SCALING IN THE DYNAMICS OF GELATIN GELS

Gelatin gels never reach equilibrium. The storage modulus increases linearly in log(time) after some initial time, typically one hour. This behaviour, which is characteristic of physical ageing, makes it hard to define the gelation concentration, gelation temperature and gel melting temperature in a non-arbitrary way. For instance, after slight heating close to the gelation temperature certain gels melt, but then reform several hours later. The gelation kinetics for all temperatures, concentrations, molecular weight distributions and times can be parameterised by forming master curves in log(elasticity)/log(time) space. The effects of temperature and concentration are independent. For concentrations between 7.5 and 100 g/kg we find no evidence for a critical gelation concentration. For a given sample, all the data can be fit using a single critical temperature, which shows the characteristics of the critical temperature of a second order phase transition. There are two regimes with different critical exponents. We call these “close-to-critical” and “far-from-critical”. Cuts made in a gel during the close-to-critical regime heal, whereas cuts made once a gel is in the far-from-critical regime do not heal. The effects of temperature jumps in the two regimes are qualitatively different. Both show scaling properties. These results are discussed in the context of the physics of out of equilibrium systems