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