Nanodrop Coalescence: Edinburgh-Warwick Collaboration Published in PRL

Nanodrop Coalescence:  Edinburgh-Warwick Collaboration Published in PRL

I have had the privilege of being involved in a collaboration between Edinburgh (Sree Hari, Matthew & Jason) and Warwick (Mykyta and me) working on understanding how liquid drops first come into contact.  This research has recently been published in Physical Review Letters and a Synopsis was chosen to feature in the APS’s online Physics Magazine.

Initially our intention was to study the singularities inherent in the classical modelling of drop coalescence, but like much fundamental research of worth our eventual findings involved entirely different physics.  Specifically, Sree Hari’s molecular simulations (see below) showed that the classical picture of ‘spherical drops touching at a point’ is not observed at the nanoscale, and instead one observes multiple off-centre contacts.

Merging of water nanodrops

Sree Hari then showed that these off-centre contacts were caused by waves on the drop’s boundary that deform it away from its usual spherical shape.  These waves are driven by thermal fluctuations that become prominent at the nanoscale, where they balance with the restoring force of surface tension to create nanometric amplitude disturbances.  Thus, in the light of ‘experimental findings’ from MD our research changed direction towards trying to characterise these waves.

At this point Mykyta Chubynsky became involved in the project and he was able to derive expressions for the properties of these waves, both for spheres (practical case of interest) and for liquid cylinders/disks (useful in MD to enable larger domains).  After a number of large MD simulations using ARCHER, Sree Hari was able to show that Mykyta’s new theory did indeed capture the new effects.  Most importantly, the theory can now be used to make predictions for larger droplets where MD is computationally intractable.

Remarkably, due to the cusp-like geometry of the gap between two approaching droplets, the nanoscale effects discovered are predicted to be dominant even for much larger micron-sized droplets.  This calls into question existing theories and computations which focus on a classical model that does not contains any information about thermal fluctuations; deriving new macroscopic models that go beyond the classical paradigms are areas of current work that fit in perfectly with the research philosophy of the Micro and Nano Flows group.

Finally, and most notably, I would like to mention that this was my first publication with Jason Reese who passed away just a week before this work was published.  His recent funeral made clear the depth of affection for him and the many people, myself included, who have admired and learnt from him.  I think he would have been very proud of this work as it captures everything he was about:  working across traditionally distinct fields, collaborating with other disciplines, encouraging young researchers, publishing in the top journals and publicising findings in an accessible manner (current Altmetric score of 80).