Hi,

I have a microfluidic channel with a very low aspect ratio (100 um tall x 2 mm wide). When I flow cells though this channel I notice cells move at two separate speeds. I am trying to figure out exactly why this is and how to resolve the issue.

The channel is made on 3 sides with PDMS and on the last side by SU-8.

My thinking thus far is that in a low aspect ratio channel the cells will inertially focus into two equilibrium positions in the vertical axis of the channel: But these two positions are not at the same spot on the parabolic velocity profile and thus have different speeds.

Where the cells find an equilibrium is a function of the wall interaction and the sheer gradient lift force. Cells at each equilibrium will have the same fluid velocity, density and size. Thus there must be a difference in the velocity profile to cause this discrepancy. I assume this would be because the materials are different. Something to do with the slip length, the surface charge (zeta potential) or surface roughness.

Has anyone had a similar issue? Any suggestions for how I could resolve this issue (excluding changing the geometry much)? Does my thinking make sense, perhaps there is another way to explain why I see cells moving at two different speeds?

Best,

David