Developing thermal processing strategies for colloidal gels

Matthew Helgeson, Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA, USA
Juntae Kim, Chemical Engineering, University Of California, Santa Barbara, Santa Barbara, Ca, Usa
Yongxiang Gao, Chemical Engineering, University Of California, Santa Barbara, Santa Barbara, Ca, Usa
Tuan Nguyen, Chemical Engineering, University Of California, Santa Barbara, Santa Barbara, Ca, Usa

The use of thermal processing to control phase separation in atomic and molecular solids is a conserved motif for designing mesostructured materials with tailored mechanical properties that spans antiquity, modern technology and the natural world. However, developing such processing strategies for colloidal solids poses significant challenges, including relatively slow dynamics and a lack of scalable material systems in which colloidal interactions can be actively tuned and quenched. To overcome these challenges and demonstrate thermal processing of colloidal solids, we have developed a model system based on nanoemulsions containing thermoresponsive polymers that allows for fine control over their interdroplet attractions and resulting colloidal gelation and phase separation. Using a combination of microscopy, rheology and neutron scattering methods, we show how this thermoreversible behavior allows access to a range of gelation mechanisms including percolation, phase separation and glass formation, whose outcome can be selected through the thermal path to the gelled state. In cases involving arrested phase separation, we identify the mechanisms of coarsening and arrest, which enable control over the arrested length scale of phase separation. Ultimately, we show how the sophisticated control of gelation and arrested structure in these thermoreversible colloids can be used to tailor the linear and nonlinear rheological properties of colloidal gels.