Dear all,
I am having severe problems with bubbles occuring during the curing of PDMS. I mix part A and part B of PDMS (Sylgard, Dow corning) thoroughly with a ratio of 10:1 and degas under vacuum in a desiccator until all visible bubbles go away. After pouring the mix into the molds made of adhesive tapes sticked on glass, I continued degassing. However, during degassing, lots of air bubbles emerged over the adhesive tapes as you can see from the attached images. I kept degassing under vacuum for 1 hour more to get rid of these bubbles, however, a few bubbles either kept forming or if no bubbles were left I started curing process by putting the molds at 65 C in the oven for 1h. But still there were bubbles confined inside the PDMS after curing.
Why do you think these bubbles occur? And, what is the solution to overcome this problem?
I would appreciate to hear your comments and suggestions.
Thank you.
Hi,
It is a typical behavior of this system!!! The moment we use adhesive tapes either for making molds or part of the mold which is exposed to PDMS, one will get bubbles for sure. The tape edges if not properly attached to the petri dish/substrate it traps air. The magnitude of the vacuum pressure in the degassing process is not big enough to suck out this air but the thermal treatment used for curing PDMS provides enough heat to the trap air for it to under go expansion and give the results as shown in your images. The air can also get trap while one pours the liquid viscous elastomer in the mold as well as the microbubbles trapped while mixing the the two components. Due to high viscosity of the elastomer and low buoyancy force on the micro volume air bubbles (try to observe them closely under some microscope), they take considerable time to surface and escape in the degassing process.
Solution for such problem is equally skill based and technical tinkering. While making the mold make sure there are absolutely no air trapping in between the adhesive tape and the glass petri dish. If you cannot avoid that try using coverslips if your wall height is 100um and above or some novel out of the box idea. Next while pouring elastomer, make sure to pour it out as gentle (lowest manually achievable flow rate) as possible and without breaking the flow stream. Start the pouring position from one of the corners of the mold and let the flowing elastomer occupy the available volume by itself. Last but not the least, cure the elastomer at room temperature for 24 hours (it gets cured roughly in 15hrs). 24 hrs to make sure that all of it is cure. The room temperature curing does not expand air even if it is trapped in the form of microbubbles. The room temperature cured can latter be put inside an oven for few hours at 70-80degrees to remove the surface stickiness if at all it is remaining!!!
These few procedures worked out very well for me for wall heights from 30um to well above 1cm and in all possible mold geometries created using numerous materials as the molds!!! Practically adhesive tapes themselves will not degas at the vacuum pressures that we employ for degassing! These are the micro cavities that are usually formed while creating the molds out of adhesive tapes.
Regards
Actually Amit Morarka said it well. I just add a few words.
If possible don't use tape, tape always retains some gas. If you really have to use tape, once let PDMS cure in room temperature for a long time (1 day maybe) then remove the cured and bubbly PDMS and use the mold now. This way there is enough time for most cavities in the tape to be filled with PDMS. By the way, after you stick tape on the glass do you put it on hot plate? If not then try that, it might help.
Maybe this article could be helpful to you:
Article Adhesive-based bonding technique for PDMS microfluidic devices
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