Vishwa,

At first glance, I'd guess that you're seeing the effects of optical
inteference. Within the mixing/rotating portion with herringbone pattern,
there will be changing densities of particles at different distances.
Fluorescence from this region will probably tend to cancel itself out a bit
due to destructive interference. In the flat portions, the particles are
evenly distributed and there would be less destructive interference.

That is assuming you've imaged the channels without flowing particles
through them and did NOT see this same effect. If you do see it without the
particle flow, then you've got the same situation, but due to the PDMS
thickness differences.

Or, if you're not using an optical filter to block the YAG wavelengths,
you've got reflections from the laser light into the detector.

Mike


Dr. Michael H. Beggans
Naval Surface Warfare Center, Indian Head
101 Strauss Ave. Bldg. 302            Phone: (301) 744-1927
Indian Head, MD 20640                  Fax:    (301) 744-6406

-----Original Message-----
From: Vishwa [mailto:dagarshali@gmail.com]
Sent: Friday, March 16, 2007 1:19 PM
To: General MEMS discussion
Subject: [mems-talk] Bright Spots on PDMS during microPIV

Hi,
The PDMS microchannel looks good. I am using PDMS bonded with Glass slide. I
use it for flow  visualization study using microPIV.  The particles that I
use are from Invitrogen and they are 1um fluorescent based particles.

The glass slide forms the bottom wall of the channel. The other 3 walls are
made from PDMS. The ceiling of the channel has an array of
grooves(herringbone pattern). So, if you are traversing along the cieling,
you will encounter a grove which is 20um x 50 um and spans the entire
channel(2.5mm), which is then followed by the ceiling. This pattern
continues.

When I pass the particle solution and take images using a Nd:YAG laser, The
portions of the ceiling where there are no grooves appear saturated(very
bright).. I am not sure as to why this is happening.

If you need more info, I can email the picture so that you will get a better
understandig of the problem as my explanation could be hard to follow.

Thanks a bunch in advance.

Vishwa