You could easily do in-situ reflective spectroscopy on the silicon
substrates and fairly easily on the glass.  The index and thickness of
the hydrogels will change with water absorption changing the reflected
spectra (optical interference will change).  This should be fairly
easily modeled.

I suspect that you could also do some type of acoustic damping but I
don't know enough about that to help much.

Good luck and have fun,
Matt Coda

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Rajib Ahmed
Sent: Wednesday, May 30, 2007 9:15 AM
To: General MEMS discussion
Subject: [mems-talk] amount of swelling

Hello everyone,

We have a problem and were wondering if someone could kindly offer some
assistance.  We deposit hydrogels on silicon wafers or glass substrates.
 Then we dip the substrate in water or some other solvent to see how
much it swells up from absorbing the water.  Is there a reliable way to
measure the swelling in-situ?  If we dry it, then it defeats the
purpose.  Any help you can offer would be greatly appreciated.