What kind of material is the channel made out of?  If PDMS or any
elastic material, the higher pressure used to drive the higher flowrate
will distend the channel.  This will effectively increase the local
volume of dye, increasing the observed intensity.

I also agree with Gareth.  Rhodamine is fairly temperature sensitive, so
if you can try to control the temperature of your setup somewhere above
ambient.  If it is photobleaching, try capturing a few images, then
pausing, to let the flow replenish the local unbleached dye
concnetration, and capture another set of images.  If the intensity
changes between acquisition sets then you might be bleaching.  If not,
then you have either of the other issues listed above.

Lee

Vishwanath Somashekar wrote:
> Hi All,
> I am using rhodamine 6g mixed in a tiny bit of alcohol, which is then
> dissolved in DI water. I pass this dye solution into a micochannel( the
> microchannel cross section dimensions are 2.5mm x 100 microns)by means of a
> syringe pump. I have used both a xenon bulb and nd:yag laser as the light
> source. What I find is that the when the flow rate is faster, the intensity
> of the dye image is higher and it is lower when i decrease the flow rate. I
> find this strange as it is the same dye solution that i am using at both
> flow rates and the channel is completely filled with dye solution. Could any
> one of you explain what might be going on and how to overcome this?