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Research Highlights

Emulsions: Factoring in bubble bounce

Published online 27 October 2019

High speed cameras reveal details about the interactions of bubbles and droplets in emulsions, with implications for industrial processing

Tim Reid

Image from a numerical simulation showing the fluid velocity during a collision between two water droplets in oil. The immobile surface droplets (upper pair) re-coalesce after an initial bounce, whereas the mobile surface droplets (lower pair) bounce apart without coalescing.
Image from a numerical simulation showing the fluid velocity during a collision between two water droplets in oil. The immobile surface droplets (upper pair) re-coalesce after an initial bounce, whereas the mobile surface droplets (lower pair) bounce apart without coalescing.
Ivan Vakarelski (KAUST)
Gas and liquid emulsions are fundamental to industrial processes related to food, pharmaceuticals and crude oil. Ensuring the stability of emulsions remains challenging, because the dynamic interactions between gas bubbles or liquid droplets manifest on micro- and nano-scales, and are influenced by factors including bubble and droplet size, surface chemistry and the diverse forces acting upon them. 

Now, Ivan Vakarelski at King Abdullah University of Science and Technology (KAUST), Saudi Arabia, and co-workers have used high-speed cameras to examine how bubbles and droplets collide and coalesce at different interfaces. 

Crucially, the team were interested in the difference in behaviours between interfaces that are mobile and immobile – with their level of mobility determined by the molecular behaviour at the interface. At mobile interfaces, bubbles and droplets should coalesce quickly because of the low hydrodynamic resistance.

The team made use of a high-purity perfluorocarbon liquid as their medium, which has a viscosity close to water. They found that bubbles and droplets at mobile interfaces do indeed coalesce faster than those at immobile interfaces. However, the bubbles and droplets were also found to bounce off the mobile interface very strongly before the rapid coalescence. The team verified these findings with numerical modelling. 

“The bounce enhancement with increased surface mobility is counter-intuitive to the rapid coalescence,” state the researchers in their paper in Science Advances. “These observations require a reassessment of the role of surface mobility for controlling the dynamic stability of gas or liquid emulsion systems, relevant to a wide range of processes.” 

doi:10.1038/nmiddleeast.2019.144

Vakarelski, I. U. et al. Mobile-surface bubbles and droplets coalesce faster but bounce stronger. Sci. Adv. 5, eaaw4292 (2019).