The rheological properties of non-Brownian carbon nanotube suspensions are measured over a range of nanotube volume fractions spanning the transition from semidilute to concentrated. The polymer-stabilized nanotubes are "sticky" and form a quiescent elastic network with a well-defined shear modulus and yield stress that both depend strongly on nanotube volume fraction with different but related critical exponents. We compare controlled-strain-rate and controlled-stress measurements of yielding in shear flow, and we study the effect of slow periodic stress reversal on yielding and the arrest of flow. Our measurements support a universal scaling of both the linear viscoelastic and steady-shear viscometric response. The former allows us to extract the elastic shear modulus of semidilute nanotube networks for values that are near or below the resolution limit of the rheometers used, while the latter provides a similar extrapolation of the yield stress. A simple scaling argument is used to model the dependence of yield stress and elastic modulus on concentration.

We report a universal phase diagram describing the evolution from solidlike networks to flowing nematics for "sticky" nanotube suspensions under an applied shear stress. Although the nanotubes are strongly non-Brownian, we find features characteristic of first-order phase transitions, including a discontinuity in the nematic order parameter at the isotropic-(para)nematic phase boundary. Using simple physical arguments, we account for the shape of the coexistence curves, as well as the dependence of the order parameter on concentration and stress.

We report the observation of a novel phase of soot in an acetylene/air laminar diffusion flame. As was measured by a small-angle light scattering apparatus, the phase consists of about 10-μm superaggregates with a fractal dimension of D ≃ 2.6. This phase coexists with submicrometer D ≃ 1.8 soot fractal aggregates typical for most flames. Both percolation and shear-induced restructuring are considered as possible mechanisms of formation for those superaggregates.

Aggregation kinetics and cluster-size distributions are studied with off-lattice, diffusion-limited cluster-cluster simulations. With increased cluster crowding (occurring at late times) as measured by the normalized free volume, Omega, both the kinetics speeds up and the size distribution broadens. The exponents characterizing each, z and lambda, respectively, are found to be universal functions of Omega. Moreover, the relation z=(1-lambda)(-1) continues to hold up to omega=0 (the ideal gel point), implying mean-field kinetics still applies despite the crowding.

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