Nathan,

To get PDMS to come off, you don't need a very well controlled
hydrophobic layer. Just use perfluorodecyltrichlorosilane (FDTS)
applied in a vacuum desiccator, and then baked at ~110. You can find
FDTS suppliers here: http://www.chemexper.com/

1) Place your Si wafers (with no PDMS on them, this is just so you end
up with a long fluorinated molecule sticking up from the surface so
the PDMS will peel off easily later) in a vacuum desiccator connected
to house vacuum (do a google image search for vacuum desiccator to see
what these look like). 2) Place 10-20 uL of FDTS in a small container
(the top of an eppendorf tube works well) in the bottom of the chamber
(location doesn't matter much). 3) Pull house vacuum, and leave them
for ~2 hours (exact time will be more or less depending on the vacuum
strength and container size). 4) Bake the wafers at 110 for ~2 hours.

There are lots of methods out there to get higher contact angles, but
that will be good enough.

If you have a goniometer, measure the static contact angle and check
that it's at least 90 with DI water. If you don't have a goniometer
handy, just pipette (or somehow dispense) a few microliters of DI
water on the surface, and make sure that it forms tight balls that
roll around easily.

Also, keep in mind that most people use PDMS precisely because you
don't need to ablate, etch, or otherwise chemically attack it. You can
just use "soft lithography" which uses a stamp to mold the PDMS as it
cures. Si molds are common, but glass and metal (even CNC machined
brass molds are good enough for some microfluidic applications). There
are hundreds of papers out there describing it, if you're interested.

Kevin P Nichols, Ph.D.

Postdoctoral Scholar
Ismagilov Group
Department of Chemistry
Gordon Center CIS E305
University of Chicago
929 East 57th Street
Chicago, IL 60637

E: kpnichols@uchicago.edu
T: 773-834-8474
F: 773-834-3544

On Fri, Mar 26, 2010 at 11:31 PM, Nathan McCorkle  wrote:
> I am using a laser to ablate PDMS to make channels. I tried peeling
> off the PDMS layer, but it was attached quite well to both polished
> aluminum wafers as well as silicon wafers. I am not sure what the best
> method of getting the PDMS to release will be, please help!
>
> -Nathan McCorkle