37 research outputs found
Irreversible Aging Dynamics and Generic Phase Behavior of Aqueous Suspensions of Laponite
In this work we study the aging behavior of aqueous suspension of Laponite
having 2.8 weight % concentration using rheological tools. At various salt
concentration all the samples demonstrate orientational order when observed
using crossed polarizers. In rheological experiments we observe inherent
irreversibility in the aging dynamics which forces the system not to rejuvenate
to the same state in the shear melting experiment carried out at a later date
since preparation. The extensive rheological experiments carried out as a
function of time elapsed since preparation demonstrate the self similar trend
in the aging behavior irrespective of the concentration of salt. We observe
that the exploration of the low energy states as a function of aging time is
only kinetically affected by the presence of salt. We estimate that the energy
barrier to attain the low energy states decreases linearly with increase in the
concentration of salt. The observed superposition of all the elapsed time and
the salt concentration dependent data suggests that the aging that occurs in
low salt concentration systems over a very long period is qualitatively similar
to the aging behavior observed in systems with high salt concentration over a
shorter period.Comment: 27 pages, 8 figures. Langmuir, in pres
Magnetorheology in an aging, yield stress matrix fluid
Field-induced static and dynamic yield stresses are explored for magnetorheological (MR) suspensions in an aging, yield stress matrix fluid composed of an aqueous dispersion of Laponite® clay. Using a custom-built magnetorheometry fixture, the MR response is studied for magnetic field strengths up to 1 T and magnetic particle concentrations up to 30 v%. The yield stress of the matrix fluid, which serves to inhibit sedimentation of dispersed carbonyl iron magnetic microparticles, is found to have a negligible effect on the field-induced static yield stress for sufficient applied fields, and good agreement is observed between field-induced static and dynamic yield stresses for all but the lowest field strengths and particle concentrations. These results, which generally imply a dominance of inter-particle dipolar interactions over the matrix fluid yield stress, are analyzed by considering a dimensionless magnetic yield parameter that quantifies the balance of stresses on particles. By characterizing the applied magnetic field in terms of the average particle magnetization, a rheological master curve is generated for the field-induced static yield stress that indicates a concentration–magnetization superposition. The results presented herein will provide guidance to formulators of MR fluids and designers of MR devices who require a field-induced static yield stress and a dispersion that is essentially indefinitely stable to sedimentation.Petroleum Research Fund (ACS-PRF Grant No. 49956-ND9)American Chemical Society (ACS-PRF Grant No. 49956-ND9
Structure of gels and aggregates of disk-like colloids
The static structure factor () of dispersions and gels of disk-like
mineral colloids (Laponite) was investigated using time- and ensemble-averaged
light scattering. The evolution of in time after increasing
the ionic strength of well-dispersed Laponite suspensions shows that
Laponite aggregates and forms fractal clusters. The structure of the
aggregates does not depend on the ionic strength, but the rate of growth
increases very strongly with the ionic strength. At concentrations below
about 3 g/l (0.12% v/v) the aggregates sediment while at higher
concentrations space-filling gels are formed. The gels are homogeneous on
length scales larger than the correlation length which decreases strongly
with decreasing ionic strength and increasing concentration. However, the
local structure is the same, independent of the concentration and the ionic
strength
Regional and Local Sea Level variations
Sea Level Change and Coastal Processes: implications of Europe , Energy, Enviro