Numerical analyses of the coupled-dynamic behavior of droplets on a vibrating membrane with a microsphere array at a three-phase interface

Study effects of particle size in metal nanoink for electrohydrodynamic inkjet printing through analysis of droplet impact behaviors

This paper describes an experimental investigation of the behavior of water droplets in axial acoustic fields. It was motivated by the increasing interest in the use of pulsations to improve the performance of energy intensive, industrial processes. The presence of an acoustic field is believed to enhance heat and mass transfer to and from the droplets, probably because of the relative motion between the droplets and the gas phase. This relative motion is characterized by the ratio of the amplitude of the oscillatory droplet velocity to that of the acoustic velocity (entrainment factor), and by the phase between the droplet and gas phase oscillations. An experimental set-up was developed to investigate the effect of acoustic oscillations on the motion of individual droplets. In these experiments a droplet produced by a piezo-ceramic droplet generator, is allowed to fall through a transparent test section in which an acoustic field has been set up using a pair of acoustic drivers. Images of the droplets in the test section acquired at consecutive instants using a high speed, intensified imaging system were used to determine the time dependent droplet trajectory and velocity. The acoustic velocity was calculated from measured acoustic pressure distributions. The entrainment factor and the phase difference were then determined from these data. The results show how the entrainment factor decreases and the phase difference increases with increasing droplet diameter and frequency, indicating that larger diameters and higher frequencies reduce the “ability” of the droplets to follow the gas phase oscillations. The measured data are in excellent agreement with the prediction of the Hjelmfelt and Mockros model. Both, theoretical predictions and measured data were correlated with the Stokes number, which accounts for the effects of droplet diameter and frequency. It was also shown that acoustic oscillations decrease the mean terminal velocity of the droplets.

Wetting behavior of sessile droplet affected by chemical heterogeneity size: A theoretical and simulative analysis with consideration of contact line width

The static shape of droplets under electrowetting actuation is well understood. The steady-state shape of the droplet is obtained on the basis of the balance of surface tension and electrowetting forces, and the change in the apparent contact angle is well characterized by the Young-Lippmann equation. However, the transient droplet shape behavior when a voltage is suddenly applied across a droplet has received less attention. Additional dynamic frictional forces are at play during this transient process. We present a model to predict this transient behavior of the droplet shape under electrowetting actuation. The droplet shape is modeled using the volume of fluid method. The electrowetting and dynamic frictional forces are included as an effective dynamic contact angle through a force balance at the contact line. The model is used to predict the transient behavior of water droplets on smooth hydrophobic surfaces under electrowetting actuation. The predictions of the transient behavior of droplet shape and contact radius are in excellent agreement with our experimental measurements. The internal fluid motion is explained, and the droplet motion is shown to initiate from the contact line. An approximate mathematical model is also developed to understand the physics of the droplet motion and to describe the overall droplet motion and the contact line velocities.

We humans, characterize for our pursuit of a deep understanding of nature and also for our way of using this knowledge in order to alter our environment (sometimes also to protect and preserve it) in the most convenient way for us. Droplets are present in a great variety of natural processes and also in a wide collection of human applications, and thus they have captured the attention of the scientific community already for very long. In particular, the study of some animals and plants strategies, have shown us how to control the adhesion of liquids like oils and water, and these strategies are now used and adapted to many industrial processes and manufactured tools. These strategies that in animals and plants are a result from the evolution of the species, have led to microscopic and intricate geometries and these materials have been taken as inspiration and reproduced by the scientific community. In this thesis, we provide a collection of experimental studies of the fundamentals of the droplet behavior on substrates with different geometries and materials. In all of these studies, the droplet’s volume evolves in time by means of natural diffusion and convection and is forced by the properties of the materials in which they sit to adopt changes in topology to minimize their surface free energy. The knowledge acquired through these experiments allows us to understand the behavior of droplets and their contact lines under different conditions imposed by the topology of the substrates. In particular, we have studied the behavior of drying and dissolving drops on substrates engraved with concentric rings and spiral patterns. These experiments show the special motion of the contact line between the rings and spiraling groves that we name zipping-dewetting, as it opens the gap between the grooves with its advance. This zipping-dewetting is shown to be a succession of stable states controlled by the diffusion process and the geometry of the substrates. Additionally, we have studied the effect of chemical heterogeneities of the substrate with elliptical shape. The experimental results compared to the computational calculations show the strengths and weaknesses of our computational methods to predict the behavior of the droplets on real experimental conditions. Finally, we studied a very eye-catching effect that an entrapped bubble causes in some dissolving drops. Its heartbeat-like motion and the waves expelled by the system, besides being amusing to see, highly foment mixing and dissolution. This phenomenon that we call puffing, and that only happens for some combination of liquids, is just the previous step for a convective dissolution of the drops driven by Marangoni tensions which dramatically enhance mixing. The studies in this book offer a step forward in the fundamental understanding of droplets that can open the door to the optimization of processes as, for example, microfluidics mixing components, nano-architecture, advanced printing and 3D-printing technology or droplet-based diagnostics.

Experimental and theoretical analysis of microjet droplet behavior

The evaporation and dynamics of a multicomponent droplet on a heated chemical patterned surface were presented. Comparing to the evaporation process of a multicomponent droplet on a homogenous surface, it is found that the chemical patterned surface can not only enhance evaporation by elongating the contact line, but also change the evaporation process from three regimes for the homogenous surface including constant contact line (CCL) regime, constant contact angle (CCA) regime and mix mode (MM) to two regimes, i.e. constant contact line (CCL) and moving contact line (MCL) regimes. The mechanism of contact line stepwise movement in MCL regimes in the microscopic range is investigated in detail. In addition, an improved local force model on the contact line was employed for analyzing the critical receding contact angles on homogenous and patterned surfaces. The analysis results agree well for both surfaces, and confirm that the transition from CCL to MCL regimes indicated droplet composition changes from multicomponent to monocomponent, providing an important metric to predict and control the dynamic behavior and composition of a multicomponent droplet using a patterned surface.

When the molten fuel with high temperature falls into the cavity water, it will be dispersed into droplets which are covered with vapor films due to the rapid heat transfer with phase transition. This situation cannot be simply described by liquid-liquid or gas-liquid systems. And there are no sufficient experimental studies on the behavior of droplet covered with vapor film because of the rapid reaction and the difficulty in capture of the film configuration. In this paper, a multiphase code with the volume of fluid (VOF) method is used to simulate the earlier behavior of droplet when vapor film exits. The earlier behavior is defined as behavior of the droplet before its disintegration. Thermal effect and pure hydrodynamic effect are, respectively, considered. The simulation results indicate that the film thickness and material density have significant effect on the earlier behavior of droplet. The situation assumed in Ciccarelli and Frost’s model (1994) is observed in current simulation of earlier thermal droplet behavior. The effect of triggering pressure pulse on earlier hydrodynamic behavior is also discussed and it indicates that vapor film has little effect on the hydrodynamic droplet deformation when the intensity of the pressure pulse is very high.

Based on FLUENT software, the VOF model was selected to numerically simulate the transfer behavior of ultrasonic-assisted MIG welding droplet under the effect of gravity, surface tension, electromagnetic force and plasma flow force. The influence of ultrasonic vibration on the necking radius of the plasma flow transfer of conventional MIG welding droplets is analyzed, and the effects of ultrasonic vibration transmission and radiation on the internal pressure, vector and the entire calculated flow field of the droplet are explored. The results of comparison with conventional MIG welding show that ultrasonic vibration causes rapid necking of droplet, the necking radius decreases and the plasma flow force increases, which promotes the transfer process of droplet; the internal dynamic pressure of the droplet increases and the absolute pressure in the flow field decreases. To verify the accuracy of the simulation results, welding tests were carried out, and the droplet transfer process was filmed by high-speed camera equipment and the droplet size was obtained by image processing method, so as to effectively and quantitatively verify the accuracy of the simulation results. The comparison results show that the simulation results are in good agreement with the experimental results, and the mathematical model of the droplet transfer is reasonable.

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

High-temperature heat pipe is a passive and efficient heat transfer element, which is generally filled with a certain mass of alkali metal as the heat transfer medium, and utilizes vapor-liquid phase change to efficiently transfer heat from the evaporation section to the condensation section of the heat pipe. In the normal operation of high-temperature heat pipe, the alkali metal will experience evaporation in the evaporation section, vapor diffusion in the adiabatic section, condensation in the condensation section, and reflux under the action of capillary force of the wick structure and other complex heat and mass transfer behaviors. This study focuses on the evaporation behavior of sodium alkali metal inside a high-temperature heat pipe. First of all, an experimental rig is constructed to visualize the evaporation behavior of alkali metal sodium droplets, which mainly includes the alkali metal sodium purification system (to remove the kerosene and impurities inside the commercial sodium to ensure the high purity of sodium), the visual measurement system (to record the droplet morphology), and the constant-temperature and constant-pressure argon system (to provide a high-purity argon environment to avoid the chemical reaction between sodium and the water vapor and oxygen in the air during the experimental process and to provide different temperature and pressure environments for the evaporation of sodium). Then, the evaporation behavior of alkali metal sodium droplets in a circular tube with different wall temperatures and pressure is experimentally investigated by using the previously constructed experimental rig, and the morphological changes of the droplets during the evaporation process were recorded by a high-speed camera. After that, the image analysis software ImageJ is used to analyze the droplet morphology to obtain the change curves of droplet height, contact line length, and contact angle with time during the evaporation of sodium alkali droplets. And the curves are further analyzed to obtain the information of sodium droplet evaporation mode and evaporation rate. By comparing and analyzing the experimental data of sodium droplet evaporation from round tubes with different temperatures and ambient pressures, the influence of the temperature and ambient pressures on the evaporation behavior of sodium droplets is obtained. In this study, the first experimental data of sodium alkali droplet evaporation in a high-temperature heat pipe is obtained by visualization experimental means, which reveals the mechanism of sodium droplet evaporation and lays a foundation for further improvement of the existing heat pipe simulation model.

Lattice Boltzmann Modeling (LBM) simulations were performed on the dynamic behavior of liquid droplets on chemically striped patterned surfaces, ultimately with the aim to develop a predictive tool enabling reliable design of future experiments. The simulations accurately mimic experimental results, which have shown that water droplets on such surfaces adopt an elongated shape due to anisotropic preferential spreading. Details of the contact line motion such as advancing of the contact line in the direction perpendicular to the stripes exhibit pronounced similarities in experiments and simulations. The opposite of spreading, i.e., evaporation of water droplets, leads to a characteristic receding motion first in the direction parallel to the stripes, while the contact line remains pinned perpendicular to the stripes. Only when the aspect ratio is close to unity, the contact line also starts to recede in the perpendicular direction. Very similar behavior was observed in the LBM simulations. Finally, droplet movement can be induced by a gradient in surface wettability. LBM simulations show good semiquantitative agreement with experimental results of decanol droplets on a well-defined striped gradient, which move from high- to low-contact angle surfaces. Similarities and differences for all systems are described and discussed in terms of the predictive capabilities of LBM simulations to model direction wetting.

To simulate the interfacial behaviors in real heterogeneous systems, the point contact condition is constructed to study the classical immiscible displacement problem in this work. Specifically, the interfacial dynamics during the water droplet passing through the oil capillary bridge formed under the point contact condition is investigated. Emphasis is put on the influences of the wettabilities and the relative separation motion of the solid surfaces on the dynamic behavior of the droplets. The observations suggested that the capillary pressure had negligible effect on the movement of the water droplet when it was passing though the oil capillary bridge. The wettability and the relative separation of the disk and ball would influence the final adhesion behaviors of the water droplet after the droplet passed through the oil capillary bridge. Surface tension and adhesion energy were used to interpret these observations.

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.

The distinct physical properties of liquid nitrogen make liquid nitrogen spray cooling a promising technique in aerospace engineering, the electronic industry, superconductor cooling, cryobiology, etc. In-depth study of the dynamics and thermodynamic behavior of liquid nitrogen droplets impinging on the wall surface is helpful to understand the heat transfer mechanism of spray cooling technology with liquid nitrogen. Therefore, the mathematical model of single-liquid nitrogen droplet impacted solid surface is developed by Level Set-VOF method. The effects of wall wettability (30°－150°), initial velocity (0.1, 1.6 m/s) and wall temperature (300－500 K) on the phase change behavior during the evolution of droplets are investigated, and the mathematical model of film thickness is established. The results show that enhancing the wall wettability and increasing the impact speed facilitate the spreading of the droplets in the radial direction, thereby increasing the heat exchange area and reducing the thermal resistance. Ultimately, the heat exchange performance is significantly improved. Increasing the wall temperature results in an increase in the difference between temperatures of the solid surface and the liquid, thereby significantly increasing the wall heat flux density. The lower thermal resistance at the three-phase contact line results in a higher heat flux density at the edge than in the center; the difference among the heat flux distributions on different wetted walls decreases due to the increase of initial velocity, showing a significant velocity effect. In the film boiling region, the heat transfer process is mainly concentrated in the initial stage of impact, and the gas film is the main heat transfer resistance. Based on conservation of mass and energy, a numerical model of film thickness is developed in this paper. The model predictions are in good agreement with the simulation results of this paper and others.