(142dd) Probing the Microstructures of Adhesive Hard-Spheres Under Large Amplitude Oscillatory Shear (LAOS) Deformation with Time-Resolved Small-Angle Neutron Scattering (tOR-SANS)

Large amplitude oscillatory shear (LAOS) is becoming a popular method to study the nonlinear response of complex fluids and polymers. Many reports attempt to rationalize the LAOS response in terms of proposed microstructural changes. However, interpretation of the measurements is largely qualitative.  To address this, we have developed new experimental methods to directly measure the microstructure during LAOS. The yielding and flow of colloidal gels and glasses is of significant academic and industrial interest.  Here, we synthesize a model adhesive hard-spheres (AHS) system comprised of octadecyl-grafted silica in tetradecane, which thermoreversibly gels upon cooling [1]. Rheology and light scattering are used to determine the point of dynamic percolation (gelation and glass formation) as a function of particle concentration and temperature. Steady shear flow and LAOS rheological experiments are performed and the microstructure is probed simultaneously via time-resolved small-angle neutron scattering (tOR-SANS) in the 1-3 plane. With decreasing temperature the dispersion gels and the strength of the attraction is extracted by modeling the structure facture determined by SANS. Under strong shear, the systems near and in the gelled state exhibit highly anisotropic 2-D SANS patterns that show a “butterfly” structure consistent with previous reports [2, 3].  This corresponds to the gel microstructure stretching along the flow direction.  Under LAOS deformation the butterfly patterns are observed to oscillate.  Analysis of the patterns reveals how the oscillatory shear flow organizes the microstructure.  Identification of a structural order parameter enables constructing the first structural-Lissajous plots for this class of materials, which presents the microstructure as a function of the instantaneous strain and strain-rate of the shear deformation.  A detailed analysis of the third harmonic using a recently developed theory applicable for the onset of nonlinearity, the so-called medium amplitude oscillatory shear, or MAOS regime is performed. As the model system can be varied from near hard sphere behavior to a strong gel, this analysis provides insight in the relative importance of hydrodynamic and thermodynamic forces to the microstructure and rheology of colloidal dispersions. This new experimental technique and analysis method provides a new pathway to link rheology and microstructure under time-dependent deformation and provides valuable new insights into suspension micromechanics.

1.         Eberle, A.P.R., N.J. Wagner, and R. Castaneda-Priego, Dynamical arrest transition in nanoparticle dispersions with short-range interactions. Physical Review Letters, 2011. 106(10).

2.         Hoekstra, H., et al., Multi length scale analysis of the microstructure in sticky sphere dispersions during shear flow. Langmuir, 2005. 21(24): p. 11017-11025.

3.         Thareja, P., et al., Shear-induced phase separation (SIPS) with shear banding in solutions of cationic surfactant and salt. Journal of Rheology, 2011. 55(6): p. 1375-1397.