I've noticed something similar in experiments I've done and I believe
the cause is dye adhering to the wall in the new position. Try moving to
the new position and leaving the laser there for a while (up to a couple
minutes). You'll probably see the dye bleach and the signal settle to
some lower level. then if you step a micron over, you'll see the signal
spike again because there is new dye stuck to the wall there.

Also, can you say what the dimensions of your channel are and how far
down you are moving to check for mixing, because that will determine
whether or not it is even appropriate to expect appreciable mixing.

-Joe Grogan

Vishwanath Somashekar wrote:
> Hi all,
> I have posted on memsnet before regarding some problems that i have
> been facing when I use Rhodamine 6g..
>
> My experiment deals with micromixing. I use a microchannel, with
> herringbone patterned structures on one wall to enchance mixing. To
> characterize mixing, I have 3 inlets for this channel and perform 4
> experiments, which can be listed as follows
> 1) D-ND-D
> 2) D-D-ND
> 3) D-ND-ND
> 4)ND-D-ND
>
> where ND = no dye (ethanol) and D( rhodamine 6g with a concentration
> of 2g/L in ethanol). I have used Xenon bulb with a green filter and
> 532nm Nd:Yag laser as illuminating medium
>
> for experiment 1, when i take an image (using nikon eclipse inverted
> microscope) at the entrance region, i get a square wave kinda profile.
> Where there is dye, i get say X counts and where is no dye, I get Y
> counts. This Y counts is basically the noise and should go to zero
> when I subtract the background image from the raw image.