Gelation under shear: A dynamic phase transition

Gelation is the process of aggregation which transforms a solution of particles or polymers into a solid in which the solvent is trapped. Gelation is well known essentially for solutions at rest where the aggregation can proceed without disturbances produced by mechanical actions, such as stirring, pouring, pumping. However, in many industrial applications gels are not prepared in quiescent conditions, a flow is imposed to the solution during gelation. A competition arises then between aggregation (formation of clusters) and disruption of the clusters by the flow. In this chapter we present an experimental investigation of the gelation of a polymer gel (gelatine gel) under continuous flow: the gelling solution is submitted to a shearing, with an imposed shear stress or an imposed shear rate. The critical conditions for either allowing or impeding the formation of a gel, and the structure of the gel made under flow (particulate gel) are discussed. The differences between gels made in quiescent conditions and gels made under flow are underlined.