What is the length of the channel to your outlet? It could be a
back-pressure issue (you can use the Hagen-Poiseuille law to estimate the
flow rate you should be able to achieve with your vacuum pump).
Alternatively, your channel could simply be blocked or it could be poor
sealing between the SU-8 and PDMS (perhaps more likely). Remember also that
PDMS is gas-permeable so with a thin layer, under some conditions you may
simply be drawing air through the PDMS rather than sucking the liquid out.
Another tip: try filling with ethanol first and then flushing through with
water.


On Thu, Aug 13, 2009 at 17:26, Yifan Wu  wrote:

> Dear all,
>
> I was fabricating a microfluidic device which consists of glass substrate,
> SU-8 channel (30um thick) and PDMS cover lid. The device has 2 inlets and 1
> outlet. 2 inlet channel are 100um wide and once they mix into one outlet
> channel, the channel shrink to 24 um. It is easy to suck the water from 2
> inlet tubes into the channel. But after liquid reach the channel it is
> impossible to suck it out from the outlet. The liquid can go from inlet to
> inlet easily but not to outlet. Is this related to the hydrophobic property
> of SU-8 and PDMS? How could I solve it? Thank you very much.
>
> Yifan
> University of Missouri