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Re: [Phys-l] Coalesing drops

On 10/08/2010 04:33 PM, Bernard Cleyet wrote:
Becoming pop. is photographing splashing and drop collisions -- my
thinking such is a development of an interactive exhibit at the

You will have no time in 3 or 4 feet for a small drop to "catch up"
to a larger drop. The collisions are caused only by a later drop
colliding with the spout of an earlier drop.

I'm not sure the following is relevant to the spirit of
the original question ... but if you want to know about
"coalescing drops" in general, then
-- exploratorium-style demos are not the only game in town
-- drops in free fall are not the only game in town.

It turns out that small droplets can be suspended in in inert
fluorocarbon oil. Individual droplets can be studied for extended
periods of time. They can be captured by means of laser trapping
aka optical tweezers. When two droplets are brought together
they immediately coalesce.

This has tremendous importance because
*) molecules are too small
*) test tubes are too big
*) droplets are baby-bear just right

That is, individual molecules of interesting sizes are too small to
be caught in an optical trap. Even biopolymers such as messenger RNA
are too small. Ordinary test tubes are too large, since at anything
approaching physiological density they contain huuuge numbers of
molecules so all we get to see are average properties. A femtoliter
droplet will hold one molecule under physiological conditions to a
reasonable approximation, and can be trapped. When droplets coalesce
reactions take place. We can watch the reactions on a molecule-by-
molecule basis using techniques such as FRET i.e. fluorescent resonant
energy transfer.

This technique is in its infancy. The upside potential is enormous.
I have some papers on the subject I can send to anybody who is


Even if you confine yourself to droplets in air, free fall is not the
only option. You can entrain the droplets in an upward jet and study
them in the quasi-steady state.

A tremendous amount is known about this; see e.g.