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After looking at the video, the "movements" look to me like a Schlieren effect; 
A wave of higher-RI solution rolls over the sample from left to right. Do you 
know what is the difference (if any) in refractive indices between the two 
solutions that you are switching?
If that is indeed the effect of RI, you should not see any movements when 
you use two identical solutions and perform the same valve switching.  If the 
movement is still there, 

A change in RI will cause Z-axis compression/expansion (focus shift); 
Perhaps you could tune the first buffer to the same RI as the propionate 
solution by some "inert" additive.  BSA, gelatine, or some other non-osmotic 
agent comes to mind (but I know nothing about what might interfere with your 
samples).  

I am just guessing here, but  another explanation may be that switching to the 
propionate buffer ('open-system buffer') enables osmotic effects of other 
media components and leads to cell shrinkage and intracellular pH changes.  
Something like in this paper: Thomas A. Heming, Gregory Boyarsky, Divina M. 
Tuazon, and Akhil Bidani:  pHi responses to osmotic cell shrinkage in the 
presence of open-system buffers J Appl Physiol October 1, 2000 89:(4) 1543-
1552 

Stan Vitha
Microscopy and Imaging Center
Texas A&M University