Fluorescein should self-quench when concentrated (i.e. dried out).  That dye or almost any other water-soluble fluor should show that property.  But as far as changing color when dry, you'll probably have to experiment.  What about something like bodipy disulfonate, which is very water-soluble and forms excimers with red fluorescence when concentrated?  (Articles by RE Pagano's group exploited that property of bodipy.)

Richard


> [log in to unmask] 10/13/01 11:45:13 AM >>>
Hi everyone,

The Problem:

A plastic part is steam sterilized and can have a residual water drop in a small depression in the part.  All the water has to be removed before the mfg process can continue.  The proposed solution is to apply pressurized air (or some other gas) to remove the water drop.  However, when a jet of pressurized air is applied it will blow out most of the water but leave behind small 20-50 micrometer microdroplets.  These are still a problem.

I thought that I could use a dye in the test droplet of water applied to the part before it is subjected to the proposed air blast in an experimental test setup.  The idea would be that if any microdroplets remained I would be able to see them under a simple macroscope.  However, for such things as food dye, when it dries, the color remains and I can't tell if it is wet or dry.

One of the acceptable situation may be that most of the droplet is blown off and the remaining microdroplets dry within seconds using heat lamp, dry air, low RH in the test area, etc.  So what I'm looking for is a dye that is one color when wet and either another color when dry or no color at all (clear, or white powder).   My first attempts have been with cobalt chloride but driving off all the mositure is difficult.   Another possibility is using a fluorescent dye that fluoresces one color when wet and has a very much reduced fluorescence when dry or better yet a different color.

I certainly appreciate you help and all suggestions will be considered, even if you have a totally different idea on how to determine when all the water is gone.

Damian Neuberger