Did you let the sample just dry in air after cleaning it with DI water? Sometimes specks develop as dried patches of water. Although these look more like pockmarks in the resist, if the resist is still there on the sample.

What do you mean by "cleaned it with plasma and methanol for alignment"? O2 plasma can introduce damage to the resist, which may well look like pockmarks. Do you have before/after images to compare?

Hello Maggie,

I'm afraid I have more questions rather than answers! It looks like a lot of speckles! You haven't provided a scale or mentioned the size of the channels, which could narrow down the cause of contamination. Am I right in thinking you didn't see these before you left the device for the weekend?

When you left the device with tubing connected, were the ends of the tubing sealed? Or did you leave them exposed to the air? When you returned was there still plenty of water in the tubing?

When sonicating, are you sonicating in a smaller beaker of DI water? Again, is there much exposure to air? How clean is the surrounding environment?

Jules

@Tanuj, O2 plasma surface treatment/activation (for PDMS-glass) then using methanol as a lubricant for alignment is fairly common practice.

How do you sonicate? PDMS is permeable to a variety of substances, maybe you're inducing gas bubbles.

Hi Jules, here are the answers:

The channel dimensions are 110uM wide and ~ 150uM high.  

No, I do not see these speckles before I use the device.

The tubings were in a sealed vial, still with some water in there. But I did not take notice whether there's water in the tubing itself.

I sonicated the device inside a fresh zip-lock bag once with hand-washing liquid, then once in methanol, and finally once with DI-water. I did all of these in a standard lab (not cleanroom environment).

Thank you.

Hi Wenyang,

I sonicated the PDMS device in an ultrasonic water bath, inside a fresh zip-lock bag. Please see my answer to Jules on how I do it.

My guess is stuff getting inside the PDMS. Since sonication seems to cause the same thing, as a test, I would try eliminating the soap first and see if you can still reproduce it. I've used soap before, but not without diluting and it has to be filtered. You can also try eliminating methanol. DI-water itself should not be a problem. Do this to confirm if the substances that you're using for cleaning are the cause.

Hi,

My money is on the dust particles. As you did not used clean room for the fabrication process, it is likely that pdms has acquire (due to static property) those dust particles from the surrounding atmosphere before it was sealed. Did you observed the pdms substrate under the microscope before sealing them together? Looking from the image, with respect to the 100um channel, the speck dimensions are around tens of micron which normally corresponds to dust particles in our regularly cleaned labs.

Try to observe the newly fab chip under microscope without plasma treatment for both the surfaces by contacting them together.

Regards

I'm with Amit, I also think it's dust particles.

Do you have access to a laminar flow cabinet? Maybe that could help reduce contamination?

Hope you find a solution.

Jules

Hi Amit, I did the entire fabrication process in the cleanroom. Only when doing the microfluidics and the sonication process that is in a standard laboratory. But I will try filtering the solutions for sonication cleaning.