Researchers have reported unexpected results from studies into the behavior of oppositely charged liquid drops. Published in Nature (doi: 10.1038/nature08294), the team from the University of California, Davis, the School of Engineering and Applied Sciences at Harvard and Pennsylvania State University assumed that drops would always be attracted to each other and stick together on contact; however, they found – using high-speed videography to observe the bouncing drops in the act – that they appear to bounce off each other instead.

With the electrically induced attraction of liquid drops, and their coalescence, being involved in printing technology, the use of petroleum, and even the formation of storm clouds, this research could open up whole new areas of study. One of the biggest applications for this research is in the electrostatic dehydration of petroleum and other oils, such as biofuels and vegetable oils.

Bill Ristenpart and the research team actually started on this line of experimentation by accident – Ristenpart was studying Taylor cones in an oil/water system when he accidentally applied too high an electric field strength, with several kilovolts passing through the water. This shorted the circuit, causing a small explosion in the water, with many small water droplets being ejected. When the field strength was turned down, it was observed that the drops were bouncing back and forth. This led to the obvious question of why the positively charged drops were bouncing off the negatively charged interface – three years later they have now answered the question.

For years, different investigators have reported that the efficiency of coalescence plateaus above a critical field strength, without anybody quite knowing why this was happening. As Ristenpart explains, “We think that the results in this work shed some light on why this might be.” The research shows that stronger fields distort the geometry of the aqueous bridge that forms between drops – this stronger field acts to pinch off the drops from each other.

One of the most significant advances in the research is the proposed mechanism for explaining why the drops bounce.  It works in a very counterintuitive way – if you increase the force that brings the drops together, you decrease the probability they will coalesce.  So the mechanism being used, which is based on capillary pressure effects, provides a framework for thinking about why they bounce.

As for further developments, the team are now looking at different microfluidic technologies and investigating bouncing in petroleum. Another exciting phenomenon that may lead to future research involves partial coalescence, where only some part (such as a half) is ejected, rather than the entire drop bouncing.