The tiny droplets that bounce without bursting

If you’ve ever added liquid to a hot frying pan, maybe you noticed how the droplets bubbled up and skittered across the sizzling surface, rather than immediately flattening and wetting. This happens because the pan’s heat starts boiling the undersides of the droplets, producing vapor that acts as an insulating cushion on which they can – momentarily – dance.

Previously, scientists have produced a room-temperature version of this phenomenon – known as the Leidenfrost effect – by replacing the hot surface with a rapidly vibrating liquid bath. In these experiments, the vibrations produced a thin film of air on which the liquid droplets could bounce and hover perpetually.

Now, researchers in EPFL’s School of Engineering have shown experimentally that an oil droplet can bounce on a vibrating solid surface at room temperature for up to five minutes.

“What’s interesting here is that previous observations of perpetually bouncing droplets were determined by the changing surface of the vibrating liquid bath, but in our case the surface is solid, so the drop’s own deformations are driving its unique behavior,” explains John Kolinski, head of the Engineering Mechanics of Soft Interfaces Lab. “Our work provides new physics insights and highlights the potential for precision manipulation of small liquid quantities in air.”

The scientists have published their observations, along with a model to explain and predict them, in the journal Physical Review Letters.

Pharmaceutical precision

In their experiments, the researchers released a 1.6-millimeter droplet of silicon oil over a solid surface, beneath which a stage produced controlled vibrations. First author and PhD student Lebo Molefe likens it to keeping a ball bouncing on a table tennis paddle. “If we replace the ball with a liquid drop, we find that it can perpetually bounce above a thin air layer on a vibrating ‘paddle’, which in our case is made of mica – a special material that is atomically smooth,” she says.

By playing with the frequency and amplitude of these vibrations, the researchers produced two distinct droplet behaviors: some frequencies made the droplet appear to bounce like a basketball, while others made it move rapidly up and down without ever leaving the thin air cushion above the mica. As Molefe explains, the transition between these two states is linked to the way the droplet’s surface bulges and deforms as it interacts with the rigid surface below: “To ‘jump off’ the surface, the drop needs enough time to flatten first, so surface tension causes it to store energy like a coiled spring. At high vibration frequencies, there’s not enough time for this to happen, so the drop appears to be stuck near the surface.”

To interpret their observations, Laboratory of Fluid Mechanics and Instabilities researcher Tomas Fullana led numerical simulations aimed at characterizing the complex dynamics of the droplet’s rebound, facilitating the development of a model that allowed the team to simulate and accurately predict their measured bouncing behaviors.

Interestingly, they found that the droplet’s hover time appeared to be limited only by its lateral progress across the mica surface, as it would eventually encounter a defect that ruptured the air film beneath, triggering the usual ‘splat’. Otherwise, says Fullana, “our numerical simulations show that a drop could retain enough kinetic energy to bounce for an extended period, and possibly indefinitely.”

The researchers say their findings could change how scientists think about handling extremely small quantities of liquid in air at room temperature – an important challenge in the pharmaceutical industry, where chemical purity and precision are paramount. For example, in a proof-of-concept experiment, the EPFL team succeeded in controlling the sideways movement of their bouncing droplet on the mica surface, using ‘tweezers’ made of tiny jets of compressed air.