Droplet Collision Research Articles

Collision dynamics of binary liquid metal droplets under horizontal magnetic field

We conduct experimental research on binary liquid metal droplet collisions under the influence of a horizontal magnetic field. For magnetic intensity smaller than 1.5 T, only reflexive separation is obviously facilitated, but other collision regimes show little difference with zero magnetic intensity. We infer that when magnetic intensity is larger than a critical value of 4 T, remarkable influences on all regimes would be observed.

Head-on Collision of Two Nanodroplets on a Solid Surface: A Molecular Dynamics Simulation Study.

Most researchers focus on the collision of a single droplet with a solid surface, while it is common for a droplet to collide with a sessile droplet on a solid surface in reality. This study performed the head-on collision of two nanodroplets on a solid surface using the molecular dynamics simulation method. The effects of impact velocity, interaction intensity between solid and liquid atoms, and the solid fraction of the surface on the collision process are studied with independent simulation cases. The maximum spreading factor and the dimensionless maximum spreading time are recorded and calculated to describe the collision process quantitatively. The simulation results indicate that the maximum spreading factor depends more on the solid fraction than the interaction intensity since it does not fundamentally change the wetting state of the droplet at its maximum spreading state. Because of two different effects, the maximum dimensionless spreading time decreases first and then increases with the interaction intensity, and both effects weaken with the increase of impact velocity. As the solid fraction increases, the maximum spreading factor increases significantly at high impact velocity, and the maximum dimensionless spreading time first decreases and then increases because the wetting state of the coalescent droplet at the maximum spreading moment gradually changes from the Wenzel state to the Cassie state. In general, the initial wetting state of the sessile droplet and the wetting state of the coalescent droplet at the maximum spreading moment have important effects on the maximum spreading factor and the maximum spreading time. We establish a theoretical prediction model for the maximum spreading factor on a smooth surface based on energy conservation with quite good accuracy. This research has improved our understanding of the head-on collision process of two nanodroplets on a solid surface.

Study of Stratus-Lowering Marine-Fog Events Observed During C-FOG

Two stratus-lowering marine fog events observed on 28 September and 4 October 2018 during the Coastal Fog (C-Fog) field campaign that took place offshore of eastern Canada from 1 September to 6 October 2018 are described. In situ, profiling, and remote sensing observations were made at selected land sites in eastern Newfoundland, Nova Scotia, and aboard the research vessel Hugh R. Sharp cruising in adjoining coastal waters. Synoptic-scale analysis shows that both fog episodes result from the interaction between synoptic-scale surface-level low-pressure systems and a contiguous high-pressure system. At the same time, back trajectories reveal that the bulk of the fog layer is formed due to differential advection. The diameter of the fog droplets at the surface gradually decreases from the centre of the fog layer to its leading/trailing edges. The bimodal fog-droplet diameter distribution with peaks at 5–10 µm and 20–25 µm provide clues on droplet collision and coalescence processes. The observed difference between microphysical variables and droplet distribution between the two fog events and within the same fog layer might be governed by the atmospheric-boundary-layer (e.g., humidity conditions and turbulence) that prevailed in the fog layer. Overall, it is concluded that the life cycle of observed stratus-lowering coastal-fog episodes depends on synoptic conditions and atmospheric-boundary-layer characteristics such as stability, cloud-top cooling, and entrainment.

Transitions of bouncing and coalescence in binary droplet collisions

In droplet impacts, transitions between coalescence and bouncing are determined by complex interplays of multiple mechanisms dominating at various length scales. Here we investigate the mechanisms and governing parameters comprehensively by experiments and scaling analyses, providing a unified framework for understanding and predicting the outcomes when using different fluids. Specifically, while bouncing had not been observed in head-on collisions of water drops under atmospheric conditions, it was found in our experiments to appear on increasing the droplet diameter sufficiently. Contrarily, while bouncing was always observed in head-on impacts of alkane drops, we found it to disappear on decreasing the diameter sufficiently. The variations are related to gas draining dynamics in the inter-droplet film and suggest an easier means for controlling bouncing as compared to alternating the ambient pressure usually sought. The scaling analysis further shows that for a given Weber number, enlarging droplet diameter or fluid viscosities, or lowering surface tension contributes to a larger characteristic minimum thickness of the gas film, thus enhancing bouncing. The key dimensionless group $(O{h_{g,l}},\;O{h_l},\;{A^\ast })$ is identified, referred to as the two-phase Ohnesorge number, the Ohnesorge number of liquid and the Hamaker constant, respectively. Our thickness-based model indicates that as ${h^{\prime}_{m,c}} > 21.1{h_{cr}}$ , where ${h^{\prime}_{m,c}}$ is the maximum value of the characteristic minimum film thickness $({h_{m,c}})$ and ${h_{cr}}$ is the critical thickness, bouncing occurs in both head-on and off-centre collisions. That is, when $1.2O{h_{g,l}}/(1 - 2O{h_l}) > \sqrt[3]{{{A^\ast }}}$ , a fully developed bouncing regime occurs, thereby yielding a lower coalescence efficiency. The transitional Weber number is found universally to be 4.

Interaction between Strong Sound Waves and Cloud Droplets: Theoretical Analysis

AbstractRecently, strong sound wave was proposed to enhance precipitation. The theoretical basis of this proposal has not been effectively studied either experimentally or theoretically. On the basis of the microscopic parameters of atmospheric cloud physics, this paper solved the complex nonlinear differential equation to show the movement characteristics of cloud droplets under the action of sound waves. The motion process of an individual cloud droplet in a cloud layer in the acoustic field is discussed as well as the relative motion between two cloud droplets. The effects of different particle sizes and sound field characteristics on particle motion and collision are studied to analyze the dynamic effects of thunder-level sound waves on cloud droplets. The amplitude of velocity variation has positive correlation with sound pressure level (SPL) and negative correlation with the frequency of the surrounding sound field. Under the action of low-frequency sound waves with sufficient intensity, individual cloud droplets could be forced to oscillate significantly. A droplet smaller than 40 μm can be easily driven by sound waves of 50 Hz and 123.4 dB. The calculation of the collision process of two droplets reveals that the disorder of motion for polydisperse droplets is intensified, resulting in the broadening of the collision time range and spatial range. When the acoustic frequency is less than 100 Hz (at 123.4 dB) or the SPL is greater than 117.4 dB (at 50 Hz), the sound wave can affect the collision of cloud droplets significantly. This study provides a theoretical perspective of the acoustic effect on the microphysics of atmospheric clouds.

Collisions of Liquid Droplets in a Gaseous Medium under Conditions of Intense Phase Transformations: Review

The article presents the results of theoretical and experimental studies of coalescence, disruption, and fragmentation of liquid droplets in multiphase and multicomponent gas-vapor-droplet media. Highly promising approaches are considered to studying the interaction of liquid droplets in gaseous media with different compositions and parameters. A comparative analysis of promising technologies is carried out for the primary and secondary atomization of liquid droplets using schemes of their collision with each other. The influence of a range of factors and parameters on the collision processes of drops is analyzed, in particular, viscosity, density, surface, and interfacial tension of a liquid, trajectories of droplets in a gaseous medium, droplet velocities and sizes. The processes involved in the interaction of dissimilar droplets with a variable component composition and temperature are described. Fundamental differences are shown in the number and size of droplets formed due to binary collisions and collisions between droplets and particles at different Weber numbers. The conditions are analyzed for the several-fold increase in the number of droplets in the air flow due to their collisions in the disruption mode. A technique is described for generalizing and presenting the research findings on the interaction of drops in the form of theoretical collision regime maps using various approaches.

Inertial stretching separation in binary droplet collisions

Binary droplet collisions exhibit a wide range of outcomes, including coalescence and stretching separation, with a transition between these two outcomes arising for high Weber numbers and impact parameters. Our experimental study elucidates the effect of viscosity on this transition, which we show exhibits inertial (viscosity-independent) behaviour over an order-of-magnitude-wide range of Ohnesorge numbers. That is, the transition is not always shifted towards higher impact parameters by increasing droplet viscosity, as it might be thought from the existing literature. Moreover, we provide compelling experimental evidence that stretching separation only arises if the length of the coalesced droplet exceeds a critical multiple of the original droplet diameters (3.35). Using this as a criterion, we provide a simple but robust model (without any arbitrarily chosen free parameters) to predict the coalescence/stretching-separation transition.

CFD simulation of agglomeration and coalescence in spray dryer

Particle size distribution is of utmost importance in spray dryers. Controlling the degree of agglomeration and coalescence is vital for manufacturing products that conform to the requirements. To predict the particle size accurately, this study presents a CFD model of particle agglomeration and droplet coalescence in a counter-current spray dryer. The stochastic agglomeration model was modified to incorporate more realistic droplet-droplet, droplet-particle and particle-particle interactions. Binary droplet collisions in the atomization zone of a spray dryer can result in either coalescence or separation. The boundaries between collision regimes were experimentally determined, and existing binary collision models were validated for water and milk concentrate to include the effect of viscous forces on collision outcomes. A high-speed camera was used to record the kinematic and physical properties of the droplets to identify various collision regimes. A suitable model was then incorporated in CFD simulations of spray drying. Using existing experimental data from a counter-current spray dryer with strong agglomeration, it is shown that the model can accurately predict product particle properties under different process conditions.

Experimental studies on binary water droplet collisions considering size ratio effects

Experimental studies on binary water droplet collisions considering size ratio effects

An improved algorithm to estimate droplet collision characteristics for Lagrangian simulation of impinging spray under different ambient pressures

An improved algorithm to estimate droplet collision characteristics for Lagrangian simulation of impinging spray under different ambient pressures

Suppressing Breakups in Binary Droplet Collisions by Surfactant

Suppressing Breakups in Binary Droplet Collisions by Surfactant

Euler/Lagrange Computations of Sprays Produced by Different Liquids using a Generalised Stochastic Droplet Collision Model

Euler/Lagrange Computations of Sprays Produced by Different Liquids using a Generalised Stochastic Droplet Collision Model

Size effects on dynamics of nanodroplets in binary head-on collisions

Head-on collision dynamics of 10, 50 and 100 nm droplets are investigated in vacuum by molecular dynamics, involving 35,858, 4,506,410 and 36,051,466 molecules, respectively. A variety of droplet collision dynamics are observed, such as coalescence, hole formation and shattering, as a function of the Weber number. It is found for the first time that the collision and reflexive separation can occur in the nanodroplet regime when the droplet diameter reaches 100 nm but not for 10 or 50 nm droplets. The size effect in droplet collisions is studied based on the analysis of stretching factors, energy dissipation and collision outcomes for droplets of different diameters. The kinetic energy dissipation due to the atomic interactions at nanoscales is identified to significantly influence the occurrence or otherwise of reflexive separation. Through quantitative analysis of the evolution of the internal structure of the 100 nm nanodroplets collision at the Weber number of 277, it is revealed for the first time that molecules from both parent nanodroplets have penetrated the full length of the merged nanodroplet in the direction of collision, due to a combination of molecular mixing and internal currents. Consequently, all three child nanodroplets have molecules from both parent nanodroplets, contrary to the perception gained from common imaging techniques. The results show that the dynamics, outcomes and mechanisms of nanodroplet collisions have both similarities and differences compared with their micro- and macro-counterparts.

Characterization of clay-stabilized, oil-in-water Pickering emulsion for potential conformance control in high-salinity, high-temperature reservoirs

In this study, a Na-montmorillonite clay-stabilized, oil-in-water (O/W) Pickering emulsion system was prepared and tested under high-salinity and high-temperature conditions. The emulsifying ability of the clay particles, emulsion stability against creaming and coalescence, and emulsion rheological behavior at different NaCl concentrations and temperatures were investigated. The results showed that decane could be effectively emulsified by clay particles into small emulsion droplets at salinities up to 25 wt% NaCl. The emulsion had a relatively narrow drop size distribution. However, the emulsion stability against creaming decreased with increases in NaCl concentration. At low NaCl concentrations, clay particles connected with each other by edge-edge and edge-face interactions, forming a three-dimensional network that stabilized the emulsion against creaming. A higher concentration of NaCl caused some clay particles to connect through face-face interactions, forming clay clusters. At high NaCl concentrations, the temperature had only a slight effect on the emulsion stability against creaming. However, emulsion stability against coalescence decreased with increasing temperature due to increased droplet collision caused by Brownian motion. A higher concentration of NaCl moderated this temperature effect of coalescence. The viscosity of emulsion decreased with increased NaCl concentration. Temperature increases caused emulsion viscosity to decrease at low NaCl concentration conditions. When the NaCl concentration was higher than 10 wt%, the temperature had little effect on the viscosity. In summary, this study identifies an O/W Pickering system with the potential for conformance control in high-salinity and high-temperature reservoirs.

Numerical analysis of collision characteristics between charged drop and neutral droplet under uniform electric field

Dissipatingfog and fog water collection have been of great concern recently. Corona discharge can produce charge and electric field, and accelerate the above process by promoting fog droplets collision. Collision efficiency and kernel are introduced toanalyze the effect of electric charge and electric field on fog droplets collision in this paper. For fog droplets, the efficiency and kernel represent the probability and rate of collision, respectively. In addition, an extended trajectorymodel, including drag force, gravity, electrostatic force and applied electric field force, is proposed. The results indicate that the charge promotes the efficiency and kernel significantly. However, the collision efficiency between charged drop and neutral droplet under uniform electric field is reduced. When the applied electric field strength increases, the collision efficiency first decreases and then increases to 1.0, whereas the collision kernel increases at all times. This is because the electrostatic force, hydrodynamic force and inertial force vary with electric field. The calculatedresults can be used to optimize the efficiency of fog dissipation and fog water collection.

Elasto-hydrodynamics of non-Newtonian droplet collision with convex substrates

In this article, we report the post-collision elasto-hydrodynamics of non-Newtonian elastic or Boger fluid droplets [polyacrylamide (PAAM) solution in water] on convex or cylindrical targets of various diameters. Both hydrophilic and superhydrophobic (SH) surfaces were studied to deduce the role of wettability. Different governing parameters, such as cylinder diameter, Weber number, and fluid elasticity (different polymer concentrations), were systematically varied to understand various hydrodynamic outcomes. In contrast to the Newtonian water droplets on hydrophilic surfaces, PAAM droplets resisted capillary breakup and exhibited formation of long lasting, slender, fluid filaments. In certain cases, these filaments showed the existence of satellite beads during stretching, which are generated through blistering or pearling instability (known as beads-on-a string). In the case of SH surfaces, PAAM droplets rebound at larger cylindrical diameters and higher Weber number compared to water. Thin transient filaments attached to the cylinder surface eventually suppress droplet rebound. Such rebound suppression is essentially a non-Newtonian feature, as water droplets on a cylindrical SH surface always exhibited rebound and fragmentation. Finally, we illustrate phase maps where the different regimes of post-impact elasto-hydrodynamics are correlated as functions of a proposed elastic Weber number (which incorporates the effects of both the Weber and the Weissenberg numbers) and the non-dimensional diameter D*. We show that distinct scaling regimes appear in the elasto-hydrodynamic behavior of the post-impact droplets of elastic fluids.

Maximal spreading of droplet during collision on particle: Effects of liquid viscosity and surface curvature

This study uses the level contour reconstruction method to numerically investigate the maximum spreading due to droplet collision with a dry, stationary, spherical particle. We consider a broad range of impact conditions: Weber number 30–90, Ohnesorge number 0.0013–0.7869, and droplet-to-particle size ratio 1/10–1/2, and quantitatively and systematically analyze 120 collision cases to understand how liquid viscosity and surface curvature affect the maximum spreading. The maximum spreading increases on the smaller particles for both the capillary and viscous regimes, but the underlying physics clearly differ. The increase in maximum spreading is governed mainly by the surface deformation of the rim for the capillary regime and viscous dissipation for the viscous regime. An empirical correlation that can be applied to the droplet impact on both a particle and a flat surface is also presented. The model shows good agreement with existing experimental data as well as our simulation results within a deviation range of ±15%.

Determination of dynamic interactions of droplets in continuous fluids using droplet probe

HypothesisInteractions between droplets are of fundamental importance for understanding phenomena involving droplet collision and coalescence that determine multiphase flow behavior. The quantitative understanding of these interactions is essential for the manipulation and control of emulsions or complex fluids. The existing methods for interaction force determination are typically based on expensive mechanical probes and fine distance control. Therefore, further development of new techniques for interaction force determination is expected to be beneficial for research on surface force. ExperimentsIn this study, droplet deformation during the interaction between two droplets was captured and analyzed to determine the interaction force. The approach speed of the two droplets was controlled by the injection rate of the fluid. The dynamic interaction force between two tetradecane droplets in various aqueous solutions was determined using the newly developed method, and the effects of two-phase physical properties and operating conditions on the measurement errors were investigated. FindingsThe droplet profile deformation was first applied as a probe to detect the interaction force. The measurement results were in good agreement with those obtained using the precise weighing sensor of a commercial interfacial tensiometer (K100, Kruss, Germany). The newly developed method was reliable, simple, and did not require the use of expensive devices. Furthermore, droplet deformability was found to be the key parameter in determining the total interaction force between the droplets.

Experimental study on coalescence of fog droplets in cloud chamber under low-frequency sound waves

The impact of foggy weather on the national economy has become more and more serious with the development of society. Exploring cost-effective artificial fog elimination technology has become a practical need. Acoustic coalescence of droplets is a new atmospheric interference technology, which has the advantages of no chemical pollution, easy operation, and so on. Existing field experiments have successfully observed artificial rainfall or defogging promoted by sound waves. However, there is still a lack of systematic experiments to study the influence of different parameters (frequency, sound power, lasting time of sound waves and initial mass concentration, etc) on the collision-coalescence of droplets under sound waves. In this paper, a series of experimental studies have been carried out in a cloud chamber with 1.7 × 0.52 × 0.52 m. The results show that frequency is an important factor affecting the acoustic coalescence of droplets, and there is an optimal frequency of 300 Hz corresponding to droplets with mode diameters of 2.88 μm in our experiment. Increasing the sound power and lasting time can effectively promote the collision of droplets to form larger droplets. Interestingly, the acoustic coalescence effect of droplets does not increase significantly at the optimal frequency of 300 Hz when the sound power exceeds 75 W and the lasting time exceeds 20 s in our device, which is attributed to the breaking of the larger droplets. The initial mass concentration of the droplet is positively correlated with the droplet growth ratio. In this paper, the frequency of 300 Hz, the sound power of 75 W and the lasting time of 20 s are more reasonable to achieve a cost-effective goal.

Design and analysis of de-oiling coalescence hydrocyclone

ABSTRACT De-oiling hydrocyclones are extensively applied in oilfield wastewater purification for fluid separation, because of their efficient and rapid processing performance. However, it is difficult for a hydrocyclone to separate tiny oil droplets owing to the inadequate centrifugal force. A type of coalescence hydrocyclone (CHC) is designed based on the principle of centrifugal separation and hydraulic droplet collision coalescence. The structure, coalescence theory, and separation principle of the CHC are introduced. Based on the Euler–Euler method, the population balance model (PBM) is applied to a continuous equation to simulate the distributions of the velocities, oil volume fraction, and oil droplet size in the CHC. The simulation method is verified by conducting particle image velocimetry tests in a polymethyl methacrylate hydrocyclone. The results indicate that the CHC can improve the separation performance by enlarging the oil droplet size by collision coalescence before entering the hydrocyclone. The results of separation efficiency experiment indicate that the CHC can not only improve the oil–water separation efficiency but also clearly enhance the processing capability. The optimal inlet flow rate is 5.5 m3·h-1 and the split ratio is 25%. The feasibility of the structure design and the reliability of the separation performance are validated by experiments.