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Phase diagram of droplet impact on surfaces with different CA.
Source publication
The effect of surface wettability on droplet impact on spherical surfaces is studied with the CLSVOF method. When the impact velocity is constant, with the increase in the contact angle (CA), the maximum spreading factor and time needed to reach the maximum spreading factor (tmax) both decrease; the liquid film is more prone to breakup and rebound....
Contexts in source publication
Context 1
... phase diagram of droplets impacting spherical surfaces is shown in Figure 3. The curvature ratio is 0.2, the impact velocity is 0.7 m/s and CA is 60 • , 90 • , 120 • and 150 • , respectively. ...
Context 2
... curvature ratio is 0.2, the impact velocity is 0.7 m/s and CA is 60 • , 90 • , 120 • and 150 • , respectively. From Figure 3, the dynamic characteristics of droplet after impact are different with different contact angles. With the increment in CA, the liquid film is more prone to breakup and rebound. ...
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Citations
... The wetting contact occurred when the surface temperature was below or close to the saturation temperature, while non-wetting contact was observed when the surface temperature exceeded the saturation temperature. Liu et al. 35 examined the effect of surface wettability on dynamic droplet characteristics using the volume of fluid method. They compared their findings with the experiments conducted by Mitra et al. 34 This approach involved combining the VOF method with level-set techniques, allowing for a more accurate determination of the phase interface parameters. ...
This study examined the impact of Boger droplets on spherical surfaces, considering the various properties of liquids and surfaces for the first time. The experimental setup involved spheres with three different diameters composed of hydrophilic and hydrophobic surfaces. The main innovative aspect of the present study lies in the examination of the effects of rheological properties including elasticity, elongational viscosity, and the first normal stress difference on the impact of Boger droplets on solid surfaces via stress analysis. For this purpose, the results of the impact of Boger droplets are compared to the impact of equivalent Newtonian droplets with the same viscosity and surface tension coefficient. The research also explored the influences of Weber number, diameter ratio, and surface wettability on the dynamic behaviors of viscoelastic droplets during the spreading and receding stages. The results suggested that increasing the Weber number increases the maximum spreading factor of the droplets. A smaller diameter ratio required more time to reach this factor. The surface wettability significantly affected the receding phase, while its effect on the spreading stage was minimal. Droplets exhibited more retraction on surfaces with higher contact angles. The Newtonian fluids showed greater spreading and retraction than viscoelastic fluids with the same viscosity, highlighting the influence of fluid elasticity. It is shown that the normal stress caused by extensional viscosity during droplet spreading is significantly higher than the normal stress difference resulting from shear deformation, showing different behaviors between these two types of stress concerning droplets.
... From the aspect of feature parameters, the maximum spreading factor is frequently focused on. Liu et al. 50 found the maximum spreading factor increases with the diameter ratio of solid spheres to droplets in a wide contact angle range from 30 to 150 but shows a contrary tendency in the extreme hydrophilic range from 0 to 30 . Based on energy conservation or scaling analysis, several models of b max have been established by incorporating the diameter ratio between solid spheres and droplets. ...
Rising nanoscale technologies arouse interest in investigating the impact dynamics of nanodroplets. In this work, the impact of nanodroplets on solid spheres is investigated by the molecular dynamics simulation method, to comprehensively report outcome regimes and reveal the curvature effect, in wide ranges of Weber numbers (We) from 1.5 to 235.8, diameter ratios (λ) of nanodroplets to solid spheres from 0.3 to 5, and contact angles (θ) from 105° to 135°. Five outcomes are identified, including deposition, bouncing, splash, covering, and dripping. The former three outcomes are found in the high diameter ratio range (λ > 1), showing similar dynamic behaviors with impacts on flat surfaces, whereas in the low diameter ratio range (λ ≤)1, splash disappears, and covering and dripping take place additionally. At each contact angle, the outcomes are recorded in λ-We phase diagrams. It is found that the bouncing, splash, covering, and dripping are all promoted by decreasing diameter ratios; in addition, the critical Weber numbers for trigging bouncing and splash increase with decreasing θ. However, the critical We of the boundary between the bouncing to other regimes in the low diameter ratio range is not sensitive to wettability owing to the relatively small diameter of solid spheres. For quantitatively describing the curvature effect, the boundaries between the deposition and bouncing regimes in the high diameter ratio range and between the bouncing and other regimes in the low diameter ratio range are established. Both the established models show satisfactory agreement with the boundaries in the phase diagrams.
... It was found that when the impact velocity was constant, the maximum expansion factor and the time required to reach the maximum expansion factor decreased with the increase of contact angle. 33,34 Banks et al. studied the process of spray droplets impacting the solid surface and found that the kinematic viscosity, impact velocity, and surface tension effect of droplets played an important role in the oscillation. High viscosity droplets often stopped oscillation faster than low viscosity droplets. ...
The dynamic wetting behavior of droplets impacting the coal surface directly affects the efficient application of water-based dust suppression materials in coal-related industrial production. In this paper, ultrapure water, Tween-80, and sodium carboxymethyl cellulose are taken as the research objects. Using high-speed photography technology, the spreading, oscillation process, and splash morphology of many kinds of droplets during impacting the coal surface are captured. The effects of viscosity, surface tension, and impact velocity on dynamic wetting characteristics were studied. The results show that with the decrease of surface tension, the retraction and oscillation of droplets are significantly reduced. For the same kind of droplets, the greater the impact velocity, the faster the droplet spread, and the dimensionless maximum spreading coefficient (βmax) and dimensionless steady-state spreading coefficient (βe) of droplets are bigger. With the increase of velocity, the time for different kinds of droplets to reach the βmax increases. At the same impact velocity, βmax and βe of droplets (0.2% Tween-80 + 0.1% sodium carboxymethyl cellulose) are the largest, indicating that adding a small amount of sodium carboxymethyl cellulose can promote droplet spreading. With the increase of sodium carboxymethyl cellulose content, βmax and βe decreased gradually. The results have a great significance to the research, development, and scientific utilization of water-soluble polymer dust inhibitors.
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... A drop hitting a flat surface retains a circular symmetry throughout the impact process [71]. Flat surfaces with a smaller θ c have a higher droplet spreading factor (ratio of length of twodimensional wetting arc and initial droplet diameter [72]) and reach equilibrium sooner than flat surfaces with a large θ c [9]. The rate of droplet spreading becomes slower with decreasing curvature ratio (the ratio of the initial droplet diameter to the surface diameter) [73]. ...
Complex and at times extreme environments have pushed many bird species to develop unique eggshell surface properties to protect the embryo from external threats. Because microbes are usually transmitted into eggs by moisture, some species have evolved hydrophobic shell surfaces that resist water absorption, while also regulating heat loss and the exchange of gases. Here, we investigate the relationship between the wettability of eggshells from 441 bird species and their life-history traits. We measured the initial contact angle between sessile water droplets and the shell surface, and how far the droplet spread. Using phylogenetic comparative methods, we show that body mass, annual temperature and eggshell maculation primarily explained variance in water contact angle across eggshells. Species nesting in warm climates were more likely to exhibit highly hydrophobic eggshells than those nesting in cold climates, potentially to reduce microbial colonization. In non-passerines, immaculate eggs were found to have more hydrophobic surfaces than maculate eggshells. Droplets spread more quickly on eggshells incubated in open nests compared to domed nests, likely to decrease heat transfer from the egg. Here, we identify clear adaptations of eggshell wettability across a diverse range of nesting environments, driven by the need to retain heat and prevent microbial adhesion.
Though single droplet impact dynamics was extensively investigated, the complex hydrodynamics involved in the simultaneous interaction of droplets with a loosely held dense solid particle on a hydrophobic substrate has received less attention. In this paper, the authors report the impingement response of a water droplet colliding on a non-fixed spherical steel particle placed on a hydrophobic surface and the subsequent creation of a particle-laden compound droplet. Utilizing the kinetic energy of the water drop, the heavy metal bead is picked up from the surface during the droplet bouncing. Here, a dense metal spherical mass is located on a hydrophobic substrate having contact angle, θc=140° and drop collides with a Weber number range of 8.20 ≤We≤ 38.07. During the droplet spreading, a thin film is developed between the particle and the hydrophobic plate due to capillary action and the particle is engulfed inside the droplet while it recoils, thereby a compound droplet is created. For instance, during the rebound of a composite drop, the metal bead, having one-third the mass of the droplet, is elevated to a height of 2.5 times its diameter at We = 24.5. Phenomenological models are developed for the prediction of compound droplet rebound height and the minimum Weber number required for the composite drop creation, and it exhibited good accord with experimental observations. These results shed more light on the self-cleaning mechanism involving dense particles and provided a promising strategy for the production of a solid–liquid composite droplet.
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Droplet impact is a common but significant phenomenon in industry. The CLSVOF (coupled level set and volume-of-fluid) method is used to numerically study the successive dual-droplet impacting a super-hydrophobic tube. For the impact velocity of 1.0 m/s, the effect of the eccentric distance on dynamic characteristics is analysed, the corresponding eccentric distances are 0.5, 1.0 and 2.0, respectively. In addition, the break-up during rebound is analysed with velocity field and pressure nephogram. Results show that, the eccentric distance hinders the spread during the initial period of spreading. With the increase in eccentric distance, more liquid gathers at the eccentric side and the liquid film might rebound easily without break-up under the same impact velocity. The break-up during rebound mainly depends on the local airflow and pressure difference. The high-pressure zone near the solid-liquid interface moves towards the eccentric side with the increase in eccentric distance.
In this paper, the impact of successive double droplets on a super-hydrophobic tube surface is numerically studied using a three-dimensional model by the coupled level set and volume of fluid method. The effect of impact velocity on double droplets impact under different curvature ratios is studied. With the increase in impact velocity, two kinds of impact models (out-of-phase and in-phase impact) are obtained, and the coalescent liquid film presents rebound, breakup-rebound, and splash-rebound. With the increase in the curvature ratio, the spread of the liquid film along the circumferential direction is promoted, and the rim of leading liquid film up-warp can be observed earlier. The breakup of an extending liquid film is discussed in detail under the curvature ratio of 1, which is mainly influenced by the combined effects of local negative pressure, air flow motion, surface tension, and gravity of the gathering liquid. The air entrainment occurs for the impact velocity between 0.75 m/s (We = 15.4) and 1.25 m/s (We = 42.9), while no air entrainment occurs for the impact velocity exceeding 1.5 m/s (We = 61.7). The entrapped air bubbles would cause a center breakup of the liquid film or escape from the liquid film. The escaping direction is mainly determined by the pressure distribution in the liquid film.
