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Explained: Why Water Droplets ‘Bounce off the Walls’

Research published in Physical Review Letters (image courtesy of Duncan Lockerby)

Bouncing off the Walls: The Influence of Gas-Kinetic and van der Waals Effects in Drop Impact has been published in Physical Review Letters!

Collisions between liquid drops and surfaces, or other drops, happen all the time. For example, small water drops in clouds collide with each other to form larger drops, which can eventually fall and impact on a solid, like your car windscreen.

Drops can behave differently after the point of collision, some make a splash, some coat the surface cleanly, and some can even bounce like a beach ball.

Our article, published today in Physical Review Letters, has discovered why some droplets bounce of wettable surfaces, as discovered independently in Kolinksi et al 2014 (see video below from this group) and de Ruiter et al 2014, and why some wet these solids.

Drop bouncing off smooth wettable surfaces due to the presence of an intervening gas nanofilm.

Remarkably, the fate of the drop is determined by the behaviour of a tiny cushion of air whose height can reach the scale of nanometres. To get a sense of scale, think of something the size of the moon bouncing from a garden trampoline. To describe the process requires an unconventional approach, which couples gas kinetic theory to the Navier-Stokes equations for the liquid drop’s dynamics.

Even if the surface is perfectly smooth, like in laboratory conditions, an affinity between drop molecules and the wall molecules (known as van der Waals attraction), will mean that in most cases the drop will be pinched down onto the surface, preventing it from bouncing.

The research reveals, through highly detailed numerical simulations (see above), that for a droplet to bounce the speed of collision must be just right. Too fast, and the momentum of the drop flattens the air cushion too thinly. Too slow, and it gives the van der Waals attraction time to take hold. At the perfect speed, though, the drop can perform a clean bounce, like a high jumper just clearing the bar.

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