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We report measurements of the frequency-dependent shear moduli of aging colloidal systems that evolve from a purely low-viscosity liquid to a predominantly elastic glass or gel. Using microrheology, we measure the local complex shear modulus G*(ω) over a very wide range of frequencies (from 1 Hz to 100 kHz). The combined use of one- and two-particle microrheology allows us to differentiate between colloidal glasses and gels—the glass is homogenous, whereas the colloidal gel shows a considerable degree of heterogeneity on length scales larger than 0.5 μm. Despite this characteristic difference, both systems exhibit similar rheological behaviors which evolve in time with aging, showing a crossover from a single-power-law frequency dependence of the viscoelastic modulus to a sum of two power laws. The crossover occurs at a time t0, w...
Using x-ray diffraction from microfluidic channel arrays, we have determined concentration profiles of charge-stabilized silica colloids (radius 60±2 nm) confined between two like-charged dielectric walls at a few hundred nanometer distance. In solutions of very low ionic strength, strongly repulsive Coulomb interactions drive the colloids toward the central region between the walls. The addition of a small quantity of salt ions (0.2 mM) causes a dense colloidal monolayer to be trapped near the walls.
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