Drop Breakup Research Articles

Drop Interactions with the Conical Shock Structure Generated by a Mach 4.5 Projectile

This work presents measurements of liquid drop deformation and breakup time behind approximately conical shock waves and evaluates the predictive capabilities of low-order models and correlations developed using planar shock experiments. A conical shock was approximated by firing a bullet at Mach 4.5 past a vertical column of water drops with a mean initial diameter of . The time-resolved drop position and maximum transverse dimension were characterized using backlit stereo images taken at 500 kHz. The gas density and velocity fields experienced by the drops were estimated using a Reynolds-averaged Navier–Stokes simulation of the bullet. Classical correlations predict drop breakup times and deformation in error by a factor of 3 or more. The Taylor analogy breakup (TAB) model predicts deformed drop diameters that agree within the confidence bounds of the ensemble-averaged experimental values using a dimensionless constant compared to the accepted value . Results demonstrate existing correlations are inadequate for predicting the drop response to the three-dimensional relaxation of the flowfield downstream of a conical-like shock and suggest the TAB model results represent a path toward improved predictions.

A consistent volume-of-fluid approach for direct numerical simulation of the aerodynamic breakup of a vaporizing drop

A novel simulation framework has been developed in this study for the direct numerical simulation of the aerodynamic breakup of a vaporizing drop. The interfacial multiphase flow with phase change is resolved using a consistent geometric volume-of-fluid method. The bulk fluids are viscous and incompressible with surface tension at the interface. The newly-developed numerical methods have been implemented in the Basilisk solver, in which the adaptive octree/quadtree mesh is used for spatial discretization, allowing flexibility in dynamically refining the mesh in a user-defined region. The simulation framework is extensively validated by a series of benchmark cases, including the 1D Stefan and sucking problems, the growth of a 3D spherical bubble in a superheated liquid, and a 2D film boiling problem. The simulation results agree very well with the exact solution and previous numerical studies. 2D axisymmetric simulations were performed to resolve the vaporization of a moving drop with a low Weber number in a high-temperature free stream. The computed rate of volume loss agrees well with the empirical model of drop evaporation. Finally, the validated solver is used to simulate the aerodynamic breakup of an acetone drop at a high Weber number. A fully 3D simulation is performed and the morphological evolution of the drop is accurately resolved. The rate of vaporization is found to be significantly enhanced due to the drop deformation and breakup. The drop volume decreases nonlinearly in time and at a much higher rate than the empirical correlation for a spherical drop.

The effect of disperse phase viscosity in the emulsification of a semi-dairy beverage–combining emulsification experiments and numerical single drop breakup simulations

Due to the rise in the demand for semi-dairy beverages (vegetable fat in a de-fatted dairy matrix) as well as for fully plant-based formulations, there is a growing interest in better understanding how emulsification processes are influenced by raw material choices. Semi-dairy beverages, for example, utilize a wide variety of vegetable oils in their formulation. This contribution combines traditional emulsification experiments with high-resolution numerical drop breakup simulations to improve the understanding of how oil viscosity influences the emulsification process. It is concluded that the experimentally observed increase in drop diameter with oil viscosity is well predicted by the viscosity-corrected Kolmogorov Hinze theory. The numerical simulations are able to reproduce this trend and allow further insight into breakup mechanism; suggesting that it is the prolonging of the deformation phase that make more viscous oils more challenging to homogenize. From a beverage processing perspective, the homogenizing pressure (and energy cost) necessary to achieve the same physical stability needs to be 73% higher if using a typical high-viscosity vegetable oil than if using a typical low viscosity vegetable oil.

Emulsifier adsorption kinetics influences drop deformation and breakup in turbulent emulsification.

Turbulent drop breakup is of large importance for applications such as food and pharmaceutical processing, as well as of substantial fundamental scientific interest. Emulsification typically takes place in the presence of surface-active emulsifiers (natural occurring and/or added). Under equilibrium conditions, these lower the interfacial tension, enabling deformation and breakup. However, turbulent deformation is fast in relation to emulsifier kinetics. Little is known about the details of how the emulsifier influences drop deformation under turbulent conditions. During the last years, significant insight in the mechanism of turbulent drop breakup has been reached using numerical experiments. However, these studies typically use a highly simplistic description of how the interface responds to turbulent stress. This study investigates how the limited exchange rate of emulsifier between the bulk and the interface influences the deformation process in turbulent drop breakup for application-relevant emulsifiers and concentrations, in the context of state-of-the-art single drop breakup simulations. In conclusion, if the Weber number is high or the emulsifier is supplied at a concentration giving an adsorption time less than 1/10th of the drop breakup time, deformation proceeds as if the emulsifier adsorbed infinitely fast. Otherwise, the limited emulsifier kinetics delays breakup and can alter the breakup mechanism.

Influence of rheology and micropatterns on spreading, retraction and fingering of an impacting drop.

Rheology and surface microstructure affect many drop impact processes, including in emerging printing and patterning applications. This study reports on experiments systematically addressing the influence of these parameters on drop impacts. The experiments involved drop impacts of water, glycerol, and shear-thinning carbopol solutions on ten different microstructured surfaces, captured using high-speed photography. The impact Weber number (We) was varied from 70 to 350, and the microstructures consisted of 20 μm wide pillars with circular and square cross sections arranged in square arrays. The data focus on maximum spreading, retraction rates, threshold conditions for asymmetric (non-circular) spreading, and fingers protruding from the spreading rim. The extent of spreading was reduced by the presence of micropillars, and was well-explained using a hybrid scaling model. The drop retraction rate () showed moderate agreement with the inertial regime scaling  ∝ We-0.50, but did decrease with effective viscosity. Retraction was slower when the contact line was pinned on surfaces that were flat or had relatively tall or closely-spaced pillars, and was disrupted by drop break-up at We ≳ 250 for low-viscosity fluids. Impact velocities at the onset of asymmetric spreading had weak dependence on viscosity. Fingers were more numerous for greater We, lower effective viscosity, lower pillar height, and for pillars with square cross-sections. Fingers were favoured in directions parallel to the rows of the pillar array, especially near the onset of finger formation. Consistent comparisons between Newtonian and non-Newtonian fluids were enabled by calculating an effective Reynolds number.

Experimental study of the stable droplet formation process during micro-valve-based three-dimensional bioprinting

Three-dimensional (3D) bioprinting offers great potential for the fabrication of complex 3D cell-laden constructs for clinical and research applications. The droplet formation process is the important first step in droplet-based 3D bioprinting, affecting the positional accuracy and printing fidelity. In this paper, the drop ejection behavior, thresholds for stable droplet generation, and formation of satellite drops are studied, under various ink properties, printing conditions, and input cell concentrations using a micro-valve-based 3D bioprinter. Three droplet ejection behaviors are identified under different conditions: an isolated stable droplet, satellites coalescing into a single droplet, and the presence of one/multiple satellites. The droplet state is represented by a phase diagram bounded by a dimensionless Z number (the inverse of the Ohnesorge number) and a jet Weber number, Wej, to define the printability of the utilized bioprinter. The printability range is defined as 2 < Z < 15 and 10 < Wej < 25 by considering characteristics, such as stable single droplet formability and sufficient drop falling velocity. There is no fatal damage on cells within this printability range. The results show there is no strong influence of an actuation system on droplet-based bioprinting printability. As the input cell concentration increases, the bioink's density and viscosity increases, and surface tension decreases, which, in turn, causes the Z number to slightly decrease. The change in the cell concentration (from 0 to 1×107 cells/ml), within a Newtonian bioink, has negligible impact on the droplet volume, falling velocity, drop ejection behavior, breakup time, and ligament length in microvalve-based bioprinting.

FRAGMENTATION OF WATER DROPS IN COLLISION WITH A SMALL OBSTACLE

The study is dedicated to the general features of the processes of deformation and fragmentation of liquid drops have been studied when they collide with obstacles. Masks and filters, protecting against airborne infections, are among the possible obstacles. Coughing, sneezing, and talking cause the ejection of drops of saliva and bronchial mucus. A local drop-mask or drop-filter collision is modeled by the impact of a drop on a small obstacle as the simplest hydrodynamic case with a minimum number of influencing factors. For water-based oral and bronchial drops with a typical diameter <i>d<sub>i</sub></i> = 100 μm and impact velocity of the order of <i>v<sub>i</sub></i> = 10 m/s, the impact Weber number is about We<sub><i>i</i></sub> = ρ<i>v<sub>i</sub></i><sup>2</sup><i>d<sub>i</sub></i>/γ = 139. As a starting point in the problem of the drop breakup in a collision with a solid obstacle, we consider the coaxial impact of an inviscid liquid drop with a diameter of 2.8 mm on a disk with a diameter of 4.0 mm. In laboratory experiments, the similarity was provided by impact velocities of 1.88-3.57 m/s, which gives impact Weber numbers We<sub><i>i</i></sub> = 137-496. Such collisions are controlled only by inertia and capillarity, while the influence of all other factors is negligible. A round liquid lamella with a torus-shaped rim is formed upon the collision. The rim first expands and then retracts, forming radially directed liquid fingers in the rim. At low impact velocities, the fingers retained continuity, while at sufficiently high velocities, the fingers spattered into secondary droplets. Experiments have shown that the critical Weber number corresponding to the transition to spattering lies between 137 and 206. Approximately the same values of the Weber number occur when infected drops hit masks or filters.

Memoryless drop breakup in turbulence.

The breakup of drops and bubbles in turbulent fluids is a key mechanism in many environmental and engineering processes. Even in the well-studied dilute case, quantitative descriptions of drop fragmentation remain elusive, and empirical models continue to proliferate. We here investigate drop breakup by leveraging a novel computer code, which enables the generation of ensembles of experiments with thousands of independent, fully resolved simulations. We show that in homogeneous isotropic turbulence breakup is a memoryless process whose rate depends only on the Weber number. A simple model based on the computed breakup rates can accurately predict experimental measurements and demonstrates that dilute emulsions evolve through a continuous fragmentation process with exponentially increasing time scales. Our results suggest a nonvanishing breakup rate below the critical Kolmogorov-Hinze diameter, challenging the current paradigm of inertial drop fragmentation.

The largest diameter of falling drop in the up-gas flow

This study aimed to investigate the largest diameter of falling drop in the up-gas flow. Due to gravity, the falling drop is common in nature and industry. Drop breakup in the airflow is referred to as secondary atomization, found in numerous practical applications. Critical Weber number is the key dimensionless number marking the beginning of drop breakup. Herein, we explain the different critical Weber numbers between the free-fall raindrops breakup and secondary atomization based on the Rayleigh-Taylor (RT) instability. The theoretical criterion of largest raindrop size is proposed by RT wave number model. In addition, we suggest a map of falling drop size regions, which can well explain the phenomena of the super-large raindrop. The current study is also helpful in the remission of drop entrainment in the washing tower.

Comparison of turbulent drop breakup in an emulsification device and homogeneous isotropic turbulence: Insights from numerical experiments

Turbulent emulsification is of considerable industrial interest. Nevertheless, numerical experiments (direct numerical simulations, DNS, with highly resolved interface tracking) have been mainly used to study drop breakup in idealized flows. This study, therefore, compares drop breakup in two different settings (homogenous and isotropic flow, and a simplified high-pressure homogenizer) with the intention of better understanding how insight gained from the idealized systems can be applied to industrially relevant devices. The flow differs between the two cases, with highly anisotropic and inhomogeneous turbulence in the latter. Results show similarities between the two cases regarding morphology of breakup, suggesting that the underlying mechanism, as a function of Weber number, is similar. However, differences are also observed, e.g., in terms of breakup time and deformed morphology, which are associated with the locality of the turbulence in the homogenizer. Implications for an improved understanding of turbulent breakup in industrially relevant devices are discussed.

Assessing Raindrop Breakup Parameterizations Using Disdrometer Observations

Abstract An intercomparison of raindrop mean diameter frequency distribution (RDFD) is performed for numerical simulations of precipitating cloud systems using an array of models and microphysics schemes. This includes results from the Regional Atmospheric Modeling System (RAMS) double-moment microphysics, the Hebrew University Cloud Model bin microphysics (HUCM) interfaced to the RAMS parent model, and the Weather Research and Forecasting (WRF) Model with the Thompson, Morrison, double-moment 6-class (WDM6), and National Severe Storms Laboratory (NSSL) double-moment schemes. Simulations are examined with respect to the raindrop size distribution (DSD) volume-number mean diameter (Dm) and intercept parameter (Nw). When compared to a suite of disdrometer observations, the RDFD resulting from each microphysics scheme exhibits varying degrees of mean drop size constraints and peaks in the frequency distribution of Dm. A more detailed investigation of the peaked RDFD from the RAMS simulations suggests that the parameterization of raindrop collisional breakup can impose strong limitations on the evolution of simulated drop growth. As such, a summary and comparison of the drop breakup parameterizations among the aforementioned microphysics schemes is presented. While some drop breakup parameterizations are adjusted toward the observations by modifying the threshold diameter for the onset of breakup, this study explores the use of a modified maximum breakup efficiency. This method permits the parameterization to retain its threshold breakup diameter, while limiting the strength of drop breakup and permitting a broader range of drop sizes. As a result, the simulated mean drop sizes are in better agreement with observations.

Identification and Mapping of Three Distinct Breakup Morphologies in the Turbulent Inertial Regime of Emulsification—Effect of Weber Number and Viscosity Ratio

Turbulent emulsification is an important unit operation in chemical engineering. Due to its high energy cost, there is substantial interest in increasing the fundamental understanding of drop breakup in these devices, e.g., for optimization. In this study, numerical breakup experiments are used to study turbulent fragmentation of viscous drops, under conditions similar to emulsification devices such as high-pressure homogenizers and rotor-stator mixers. The drop diameter was kept larger than the Kolmogorov length scale (i.e., turbulent inertial breakup). When varying the Weber number (We) and the disperse-to-continuous phase viscosity ratio in a range applicable to emulsification, three distinct breakup morphologies are identified: sheet breakup (large We and/or low viscosity ratio), thread breakup (intermediary We and viscosity ratio > 5), and bulb breakup (low We). The number and size of resulting fragments differ between these three morphologies. Moreover, results also confirm previous findings showing drops with different We differing in how they attenuate the surrounding turbulent flow. This can create ‘exclaves’ in the phase space, i.e., narrow We-intervals, where drops with lower We break and drops with higher We do not (due to the latter attenuating the surrounding turbulence stresses more).

Sharp Interface Methods for Simulation and Analysis of Free Surface Flows with Singularities: Breakup and Coalescence

A common feature of many free surface flows—drop/bubble breakup or coalescence and film/sheet rupture—is the occurrence of hydrodynamic singularities. Accurately computing such flows with continuum mechanical, multidimensional free surface flow algorithms is a challenging task given these problems’ multiscale nature, which necessitates capturing dynamics occurring over disparate length scales across 5–6 orders of magnitude. In drop breakup, the thinning of fluid threads that form and eventually pinch-off must be simulated until the thread's radius is about 10 nm. When two drops approach one another, the thickness of the fluid film separating them must fall below 10 nm before coalescence is said to have occurred. If the initial drop radii are 1 mm, simulations must remain faithful to the physics as thread radius or film thickness falls from 10−3 m to below 10−8 m. Here we review significant findings in interfacial flows with hydrodynamic singularities spearheaded by sharp interface algorithms. These multidimensional algorithms can achieve resolution that to date has only been possible with the use of simple 1D evolution equations.

Three-Dimensional Self-Similarity of Coalescing Viscous Drops in the Thin-Film Regime.

Coalescence and breakup of drops are classic problems in fluid physics that often involve self-similarity and singularity formation. While the coalescence of suspended drops is axisymmetric, the coalescence of drops on a substrate is inherently three-dimensional. Yet, studies so far have only considered this problem in two dimensions. In this Letter, we use interferometry to reveal the three-dimensional shape of the interface as two drops coalescence on a substrate. We unify the known scaling laws in this problem within the thin-film approximation and find a three-dimensional self-similarity that enables us to describe the anisotropic shape of the dynamic interface with a universal curve.

Hydrophobic Antiwetting of Aquatic UAVs: Static and Dynamic Experiment and Simulation

The adhesion of water to the surfaces of unmanned aerial vehicles (UAVs) adversely affects the function. The proposed UAVs will have underwater as well as flight capability, and these aquatic UAVs must shed water to resume flight. The efficient separation of the adhering water from aquatic-UAV surfaces is a challenging problem; we investigated the application of hydrophobic surfaces as a potential solution. Using aquatic-UAV models, one with hydrophilic surfaces and the other with superhydrophobic anisotropic textured surfaces, the antiwetting mechanism of the hydrophobic surfaces was investigated using a simulated-precipitation system and instrumentation to measure the load of the water adhering to the aquatic UAV, and to measure the impact energies. When the model was stationary (passive antiwetting), no adhesion occurred on the superhydrophobic surfaces, while continuous asymmetric thick liquid films were observed on the hydrophilic surfaces. The superhydrophobic surfaces reduced the rain loading by 87.5%. The vibration and movement of the model (dynamic antiwetting, simulating flight motions) accelerated the separation process and reduced the contact time. The observed results were augmented by the use of computational fluid dynamics with lattice Boltzmann methods (LBM) to analyze the particle traces inside the droplets, the liquid phase velocity-field and pressure-field strengths, and the backward bouncing behavior of the derived droplet group induced by the moving surface. The synergy between the superhydrophobic surfaces and the kinetic energy of the droplets promotes the breakup of drops, which avoids the significant lateral unbalance observed with hydrophilic surfaces during simulated flight.

An Investigation on Super- and Sub-Terminal Drops in Two Different Rain Categories and Climate Regimes

The pressing need for accurate and reliable precipitation measurements and forecasting poses theoretical and technological problems. Remote-sensing instruments with increased coverage and sensitivity (such as space-borne and ground-based radar) are available; however, their full exploitation requires physical calibration and validation based on a deep knowledge of precipitation microphysics. This study reports a detailed analysis of the evidence of non-terminal velocities in a mid-latitude (Bologna, Italy) and a tropical location (Kolkata, India). The data from two identical disdrometers OTT-Parsivel2 were analyzed to shed light on the nature of the raindrops that fall at a velocity that is significantly higher (i.e., super-terminal drops) or lower (sub-terminal drops) than the terminal velocity expected for the raindrop sizes. The results show a significant fraction of super- and sub-terminal drops in both locations. The percentages of both super- and sub-terminal drops were higher in Kolkata. However, the difference was more notable for convective rain. The percentages of both super- and sub-terminal drops were found to be high within a drop diameter of 1 mm. The number of sub-terminal drops seemed to increase with an increase in diameter for drops larger than ~2.5 mm. The natural rain in Bologna showed stronger evidence of drop break-up in correspondence with the evolution of non-terminal velocities. Moreover, this study once again pointed toward the fact that the process of break-up cannot be neglected in natural rain of tropical or mid-latitude locations. We found that 7% and 10% of rain samples in Bologna and Kolkata seemed to be subjected to drop break-up. The results indicate that radar measurements of rain in the tropics or mid-latitude regions, relying on the Gunn–Kinzer relationship between velocity and diameter, should be verified by observations of disdrometers for a high precision QPE.

Effect of conductivity on the mechanism of charge ejection in Rayleigh breakup of a charged drop

A critically charged drop is known to undergo Rayleigh fission by forming oppositely directed symmetric jets, which further disintegrate into progeny droplets. In this work, we present numerical simulations based on the axisymmetric boundary integral method and show that the electrical conductivity of the droplet not only affects the characteristics of the progeny droplets but also modifies the mode of drop breakup. It is observed that, at higher conductivity, the droplet undergoes fission by jet detachment from the mother drop, losing higher charge, while at lower conductivity, the jet relaxes back after ejecting a few progeny droplets that carry a lower charge.

Mechanical Properties of Acoustically Levitated Granular Rafts

We investigate a model system for the rotational dynamics of inertial many-particle clustering, in which submillimeter objects are acoustically levitated in air. Driven by scattered sound, levitated grains self-assemble into a monolayer of particles, forming mesoscopic granular rafts with both an acoustic binding energy and a bending rigidity. Detuning the acoustic trap can give rise to stochastic forces and torques that impart angular momentum to levitated objects. As the angular momentum of a quasi-two-dimensional granular raft is increased, the raft deforms from a disk to an ellipse, eventually pinching off into multiple separate rafts, in a mechanism that resembles the breakup of a liquid drop. We extract the raft effective surface tension and elastic modulus and show that nonpairwise acoustic forces give rise to effective elastic moduli that scale with the raft size. We also show that the raft size controls the microstructural basis of plastic deformation, resulting in a transition from fracture to ductile failure.3 MoreReceived 17 June 2021Revised 14 February 2022Accepted 4 March 2022DOI:https://doi.org/10.1103/PhysRevX.12.021017Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAcoustic interactionsClusteringGranular materialsPolymers & Soft Matter

Drop breakup and entrainment in the updraft

AbstractThis study aims to investigate the characteristics of gas–liquid countercurrent contact processes. In spray towers or other applications, several drops containing pollutants are entrained by the updraft flue gas, which can easily cause environmental pollution. Traditionally, this drop entrainment phenomenon is alleviated by increasing the diameter of the drops. However, the breakup of a large drop would also cause drop entrainment to become serious, a process referred to as secondary atomization. Herein, we propose the boundary of three drop modes in the updraft: drop falling mode, reverse entrainment mode, and breakup entrainment mode. The critical Weber number (We) is the key dimensionless number marking the beginning of the drop breakup. The ratio of the drag force to gravity and We are proposed as criteria for the drop entrainment.

Effect of inlet chamber design and operation conditions on laminar drop deformation in a production-scale high-pressure homogenizer—A hydrodynamic investigation

Due to their wide utilization and low thermodynamic efficiency, there is a large interest in designing more efficient high-pressure homogenizer valves. Drop breakup in these devices is by turbulent interaction downstream of the gap exit. However, recent investigations in scaled systems suggest that the drops can be ‘primed’ for breakup by elongation upstream of the gap entrance. This study uses hydrodynamic modelling to investigate (1) if this effect is expected in production-scale valves and (2) if it can be enhanced by simple design modifications. Results confirm the high velocity gradients formed upstream of the gap (and proposes a model for estimating them based on design and operating conditions). However, regardless of which of the three deformation/relaxation models that is applied, no sizable drop deformation is predicted at the gap exit under standard conditions (e.g., in dairy processing, the single largest application). The drops are too small to deform substantially. Under extreme conditions (higher pressures, flowrates and inlet angles, and shorter gaps), a lightly deformed drop exits the gap. However, when taking into account the constraints set by applications, energy cost and valve manufacturing, no sizable deformation is possible to achieve by simple design modifications to the inlet chamber.