The waltz of tiny droplets and the fluid they live in*

A microbe swimming in the ocean cannot force the mighty waves to change. Are tiny droplets in a turbulent cloud different? I will answer this with a yes, and show how. Evaporation and condensation inject buoyancy into the flow and change the surrounding turbulence by creating small-scale eddies and new instabilities. Besides bringing in new physics, these instabilities provide aesthetic delight through breathtaking cloud formations such as mammatus and asperitas.  We will show how these arise and how wind shear affects them. We will also see that droplets that potentially carry infection might spread less in a conversation than in a monologue.

 

Can tiny droplets always be described by the steady Stokes equation? We will argue for a ‘no’, especially since the Basset-Boussinesq history force slows down the approach to a steady state. We will talk about a way to incorporate this force exactly, without unrealistically high data-storage demands.

 

Raindrops form when larger droplets collide and coalesce. Can the tiniest droplets in a dilute suspension grow thus? Yes, especially if they participate in caustics near a vortex.  I will present a universal description of this phenomenon. Much more can happen in denser suspensions when droplets and particles interact with each other and can force the flow, as we will see from examples.