Collision efficiency of cloud droplets in quiescent air considering lubrication interactions, mobility of interfaces, and noncontinuum molecular effects

Ababaei, Ahmad; Rosa, Bogdan

doi:10.1103/physrevfluids.8.014102

Cited by 5 publications

(10 citation statements)

References 46 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Sundararajakumar & Koch (1996) went beyond the small Kn assumption using the linearized Boltzmann equation in a lubrication framework. Extensions of rigid sphere work to nonaxisymmetric collisions have recently been provided by How et al (2021), as have considerations of the effect of internal flow by Ababaei & Rosa (2023).…”

Section: Gas Kinetic Effectsmentioning

confidence: 99%

Gas Microfilms in Droplet Dynamics: When Do Drops Bounce?

Sprittles

2024

Annu. Rev. Fluid Mech.

View full text Add to dashboard Cite

In the last ten years, advances in experimental techniques have enabled remarkable discoveries of how the dynamics of thin gas films can profoundly influence the behavior of liquid droplets. Drops impacting onto solids can skate on a film of air so that they bounce off solids. For drop–drop collisions, this effect, which prevents coalescence, has been long recognized. Notably, the precise physical mechanisms governing these phenomena have been a topic of intense debate, leading to a synergistic interplay of experimental, theoretical, and computational approaches. This review attempts to synthesize our knowledge of when and how drops bounce, with a focus on ( a) the unconventional microscale and nanoscale physics required to predict transitions to/from merging and ( b) the development of computational models. This naturally leads to the exploration of an array of other topics, such as the Leidenfrost effect and dynamic wetting, in which gas films also play a prominent role. Expected final online publication date for the Annual Review of Fluid Mechanics, Volume 56 is January 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

show abstract

Section: Gas Kinetic Effectsmentioning

confidence: 99%

Gas Microfilms in Droplet Dynamics: When Do Drops Bounce?

Sprittles

2024

Annu. Rev. Fluid Mech.

View full text Add to dashboard Cite

show abstract

“…Davis (1972) and Jonas (1972) introduced slip flow to the Stimson & Jeffery (1926) solution and removed the need of an arbitrary cut-off. For Stokes flow, Ababaei & Rosa (2023) compared the twin multipole expansion with an analytical solution in bispherical coordinates and the non-continuum lubrication of Reed & Morrison (1974). Rother, Stark & Davis (2022) also made use of bispherical coordinates, considering flow inside the drops, slip as the only non-continuum effect and the effect of van der Waals forces.…”

Section: Dynamical Mechanismsmentioning

confidence: 99%

Gap in drop collision rate between diffusive and inertial regimes explains the stability of fogs and non-precipitating clouds

Poydenot,

Andreotti

2024

J. Fluid Mech.

View full text Add to dashboard Cite

Rain drops form in clouds by collision of submillimetric droplets falling under gravity: larger drops fall faster than smaller ones and collect them on their path. The puzzling stability of fogs and non-precipitating warm clouds with respect to this avalanche mechanism has been a longstanding problem. How can droplets of diameter around 10 $\mathrm {\mu }$ m have a low collision probability, inhibiting the cascade towards larger and larger drops? Here we review the dynamical mechanisms that have been proposed in the literature and quantitatively investigate the frequency of drop collisions induced by Brownian diffusion, electrostatics and gravity, using an open-source Monte Carlo code taking all of them into account. Inertia dominates over aerodynamic forces for large drops, when the Stokes number is larger than $1$ . Thermal diffusion dominates over aerodynamic forces for small drops, when the Péclet number is smaller than $1$ . We show that there exists a range of size (typically 3–30 $\mathrm {\mu }$ m for water drops in air) where neither inertia nor Brownian diffusion are significant, leading to a gap in the collision rate. The effect is particularly important, due to the lubrication film forming between the drops immediately before collision, and secondarily to the long-range aerodynamic interaction. Two different mechanisms regularise the divergence of the lubrication force at vanishing separation: the transition to a non-continuum regime in the lubrication film, when the separation is comparable to the mean free path of air, and the induction of a flow inside the drops due to shear at their surfaces. In the gap between inertia-dominated and diffusion-dominated regimes, dipole–dipole electrostatic interactions becomes the major effect controlling the efficiency of drop collisions.

show abstract

“…In previous work, where the drops were treated as solid particles (Hocking, 1959;Davis, 1984), due to the large drop-to-medium viscosity ratio, either van der Waals forces (Davis, 1984) or Maxwell slip (Davis, 1972;Hocking, 1973) needed to be incorporated in the analysis for coalescence to occur. More recent raindrop analysis has accounted for droplet viscous effects, including internal circulation (Rother et al, 2022;Ababaei and Rosa, 2023). For viscous drops, coalescence is possible without any additional considerations, since the drainage between the drops is increased by internal circulation (Zinchenko, 1982;Davis et al, 1989;Zhang and Davis, 1991;Rother et al, 2022).…”

Section: Introductionmentioning

confidence: 99%

“…Maxwell slip has been studied for solid spheres using asymptotic methods (Hocking, 1973;Davis, 1984) and more rigorously with bispherical coordinates (Davis, 1972). The case of slip for viscous drops has also been examined asymptotically (Zinchenko, 1981) and by bispherical coordinates (Grashchenkov, 1996;Rother et al, 2022;Ababaei and Rosa, 2023), and has a significant impact over a wider range of drop sizes than van der Waals forces.…”

Section: Introductionmentioning

confidence: 99%

Gravitational Interactions of Two Small Evaporating Drops

Rother

2024

Preprint

View full text Add to dashboard Cite

Relative trajectories are determined for a pair of water drops falling in air with radii between 2 μm and 30 μm. The droplets are small enough that diffusion dominates convection in evaporation, but significant drop inertia at low Reynolds number is considered. In addition to hydrodynamic and lubrication forces, attractive van der Waals forces and Maxwell slip are taken into account. Because the loss of mass is not uniform due to the presence of a second drop, and mass transfer and momentum transfer are effectively decoupled, the droplet position needs to be assessed. Three outcomes are compared. The isolated drop result for both droplets is employed as a base case, since the mass loss is constant over each drop surface. Alternatively, the bispherical coordinate solution for two evaporating drops is applied, with the drop positions 1) fixed by the hydrodynamics or 2) allowed to move based on the nonspherical mass loss. In all three cases, evaporation leads to weaker inertial effects and stronger hydrodynamic effects. The single sphere model has the largest drop gap, followed by the bispherical coordinate solutions with a fixed and movable center, respectively. Critical horizontal offsets for coalescence are also calculated with a finite vertical offset. With both attractive molecular forces and slip, all three approaches to evaporation lead to similar results, making the choice of method nearly inconsequential. Moderate agreement between previous experiments and an approximate, generalized form of the current theory is obtained for the evaporation rate of falling drops.

show abstract

Collision efficiency of cloud droplets in quiescent air considering lubrication interactions, mobility of interfaces, and noncontinuum molecular effects

Cited by 5 publications

References 46 publications

Gas Microfilms in Droplet Dynamics: When Do Drops Bounce?

Gas Microfilms in Droplet Dynamics: When Do Drops Bounce?

Gap in drop collision rate between diffusive and inertial regimes explains the stability of fogs and non-precipitating clouds

Gravitational Interactions of Two Small Evaporating Drops

Contact Info

Product

Resources

About