A series of viscosimetric and small-angle neutron scattering
experiments on asphaltenes diluted in
mixed toluene/heptane solvents has been conducted, with the purpose of
characterizing the size, molecular
weight, and internal structure of asphaltene aggregates as a function
of solvent conditions. With increasing
flocculant (i.e., heptane) content in the solvent, the intrinsic
viscosities of asphaltene aggregates first
decreased, went through a minimum for heptane fractions ≈ 10−20%,
and then increased at the approach
of flocculation. These variations paralleled those of the volume
of aggregate occupied per unit mass of
asphaltene, a behavior reminiscent of the Flory−Fox relationship for
polymers in a solvent. This volume,
proportional to the cubed radius of gyration of the aggregates divided
by their molecular weight, was
determined from the neutron scattering data. For increasing
heptane fractions in the solvent, the molecular
weight of the aggregates increased with their radius of gyration
according to a power law, the exponent
being in the range of 2. This exponent also characterized the
self-similar internal structure of the asphaltene
aggregates. With due care to the possible systematic effects of
the strong polydispersity of these aggregates,
these results are discussed in light of recent models of colloidal
aggregation.
We present experiments on slow granular flows in a modified (split-bottomed) Couette geometry in which wide and tunable shear zones are created away from the sidewalls. For increasing layer heights, the zones grow wider (apparently without bound) and evolve towards the inner cylinder according to a simple, particle-independent scaling law. After rescaling, the velocity profiles across the zones fall onto a universal master curve given by an error function. We study the shear zones also inside the material as a function of both their local height and the total layer height.
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