In the Case of Droplets, Opposites Repel
Coulomb’s law states that an opposite electric charges--positive and negative--attract each other. According to this law, oppositely charged water droplets mutually attract each other. This attraction pulls the droplets together, deforming them and forming a shape called a Taylor cone on its surface. Usually the cones touch to form a bridge between the droplets, which then merge. A recent discovery showed a different result than expected from Coulomb’s law. Droplets with a whole lot of charge bounce off each other. In 2005, chemical engineer William Ristenpart was studying the effects of electric charge on droplets of water in oil. His team discovered that if the droplets are carrying low or moderate charges then the cones are short and wide, with a large angle at the top. The surface tension pulls the drops together into one big droplet. However, if the droplets are highly charged, then they pull on each other so strongly that the cones become tall and skinny, with a small angle at their tips. The high surface tension causes the bridge to break, thus making the electric field become virtually negligible. There is no electric charge between them, so they rebound each other. The researchers’ state there is a “critical” angle which droplets do not merge. This finding could cause a rethinking on some important industrial processes, such as the electrostatic separation of water from crude oil.
P. Berardelli. “In the Case of Droplets, Opposites Repel .” Science, 10/16/2009. 2/18/10
http://news.sciencemag.org/sciencenow/2009/09/16-02.html.
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