I would think this is either photobleaching or a temperature effect. Try
some neutral density filters - if it is photobleaching the effect should
diminish.
If somehow the solution is being heated whilst in the channel such that
a slower flow rate increases the relative temperature, the fluorescence
will drop with lower flow rates.
Hope this helps

Gareth


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?