32 research outputs found
Thixotropy in macroscopic suspensions of spheres
An experimental study of the viscosity of a macroscopic suspension, i.e. a
suspension for which Brownian motion can be neglected, under steady shear is
presented. The suspension is prepared with a high packing fraction and is
density-matched in a Newtonian carrier fluid. The viscosity of the suspension
depends on the shear rate and the time of shearing. It is shown for the first
time that a macroscopic suspension shows thixotropic viscosity, i.e.
shear-thinning with a long relaxation time as a unique function of shear. The
relaxation times show a systematic decrease with increasing shear rate. These
relaxation times are larger when decreasing the shear rates, compared to those
observed after increasing the shear. The time scales involved are about 10000
times larger than the viscous time scale and about 1000 times smaller than the
thermodynamic time scale. The structure of the suspension at the outer cylinder
of a viscometer is monitored with a camera, showing the formation of a
hexagonal structure. The temporal decrease of the viscosity under shear
coincides with the formation of this hexagonal pattern
Interactions of distinct quadrupolar nematic colloids
The effective interaction between spherical colloids in nematic liquid
crystals is investigated in the framework of the Landau-de Gennes theory. The
colloids differ through their interaction with the nematic. While both
particles induce quadrupolar far-field distortions in the nematic matrix, with
unlike quadrupole moments, one favours homeotropic and the other degenerate
planar anchoring of the nematic director. In the strong anchoring regime the
colloids with homeotropic anchoring are accompanied by an equatorial
disclination line defect, known as "Saturn-ring", while the colloids with
degenerate planar anchoring nucleate a pair of antipodal surface defects,
called "Boojums". In the linear (large-distance) regime the colloidal
interactions are of the quadrupolar type, where the quadrupoles have opposite
signs. These are attractive when the colloids are aligned either parallel or
perpendicular to the far-field director. At short distance, non-linear effects
including "direct" interactions between defects give rise to a repulsion
between the particles, which prevents them from touching. This finding supports
the stability of nematic colloidal square crystallites the assembly of which
has been reported recently.Comment: 7 pages, 5 figure