Effect of the range of attraction on the rheology, microstructure, and thermodynamics of thermoreversible gels with adhesive hard-sphere interactions.

摘要

Dispersions of colloidal particles with short-range attraction exhibit a rich spectrum of phase behavior including a transition between gel and liquid in the semi-dilute regime upon changing either the potential of mean force or the volume fraction. Studying this transition helps with understanding both the static and flow properties of various complex fluids that are currently processed in industry. However, the exact location of the gel-liquid transition in the state diagram is still a topic under debate. In this dissertation, the location of gel line is studied by determining dynamic arrest and determining the interaction strength from the gel structure for model thermoreversible colloidal dispersions with different relative ranges of attraction. The study is extended by investigating structural anisotropy under steady shear and large amplitude oscillatory strain (LAOS) deformations using time-resolved oscillatory rheo-small-angle neutron scattering (tOr-SANS) and linking the bulk flow properties to the instantaneous microstructure.;Single-particle properties of the model colloids such as particle size, shape, density, and grafting density are characterized with transmission electron microscopy, dynamic light scattering, SANS, gravimetric densitometry, and thermogravimetric analysis. The gel transition temperature is pinpointed as a function of particle size and volume fraction via performing small-amplitude oscillatory rheometry and finding the temperature at which tan δ < 1 over several orders of frequency. The strength of attraction, expressed with the Baxter parameter, is extracted by analyzing SANS and ultra-small-angle neutron scattering (USANS) data. The reliability of the analysis scheme is validated by performing an independent study using a thermodynamically self-consistent closure. A hypothesized competition between Brownian motion and gravitational settling during gelation is quantified through a gravitational Péclet number. Finally, the micromechanics of structural anisotropy under steady shear and LAOS deformations is studied with rheo- and tOr-SANS and analyzed in the framework of an alignment factor.;Based on the single-particle properties, the range of attraction is decreased by a factor of 3 (1.01%, 1.003%, and 1.001%) from system to system as the particle size is increased (30 nm, 100 nm, and 300 nm). The gel transition temperature was found to increase with either increasing volume fraction for a specific particle size or decreasing particle size at a fixed volume fraction. The constructed gel line shows a discrepancy across different systems, which is believed to occur as a consequence of a competition between Brownian motion and gravitational settling. The critical gravitational Péclet number of ∼0.01 defines the threshold between gelation and phase separation. In addition, the tOr-SANS results reveal that the microstructures under steady shear and LAOS deformations are fundamentally different. In conclusion, findings of this research provide a deeper, quantitative understanding of gelation in adhesive hard sphere dispersions and these can aid in rational design of nanomaterials and complex fluids.