Droplet Train Research Articles

Experimental study of surface temperature non-uniformity and fluctuation in dense controllable droplet trains cooling

Experimental study of surface temperature non-uniformity and fluctuation in dense controllable droplet trains cooling

Morphology and heat transfer of a train of microdroplets impinging on the heated surface for spray cooling

Owing to the superior cooling performance of the droplet impingement process, the spray cooling is widely used in various industries including aerospace, e-motors, data centres and so on. In the present study, the morphological evolution and heat transfer characteristics of vertical and oblique impacts on the heated surface were investigated experimentally and numerically. A droplet generation system and a substrate heating system were designed to observe continuous vertical and oblique droplets impinging on the hot surface by a high-speed camera. The coupled Level Set and VOF (CLS-VOF) model and adaptive mesh refinement technique were used to establish a mathematical model for a train of micro-sized droplets impinging on the heated surface, which was validated by experimental data. The film that spreads on the surface exhibits asymmetry due to the horizontal velocity of the droplets. When continuous droplets impinge vertically on the heated wall, the evolution of the crown and the wall temperature appear periodic. The effects of impingement angles on morphological patterns and heat transfer of a train of micro-sized monodispersed droplets with the same Weber numbers were analyzed further. The results indicated that with a smaller angle of impingement, the crown is more likely to splash, and the crown asymmetry is less obvious.

Simple Method for On-Demand Droplet Trapping in a Microfluidic Device Based on the Concept of Hydrodynamic Resistance.

We demonstrate an innovative method to catch the desired droplets from a train of droplets and immobilize them in traps located in an integrated microfluidic device. To this end, water-in-oil droplets are generated in a flow-focusing junction and then guided to a channel connected to chambers designated for on-demand droplet trapping. Each chamber is connected to a side channel through a batch of microposts. The side channels are also connected to the flexible poly(vinyl chloride) tubes, which can be closed by attaching binder clips. The hydrodynamic resistance of the routes in the device can be changed by opening and closing the binder clips. In this way, droplets are easily guided into individual traps based on the user's demand. A set of numerical simulations was also conducted to investigate the authenticity of the employed idea and to find the optimal geometry for implementing our strategy. This simple method can be easily employed for on-demand droplet trapping without using on-chip valves or complex off-chip actuators proposed in previous studies.

Erosion characteristics of water droplet machining

Water Droplet Machining (WDM) is a new manufacturing process, which uses a series of high-velocity, pure-water droplets to impact and erode solid workpieces, for the purpose of through-cutting, milling and surface profiling. The process is conducted within a vacuum environment to suppress aerodynamic drag and atomization of the waterjet and droplet train. This preserves droplet momentum and allows for a more efficient transfer of energy between the water and workpiece, than in standard atmospheric pressure. As a new manufacturing technique, parameter-specific details and characteristics of this process are absent from the scientific literature. Therefore, this study aims to elucidate the capabilities of WDM, and uncover its erosion characteristics. Experiments are performed using a custom-fabricated machine, where a range of waterjet-types (and droplet trains) are produced. The experiments are performed on two industrial-relevant metals: stainless steel SS 316 L and aluminum AA 6022-T4. The target shape is a partial-cut through the material, i.e., a trench. A parametric study investigates the effects of stand-off distance, orifice diameter, and feed-rate on the trench shape, material-removal-rate, and reveals the conditions which render WDM an effective machining process. It was found that, for orifice diameters of 100 μm, an effective stand-off distance is between 457 and 686 mm; below that, no material removal is observed. In this configuration, accurate through-cuts are produced and can be used for manufacturing purposes. For large orifice diameters, e.g., 250 μm, an atomized spray is produced, which features a comparatively large material removal rate, and can be used for surface profiling and milling.

Design of microfluidic chromatographs through reinforcement learning

Chromatography is one of the most valuable techniques chemists possess at their disposal, conducive to everything from developing vaccines, food, beverage, and drug testing to catching criminals. The diverse applications allow it to be used for analytical and preparative purposes. On the other hand, droplet microfluidics has significantly evolved from simple droplet generators to complex and integrated tasks through specially designed networks. Microfluidics finds itself at the center of various Lab-on-Chip studies, enabling single-cell analysis, biochemical synthesis, etc. We demonstrate a microfluidic chromatograph machine that can produce an ordered droplet arrangement for a large number of drops. The droplets are sent into the device using a novel methodology where the conventional droplet train is made into smaller batches. The study describes the use of droplet batch methodology and compares it with the traditional droplet train approach. Using this platform, different droplet sequences are sent through the chromatograph, which preferably allows some droplets to exit first while others take a longer time to flow across the chromatograph based on the droplet properties and device design. The droplet sequences contain various drops; however, the type of drops in these sequences is limited to 2. The chromatograph can handle any number of drops in a single machine is enough for handling diverse droplet sequences. The stability of the microfluidic chromatography is also studied by varying the droplet properties and the droplet batch size.

Experiment and modeling of stochastic ignition and combustion of fuel droplets impacting a hot surface

The ignition dynamic of liquid fuel droplets impacting hot surfaces is critical for fire-safety analysis in engineering systems, as well as for controlling wall-filming effects in IC engines. The scenario of slow fuel-leakage rates poses challenges associated with the stochastic processes of droplet splashing, break-up, turbulent mixing, and combustion. To address this, we conduct controlled experiments of liquid n-heptane droplets impacting a heated surface in the Leidenfrost regime, targeting the individual-droplet-deposition conditions. The experiment encompasses a range of surface temperatures and droplet-deposition rates. The experiments are complemented by theoretical analysis, where we developed a stochastic low-order numerical model, demonstrating good accuracy for predicting ignition probability and overall combustion dynamics. Notably, we observe a broad region of intermittent combustion behavior, with ignition probability varying based on surface temperature and droplet deposition rate. Additionally, we find that the transition to consistent ignition relies heavily on both surface temperature and deposition rate. Experimental and numerical model results shed light on the roles of the complex interplay between droplet breakup, chemical kinetics, and evaporation and mixing time scales, as well as the interaction among subsequent droplet combustion events, in governing the ignition and combustion of impacting droplet trains. The revealed dynamic of droplet/hot-surface ignition and the proposed stochastic model hold promise for advancing predictive capabilities of hot-surface-induced ignition and combustion arising from accidental leaks in flammable-liquid piping and wall-filming, particularly in the stochasticity-dominated individual-droplet-deposition regime.

Heat transfer characteristics of controlled droplet trains impacting solid surfaces

The unpredictable spray characterization caused by different manufacturing standards still limits the implementation of spray cooling. To obtain a predictable spray, a droplet generator was in-house designed and fabricated, which can generate predictable monodisperse droplets or multiple droplet trains at a certain droplet generation frequency (fG). Based on the measurement of the droplet flux (N), the controlled droplet trains are classified into the controlled sparse spray and dense spray. In the experiments, deionized water was used as the liquid coolant, and heat transfer characteristics were analyzed in the controlled sparse spray and dense spray. The experimental results show at the same volumetric flow rate the heat transfer curve of dense spray is always above the heat transfer curve of sparse spray. In the cooling area of controlled dense spray, the average Nusselt number Nu‾L was correlated as logNu‾L∝−(fG/N)1/2. This study would benefit the application of controlled spray cooling technology in high-heat-flux thermal management.

Quantum droplets in one-dimensional Bose-Bose mixtures: beyond the Lee-Huang-Yang description

The static and dynamic properties of self-bound quantum droplets in a one-dimensional Bose-Bose mixture are discussed in the spirit of the Hartree–Fock-Bogoliubov theory. This latter enables us to provide beyond the Lee-Huang-Yang (LHY) quantum corrections to the equation of state at both zero and finite temperatures. In the uniform case our results for the ground-state energy and the critical temperature are confirmed through comparison with Quantum Monte-Carlo simulation and with available theoretical results. The density profiles are supported by numerical simulations of the generalized Gross-Pitaevskii equation which selfconsistently includes higher-order terms originating from the normal and anomalous fluctuations under the local density approximation. We show that the density exhibits a dip near its center in the flat-top plateau region for large interspecies interactions. We exemplify the impact of the beyond LHY corrections on the spatiotemporal evolution of the self-bound droplet in the presence of excitation induced by periodic density modulation. It is found that higher-order corrections may lead to the formation of a train of small droplets. We then extend our study for the case of inhomogeneous droplets in quasi one-dimensional Bose mixtures.

Multiparametric, Label-Free Analysis of Microfluidic Droplets via a Dynamic Phase Grating Approach.

Droplet-based microfluidic systems possess many fundamental advantages as a platform for the analysis of chemical and biological species. However, whereas on-chip operations have rapidly developed over the past decades, approaches for analyzing target molecules within droplets have largely remained limited to methods requiring bulky and expensive instrumentation. In this work, we describe a droplet analysis approach whereby the droplet train itself is the sensing construct. Specifically, the droplet train is interrogated as a transmission phase grating, allowing high-throughput, label-free, solution-phase, and multi-parametric analysis of droplet contents. Importantly, three distinct properties of generated droplets can be simultaneously extracted using this conceptually simple and experimentally straightforward measurement approach. Under constant droplet generation conditions, measurement of droplet viscosity is achieved by monitoring changes in zero order to first order peak separation in the far-field diffraction pattern, with a sensitivity of 2.28 × 10-4 cSt per μm change in peak separation. In parallel, measurement of droplet refractive index (RI) is achieved by measuring changes in the ratio of the zero order to first order peak intensity, with a sensitivity of 2.14 × 10-4 RI units per unit change in a diffracted peak intensity ratio. Finally, droplet generation frequency is determined from the time-varying oscillation of the peak height ratio, yielding comparable results to an expensive high-speed camera commonly used for droplet imaging. Importantly, the experimental strategy for this approach is straightforward and does not require expensive instrumentation; therefore, it may find utility in affordable and portable analysis approaches applied to diverse droplet microfluidic assays.

Flow Visualization and Heat Transfer Characteristics of Droplet Train Obliquely Impinging on a Moving Hot Solid

Flow Visualization and Heat Transfer Characteristics of Droplet Train Obliquely Impinging on a Moving Hot Solid

Nanoscopic jets and filaments of superfluid 4He at zero temperature: A DFT study.

The instability of a cryogenic 4He jet exiting through a small nozzle into vacuum leads to the formation of 4He drops, which are considered ideal matrices for spectroscopic studies of embedded atoms and molecules. Here, we present a He-density functional theory (DFT) description of droplet formation resulting from jet breaking and contraction of superfluid 4He filaments. Whereas the fragmentation of long jets closely follows the predictions of linear theory for inviscid fluids, leading to droplet trains interspersed with smaller satellite droplets, the contraction of filaments with an aspect ratio larger than a threshold value leads to the nucleation of vortex rings, which hinder their breakup into droplets.

Morphology and kinematics of a train of power-law droplets in a corrugated microchannel

We numerically investigate the deformation of a train of power-law droplets in a highly viscous suspending medium in nonlinear microfluidic geometry. We examine how viscous stresses conspire against surface tension in two-dimensional incompressible liquid–liquid flow in a corrugated microchannel using the finite volume technique with the volume of fluid method. To instill the influences of shear thinning, Newtonian, and shear-thickening fluids, the range of power-law indices (n) is varied from 0.7-1.3. We validate our numerical model with the available literature data in good agreement. Interestingly, the corrugated cavities are found to have an elongating effect on the non-Newtonian droplets, whereas literature delineates a compressive effect of the nonlinear geometry on bubbles. Specific orbital trajectories are found in the (droplet length A, spacing B) space with dispersed and synchronized orbitals at low and high capillary number for all power-law droplets, respectively. Furthermore, the hysteresis in the droplet shape is found to increase with the capillary number. Consequently, the hysteretic area Ap revealed that the slope increases with (n). An interesting finding is elucidated where the hysteresis loops are found to rotate from low capillary to high capillary number for all power-law droplets. A new finding is reported where the evolution of the droplet perimeter revealed the development of secondary peaks as the capillary number increases. The power-law droplets are found to pack themselves up in the corrugated section in two distinct regimes: smaller droplets obey the linear deformation regime. In contrast, large droplets lie in the flow-induced deformation regime. The instantaneous cap velocities produce a nonmonotonic behavior for the front velocity as the capillary number increases for all power-law droplets. We develop functional relationships between flow parameters of segmented flows at the pore scale for every non-Newtonian droplet. Given the implications of this setup with industrial and biomedical applications, the illustrations shown herein could be beneficial in tackling problems at the pore scale with nonlinear fluids.

Experimental study of dripping, jetting and drop-off from thin film flows on inclined fibers

Gravity driven film flows on vertical fibers are known to exhibit a variety of flow dynamics including the formation of droplet trains induced by the hydrodynamic (Kapitza) and Plateau–Rayleigh instability mechanisms. Through an experimental study, it is shown how inclination of the fiber from the vertical influences these dynamics. The formation of waves, regime transitions from dripping to jetting regimes, as well as the onset of drop-off in the form of droplet detachment from the fiber are illustrated and described in dependence of the fiber inclination angle and the liquid mass flow rate. Additionally, the influence of fiber diameter and nozzle geometry on regime transitions and the onset of drop-off from the substrate are examined. It is shown that the onset of drop-off is strongly related to the transition from a regime characterized by a regular wave pattern to a regime characterized by an irregular wave pattern. It is also demonstrated that this regime transition depends not only on flow rate and fiber geometry, but also strongly on the inclination angle. Interestingly, a stabilizing effect of increasing the fiber inclination is detected for constant fiber geometry and film flow rate.

Analysis of the evaporation coefficients of water, heavy water, and methanol in a high vacuum environment

The evaporation coefficient of liquids is an important factor for many practical situations but has been poorly understood due to the complexity of the experiments and uncertainty in the measurements. Indeed, the lack of current knowledge about the nature of the evaporation coefficient prevents accurate confirmation of its value. We have determined the evaporation coefficients of three liquids, water, heavy water, and methanol, by fitting Knudsen's evaporation rate equation to past measurements of the temperature of droplet trains and liquid jets evaporating in a high-vacuumed environment. Analysis of available experimental data suggests the value of 1 for the evaporation coefficient (αe) of water, and 0.725 for that of heavy water. A small-jet methanol study predicts αe≈1; however, analysis of experimental data from a large-jet methanol study was an exception—indicating coefficients well over unity (1.5<αe<9). These exceed the suggested maximum theoretical value of unity. We found that considering αe=1 for water, and most of the test fluids, is a fair approximation in a high-vacuumed environment that presents minimal kinetic barriers to evaporation. These results confirm that water should not be considered an anomalous liquid with respect to evaporation. This study provides insight that will help improve the design and development of the many processes in which evaporation plays a key role.

Offset impacting of a liquid aluminum droplet train on a molten pool on a horizontally moving substrate during TIG-assisted droplet deposition manufacturing

Internal metallurgical defects and poor mechanical properties and surface quality of the formed parts are the most important challenges of metal micro-droplets deposition manufacturing technology. Lack of strong coalescence between neighboring droplets and the blocking effects of periodic ripples on the fusion process are major sources of abovementioned problems. To address this issue, a tungsten inert gas (TIG) welding arc was introduced to pre-melt the material below the droplet impacting location, thus, one droplet train will be deposited on a curved molten pool surface to fabricate deposited layers in an additive approach. In the present work, a numerical model based on the computational fluid dynamics (CFD) method was developed to investigate the dynamic behaviors of successive droplets’ impact and coalescence on the molten pool created by a tilted TIG welding arc, and focuses on the effect of the offset distance between the impacting point of droplet and the longitudinal plane of molten pool on the deposition morphology and the impact-induced molten pool behaviors. The simulated tracks exhibit regular scaly shapes at different offset distances, which is validated against the corresponding experiments. The results indicated that as the lateral offset distance increases, the symmetric spreading and recoiling of the subsequent droplets on the pre-existing bead will be broken due to the inclined and curved solidification front morphologies, the inclined front morphologies are caused by the increased growth rate of the solidification front at the impact zone. With an increased lateral offset distance, the volume of the molten pool decreases, the width of the rear molten pool increases slightly. The calculated solidification parameters are found to be dependent of the offset distance, and vary significantly at the onset of solidification at different portions of the molten pool. The scientific findings provide useful insights for further researches of TIG-assisted DDM.

Thermographic Observation and Hydrodynamic Patterns of Inclined Ethanol Droplet Train Impingement on a Non-Uniformly Heated Glass Surface

Droplet train impingement is a fundamental approach to mimic the complicated interactions between the fluid and the substrate in advanced thermal engineering applications in industry. Differently from previous studies, the main original contribution of this study is to perform an inclined droplet train impingement on a non-uniformly heated surface. Ethanol was used as the liquid for droplet train impingement applications, while glass substrate was selected as the target surface. The inclined flow angle was 63 degrees. Both optical and thermographic observations were performed on the target surface by focusing on the droplet impact area. Three experimental sets were created with the Weber numbers 667.57, 841.90, and 998.01. A surface temperature range was selected between 85.00 °C and 200.00 °C, which was above the boiling point of the ethanol. The maximum spreading length was measured at 0.97 mm at the surface temperature of 82.00 °C for the experiment with the Weber number of 998.01, whilst the minimum spreading length was found at 0.18 mm at the highest surface temperature for the experiment with the Weber number of 667.57. A uniform splashing direction was observed above 170.00 °C for all experiments, which meant that the sign of the transition regime appeared.

Experimental research on the dynamics of a train of droplets impacting, from droplets to liquid film, continuity and inheritance

The propagation of the droplet inside the air treatment can be simplified as the droplet impact the wall, liquid film, or collide with each other. We build a visual experimental platform for a train of droplets using the micro-vertical injection pump and the high-speed imaging system in order to examine the variations in the impact behavior of the falling droplets. The variables are the impacting velocity and the contact angle. The dynamic process of the first impact (the droplet impact on the plate surface) and, second impact (the droplet impact on the static one resting on the surface) and the final impact (the droplet impacts on the liquid film converged by the previous droplets) are captured and analyzed respectively. The results indicate that the impact velocity and the contact angle are the driving factors of the impact behavior promoting the formation of the tip sub-droplet and the sub-droplet of adhesion respectively. When the droplet impacts the liquid film, there is no generation of the sub-droplet, whose behavior is not dependent on the contact angle but rather the initial velocity. The splashing and protrusion height are converging as the velocity increases. While the velocity retards the splashing process.

Interaction between droplets and co-flow interface in a microchannel: Droplet migration and interfacial deformation

We investigate hydrodynamic interactions between droplets and the interface of two co-flowing immiscible streams in a microchannel. We find that small droplets of confinement ratio (drop size to suspending stream width), $\ensuremath{\beta}&lt;0.5$ migrate towards the co-flow interface while larger droplets of $0.5&lt;\ensuremath{\beta}&lt;1$ unexpectedly drift away from the interface. Large droplets with $\ensuremath{\beta}&gt;1$ cause deformation waves in an initially stable and flat interface that propagate downstream akin to traveling peristaltic waves. The deformation wave amplitude associated with a droplet grows spatially downstream but remains the same for a train of droplets crossing a fixed location, indicating convective instability.

Elevating Leidenfrost temperature by orderly droplet stream impingement boiling

Impinging high frequency microscale droplet stream on a high temperature substrate is an effective method to realize point heat sink. The boiling patterns and the heat transfer characteristics of droplet impingements were investigated based on the captured images and the measured heat transfer. In the experiments, six different hydrodynamic patterns could be identified, base on which the phase diagram was drawn. The boundaries of different boiling patterns varied with the flowrate as well as the frequency of the droplet stream. The substrate temperature threshold for the transition from the contact boiling regime to the levitation transition boiling regime was influenced by flowrate apparently, but it was independent of droplet frequency. However, both the hydrodynamic patterns and the heat transfer measurements showed that the Leidenfrost temperature was elevated by increasing the flowrate or the droplet frequency. Compared to the droplet stream with random size and spacing, the Leidenfrost temperature could be increased by impinging orderly droplet stream without using more coolant. The reason was inferred that periodical collisions of droplet train released vapor in an efficient manner.

Numerical Study of Consecutive Drop/Wall Impacts Using Smoothed Particle Hydrodynamics

A fundamental understanding of drop/wall interactions is of vital importance to many engineering applications, particularly in internal combustion engines; the outcomes of drop/wall interactions directly affect an engine's overall performance. Numerical models developed for drop-wall interactions are mostly based on the impact of a single drop on dry or wet walls. To accurately mimic and model real engine conditions, it is necessary to have a comprehensive study of drop/wall interactions with consecutive droplets. A theoretical splashing criterion based on the K parameter incorporating the impingement frequency is presented in this paper. In this study, a numerical method, based on Smoothed Particle Hydrodynamics (SPH), is used to simulate the impact of a droplet train on a heated wall at different impingement frequencies. The present SPH method is validated against the experimental results on the propagation of the crown rim's diameter as a function of the non-dimensional time. Impact regimes are identified for various impact conditions based on the study of the time evolution of the post-impingement process. In this work, simulations were conducted for n-heptane drops at different impingement frequencies and diameters. It is found that the K parameter provides a better prediction for the impingement outcomes and the effect of impingement frequency is seen as the kinematic discontinuity among the spreading lamellae, film formation, and rapidness of drop impact. Additionally, the inhibition by high ambient pressure on the crown rim propagation is also quantified. The results of this study can be used to further improve spray/wall interactions models at realistic engine conditions.