Rheology, microstructure, and thermodynamics of thermoreversible gels with tunable interactions
Colloidal dispersions can undergo an abrupt phase transition between solid and liquid when either the volume fraction or interparticle potential is varied. Aside from fractals at sufficiently low concentrations and attractive- or repulsive-driven glass at high concentrations, the particles may percolate to form a network that spans the whole macroscopic system in the intermediate concentrations, where such behavior is called gelation. Gels possess many academic and industrial interests as it can drastically alter material properties of many complex fluids; however, no consensus has been reached on the mechanism of gelation.
To extend our understanding on the mechanism of gelation and to establish the gel line on the state diagram, we have synthesized a model thermoreversible gel system by chemically grafting a silica core with octadecyl (C18) hydrocarbon chains on the surface. Suspending the particles in a poor solvent such as n-tetradecane facilitates gelation upon lowering the temperatures. We have varied the core size while keeping the brush layer thickness constant (i.e. varying the relative range of attraction) to elucidate the effect of particle size on the dynamical arrest of the system. To answer this question, we rheologically characterize the gel temperatures, which are corroborated with light/neutron scattering experiments. Also, the static structure of the system is directly measured via small angle neutron scattering (SANS) and SANS intensity is fitted to extract the stickiness parameter. For intensity fitting, we use a thermodynamically self-consistent scheme with double-Yukawa potential model, which is to be compared against the force curve via atomic force microscope. We look forward to explaining the mechanism of aggregation and dynamical arrest of colloidal particles in dispersions at the completion of this project.