Concentration dependence of the dynamics of microgel suspensions investigated by dynamic light scattering

Abstract

The dynamics of colloidal gel particle suspensions, i.e., microgel suspensions, has been investigated by dynamic light scattering (DLS) over a wide concentration range from the (I) dilute (φ < φ_cp) to the (II) intermediate (φ ≈ φ_cp) and (III) high concentration regions (φ ≫ φ_cp), where φ and φ_cp are the volume fraction of the gel particles in the suspension and the random close packing fraction, φ_cp ≈ 0.64, respectively. The time–intensity correlation function exhibited a distinct change with increasing φ, i.e., from ergodic behaviour (region I and II) to nonergodic behaviour (region III). A mode transition from translational (region I) to cooperative diffusion (the so-called gel mode) (region II) was also observed due to the soft and deformable nature of the microgels. Different from the dynamics of hard colloidal glass suspensions, the gel mode remained even at φ ≫ φ_cp. By using the ensemble-averaged time–correlation function, 〈I〉_E, we quantify the relationship between φ and their dynamics, and show that the soft microgels are deswollen in the densely packed state.