  We have studied the effect of an external DC electric field (~kV/mm) on the
rheological properties of colloidal suspensions consisting of aggregates of
laponite particles in a silicone oil. Microscopy observations show that under
application of an electric field greater than a triggering electric field
E_c~0.6 kV/mm, laponite aggregates assemble into chain- and/or column-like
structures in the oil. Without an applied electric field, the steady state
shear behavior of such suspensions is Newtonian-like. Under application of an
electric field larger than E_c, it changes dramatically as a result of the
changes in the microstructure: a significant yield stress is measured, and
under continuous shear the fluid is shear-thinning. The rheological properties,
in particular the dynamic and static shear stress, were studied as a function
of particle volume fraction, for various strengths(including null) of the
applied electric field. The flow curves under continuous shearing can be scaled
with respect to both particle fraction and electric field strength, onto a
master curve. This scaling is consistent with simple scaling arguments. The
shape of the master curve accounts for the system's complexity; it approaches a
standard Herschel-Bulkley model at high Manson numbers. Both dynamic and static
yield stress are observed to depend on the particle fraction \phi and electric
field E as \phi^\beta E^\alpha, with \alpha~1.85, and \beta~1 and 1.70, for the
dynamic and static yield stresses, respectively. The measured yield stress
behavior may be explained in terms of standard conduction models for
electrorheological systems. Interesting prospects include using such systems
for self-guided assembly of clay nano-particles.
