Saturday, April 30, 2005

Picosecond flashes from mercury and glass

This paper says that mercury and glass in vacuum exhibit triboelectricity (static electricity charging due to surface contact). The electric force causes a stick-slip interaction that produces picosecond flashes of light.

That's neat—there are not very many ways to make sub-nanosecond flashes of light. Most techniques, like mode-locked lasers, are expensive and balky. This might make a good (or at least cheap) bench-top picosecond source for the lab.

Hmmm...with further thought I'm having trouble seeing the physical mechanism behind the flashes. Bulk liquid doesn't accelerate on a picosecond time scale. When a slip occurs, the gap will grow gradually. (Microseconds? Certainly no shorter than nanoseconds.) Likewise for electric arcs, which tend to grow and diminish gradually. The only fast effect I can think of is that an empty wedge forms as the mercury separates from the glass. Because the wedge is not made of matter, its tip can move fast, potentially faster than light, so particles that follow the tip can reach ludicrous energies. When the tip suddenly stops moving (the "stick" part of stick-slip), the particles catch up and crash into solid matter, making the picosecond flash.

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