Bouncing droplets

Drop impact is a crucial step in many natural and industrial processes. The impact of raindrop is responsible for the aeration of lakes and the propagation of many forms of life, including fungi and bacteria. Impact is also determinant for spray coating, fuel injection, surface cooling, ink-jet printing and self-cleaning.

We here focus on the low-velocity impact of drops on a variety of liquid surfaces: pools, thin viscous layers and soap films. In each case, drops can either coalesce or bounce, remain in one piece or split into several droplets. As a drop approaches the other liquid body, air is expelled from the intervening air layer. The drop will bounce or coalesce according to whether this layer drains to a critical thickness during impact.

This sustained air layer exerts a lubrication force that causes both the drop and the underlying liquid to deform. This stores surface energy in the same way as a spring. The drop then decelerates, and ultimately reverses direction when the surface energy is released.

 

An oil drop impacts a thin viscous layer. It merges and splits into four droplets. Each of them then bounces several times. Only the drop (not the viscous layer) significantly deforms and stores energy.

A drop can also bounce on a soap film, like a gymnast on a trampoline. Here, the soap film stores much more surface energy than the drop.

Impacting a soap film: (Left) At low impact velocity, a drop of soapy water bounces on a soap film pinned on a circular ring. (Center) At high velocity, the drop crosses the soap film without breaking it. (Right) At intermediate velocity, the drop splits in two droplets.

On a still liquid surface, the impact velocity decreases at each bounce, until the droplet settles onto and ultimately merges into the underlying liquid. To counter dissipative losses, a vertical vibration is applied: the droplet may then bounce indefinitely.

Periodic bouncing of droplets onto a vertically vibrated liquid. From left to right: (1) Simple rebound on a bath. (2) Self-propelled “roller” mode. (3) Emulsification through bouncing: oil droplets in water in oil. (4-6) Multistability: 3 drops bouncing differently on the same soap film.

 

Self-organization of bouncing droplets.

If you get excited by these bouncing drops, we’ll be happy to ship some for you. 😉

 

Related publications

• N. Vandewalle, D. Terwagne, K. Mulleners, T. Gilet and S. Dorbolo, Dancing droplets onto liquid surfaces, Phys. Fluids 18, 091106 (2006) [Postprint]
• T. Gilet, D. Terwagne, N. Vandewalle and S. Dorbolo, Dynamics of a bouncing droplet on a vertically vibrated interface, Phys. Rev. Lett. 100, 167802 (2008) [Postprint]
• S. Dorbolo, D. Terwagne, N. Vandewalle and T. Gilet, Resonant and rolling droplets, New J. Phys. 10, 113021 (2008) [Postprint]
• D. Terwagne, T. Gilet, N. Vandewalle and S. Dorbolo, From bouncing to boxing, Chaos 18, 041104 (2008) [Postprint]
• T. Gilet and J.W.M. Bush, Chaotic bouncing of a droplet on a soap film, Phys. Rev. Lett. 102, 014501 (2009) [Postprint]
• T. Gilet and J.W.M. Bush, The fluid trampoline : droplets bouncing on a soap film, J. Fluid Mech. 625, 167 (2009) [Postprint]
• T. Gilet, N. Vandewalle and S. Dorbolo, Completely inelastic ball, Phys. Rev. E 79, 055201 (R) (2009) [Postprint]
• D. Terwagne, T. Gilet, N. Vandewalle and S. Dorbolo, Metastable bouncing droplets, Phys. Fluids 21, 054103 (2009) [Postprint]
• D. Terwagne, N. Mack, S. Dorbolo, T. Gilet, J.-Y.Raty and N. Vandewalle, The mayonnaise droplet, Chaos 19, 041105 (2009) [Postprint]
• D. Terwagne, T. Gilet, N. Vandewalle and S. Dorbolo, From a bouncing compound drop to a double emulsion, Langmuir 26 (14), 11680 (2010) [Postprint]
• D. Terwagne, T. Gilet, N. Vandewalle and S. Dorbolo, Double emulsion in a compound droplet, Coll. Surf. A : Physicochem. Eng. Aspects 365 (1-3), 178 (2010) [Postprint]
• T. Gilet and J.W.M. Bush, Droplets bouncing on a wet, inclined surface, Phys. Fluids 24, 122103 (2012) [Postprint]

Press

• Chaotic bouncing trajectory on a soap film: Scientific American, Nature Physics
• Deformation modes, spherical harmonics and rollers: eFluids Gallery of Videos, New Scientist
• Droplets dancing on music
• Bouncing droplets – Gallery of Fluid Motion 2006
• Chaotic deformations
• The mayonnaise droplet

Contact @ µFL: Tristan GILET

Collaborations: Nicolas VANDEWALLE, Stéphane DORBOLO, Denis TERWAGNE, John W.M. BUSH

More in-depth

 

Normal (Left) and tangent (Right) coefficients of restitution, as functions of the normal Weber number. Blue = thin liquid film on solid, Green = soap film. Symbols refer to droplet viscosity. The normal coefficient decreases with increasing We and viscosity. Surprisingly, the tangent coefficient of restitution does not depend on viscosity. Energy loss is much lower on a soap film.

The time of apparent contact between the drop and the substrate is often measured in the literature. It is proportional to the capillary time and relatively independent on the Weber. Soap films produce larger contact times.

 

Dimensionless contact time as a function of the Weber number. Blue = thin liquid film on solid. Green = soap film.

In the case of periodic bouncing, a relevant parameter is Γ = Aω²/g where A and ω are the vibration amplitude and frequency respectively, and g is the gravity acceleration. There is a threshold value above which periodic bouncing can be observed. Many different bouncing modes are observed as Γ is increased.

We have also investigated similar dynamical behaviors in theoretical models of periodic bouncing (partially elastic ball). The special case of a perfectly inelastic ball yields a very intriguing sensitivity to the forcing parameter Γ, reminiscent of structural chaos.

 

Bouncing sequence of a perfectly inelastic ball on a vibrated surface.

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