PHYSICAL GELATION UNDER SHEAR FOR GELATIN GELS

Physical gelation is the process of crosslinking which reversibly tran sforms a solution of polymers into a gel. The crosslinks of the networ k have a physical origin (hydrogen bonding, Van der Waals forces...) a nd therefore are sensitive to variations of temperature, pH, ionic con tent, etc. (non-permanent crosslinks). Physical and chemical gelation have been extensively studied in quiescent conditions, where rheology experiments have been performed to follow the network formation withou t disturbing the process. In this study we consider gelation of a well known physical, thermoreversible, gel (gelatin gel), which proceeds u nder flowing conditions. The gelling solution is submitted to a sheari ng, with imposed, permanent shear stresses or imposed, permanent, shea r rates. Under flow, a competition arises between the formation of clu sters by physical crosslinking and their disruption by the shear force s. This investigation defines the flowing conditions which either allo w or impede gel formation. In particular, a critical shear rate (gamma ) over dot, related to the gelation temperature and gelatin concentra tion, is identified which separates the two regimes. A microscopic mod el is proposed, based on the analysis of flow curves and dynamic measu rements, which describes the structure of the gelling solution: microg el particles grow to a maximum size which depends on the flow. When th e volume fraction of particles is high enough, percolation between par ticles occurs suddenly and a yield stress fluid is formed (particulate gel). The differences between gels made in quiescent conditions and g els made under flow are underlined.