Congratulations to Chenglong, Peng and Prof Law.
Their recent paper "C. Tang, P. Zhang, C. K. Law. Bouncing,
Coalescence and Separation in Head-on Collision of Unequal-size
Droplets. Phys. Fluids, in press." was selected as a
highlighted update in physicstoday!
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New insights into droplet collisions
The performance of diesel and rocket engines
may be improved by exploiting size differences in droplets.
Ocean mists and car engines are among the countless environments
whose behavior is influenced by the dynamics and interactions of
liquid droplets, and by droplet collisions in particular. Such
collisions can have numerous outcomes-the droplets might smoothly
merge with little deformation, bounce off each other, coalesce
following large deformation, or separate after temporarily
coalescing. To date, most research has focused on collisions
between identical droplets, for which the outcomes depend on the
impact parameter and the ratio of kinetic energy to surface
tension. In new work on droplets of unequal size, Chung Law and
colleagues at Princeton University have experimentally demonstrated
that the size disparity can significantly increase the parameter
range over which the droplets permanently coalesce. The team's
theoretical model, based on energy balance and scaling arguments
and verified experimentally, reveals why: As surface tension pulls
the merged, deformed droplet back toward a spherical shape,
increased viscous dissipation through internal motion reduces the
energy available for separation. The extended coalescence region,
argue the researchers, may hold promise for rocket engines fueled
with so-called hypergolic propellants that ignite when they come
into contact, since the enhanced coalescence and mixing of fuel and
oxidizer droplets should facilitate the ignition. (C. Tang, P.
Zhang, C. K. Law, Phys. Fluids, in press.)-Richard J.
Fitzgerald