Hysteresis and incubation in the dynamics of volume transition of spherical gels

The dynamics of volume transition of a spherical gel is studied theoretically. The time evolution of the radius and the strain of the gel are calculated numerically. It is shown that (i) the threshold temperature at which the swelling starts to take place is different from the temperature at which the collapsing starts (i.e., there is a hysteresis), (ii) in the swelling process there is an incubation period during which the whole gel is swollen only slightly, followed by the main process in which a large volume change is caused by the motion of the interface between the swollen state and the collapsed state, and (iii) the incubation time becomes longer as the final temperature approaches the threshold temperature. These results are in agreement with experimental observations.