The impact characteristics of liquid droplet on the nano-textured superhydrophobic surface have been investigated experimentally and numerically to understand the underlying mechanism and select ...appropriate models to describe them. The evolution of impact process with droplet impact velocity varied form 0.11–3.9 m/s (or Weber number ranging from 0.3 to 421.4) was recorded and analyzed. Besides, a conservative level set method coupled with dynamic contact angle models was developed to track the phase interface and made a comparison with the experiment. During the spreading and receding phases, the numerical simulation results showed good agreement with the experiment. The impact of droplet satisfied the energy conservation between kinetic and surface energy, and the maximum spreading factor (βmax) can be well correlated by a scaling law of βmax ~ We0.52. Both of the contact time and non-dimensional contact time of bouncing droplets were independent of Weber number in the range of 1.5 to 121. The predicted non-dimensional contact times were in good agreement with the reference value of 2.6 ± 0.1 at low and medium Weber numbers. The dynamic contact angle models derived from the hydrodynamic theory and molecular-kinetic theory, respectively, can mainly apply to the advancing and receding phases of droplet impact on nano-textured superhydrophobic surfaces.
•Droplet impact on nano-textured superhydrophobic surfaces was investigated.•A conservative level set method coupled with dynamic contact angle models was developed.•The maximum spreading factor can be well correlated by a scaling law of βmax ~ We0.52.•The non-dimensional contact time was independent of Weber number in the range of 1.5 to 121.
A droplet can bounce in a short contact time after impacting on superhydrophobic surfaces (SHSs), which is of benefit to the applications like anti-icing, anti-frosting and self-cleaning. In this ...study, we numerically investigate the fast droplet bouncing induced by asymmetric spreading on the concave SHS with a trapezoidal cross-section and focus on the droplet morphology evolution and contact time variation. A preferential transport at the both sides of the droplet from the inclined part to the flat part will occur because of the concave structure feature. The droplet will exhibit asymmetric spreading behavior. Moreover, the effects of structure parameters and impact velocity on the droplet contact time are explored. Within this concave structure, the droplet contact time is less influenced by the impact velocity and more influenced by the structural parameters. The presence of the tangential momentum can enhance the droplet asymmetric spreading, but it is not always beneficial to the droplet contact time reduction. Therefore, there will be an optimal structure to achieve the minimum droplet contact time. A concave SHS structure with the structure width w= 1–1.2r and the inclination angle of 45° is a more desirable choice, which is expected to reduce the droplet contact time by 40~45%.
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•The attached-wall cavitation bubble evolves on chemically patterned walls is studied.•The force analysis on the dynamic contact angle evolution process is proposed.•Both wall temperature and ...wettability effects on heat flux distribution are analyzed.
In this study, we aim to investigate the contact-point characteristics of cavitation bubble evolution on different chemically patterned surfaces using the thermal pseudopotential lattice Boltzmann method (LBM). The local temperature variation is considered in the contact-point dynamics simulation. Force analysis is performed on the contact points, and two main controlling factors for contact-point dynamics are identified, namely, the local pressure difference and the unbalanced Young’s force. A dimensionless temperature is used to describe the heat exchange efficiency between the wall and fluids for different wall wettability and temperatures. The attached-wall bubble on the wet wall has an unbalanced Young’s force, which is directed toward the inside of the bubble throughout the entire growth and collapse stages at the contact point, resulting in a smaller contact radius and stronger collapse intensity and making the wet wall more suitable for wall cooling. Significant pressure differences are observed at the contact points owing to local momentum concentration. In contrast, the non-wetting wall surface is more suitable for wall heating because of its smaller collapse intensity and larger contact radius on the wall.
The droplet mode falling film evaporation was an area of intense research due to its thin liquid film and large interfacial area. A two-dimensional mathematical model based on the dynamic contact ...angle (DCA) model was developed to quantitatively analyze the effect of DCA behavior on droplet topological structure and film thickness evolutions of droplet mode in falling-film evaporation. Due to the movement of the three-phase contact line (vc) considered in the DCA model, the fact that the calculated DCA was significantly greater than the static contact angle (SCA) resulted in the phenomena of the liquid ring and droplet pulsation captured by the DCA model. The observed fluctuations of the DCA and vc might be attributed to the break-up of the liquid neck. For the impact stage, the peaks of the transient film thickness calculated by the DCA model were thicker than those by the SCA model and their differences increased with the increasing circumferential angle. For the metastable stage, the average film thickness predicted by the DCA model was thicker than that by the SCA model due to the captured droplet pulsation and calculated shorter metastable state by the DCA model.
•The dynamic contact angle was significantly greater than the static contact angle.•The phenomena of liquid ring and droplet pulsation were captured by the DCA model.•The local film thickness was closely related to the dynamic contact angle behavior.
Silwet L-77 surfactant has the unique ability to spread water droplets across surfaces to a greater extent than conventional surfactants within a second. By using Silwet, the N,N-dimethylformamide ...(DMF) droplets can exhibit self-propulsion on a glass substrate. The peculiar wetting dynamics of the Silwet-laden DMF droplets was explored on the partially wetting substrates, including the polytetrafluoroethylene slide and slippery liquid-infused porous surface with negligible contact angle hysteresis (CAH). The time evolution of the contact angle (CA) was monitored for both high and low Silwet concentrations. Moreover, the volume inflation/deflation method and inclined plane method were employed to investigate the CA relaxation. The sessile droplet exhibits long-time wetting dynamics and it spreads very slowly, depending on the Silwet concentration. The dynamic CA decreases for more than 20 min at low Silwet concentrations (e.g., 1 wt%), but the droplet reaches its equilibrium state within 4 min at high concentrations (e.g., 10 wt%). The relaxation behavior is also influenced significantly by CAH which is found to impede the initial CA relaxation. Finally, the abnormal wetting dynamics is explained by the time-varying interfacial tensions according to Young’s equation.
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•The effect of laser micro-nano processing on the surface of Ti6Al4V was studied.•The change of wetting angle of solder was investigated during dynamic heating.•The joint is strengthened by chemical ...metallurgical bonding and mechanical bonding.•The formation mechanism of sealing glass/Ti6Al4V interface was presented.
Ti6Al4V alloy and ZrO2 ceramic are both used in similar fields, including electronics, aerospace, and biomedicine. Brazing techniques can be used to join these two materials, broadening their range of applications. One effective joining method is the use of a sealing glass melt of various oxides. This approach provides a strong bond between the ceramic and metal. However, it can be difficult for the glass to wet to the metal surface requiring the alloy to undergo oxidation at high temperatures for an extended period. This study involves the use of laser surface texturing to change the alloy surface roughness and shape. This procedure results in better adhesion of the sealing glass to metal surfaces. When the number of laser scanning is 10, the surface roughness Ra reaches 5.62 μm and the surface energy reaches 46.88, which are the maximum values after comparison. The wetting angle of the sealing glass on the surface is the fastest to reach the wetting state, and the shear strength of the joint reaches the maximum of 46 MPa. This technique created an oxide layer, which contributed to strong mechanical bonding between the grooves and solder, which improved joint performance through chemical and mechanical bonding. This work provides a new approach to the connection of metals and ceramics.
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•Surfactants decrease LV and SL surface tensions at varying rates.•Surfactants induce a rapid decrease in γLV.•A slower reduction in γLS is caused by diffusion-limited adsorption of ...surfactants.•Surfactant depletion from the LV interface raises γLV and lowers the infiltration rate.•Surfactant interaction with hydrophobic molecules reduces γLS.
Surfactant molecules increase the infiltration rate into hydrophobic porous media by lowering the infiltrating water's surface tension and the interfacial tension between hydrophobic surfaces and water molecules. We investigated the relative effect of these rate-limiting processes on the infiltration rate of aqueous surfactant solution into hydrophobic porous media. Two surfactants at various concentrations were applied at a constant pressure head to 1D columns filled with hydrophobic soil, and water was applied at the same pressure head to columns filled with surfactant-pretreated hydrophobic soil. Based on the measured contact angle, surfactant pretreatment significantly reduced the hydrophobic soil's interfacial surface tension, which increased the infiltration rate compared to the direct aqueous surfactant application to the hydrophobic soil. The latter's slower infiltration rate was attributed to the depletion of surfactant molecules due to its adsorption to the hydrophobic molecules near the advancing wetting front, yielding an increase in the surface tension of the infiltrating solution. Surfactant pretreatment increased the opportunity time for surfactant adsorption to the hydrophobic molecules, resulting in interfacial tension reduction and infiltration rate increase. Diffusion-limited surfactant adsorption on the hydrophobic surfaces, leading to reduced interfacial tension between the surface and infiltrating liquid, had a greater impact on limiting infiltration into hydrophobic porous media compared to the reduction in surface tension of the infiltrating liquid due to surfactant presence.
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Rough hydrophobic surfaces have many applications in industry and technology. An experimental study was done on the spreading dynamics of different concentrations of polyethylene ...glycol (PEG) solutions on rough Teflon plates with different roughness. The experiments were conducted using Wilhelmy plate method. The advancing dynamic contact angle was found to be weakly dependent of capillary number. However, the receding dynamic contact angle decreases with increasing capillary number. The degree of roughness on rough Teflon surface has an important role on dynamic contact angle. The dynamics of receding motion was found to follow the molecular-kinetic theory. A power law relation between the receding dynamic contact angle and the capillary number was also obtained.
