May 1, 2002
Electrohydrodynamics is the study of how electric fields effect the flow of liquids. In the experiment shown in Figure 1, an electric diode is separated from a conducting fluid (water) by a noncunducting fluid (oil). When the diode is charged up, the water gets sucked toward it and explodes into tiny droplets. This phenomenon is known as an electrohydrodynamic spout.
Sidney Nagel here at the University of Chicago Materials Center has measured how the spout's shape changes over time. Previous theories had speculated that the water has a cone-like structure but in the time-lapse photographs taken by Prof. Nagel (Figure 1) you can see how water is pulled by the electric field to an infinitely sharp pointed peak. This structure is known as a cusp. It is also interesting to observe that the spout's shape is remarkably similar to the "neck" of a falling drop of water (Figure 2). As the drop separates from the water above it, the neck is streched into a cusp and comes to a point just before the drop is released.
Analyzing experiments like this is often difficult because very small unpredicatble changes can have a great effect on the result. No matter how hard you try to keep your experiments identical no two droplets will ever fall in precisely the same way. This makes it essential to look beyond the minute differences in each trial and percieve the underlying similarities.
In this case, careful analysis of factors we do understand can lend insight to the entire experiment. Understanding how the water's shape changes over time gives us a better picture of the droplets that are formed. This study is important to other areas of physics and has potential applications such as liquid injection printers or spray painting.
by Seth B. Darling, Adam Kalafarski
"Singularity during the Onset of an Electrohydrodynamic Spout" Lene Oddershede and Sidney R. Nagel, Phys. Rev. Lett. 85 (2000) 1234-1237.