Macrogel induced by microgel: bridging and depletion mechanisms

In this work, aggregation and gelation behaviour of mixed suspensions of polystyrene microspheres and poly(N-isopropylacrylamide) microgels were studied. In dilute microsphere suspensions, with increasing concentration of microgel (ϕMG), microspheres first aggregated with each other through the bridging of the microgels, then dispersed individually when saturated adsorption was achieved, and finally depletion clusters formed at even higher concentrations of microgel. The concentration of microgel for saturated adsorption (ϕ*MG) is in a linear relationship with the given concentration of microsphere (ϕMS). For given ϕMS, the largest bridging clusters formed at ϕMG ≅ 0.5 ϕ*MG. In concentrated microsphere suspensions, with increasing ϕMG, the mixed suspensions showed a liquid–“bridging gel”–liquid–“depletion gel” transition. The maximum modulus and yielding stress of the bridging gel are also obtained at ϕMG ≅ 0.5 ϕ*MG. The gelation of the microsphere can be explained by the percolation of the clusters and the elasticity of bridging gel arising from the mutual attractions between the microspheres.