Spreading Characteristics Of Droplet Research Articles

Experimental study on the dynamic behaviors and spreading characteristics of a liquid nitrogen droplet impacting superheated wall

Liquid nitrogen droplets impacting superheated wall is a fundamental phenomenon in liquid nitrogen spray cooling. Its dynamic characteristics directly affect the cooling performance. In this paper, we design and build an experimental setup to investigate a cryogenic liquid nitrogen droplet impacting superheated wall. Using a high-speed camera, we capture the dynamic behaviors of the liquid nitrogen droplet upon impact. The effects of wall temperature and Weber number on the impacting dynamics and spreading characteristics of droplets are studied. The experimental results show that the droplets exhibit six regimes of dynamic behaviors under different wall temperatures and Weber numbers, namely, contact boiling, atomization boiling, film boiling with complete rebound, film boiling with retraction and breakup, film boiling with instantaneous splashing, and film boiling with crown splashing. The Leidenfrost temperature at which film boiling occurs is found to be independent of the Weber number. The spreading characteristics of droplet under different boiling regimes are summarized. During contact boiling, droplet spreading is affected by both wall temperature and Weber number. In contrast, for atomization boiling and film boiling, the spreading is solely affected by the Weber number and is insensitive to the wall temperature. But the underlying mechanisms are different. The former is due to the efficient boiling heat transfer making it insensitive to the wall temperature, while the latter is due to the near thermal isolation caused by the vapor film. Lastly, dimensionless fitting correlations for the maximum spreading coefficient of droplets under different boiling regimes are established. This study can provide guidelines for the design and development of liquid nitrogen spray cooling.

Investigation of the spreading of a liquid metal droplet under a vertical magnetic field

In the liquid metal divertor of a magnetic confinement fusion device, the spreading characteristics of the liquid metal are crucial for ensuring the stable operation of the divertor. This study has experimentally investigated the spreading characteristics of a GaInSn alloy droplet on a solid substrate under a strong vertical magnetic field, with the magnetic field intensity ranging from 0 to 2.5 T. First, several parameters of the droplet, such as droplet shape, spreading factor, dynamic contact angle, spreading velocity, and rebound behavior after impacting, were studied without a magnetic field. The fitting relationship between maximum spreading factor βmax and Weber number We was obtained and has been compared with the scaling laws from the literature. Furthermore, the effect of the vertical magnetic field on those parameters has been investigated systematically. Quantitative results on βmax and the maximum spreading time tDmax, varied with the Hartmann number (Ha) and the We number, provide a comprehensive understanding of the spreading dynamics. The specific relationship between βmax and We number under different magnetic field intensities (B) shows that a vertical magnetic field has a great inhibiting effect on liquid metal droplet spreading. Finally, the influence of oxidation on droplet spreading characteristics also has been studied. These basic findings are important for the application of liquid metal on a divertor/limiter in a fusion reactor, offering a theoretical reference engineering design.

Interfacial Behavior and Dechlorination Reaction of Water Droplet Impact on a Heated Extracted Titanium Tailing Surface.

The interfacial behaviors of the droplet impact on a heated extracted titanium tailing surface are studied experimentally. The effects of surface temperatures and Weber numbers on the droplet spreading characteristics are examined. The factors affecting the mass fraction and dechlorination ratio of extracted titanium tailings under the action of interfacial behavior have been researched by thermogravimetric analysis. The compositions and microstructures of extracted titanium tailings are characterized using X-ray fluorescence spectroscopy and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). The interfacial behaviors on the extracted titanium tailing surface are classified into four regimes, i.e., boiling-induced break-up, advancing recoiling, splash with a continuous liquid film, and splash with a broken film. The maximum spreading factors increase with the surface temperature and the Weber number. It is found that the surface temperature has a dominant influence on the spreading factors and interfacial effect, further affecting its chlorination reaction. SEM-EDS analysis revealed that the extracted titanium tailing particles are irregular shaped. There are some fine pores on the surface after the reaction. The main concentrations are Si, Al, and Ca oxides with a certain amount of C elements. The findings of this research provide a new pathway to utilize the extracted titanium tailings comprehensively.

The jetting process and spreading characteristics of the power-law fluids for material jetting process

Materials jetting, known as one of the 3D printing technologies, is widely applied in microelectronics packaging, biology and ceramic 3D printing due to its ability to print multi-materials by drop-on-demand. However, most of the materials are power-law fluids in 3D printing applications, the generation of satellites during the jetting process and droplet spreading characteristics are unclear and they have a great effect on the quality of the printout. In this paper, a common electromechanical and fluid-solid coupling model of the jet dispenser and observation platform of the jetting process are established. This modeling method is also suitable for other needle-driven jet dispensers. A commercial UV resin is adopted to study the jetting process of power-law fluid. To reveal the mechanism of satellite generation, the effects of input signals (rising time and falling time) on the dynamic characteristics of the needle and the jetting process are analyzed. On the basis thereof, the effectiveness of the optimal control parameters is demonstrated to eliminate satellites. In addition, the simulation and experimental results show that the falling time and fluid pressure can be controlled to adjust the spreading diameter and height of the droplet. Subsequently, the minimum line width of 0.276 mm is successfully printed with a nozzle of 0.07 mm.

Spreading characteristics of droplets impact on microstructured surfaces with gradient wettability

Spreading characteristics of droplets impact on microstructured surfaces with gradient wettability

Droplet Spreading Characteristics on Ultra-Slippery Solid Hydrophilic Surfaces with Ultra-Low Contact Angle Hysteresis

Dynamic interactions of the droplet impact on a solid surface are essential to many emerging applications, such as electronics cooling, ink-jet printing, water harvesting/collection, anti-frosting/icing, and microfluidic and biomedical device applications. Despite extensive studies on the kinematic features of the droplet impact on a surface over the last two decades, the spreading characteristics of the droplet impact on a solid hydrophilic surface with ultra-low contact angle hysteresis are unclear. This paper clarifies the specific role of the contact angle and contact angle hysteresis at each stage of the droplet impact and spreading process. The spreading characteristics of the droplet impact on an ultra-slippery hydrophilic solid surface are systematically compared with those on plain hydrophilic, hydroxylated hydrophilic, and plain hydrophobic surfaces. The results reveal that the maximum spreading factor (βmax) of impacting droplets is mainly dependent on the contact angle and We. βmax increases with the increase in We and the decrease in the contact angle. Low contact angle hysteresis can decrease the time required to reach the maximum spreading diameter and the time interval during which the maximum spreading diameter is maintained when the contact angles are similar. Moreover, the effect of the surface inclination angle on the spreading and slipping dynamics of impacting droplets is investigated. With the increase in the inclination angle and We, the gliding distance of the impacting droplet becomes longer. Ultra-low contact angle hysteresis enables an impacting droplet to slip continuously on the ultra-slippery hydrophilic surface without being pinned to the surface. The findings of this work not only show the important role of the surface wettability in droplet spreading characteristics but also present a pathway to controlling the dynamic interactions of impacting droplets with ultra-slippery hydrophilic surfaces.

A 3D numerical study of a molten solder droplet's wetting and solidifying on a pillar with application to electronic packaging

The 3D problem of spreading and solidification of a molten solder droplet (with a melting temperature of Tm and a radius of Rd) on a circular pillar (with a radius Rpillar and a height Hpillar, having a contact angle θpillar at wall temperature Tpillar) above a substrate has important applications in electronic packaging. In this paper, effects of pillar's contact angle and wall temperature on droplet dynamics and solidification of the solder droplet are studied numerically based on a newly developed 3D multi-component, triple- phase-change LB model. For pillar's wall temperature equal to the melting temperature of the molten droplet (Tpillar = Tm), no solidification takes place in the droplet after its contact with the pillar, and droplet's spreading characteristics depend on the pillar's contact angle. If pillar's contact angle (θpillar) is smaller than a critical contact angle (θpillar)cr, i.e., θpillar < (θpillar)cr, the solder droplet spreads toward the edge of the pillar's top, turning around the corner of the pillar and spreading downward along the pillar's side wall. Based on LB simulated results, a map in terms of (θpillar) versus (Rd/Rpillar) is presented, showing regimes of droplet spreading and not spreading around the corner of the pillar. An analytical expression for predicting the critical contact angle of the pillar is derived, which is shown to divide these two regimes in the map. LB simulations are also carried out for the case when pillar's wall temperature is lower than the melting temperature of the molten droplet (i.e., Tpillar < Tm). Under this situation, it is shown that although no solidification takes place during the initial spreading motion, but solder bumps begin to be formed subsequently due to solidification and droplet's spreading motion is arrested as a result. Since formation of solder bumps on pillar's side wall may lead to short circuit of electric connection between solder bumps with other electric sources in the gap between the pillars, it is important to prevent the formation of solder bumps on pillar's side wall. For this purpose, a map of (θpillar) versus (Rd/Rpillar) at different Stefan numbers (denoting the effect of pillar's wall temperature Tpillar) is obtained to define the regime in which no solder bumps are formed on the pillar's side wall.

Experimental study of spreading characteristics of droplet impacting on canopy fabric surface

A new experiment based on visualization technology is designed to study the spreading characteristics of droplet impacting on canopy fabric. The processes of droplet impacting on 66 type polyamide grid silk are captured. The experimental results show that the spreading characteristics are also affected by fabric pretension and fabric permeability. The pretension is favorable for the droplet to reach the final equilibrium stage. The impact velocity determines the initial kinetic energy and plays a major role in the droplet spreading. The fabric permeability determines the wettability and has different effects on spreading characteristics under different working conditions. In addition, the above factors can enhance the two competitive processes of spreading and imbibing at the same time. The spreading characteristics depend on which process is the dominant one.

Spreading and heat transfer characteristics of droplet on a heated substrate

The spreading characteristics of a droplet on a heated substrate have direct influences on its spreading area and heat transfer, so the exploration in this aspect is of important significance for cooling electronic and aerospace equipments. In the present paper, the evolution model of a droplet on a heated solid substrate is established based on the lubrication theory, and spreading processes are simulated respectively when the wall temperature is uniform and decreases exponentially from the center to both sides. A method of assessing the heat flux and heat transfer capacity of a two-dimensional liquid droplet is proposed. Influences of spreading characteristics and heat convective condition at the liquid-gas interface on heat transfer feature of the droplet are examined, and the results are in good agreement with the published ones in the literature. The simulated results show that in the case of uniform wall temperature, the evolution of the droplet is dominated mainly by gravity and illustrates symmetrical spreading characteristics, and the thickness profile presents a single-peak feature of which the value diminishes with time. The heat flux across the droplet surface decreases from both sides to the center, and the surface area of the droplet increases with time slightly, so the performance of heat transfer is strengthened to a certain extent. When the wall temperature decreases exponentially from the center to both sides, the spreading process of the droplet manifests three obvious stages, in which a single-peak feature of thickness profile gradually evolutes into a double-peak feature after surviving for a short period of time, and the peak values of the double-peak first increase firstly and then decrease, resulting from the complex game of gravity and thermocapillary force and their alternative dominance in the evolution. The variations of the dynamic contact angle and travelling speed of the contact line with time can also reflect the above characteristics. The heat flux in the center of the droplet increases, while its values at the double-peak and contact lines decrease with time. In addition, the heat flux at the contact line has a distinct jump feature compared with that at the adjacent position. The droplet surface area expands significantly with time, so the heat transfer capability is improved apparently. Enhancing heat convective condition at the liquid-gas interface, namely greater Biot number, slows the droplet spreading process, which inhibits the expansion of the droplet surface area. However, it enables the droplet to stay in a higher temperature region, resulting in the enhancement of heat dissipation of the droplet. Therefore, the comprehensive interactions of the above aspects strengthen the heat transfer capability, and this phenomenon tends to be increasingly significant over time. Greater Biot number delays the variations of the dynamic contact angle and the travelling speed of the contact line, without changing their general characteristics.

EFFECT OF TWO-DIMENSIONAL MICROPILLAR ARRAYED TOPOGRAPHY ON SPREADING OF INSOLUBLE SURFACTANT-LADEN DROPLET

For the spreading of liquid droplet containing insoluble surfactant over two-dimensional micropillar-arrayed surface, the lubrication theory was adopted to derive the evolution equations of liquid film thickness and interfacial surfactant concentration, and then the droplet spreading characteristics and the effects of related parameters were numerically simulated. Results show that when the surfactant-laden droplet was spreading over micropillar-arrayed surface, the depression appears at the groove while the ridge is shaped at the bump, and the ridge and the depression move towards both sides gradually, and the numbers of ridge and depression increase. Furthermore, when the surfactant film is flowing over the bump, the height of the ridge presents a hump shape. The numbers of ridges and depressions in the spreading region increase and the droplet spreading velocity accelerates with the improved preset film thickness and initial surfactant concentration. Increasing the groove depth or decreasing the steepness can enhance the capillary force effects, resulting in suffering the rupture possibility of the film. Improving the groove width enlarges the film deformation and promotes the film flow.

Research on Droplet spreading on Substrate Influenced by Droplet Flying Velocity

Droplet spreading characteristics on substrate make great effect on inkjet printing quality, but the ink viscosity, substrate surface parameters all and droplet flying velocity all affect droplet spreading on substrate. In this paper, we just research on droplet spreading on Substrate been influenced by droplet flying velocity from inkjet print-head to substrate. We set up numerical model of flying droplet based on moment equation and energy conservation equation, so that we can get theoretically result, then keep ink viscosity and substrate surface parameters invariableness, just change the droplet flying velocity, and from the analysis result, we find the relationship between droplet flying velocity and droplet spreading characteristic. From the experiment result, we amend the model for droplet, so that we get a correct numerical model for droplet spread, and in this model droplet velocity is the only variable.