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The goal of this research project is to determine why oppositely charged water drops in oil fail to coalesce above a critical field strength.  The funding from the PRF has been crucial in assisting me build my research group over the past year, and as discussed below my students and I have made substantial progress towards understanding the underlying mechanism.

First, however, I am happy to report that some of the preliminary results (which served as the kernel of the grant proposal) have now been published.  A few months after submission of this grant proposal to the PRF, my colleagues and I submitted our preliminary observations (focusing on water drops in silicone oil) to the journal Nature, and happily it was accepted:

Ristenpart, Bird, Belmonte, Dollar & Stone, “Non-coalescence of oppositely charged drops,”  Nature 461, 377 (2009). 

The paper received considerable attention from the scientific media; see for example the accompanying “News and Views” highlight from the same issue of Nature:  Mugele, “To merge or not to merge…”, Nature 461, 356 (2009).   The Nature paper was submitted in January 2009 and appeared in print in September 2009; since I received the PRF award in February of 2009, this PRF grant only indirectly contributed to the article in Nature.   I also co-authored a follow-up paper in Physical Review Letters examining the non-coalescence behavior of liquid droplets in air (Bird et al., Phys. Rev. Lett. 103, 164502, 2009).

In terms of direct impacts, the PRF award has played a crucial role in supporting the first graduate student to join my group (Brad Hamlin).   Brad has been making great strides in understanding the behavior of charged drops as they approach one another.  A key component of the proposed research is accurate measurements of the charge on the droplets, which traditionally have been estimated by balancing the presumed drag force with the electrophoretic driving force.  Brad demonstrated shortly after beginning work on the project that the velocity, and hence drag force, are highly sensitive to the separation between the drops and electrodes.  Brad helped derive and corroborated an “image charge” model to account for acceleration as drops approached each other or the electrodes, and he presented his preliminary results at the APS Division of Fluid Dynamics meeting in November of 2009.  Since then, he has been probing the effect of surfactants on the drag coefficient, and he has generated convincing data that the so- called “stagnant caps” of surfactant molecules which increase the drag on the droplets can actually move around dynamically in response to contact with other drops or a sudden change in the direction of the applied field.  Brad will be presenting these results at the 2010 APS DFD meeting, and we are currently working on his first manuscript for publication.

The PRF grant has also supported two excellent undergraduate research assistants in my group. John Creasey, who is now a senior in chemical engineering, joined my group in the summer of 2009.  John has generated an amazingly detailed phase diagram of coalescence behavior for water droplets in oil, including a variety of ‘partial coalescence’ events where droplets fail to coalesce completely (i.e., a daughter droplet is emitted).  This phenomenon has profound implications for electrostatic dehydration of petroleum: since smaller droplets are more difficult to separate, any process generating smaller droplets must be minimized or prevented.  John presented his preliminary experimental results at the 2009 APS DFD meeting (a rare distinction for an undergrad, only made possible here by the funding from PRF).  Over the next few months John helped develop and experimentally corroborate a theory to help predict the size of the daughter droplets.  John will be presenting these more recent results at the 2010 DFD meeting, and we are also working on a manuscript for publication.  The second undergraduate, Graham Magill, joined my group in the summer of 2010.  Graham is a computer wizard, and he has been doing numerical simulations of charged droplets coalescing (or not coalescing) in pressure driven flows.  Graham will also be presenting some of his preliminary simulations at the 2010 APS DFD meeting.

Based on the preliminary results obtained so far, I anticipate that approximately 3 to 4 journal publications will ultimately result from this PRF award.  Moreover, the work of the PRF-supported students has been crucial for helping me solicit future funding.  I recently submitted my first NSF CAREER proposal, titled “High Speed Imaging and Chronocoulometry of Charge Transfer Events in Emulsions.”  The preliminary data obtained by Brad and John was essential for motivating and strengthening my CAREER proposal.