the Magdeburg effect in nano-cavities during vertical and horizontal movement.

Water droplet behavior in between hydrophilic and hydrophobic surfaces and dust mitigation.

An innovative method is introduced for environmental dust mitigation from a hydrophobic surface by a sessile water droplet. The sessile water droplet is located between two parallel plates having hydrophilic (at the top) and hydrophobic (at the bottom) states. The water droplet is located at the top hydrophilic plate, and the effect of the plate spacing on dust mitigation rate is examined. The droplet behavior is analyzed for different plate spacings and various droplet sizes using a high-speed camera. The fluid and the particle motions are simulated inside the droplet while adopting the experimental conditions. The findings demonstrate that the sessile droplet can effectively mitigate dust. Reducing the plate spacing increases the droplet meniscus diameter and enhances the dust removal rate. The surface tension force on the hydrophilic surface remains greater than that of the pinning force on the dusty hydrophobic surface even though the Magdeburg and surface tension forces contribute to the droplet pinning force on the hydrophobic dusty surface. Flow current is developed in the droplet fluid during the squeezing period, which considerably enhances the dust removal rate from the hydrophobic surface. The cleaned area increases with the droplet volume and plate spacing. Stria patterns are observed on the circumference of the dust-removed area. The present study provides a detailed analysis of a new method of dust removal from surfaces for self-cleaning applications.

Droplet motion on sonically excited hydrophobic meshes.

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.

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.

Sliding and Rolling Motion of a Ferro-Liquid Droplet on the Hydrophobic Surface under Magnetic Influence.

The ferro-liquid droplet manipulation on hydrophobic surfaces remains vital for various applications in biomedicine, sensors and actuators, and oil-water separation. The magnetic influence of ferro-liquid droplets on the hydrophobic surface is elucidated. The mechanisms of a newborn droplet formation under the magnetic force are explored. The sliding and rolling dynamics of the ferro-liquid droplets are assessed for the various concentrations wt % of ferro-particles. High-speed recording and a tracker program are used to evaluate the droplet sliding and translational velocities. It is demonstrated that the mode of droplet motion changes from sliding to rolling as the magnetic Bond number increases, in which case, the droplet position becomes close to the magnet surface. The translational velocity of the droplet under rolling mode increases as the ferro-particle concentration in the droplet fluid increases. A further increase of the magnetic Bond number results in the creation of a newborn droplet attached to the magnet surface.

Liquid droplet impact on a sonically excited thin membrane.

The characteristics of droplet impact on hydrophobic surfaces can be altered by introducing surface oscillations. The contact duration, spreading, retraction, and rebounding behaviors of the impacting water droplet are examined at various sonic excitation frequencies of the hydrophobic membrane. Membrane oscillation and droplet behavior are analyzed by utilizing a high-speed camera. The restitution coefficient and membrane dynamics are formulated and the findings are compared with those of the experiments. It is found that the mode of membrane oscillation changes as the sonic excitation frequency is changed. The droplet spreading and retraction rates reduce while the rebound height and restitution coefficient increase at a sonic excitation frequency of 75 Hz. However, further increase of the excitation frequency results in reduced rebound height because of the increased energy dissipation on the impacted surface. The droplet contact (transition time) duration reduces as the excitation frequency increases. Increasing droplet Weber number enhances the droplet contact period on the membrane, which becomes more apparent at low frequencies of sonic excitation.

Hydrophobized metallic meshes can ease water droplet rolling.

Rolling liquid droplets are of great interest for various applications including self-cleaning of surfaces. Interfacial resistance, in terms of pinning and shear rate, has a critical role in droplet rolling dynamics on hydrophobic surfaces. Lowering the interfacial resistance requires reducing the droplet wetting length and droplet fluid contact area on hydrophobic surfaces. The present study examines droplet rolling behavior on inclined hydrophobized metallic meshes, which facilitate reduced wetting length and contact area of droplets. Experiments are carried out using a high-speed recording facility to evaluate droplet translational and rolling velocities over various sizes of hydrophobized meshes. The flow field inside the droplet fluid is simulated in 3-dimensional space mimicking the conditions of experiments. The findings reveal that droplet translational velocity attains significantly higher values for hydrophobized meshes than plain hydrophobized metallic surfaces. Increasing the mesh size enhances the droplet velocity and reduces the droplet kinetic energy dissipation created by interfacial surface tension and shear forces. Increasing the droplet volume enhances the droplet velocity despite the fact that pinning and frictional forces increase at the liquid-mesh interface. Hence, for rolling droplets on the mesh surface, the increase in the gravitational force component becomes larger than the increase in interfacial pinning and frictional forces.

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.

Droplet Rolling Dynamics over a Hydrophobic Surface with a Minute Width Channel.

Unidirectional and stabilize droplet rolling over hydrophobic surfaces is critical for self-cleaning applications of large areas. Introducing minute size channels on hydrophobic surfaces in the droplet rolling direction can minimize droplet wobbling and enables unidirectional rolling. The droplet rolling behavior over an inclined hydrophobic surface having a minute size channel is investigated. The flow field developed inside the droplet fluid is numerically simulated in a three-dimensional domain pertinent to experimental conditions. Experiments are carried out using a high-speed facility to monitor and evaluate droplet motion over channeled and flat hydrophobic surfaces. The findings revealed that predictions of the droplet translational velocity and those obtained from the experiments are in good agreement. The presence of a minute channel on the hydrophobic surface gives rise to droplet fluid inflection into the minute channel, which in turn modifies the center of mass of the droplet during rolling. This lowers the droplet wobbling height and enables the droplet to roll unidirectionally along the channel length. Enlarging the channel width on the hydrophobic surface increases droplet kinetic energy dissipation while reducing the droplet rolling speed. The complex flow structures formed in the droplet fluid modifies the pressure along the droplet centerline; however, the droplet fluid pressure remains almost the same order as the Laplace pressure in the upper region of a rolling droplet over the channeled hydrophobic surface.

Solution Crystallization of Polycarbonate Surfaces for Hydrophobic State: Water Droplet Dynamics and Life Cycle Assessment towards Self-Cleaning Applications.

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.

On the Mechanism of Human Saliva Interaction with Environmental Dust in Relation to Spreading of Viruses.

Environmental effects such as dust mitigation can amplify the spread of viruses via inhaling infected dust particles. Infusion and the spreading rate of human saliva over the dust particles can play a critical role in contiguous virus spread. In the present study, mechanical and chemical interactions of human saliva with environmental dust particles are considered. The saliva droplet impact on dust particles is examined while mimicking saliva droplet spreading during coughing in a dusty ambience. The mechanisms of saliva infusion and cloaking on the dust particles are explored. The characteristics of saliva droplet normal and oblique impacts on a dust particle are examined experimentally and numerically to evaluate the amount of saliva residues on the impacted particle surface. The findings reveal that the saliva liquid infuses and cloaks the dust particle surfaces. The saliva droplet impact on the dust particles leaves a considerable amount of saliva residues on the impacted surfaces, which remain undried for a prolonged period in indoor environments. Weak adhesion of the saliva-infected dust particles on surfaces, such as glass surfaces, enables saliva-infected dust particles to rejoin neighboring ambient air while possessing a high potential for virus spreading.

Impacting Water Droplets Can Alleviate Dust from Slanted Hydrophobic Surfaces.

Water droplet impacting on a slanted dusty hydrophobic surface is examined in relation to dust mitigation from surfaces. Impacting droplet characteristics including droplet spreading/retraction rates, slipping length, and rebound heights are analyzed via high-speed recording and a tracker program. The environmental dust characteristics in terms of size, shape, elemental composition, and surface free energy are evaluated by adopting the analytical methods. The findings reveal that the dynamic characteristics of the impacting droplet on the slanted hydrophobic surface are significantly influenced by the dust particles. The maximum droplet spreading over the dusty surface becomes smaller than that of the nondusty surface. The presence of the dust particles on the slanted hydrophobic surface increases energy dissipation, and the water droplet slipping length over the surface becomes less than that corresponding to the nondusty surface. Impacting droplet fluid infuses over the dust particle surface, which enables mitigation of dust from the surface to the droplet fluid. A dust-mitigated area on the slanted surface is larger than that corresponding to the horizontal surface; in which case, the area ratio becomes almost six-fold, which slightly reduces with increasing Weber number. The optical transmittance of the dust-mitigated surface by the impacting droplet remains high.

Sliding Water Droplet on Oil Impregnated Surface and Dust Particle Mitigation.

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.

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%.

Environmental dust repelling from hydrophilic/hydrophobic surfaces under sonic excitations.

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.

Environmental dust repelling from hydrophobic and hydrophilic surfaces under vibrational excitation.

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).

A water droplet-cleaning of a dusty hydrophobic surface: influence of dust layer thickness on droplet dynamics.

Water droplet cleaning of a dusty hydrophobic surface is examined. Environmental dust are used in the experiments and cloaking velocity of a dust layer by a droplet fluid is measured and hemi-wicking conditions for the dust layer are analyzed adopting the pores media wick structure approach. A droplet motion on dusty and inclined hydrophobic surface is analyzed using a high speed digital imaging system. Influences of dust layer thickness, droplet volume, and surface inclination angle on the mechanisms of dust removal by a rolling droplet are evaluated. The findings revealed that dust cloaking velocity decreases exponentially with time. The droplet fluid can cloak the dust layer during its transition on the dusty surface. The transition period of droplet wetted length on the dusty surface remains longer than the cloaking time of the dust layer by the droplet fluid. Translational velocity of rolling droplet is affected by the dust layer thickness, which becomes apparent for small volume droplets. Small volume droplet (20 µL) terminates on the thick dust layer (150 µm) at low surface inclination angle (1°). The quantity of dust picked up by the rolling droplet increases as the surface inclination angle increases. The amount of dust residues remaining on the rolling droplet path is relatively larger for the thick dust layer (150 µm) as compared to its counterpart of thin dust layer (50 µm).

Carbonated Water Droplet Can Ease Dust Mitigation from Hydrophobic Surfaces.

Carbonated water droplets can ease the difficulties faced by distilled water droplets mitigating dust particles from hydrophobic surfaces. Rising of CO2 bubbles in carbonated water droplets and their interaction with the flow structure, created by Marangoni and buoyancy possessions, in droplets are investigated. Spreading and infusion (cloaking) of carbonated water on dust surfaces are analyzed, and the rate at which bubbles formed inside the carbonated water droplet, as placed on a dusty hydrophobic surface, is examined. Flow structures formed inside the carbonated water droplet are simulated, and findings are compared to those corresponding to the distilled water droplet. Dust mitigation from the hydrophobic surface toward droplet liquid inside is evaluated using the high-speed recording system, and the results are compared with those of predictions. It is found that carbonated water spreads and infuses onto dust particles at a higher rate than that at which distilled water does. The rising bubble generates wake-like flow in the fluid while modifying the flow structure inside the droplet; hence, the number of circulating structures increases from two to four in droplet fluid. The dust particles picked up by flow currents are redistributed over the entire carbonated water droplet, while mitigated dust particles remain in the lower region of the distilled water droplet. Bubbles formed inside the carbonated water droplet improve dust lifting and rate of dust mitigation from the surface.

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.