Fig. 3 displays the SEM micrographs of the foam particles. The micrographs reveal that there is a big bubble in the center of the particle and small bubbles distributed around. The bubble has basically perfect surface, and the thickness of the wall is ~ 3 μm. Presumably as drying proceeds, a more or less impervious surface tends to appear on the outside of particles; because of this, water vapor pressure builds up within the samples resulting in swelling and bubbling of the material.