The craters in this image are only a few hundred microns in diameter. They were made by tiny, unwanted air bubbles that persisted through formation of the surrounding porous polymer gel. When studying materials at this scale, such small defects can leave a relatively large impact.
This scanning electron microscope (SEM) image shows a porous poly(ethylene glycol) diacrylate (PEGDA) hydrogel that contained air bubbles as it was cast in a mold. The sample was made from an oil-in-water nanoemulsion with PEGDA, which forms a colloidal network at elevated temperatures due to polymer bridging. With the addition of a photoinitiator, the PEGDA in the water phase was crosslinked using UV light. Then, the oil droplet network was selectively washed away, leaving a porous hydrogel. [316 x 484 microns]