(242a) Measuring and Modeling Interactions between Orientable Nanoparticles in Flow

Flow processing of elongated particle dispersions produces a fluid with anisotropic material properties due to the coupling of particle alignment to the flow field. In non-dilute dispersions, the interplay between particle alignment, interparticle interactions and the flow field significantly changes the spatial structure (i.e., the particles’ center of mass spatial distribution) of such dispersions, and this flow-modification is critical for describing anisotropic material properties including rheology and dielectric properties. Previously developed rheological theories make a number of different assumptions and treatments for interparticle interactions in flow, highlighting a need for new experiments and models to isolate and characterize the effects of flow on interparticle alignment and center-of-mass structure. To fill this gap, we report measurements of the structure of rod-like colloidal cellulose nanocrystal (CNC) dispersions from in situ flow small angle neutron scattering (flow-SANS) measurements across a range of flow types (including shear, extension and mixed flows) and deformation rates (characterized by the dimensionless rate, the Weissenberg number). By employing a recently developed method to independently infer particle orientation probability distribution functions (OPDFs) [1], we isolate the contributions to the OPDF from single-particle and pair-orientations and, in doing so, the anisotropic static structure factor S(q) that captures interparticle interactions in flow. The resulting measurements are used to test assumptions about the flow-induced structure that are typically made in theories for interacting particle dispersions. In general, we find that S(q) for all flow types is accurately described by a generalized anisotropic form of the random phase approximation with a scalar, isotropic interaction parameter at sufficiently low Weissenberg numbers and concentrations suggesting that, in this regime, flow-induced structuring of the fluid is dominated by thermodynamic interactions between suspended particles. At elevated shear rate and concentration, the apparent interaction parameter becomes rate-dependent, suggesting a transition to a regime where hydrodynamic interactions become important for determining interparticle structure. The results provide a simple modeling framework for describing the effects of flow on interparticle structure using the OPDF, and provide critical insights for testing and developing models for the dynamics of elongated particle dispersions that incorporate interparticle interactions.

[1] P.T. Corona, K.S. Silmore, C. Lang, P. Lettinga, J.W. Swan, L.G. Leal and M.E. Helgeson, Physical Review Materials, 2021, 5(6): 065601.