for the SEM sample, we normally give them a very good clean, and then store in a dry container. Because a tiny contamination can be magnified under SEM. The white bubbles are probably contaminations if not oxidations.

Dear Jincheng,

Thank you for your answer. These white bubbles are on all of the SEM photos. I also do care about the membrane samples to prevent any contamination.

I think something swelled or maybe the membranes have smallpox!!

Regards

Hi Foroogh, it looks like dust just as Jincheng suggested. You could find out by taking a backscattered image.