Understanding Rheology and Microstructure of Thermoresponsive Nanoemulsions as a Model System of Colloidal Suspensions
Nanoemulsions – nanoscale droplet suspensions – in polymeric solution have been widely used in many fields from consumer products to advanced technology. Due to their nanoscopic dimensions, they exhibit unique physical behavior in terms of their rheology and microstructure, both at rest and under flow. However, mechanisms that give rise the interesting phenomena observed in nanodroplet-polymer mixtures are poorly understood. Thus, the goals of this thesis are to develop a model nanoemulsion-polymer system to impart control over polymer-droplet and droplet-droplet interactions, use it to investigate the mechanisms colloidal behavior of nanodroplets in polymeric solutions, and to determine how these behaviors influence suspension microstructure, dynamics and rheology.
For these purposes, we choose to study thermoresponsive oil-in-water nanoemulsions as a model system. The systems include polydimethylsiloxane (PDMS) oil nanodroplets dispersed in aqueous mixtures of functionalized hydrophilic polymer (polyethylene glycol diacrylate, PEGDA) and ionic surfactant (sodium dodecyl sulfate, SDS). Combining calorimetry, rheology and scattering measurements, we show how molecular self-assembly in the system can be used to control the viscoelasticity of the system, which remarkably follows time-temperature superposition, much like simpler polymer fluids, and suggests that polymer-surfactant complexation forms a transient polymer network between droplets. Systematic changes in the energy scale for complexation allows us to develop a simple model for the modulus and viscoelastic relaxation time. Furthermore, the relaxation time of the network in this model system can be varied by ten orders of magnitudes, providing advantages for fundamental studies.
We exploited the properties of this novel nanoemulsion system to study shear-induced clustering of colloid-polymer mixtures and its impact on fluid rheology. The system allows us to explore several limiting regimes of polymer and suspension dynamics. Combining rheological characterization with three-dimensionally-resolved flow-small angle neutron scattering measurements reveals that an excess of particle fluxes along compressional and vorticity axes of shear are the primary mechanism of clustering, and suggestes that short-range hydrodynamic forces dominate the clustering of Brownian suspensions in viscoelastic fluids.
Lastly, we used the thermoresponsive nanoemulsions to investigate the mechanisms of so-called “two-step” or “delayed” yielding in heterogeneous colloidal gels. At high temperatures, the model system forms colloidal gels with heterogeneous microstructure resembling arrested phase separation at elevated temperature caused by polymer-bridging interactions. Analyzing the sequence of mechanical processes during the intracycle yielding processes elucidates the detailed mechanism of frequency and strain amplitude-dependence of nonlinearlity. The nonlinear analysis also allows for characterization of the strain amplitude and rate-dependent yield stress and strain of the material. Furthermore, combining large amplitude oscillatory shear measuremetns with simultaneous small and ultra-small angle neutron scattering reveals that, contrary to previous hypotheses, large-scale microstructural processes play an important, if not dominant, role in the yielding of heterogeneous colloidal gels.
The results of this thesis demonstrate that the rheology and the microstructural processes of nanoemulsions can be controlled through thermoresponsive polymer-surfactant-droplet association, which provides for sophisticated tuning of polymer-colloid and colloid-colloid interactions and dynamics, thus providing new routes and design rules for engineering the colloidal behavior of nanoemulsions.