Gelation of large hard particles with short-range attraction induced by bridging of small soft microgels

• Published in: Soft matter 2015-03, Vol.11 (12), p.2494-253
• Main Authors: Luo, Junhua, Yuan, Guangcui, Zhao, Chuanzhuang, Han, Charles. C, Chen, Jie, Liu, Yun
• Format: Article
• Language: English
• Subjects: Attraction; Bridging; Criteria; Mathematical models; Microgels; Microspheres; Polystyrene resins; Rheological properties; Strength
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/c4sm02165g

In this study, mixed suspensions of large hard polystyrene microspheres and small soft poly( N -isopropylacrylamide) microgels are used as model systems to investigate the static and viscoelastic properties of suspensions which go through liquid to gel transitions. The microgels cause short-range attraction between microspheres through the bridging and depletion mechanism whose strength can be tuned by the microgel concentration. Rheological measurements are performed on suspensions with the volume fraction ( Φ ) of microspheres ranging from 0.02 to 0.15, and the transitions from liquid-like to solid-like behaviors triggered by the concentration of microgels are carefully identified. Two gel lines due to bridging attraction under unsaturated conditions are obtained. Ultra-small angle neutron scattering is used to probe the thermodynamic properties of suspensions approaching the liquid-solid transition boundaries. Baxter's sticky hard-sphere model is used to extract the effective inter-microsphere interaction introduced by the small soft microgels. It is found that the strength of attraction (characterized by a single stickiness parameter τ ) on two gel lines formed by bridging is very close to the theoretical value for the spinodal line in the τ - Φ phase diagram predicted by Baxter's model. This indicates that the nature of the gel state may have the same thermodynamic origins, independent of the detailed mechanism of the short-range attraction. The relationship between the rheological criterion for the liquid-solid transition and the thermodynamic criterion for the equilibrium-nonequilibrium transition is also discussed. In this study, mixed suspensions of large hard polystyrene microspheres and small soft poly( N -isopropylacrylamide) microgels are used as model systems to investigate the static and viscoelastic properties of suspensions which go through liquid to gel transitions.