Deformation Process of a Water Droplet Impinging on a Solid Surface

In order to investigate the effect of droplet impact on molten pool dynamics in hybrid laser-MIG (metal inert gas) welding, a three-dimensional mathematical model was developed, and a new approach to simulate the droplet impact was proposed. In the model, the droplet heat and impact under the influence of laser was considered, and the deformation of molten pool was calculated with considering droplet impact and arc pressure. To simplify the model, a modified body heat source model was presented for laser source to take into account multiple reflections and Fresnel absorption. Surface tension, electromagnetic force, and buoyancy were considered to calculate the fluid flow pattern. Furthermore, several dimensionless numbers were used to recognize the importance of the driving forces in molten pool. It was found that molten pool dynamics and final weld bead geometry were strongly affected by the droplet impact. To demonstrate the importance of droplet impact, the temperature, velocity distribution, and the deformation of molten pool were analyzed and compared under different conditions. With considering droplet impact, the penetration was bigger, and the width was smaller. Furthermore, the simulated weld bead geometry was in good agreement with experimental measurement.

Recent years see increasing studies of air entrapment during droplet impacting on a solid surface with many results. The dynamics of trapped air film during a droplet impact on a solid surface is investigated in this work by the phase field method in combination with a dynamic contact angle (DCA) model. The DCA model is established experimentally by capturing the droplet dynamics in analogy to the entrapped air evolution. By using the DCA model as the input, the simulation can accurately reproduce the experimental results. The effects of droplet viscosity and surface tension on the dynamics of the air film are then studied, and three possible regimes are identified, demarcated by an effective Ohnesorge number (Ohe). Regime 1 is the case where no daughter droplet is generated and the air bubble is always attached to the substrate, corresponding to the classical case at a high Ohe number (Ohe > 0.073). Regime 3 is a newly discovered regime in this work where a daughter droplet is generated and the air bubble is always detached from the substrate, corresponding to a low Ohe number (Ohe < 0.019) due to combined strong surface tension and vortex effects. Regime 2 is for moderate Ohe numbers where a daughter droplet is generated and the air bubble can either detach from or attach to the substrate. Different from conventional thought that the detachment in this regime is decided by a static contact angle, the DCA plays a leading role in determining the volume ratio of the daughter droplet to the gas bubble, and the combined effects determine the fate of the bubble. Such finding provides better insight on the entrapped air dynamics upon droplet impacting on a solid surface, an area of high engineering importance.

This paper presents an experimental and theoretical study of Newtonian and non-Newtonian (Bingham plastic) emulsion drop impact on a solid non-heated surface. The utilization of different emulsifiers at a constant concentration of continuous and dispersed phases in emulsions allows the considerable variation of the surface tension at the liquid–liquid interface. Our data for the maximum spreading diameter of water, n-decane, and emulsion drops impacting on a surface are compared with that predicted from the existing models for single-phase liquid drops. All selected models underpredict the experimental data. As a result, the importance of considering the capillary effects at the internal interfaces of the emulsion drops and the careful examination of all rheological properties in the case of Bingham plastic fluids is confirmed experimentally and is taken into account theoretically. The models of Pasandideh-Fard et al. [“Capillary effects during droplet impact on a solid surface,” Phys. Fluids 8, 650 (1996)] and Ukiwe and Kwok [“On the maximum spreading diameter of impacting droplets on well-prepared solid surfaces,” Langmuir 21, 666–673 (2005)] are modified and adapted to the emulsion drop by means of including the additional surface energy term at the liquid–liquid interface of the emulsion drop in the energy conservation equation and the non-Newtonian Reynolds number. The predictions of the maximum spreading diameter give good agreement with the measured one. Several constraints and future lines of research that relate to a specific behavior of the compound liquid drops at the impact on a solid surface are highlighted.

Summary Dynamics of droplet impact on solid surfaces is of great importance in many engineering applications such as ink-jet printing and rapid spray cooling of hot surfaces. In this study, the lattice kinetic scheme based on the lattice Boltzmann method for immiscible two-phase fluids [Inamuro (2006)] is applied to such micro-fluid problems. The present method enables us to perform stable calculations of two-phase flows with large density ratios of up to 1000. The boundary condition based on the wetting potential that is calculated according to a prescribed static contact angle is used on solid surfaces [Briant, Papatzacos, and Yeomans (2002)]. First, the dynamic behavior of a single droplet impinging on a horizontal wall is investigated for various Weber numbers. The dynamic contact angle, the contact line velocity, and the wet length are calculated, and found to be in good agreement with available experimental data. Next, the collision of a falling droplet with a stationary droplet on a solid surface is investigated. The behavior of the droplets and the mixing process during their collision are simulated for various impact velocities and several static contact angles on the solid surface. It is seen that mixing occurs around the rim of the coalescent droplet owing to the circular flows. Also, the relation between the mixing rate of the primary coalescent droplet and Weber number is obtained. Finally, adherence and bouncing of liquid droplets impacting on dry surfaces are examined for several Reynolds numbers based on the droplet diameter. These calculated results indicate that the present method can be a promising numerical approach to investigation of microscale droplet dynamics on solid surfaces.

A super-hydrophobic aluminum alloy surface with decorated pillar arrays was obtained by hybrid laser ablation and further silanization process. The as-prepared surface showed a high apparent contact angle of 158.2 ± 2.0° and low sliding angle of 3 ± 1°. Surface morphologies and surface chemistry were explored to obtain insights into the generation process of super-hydrophobicity. The main objective of this current work is to investigate the maximum spreading factor of water droplets impacting on the pillar-patterned super-hydrophobic surface based on the energy conservation concept. Although many previous studies have investigated the droplet impacting behavior on flat solid surfaces, the empirical models were proposed based on a few parameters including the Reynolds number (Re), Weber number (We), as well as the Ohnesorge number (Oh). This resulted in limitations for the super-hydrophobic surfaces due to the ignorance of the geometrical parameters of the pillars and viscous energy dissipation for liquid flow within the pillar arrays. In this paper, the maximum spreading factor was deduced from the perspective of energy balance, and the predicted results were in good agreement with our experimental results with a mean error of 4.99% and standard deviation of 0.10.

The impact of droplets on solid, microstructured surfaces is broadly relevant to many academic and industrial applications. However, previous studies have focused on the convex structures, with the post-impact behavior of droplets on concave structures receiving little attention. In this study, we combine laboratory experiments with numerical simulations to investigate the impact of droplets on a solid surface with a cavity. The influence of viscosity, wettability, and geometry in the spread and splashing of the droplets is studied for a wide range of Reynolds numbers Re, contact angle θ, cavity width W, and cavity depth H. Based on experimental observations, we map the crown formation regimes that occur on Re-W and Re-H diagrams and comprehensively evaluate the effect of the controlling parameters on the height of the crown. Our numerical simulations are in good agreement with the experimental observations and provide detailed comparative analyses of the underlying physics behind the observations, including the flow field, distribution of the streamlines, and the forces exerted by the droplet on the solid surface. The numerical simulations reveal that the higher forces that arise in narrower and deeper cavities with larger Re and θ redirect more of the liquid's momentum, leading to the formation of a higher crown. The present study improves our understanding of the underlying mechanisms governing droplet impact and thereby provides valuable guidance for the design of concave surfaces.

When a droplet impinges on a solid surface, its kinetic energy is mainly converted to capillary energy and viscous dissipation energy, the ratio of which depends on the wettability of the target surface and the liquid properties. Currently, there is no experimental or theoretical evidence that suggests which types of liquids exhibit the capillary energy-dominated impingement behavior. In this paper, we reported the droplet impingement behavior for a wide range of liquid viscosities, surface tensions and target surface wettabilities. Then, we showed that a recently developed energy balance equation for the droplet impingement behavior can be universally employed for predicting the maximum spreading contact area diameter of a droplet for Newtonian liquids in deposition process by modelling the droplet surface deformation. Subsequently, applicability limitations of recent existing models are discussed. The newly developed model demonstrated that the capillary energy-dominated impingement behavior can be observed at considerably low viscosities of liquid droplets such as that of the superfluid of liquid helium.

Dynamic behaviors of droplet impact and spreading—Water on glass and paraffin

Understanding and controlling droplet impact on flexible structures such as cantilever beams is critical for various industrial and agricultural applications, yet little is known about the force exerted by a single droplet on the solid surface. Accurate modeling of this force is particularly important for applications such as the design and characterization of micro-electro-mechanical systems (MEMS) and micro energy harvesting systems. This study aims to develop an accurate mathematical model to describe the droplet impact force applied to a superhydrophobic cantilever beam. To achieve this, we first employ computational fluid dynamics to simulate the impact of a water droplet on a superhydrophobic rigid surface, quantifying the dual-peak impact force exerted by the liquid phase on the superhydrophobic surface during the impact. Building on these numerical findings, we propose a novel force model that incorporates structural flexibility and captures the complex nature of the impact force. The proposed model includes both peaks in the dual-peak impact force generated during both the spreading and retracting phases of the droplet impact. The developed model is optimized using the experimentally measured response of the cantilever beam to a droplet impact. For this, the equation of motion of the cantilever is formulated using Euler–Bernoulli beam theory. Comparing the reconstructed and the measured responses of the beam shows the accuracy of the developed model in simulating the actual droplet impact force. Overall, this work deepens the understanding of droplet impact phenomena, provides a foundation for developing accurate droplet impact force models, and helps to improve the design of more efficient energy harvesting systems and MEMS.

An experimental study on the behavior of droplets impinging on a solid flat surface was earned out in the present study. Breakup of a liquid droplet impinging on a solid surface has been investigated experimentally for various liquids With different properties The liquid droplet temperature and incident angle were chosen as major parameters Liquid droplet temperature and incident angle vaned in the range from -20℃ to 30℃ and from 30˚ to 60˚, respectively. it was found that the variation of droplet temperature influences upon the mean diameter and uniformity of droplets which were bounced out from the solid surface With increase of incident angle the dispersion mass fraction increases, causing the decrease of liquid film flow rate As the liquid temperature increases, dispersion mass fraction increases since the surface tension decreases.

Liquid methane spray cooling technology offers high heat transfer efficiency, making it essential for the sustainable development of the aerospace industry. This work employs a volume of fluid (VOF) approach to examine the dynamic influence of liquid methane droplets on solid heated surfaces. This study investigates the spreading, vaporization, and heat transfer behavior of liquid methane droplets impacting heated surfaces at different Weber numbers and surface temperatures. Unlike previous studies on liquid nitrogen and liquid natural gas droplets, this work uniquely examines liquid methane droplet impacts, quantifying wall heat flux evolution and phase change effects. The results reveal complex interactions, highlighting the influence of droplet size, impact velocity, and surface temperature on spreading behavior, heat transfer, and vaporization. Furthermore, the study establishes new correlations between droplet impact dynamics, heat flux, and vapor volume fraction, providing critical insights for optimizing cryogenic cooling systems. These findings contribute to a deeper understanding of cryogenic droplet interactions, which is essential for thermal management in aerospace propulsion, cryogenic fuel storage, and industrial cooling applications.

A three-dimensional weld pool model has been developed to study the fluid flow and heat transfer process during gas metal arc (GMA) welding. Both droplet heat content and impact force were considered in analyzing the effect of droplets on the formation of weld pool. The fluid flow in the weld pool was induced by the presence of surface tension, electromagnetic and buoyancy force. The surface deformation of weld pool was calculated by considering arc pressure and droplet impact force. Computational results under partial and full penetration welding conditions were obtained. The effect of heat flow and fluid flow characteristics on weld pool geometry was discussed, particularly with respect to the presence of droplet heat input and impact force.

Efficiently regulating the rebound behavior of droplets post-impact is crucial for various fields, mainly including the development of self-cleaning applications, the design of surface functional materials, and the advancement of industrial techniques. By performing molecular dynamics simulations, we investigated the impact and jumping behavior of droplets on heterogeneous substrates with different wetting regions. We found that, during the impacting evolution process, the retracted droplets would move toward regions with stronger wettability due to the unbalanced force caused by the wettability difference, revealing the directional migration ability. The values of the wettability difference strongly affect the degree of oblique rebound and contact time when droplets can jump off the substrate. We then designed the surfaces with a wettability gradient and found that the oblique rebound angle could be well controlled and the contact time further reduced. Our findings may provide valuable insight into the relationship between the wettability gradient and the behavior of liquid droplets on surfaces, with broad implications for various fields such as surface engineering, materials science, microfluidics, etc.

Study of the coalescence/splash threshold of droplet impact on liquid films and its relevance in assessing airborne particle release

Droplet impact on inclined substrates under electric fields is a common behavior in electrostatic demisting applications, and understanding the droplet dynamics of this process is important for improving the performance of demisters. This study investigated the droplet impact dynamics on inclined substrates within a non-uniform electric field. Using high-speed imaging, the effects of voltage (U), substrate inclination (θ), and impact velocity (v) on the droplet behaviors were analyzed. The results revealed that at higher voltages, an upward ejection or pinch-off from the liquid column occurred during the recoiling stage, while the maximum dimensionless spreading diameter D*max increased with the voltage up to U ≤ 12 kV, then sharply decreased due to the droplet ejection for U &amp;gt; 12 kV. It was found that the electric field also intensified the droplet oscillation, with the maximum recoiling height H*max positively correlated with U. The secondary droplet ejection volume fraction η increased with the increase in U, decreased with the increase in both θ and D0, and peaked at v = 0.77 m/s. Furthermore, a critical threshold for the ejection or pinch-off and a predictive model for D*max were developed, incorporating electric Bond number (BoE), Weber number (We), and θ. Based on a profound comprehension of the electrohydrodynamic mechanisms governing the droplet impact on inclined substrates, these findings provide appropriate operating conditions to avoid the droplet pinch-off and ejection, improving the efficiency of electrostatic demisters.