Phase transition in gels of sub-millimeter size induced by interaction with stimuli

Abstract

Phase transition in response to infinitesimal change of external stimuli has been observed universally in various gels made of synthetic and natural polymers. The phase transition properties of gels observed in equilibrium and their dynamic and kinetic behavior are determined by the interaction of polymer networks and a liquid. They were extensively studied in the 1980s. Recently, simple technological improvement using gels of sub-millimeter size has made it possible to observe new phenomena, which opens a new scientific field. After a brief introduction to the recent development of investigations using such tiny gels, this paper will describe three principal ways in which gels exhibit phase transition by the interaction with external stimuli. (1) Visible light-induced phase transition in gels. Local heating of a thermoresponsive network by visible light can induce phase transition. (2) Uniaxial stress-induced phase transition in gels. Uniaxial stress by applying weight can change the transition temperature of thermoresponsive gels. (3) Salt effects on the phase transition in non-ionic gels. Adding salts can change the transition temperature and discontinuity. The experimental facts and their interpretation will be proposed on the basis of the mean field equation of state of gels. These principal experiments have been presented in order to demonstrate the recent development of investigations.