(5ar) Bridging the Gap: Multiscale Approaches to Integrated Colloids and Nanomaterials

Helgeson, M. E. - Presenter, Massachusetts Institute of Technology
Wagner, N. J. - Presenter, University of Delaware
Kaler, E. W. - Presenter, Stony Brook University

Next-generation soft materials involve the integration of molecular and colloidal components, such as self-assembled surfactants, polymers, and nanoparticles, whose function and behavior are governed by increasingly complex interactions. As such, they require a detailed understanding of the hierarchical nature of self-assembly, whereby molecular interactions, mesoscale structure, and macroscopic properties are intimately connected. Although these materials hold potential for breakthroughs in the fields of medicine, energy, and sustainability, control and scaleup of their formulation and processing poses a significant technological challenge. My research aims to develop a robust understanding of mixed colloidal systems for the synthesis of functional colloids and nanomaterials. An experimental toolbox that combines techniques such as microscopy, scattering, and rheology that bridge the micro- and macroscales provides a foundation for understanding and modeling colloidal assemblies and their response to applied fields and external stimuli. Furthermore, the combination of bottom-up and top-down approaches to material synthesis leads to versatile platforms for the production of functional soft materials. This presentation summarizes results from my doctoral (Department of Chemical Engineering, University of Delaware) and postdoctoral (Department of Chemical Engineering, Massachusetts Institute of Technology) research.

Controlling the structure and dynamics of wormlike micelles using colloidal particles (and vice versa)

Wormlike micelles (WLMs) have become widely used in a number of products and processes where they come in contact with colloidal species, yet relatively little is understood regarding the interactions between WLMs and colloids and resulting changes in macroscopic properties. This work demonstrates that the addition of model nanoparticles to WLMs allows for unique tunability of the structure and rheology of the resulting fluid. Interactions at the surfactant-nanoparticle interface can be tuned to promote the fusion of micelles to an adsorbed surfactant layer to form micelle-nanoparticle junctions. These junctions act as physical cross-links between micelles and impart significant viscosity and viscoelasticity to otherwise Newtonian micellar solutions. This rheological enhancement is understood by the formation of a so-called ?double network?, where the viscoelasticity can be tuned through two energetic scales, the micellar end cap energy and micelle-nanoparticle junction energy, which are readily manipulated by adjusting solution conditions.

Conversely, the formation of micelle-nanoparticle junctions gives rise to new colloidal interactions between nanoparticles which can be tuned through the self-assembled micellar structure, leading to thermoreversible phase separation of the colloid. A statistical mechanical model has been developed to describe these interactions in terms of experimentally measurable properties. This model allows for a priori predictions of the interaction potential between colloids dispersed in WLMs, and shows quantitative agreement with independent scattering measurements and phase behavior.

Shear-induced phase transitions in surfactant liquids

Because the structure of complex fluids typically depends on a delicate balance of forces, applied flow fields such as simple shear can result in significant changes in microstructure and phase behavior. Shear banding is a particularly prominent flow instability in complex fluids, including WLMs. Here, model shear banding WLMs near an equilibrium isotropic-nematic (I-N) transition are studied using a combination of spatially-resolved rheological and scattering measurements to determine the underlying mechanism of banding and its relation to equilibrium surfactant phase behavior. The results reveal that shear banding is due to a first order, shear-induced I-N transition upon a critical degree of micellar flow-alignment. Constitutive modeling allows for combination of rheological and microstructural data to produce a non-equilibrium phase diagram which explains both the microstructural underpinnings as well as the macroscopic flow kinematics and stability during shear banding.

One-step platforms for the synthesis of polymeric colloids and nanomaterials

The assembly of polymer and composite materials is typically performed by one of two methods: bottom-up approaches using molecular self-assembly suffer from low yields and slow kinetics, whereas top-down approaches lack precise control of final material properties. Here, two methods are investigated that combine both approaches resulting in versatile platforms for the synthesis of soft materials. The first is electrospinning, which has gained significant attention for the production of polymeric particles and nanofibers. Fundamental studies and modeling of electrospinning jets yield simplified theories for jet kinematics and rheology, ultimately resulting in correlations for the final fiber morphology in terms of measurable fluid properties and controlled process parameters. These results are used to guide the synthesis of novel nanomaterials using electrospinning, including polymer-nanoparticle composite fibers for functional membranes and textiles, and reversibly-swelling surfactant polyelectrolyte coacervate nanoparticles for encapsulation and release.

Stop flow lithography (SFL) also provides a facile route to functional colloidal materials with arbitrary shape, whereby the conditions and substituents of photopolymerization are finely controlled using microfluidic devices. As such, SFL provides an inexpensive, customizable platform for the continuous manufacture of pharmaceuticals with direct incorporation of APIs and other biomaterials. Of particular interest to this work is the synthesis of functional hydrogels with hierarchical topologies for the creation of ?patchy? and multi-compartment particles for sophisticated degredation and release profiles of encapsulated materials.