Mechanical Hysteresis of Fumed Silica Dispersions

Abstract

Soft materials often exhibit mechanical properties that depend on the past deformation history the material has undergone. Such mechanical hysteresis can arise from the material's composition and structure, which can rearrange and / or yield under deformation. Colloidal dispersions of solid, usually spherical particles, in a continuous fluid medium, have been explored widely over past decades; by contrast, the behavior of dispersions of branched nanoparticle aggregates, such as transparent fumed silica (used to thicken fluids and reinforce rubber tires) and opaque carbon black, have received comparatively less attention, in spite of their significant practical importance. Nevertheless, the highly non-spherical structure of these aggregates increases substantially the mechanical hysteresis of the dispersions that incorporate them. Therefore, to investigate the structural origins of mechanical hysteresis in branched nanoparticle aggregates, I created a model system comprising transparent dispersions of fumed silica, dispersed in either liquid PDMS or mineral oil, which match the particles' refractive index. I explored their rheological behavior with a strain-controlled rheometer as a function of silica volume fraction, oscillatory strain amplitude, and oscillation frequency. I found that repeated oscillatory shear deformation at low strain amplitudes in the linear response regime leads to a gradual increase in the plateau storage modulus, whereas deformations at higher strain amplitudes in the nonlinear response regime cause a pronounced decrease in the plateau storage modulus, as well as a local dip in the loss modulus near the strain of repeated deformation: a ``strain hole." I measured the stress as a function of strain and find that the cycle-to-cycle evolution depends on the maximum strain amplitude imposed on the samples, with the most striking changes occurring in the first cycle of deformation for strain amplitudes in the nonlinear response regime. These hallmarks persist across a range of silica volume fractions and likely originate from the heterogeneous breakup of the connected particle network. Finally, I characterized the 3D structure of these fumed silica dispersions with confocal fluorescence microscopy, which enabled direct visualization of nanopartlcle aggregates within these systems.