Jamming transition in solutions containing organogelator molecules of amino-acid type: rheological and calorimetry experiments

The liquid to organogel transition of solutions containing new organogelator molecules of the amino-acid type is studied using rheological and differential scanning calorimetry (DSC) techniques. This paper describes the formation of the organogel as a function of the temperature for various concentrations of the organogelator molecules, and the mechanical properties of the organogel as a function of concentration, frequency, thermal history and aging. We show that the organogel is not a physical gel, but a jammed suspension. The viscoelastic behavior at different extents of the jamming can be scaled onto a single master curve describing the growth of a solid network in a background fluid. The formation of the solid network exhibits a critical-like behavior that is reminiscent of elasticity percolation. Four characteristic temperatures have been identified: the temperature at which the clusters begin to form, the transition temperature between the liquid and the organogel, the onset temperature of the percolation-like behavior of the solid component of the system and the temperature at which the solid component has a pure elastic response. The comparison between the rheological measurements and the DSC measurements shows that the rheological measurements detect the fluid-to-organogel transition, whereas DSC detects the molecular associations in the material, which are at the origin of the formation of the clusters. The two temperatures differ significantly from each other and their difference gives the temperature range where the clusters are crowding. This study demonstrates for the first time that an organogel is not a physical gel, as it is currently believed, but a jammed suspension.