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DTSTAMP:20260624T175558Z
SUMMARY:SPC Seminar – Climate – Anubhab Roy (IIT Madras)
DESCRIPTION:Title: Collisions of cloud droplets: non-continuum lubrication 
 and the warm-rain size gap\n \nSpeaker: Anubhab Roy (IIT Madras)\n\nAbstr
 act: The growth of cloud droplets into raindrops hinges on collisions\, ye
 t classical microphysics leaves a stubborn “size gap”: above ~15 micro
 ns\, condensation is slow\, while gravitational coalescence becomes effici
 ent only beyond ~40 microns. A central obstacle is hydrodynamic: as two dr
 oplets approach\, the air between them must drain\, and continuum lubricat
 ion predicts a resistance that diverges\, so droplets on a collision cours
 e never actually touch. I will show that this singularity is an artefact o
 f the continuum approximation: once the gap falls below the air’s mean f
 ree path (~70 nm)\, the gas flow becomes rarefied\, and the resistance gro
 ws far more slowly\, allowing coalescence in finite time without invoking 
 van der Waals attraction. Building on the kinetic-theory analysis of Sunda
 rarajakumar & Koch (1996)\, we construct uniformly valid resistance and mo
 bility functions spanning the far-field continuum and near-contact non-con
 tinuum regimes\, and use them to compute collision rates and efficiencies 
 for droplet pairs settling in quiescent\, sheared and turbulent flows. The
  collision efficiency depends systematically on the size ratio\, the Knuds
 en number\, and droplet inertia\; turbulence is found to augment different
 ial sedimentation\; and the results condense into compact\, closed-form co
 llision kernels suited to cloud and aerosol models. I will close with the 
 effect of atmospheric electric fields on droplet collisions and the open q
 uestions these raise for rain initiation.\n\nA few references to our work 
 that the talk draws on:\n1. Dhanasekaran\, Roy & Koch\, Collision rate of 
 bidisperse spheres settling in a compressional non-continuum gas flow\, J.
  Fluid Mech. 910\, A10 (2021). https://doi.org/10.1017/jfm.2020.942\n2. Dh
 anasekaran\, Roy & Koch\, Collision rate of bidisperse\, hydrodynamically 
 interacting spheres settling in a turbulent flow\, J. Fluid Mech. 912\, A5
  (2021). https://doi.org/10.1017/jfm.2020.1113\n3. Patra\, Koch & Roy\, Co
 llision efficiency of non-Brownian spheres in a simple shear flow – the 
 role of non-continuum hydrodynamic interactions\, J. Fluid Mech. 950\, A18
  (2022). https://doi.org/10.1017/jfm.2022.817\n4. Patra & Roy\, Gravity-in
 duced collisions of uncharged cloud droplets in an electric field\, J. Atm
 os. Sci. 82\, 11 (2025). https://doi.org/10.1175/JAS-D-25-0040.1\n
URL:https://www.imperial.ac.uk/events/211047/spc-seminar-climate-anubhab-ro
 y-iit-madras/
DTSTART;TZID=Europe/London:20260710T140000
DTEND;TZID=Europe/London:20260710T150000
LOCATION:139\, Huxley Building\, South Kensington Campus\, Imperial College
  London\, London\, SW7 2AZ\, United Kingdom
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