An infinitesimal change in temperature or fluid composition can, under the right conditions, abruptly collapse a gel to 1/500th of its original volume. This appears to be a phase transition in the classic thermodynamic sense. A gel is a crosslinked polymer network that holds a fluid in its interstices—for example, a jellied consommé. Considerable theoretical and experimental work has been done since the 1940's on the sol–gel transition, the phase transition from the liquid to the gel state. But prior to the recent work of Toyoichi Tanaka and his colleagues at MIT, it was not known that a phase transition could occur within the gel state itself.
Bertram M. Schwarzschild; Gel collapse is phase transition. Physics Today 1 January 1981; 34 (1): 18–20. https://doi.org/10.1063/1.2889958
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