The review deals with drop impacts on thin liquid layers and dry surfaces. The impacts resulting in crown formation are referred to as splashing. Crowns and their propagation are discussed in detail, as well as some additional kindred, albeit nonsplashing, phenomena like drop spreading and deposition, receding (recoil), jetting, fingering, and rebound. The review begins with an explanation of various practical motivations feeding the interest in the fascinating phenomena of drop impact, and the above-mentioned topics are then considered in their experimental, theoretical, and computational aspects.

Drop Impact Dynamics: Splashing, Spreading, Receding, Bouncing ...

Turbulent dispersed multiphase flows are common in many engineering and environmental applications. The stochastic nature of both the carrier-phase turbulence and the dispersed-phase distribution makes the problem of turbulent dispersed multiphase flow far more complex than its single-phase counterpart. In this article we first review the current state-of-the-art experimental and computational techniques for turbulent dispersed multiphase flows, their strengths and limitations, and opportunities for the future. The review then focuses on three important aspects of turbulent dispersed multiphase flows: the preferential concentration of particles, droplets, and bubbles; the effect of turbulence on the coupling between the dispersed and carrier phases; and modulation of carrier-phase turbulence due to the presence of particles and bubbles.

Turbulent Dispersed Multiphase Flow

A drop hitting a solid surface can deposit, bounce, or splash. Splashing arises from the breakup of a fine liquid sheet that is ejected radially along the substrate. Bouncing and deposition depend crucially on the wetting properties of the substrate. In this review, we focus on recent experimental and theoretical studies, which aim at unraveling the underlying physics, characterized by the delicate interplay of not only liquid inertia, viscosity, and surface tension, but also the surrounding gas. The gas cushions the initial contact; it is entrapped in a central microbubble on the substrate; and it promotes the so-called corona splash, by lifting the lamella away from the solid. Particular attention is paid to the influence of surface roughness, natural or engineered to enhance repellency, relevant in many applications.

https://hal.archives-ouvertes.fr/hal-01398138/document

Drop Impact on a Solid Surface

UNLABELLED A large number of infectious diseases are believed to be transmitted between people via large droplets and by airborne routes. An understanding of evaporation and dispersion of droplets and droplet nuclei is not only significant for developing effective engineering control methods for infectious diseases but also for exploring the basic transmission mechanisms of the infectious diseases. How far droplets can move is related to how far droplet-borne diseases can transmit. A simple physical model is developed and used here to investigate the evaporation and movement of droplets expelled during respiratory activities; in particular, the well-known Wells evaporation-falling curve of droplets is revisited considering the effect of relative humidity, air speed, and respiratory jets. Our simple model considers the movement of exhaled air, as well as the evaporation and movement of a single droplet. Exhaled air is treated as a steady-state non-isothermal (warm) jet horizontally issuing into stagnant surrounding air. A droplet is assumed to evaporate and move in this non-isothermal jet. Calculations are performed for both pure water droplets and droplets of sodium chloride (physiological saline) solution (0.9% w/v). We calculate the droplet lifetimes and how droplet size changes, as well as how far the droplets travel in different relative humidities. Our results indicate that a droplet's size predominately dictates its evaporation and movement after being expelled. The sizes of the largest droplets that would totally evaporate before falling 2 m away are determined under different conditions. The maximum horizontal distances that droplets can reach during different respiratory activities are also obtained. Our study is useful for developing effective prevention measures for controlling infectious diseases in hospitals and in the community at large. PRACTICAL IMPLICATIONS Our study reveals that for respiratory exhalation flows, the sizes of the largest droplets that would totally evaporate before falling 2 m away are between 60 and 100 microm, and these expelled large droplets are carried more than 6 m away by exhaled air at a velocity of 50 m/s (sneezing), more than 2 m away at a velocity of 10 m/s (coughing) and less than 1 m away at a velocity of 1 m/s (breathing). These findings are useful for developing effective engineering control methods for infectious diseases, and also for exploring the basic transmission mechanisms of the infectious diseases. There is a need to examine the air distribution systems in hospital wards for controlling both airborne and droplet-borne transmitted diseases.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1600-0668.2007.00469.x

How far droplets can move in indoor environments--revisiting the Wells evaporation-falling curve.

The corona splash due to the impact of a liquid drop on a smooth dry substrate is investigated with high-speed photography. A striking phenomenon is observed: splashing can be completely suppressed by decreasing the pressure of the surrounding gas. The threshold pressure where a splash first occurs is measured as a function of the impact velocity and found to scale with the molecular weight of the gas and the viscosity of the liquid. Both experimental scaling relations support a model in which compressible effects in the gas are responsible for splashing in liquid solid impacts.

Drop splashing on a dry smooth surface.

Introduction Industrial Applications Energy Conversion and Propulsion Fire Suppression and Control Summary Properties of Dispersed Phase Flows Concept of a Continuum Density and Volume Fraction Particle or Droplet Spacing Response Times Stokes Number Dilute versus Dense Flows Phase Coupling Properties of an Equilibrium Mixture Summary Exercises Size Distribution Discrete Size Distributions Continuous Size Distributions Statistical Parameters Frequently Used Size Distributions Summary Exercises Particle-Fluid Interaction Single-Particle Equations Mass Coupling Linearmomentumcoupling Energy Coupling Summary Exercises Particle-Particle Interaction Particle-Particle Interaction Particle-Wall Interaction Summary Exercises Continuous Phase Equations Averaging Procedures Volume Averaging Property Flux Through a Particle Cloud Volume-Averaged Conservation Equations Equation Summary Summary Exercises Turbulence Review of Turbulence in Single-Phase Flow Turbulence Modulation by Particles Review of Modulation Models Basic Test Case for Turbulence Models Volume-Averaged Turbulence Models Application to Experimental Results Summary Exercises Droplet-Particle Cloud Equations Discrete Element Method (DEM) Discrete Parcel Method (DPM) Two-Fluid Model PDF Models Summary Numerical Modeling Complete Numerical Simulation DNS Models LES Models VANS Numerical Models Summary Experimental Methods Sampling Integral Methods Local Measurement Techniques Summary Exercises Appendix A: Single-Particle Equations Appendix B: Volume Averaging Appendix C: Volume-Averaged Equations Appendix D: Turbulence Equations 425 Appendix E: Brownian Motion References Nomenclature Index

Martin Sommerfeld

Papers

Multiphase Flows with Droplets and Particles

Droplet-wall collisions: Experimental studies of the deformation and breakup process

New simple correlation formula for the drag coefficient of non-spherical particles

Experimental analysis and modelling of particle-wall collisions

Validation of a stochastic Lagrangian modelling approach for inter-particle collisions in homogeneous isotropic turbulence