Colloids dispersed in a nonpolar solvent become charged when reverse micelles are added. We study the charge of individual sterically stabilized poly(methyl methacrylate) spheres dispersed in micellar solutions of the surfactants sodium bis(2-ethyl 1-hexyl) sulfosuccinate [AOT], zirconyl 2-ethyl hexanoate [Zr(Oct)2], and a copolymer of poly(12-hydroxystearic acid)-poly(methyl methacrylate) [PHSA-PMMA]. Although the sign of the particle charge is positive for Zr(Oct)2, negative for AOT, and essentially neutral for PHSA-PMMA, the different micellar systems display a number of common features. In particular, we demonstrate that over a wide range of concentrations the particle potential is a constant, independent of the number of micelles added and independent of the colloid size. A simple thermodynamic model, in which the particle charge is generated by the competitive adsorption of both positive and negative micelles, is in good agreement with the experimental data.

Colloidal particles immersed in liquid crystals frustrate orientational order. This generates defect lines known as disclinations. At the core of these defects, the orientational order drops sharply. We have discovered a class of soft solids, with shear moduli up to 10(4) pascals, containing high concentrations of colloidal particles (volume fraction φ ≳ 20%) directly dispersed into a nematic liquid crystal. Confocal microscopy and computer simulations show that the mechanical strength derives from a percolated network of defect lines entangled with the particles in three dimensions. Such a "self-quenched glass" of defect lines and particles can be considered a self-organized analog of the "vortex glass" state in type II superconductors.

The authors develop an ultrasensitive method for the measurement of the charge carried by a colloidal particle in a nonpolar suspension. The technique uses the phenomenon of the resonance of a particle held in an optical tweezer trap and driven by a sinusoidal electric field. The trapped particle forms a strongly damped harmonic oscillator whose fluctuations are a function of gamma, the ratio of the root-mean-square average of the electric and thermal forces on the particle. At low applied fields (gamma<<1) the particle is confined to the optical axis, while at high fields (gamma>>1) the probability distribution of the particle is double peaked. The periodically modulated thermal fluctuations are measured with nanometer sensitivity using an interferometric position detector. Charges, as low as a few elementary charges, can be measured with an uncertainty of about 0.25 e. This is significantly better than previous techniques and opens up new possibilities for the study of nonpolar suspensions.

We study the thermal fluctuations of an optically confined probe particle, suspended in an aging colloidal suspension, as the suspension transforms from a viscous liquid into an elastic glass. The micron-sized bead forms a harmonic oscillator. By monitoring the equal-time fluctuations of the tracer, at two different laser powers, we determine the temperature of the oscillator, To. In the ergodic liquid the temperatures of the oscillator and its environment are equal while, in contrast, in a nonequilibrium glassy phase we find that To substantially exceeds the bath temperature.Understanding the slow dynamics of glasses is one of the most fascinating yet difficult challenges in statistical physics. One question which has attracted considerable interest is whether the dynamical fluctuations of a glass can be characterized by a non-equilibrium temperature. A rigorous thermodynamic temperature is strictly impossible to define for an aging glass, which remains far from thermal equilibrium even on long-time scales. Nevertheless various groups [1,2] have proposed "effective" temperatures with many of the properties expected for an equilibrium temperature. These ideas lead to the surprising prediction of two distinct temperatures in an aging glass [1]. The fast rattling modes of particles inside the cage constituted by neighbors thermalize rapidly to the temperature of the environment, T bath , while the much slower structural rearrangement of these cages are supposed to be characterized by a second temperature, T eff , which mean-field models predict should exceed T bath . To date most of the support for this striking two-temperature picture has emerged from simulation results [2] on idealized glasses. Experiments have so far produced conflicting results. Studies of colloidal glasses have reported that T eff increases [3], remains unchanged [4], or even decreases [5] with age in contrast to measurements on structural [6] and spin glasses [7] which have revealed effective temperature warmer than the bath temperature.In this Letter, we report the temperature of a micrometer-sized sphere immersed in an aging colloidal suspension, as the suspension transforms from a fluid to a glass. The particle, captured in an optical trap, constitutes a microscopic harmonic oscillator whose fluctuations probe the nonequilibrium dynamics of the aging glass. We measure the equal-time fluctuations of this local oscillator and show that, in an ergodic phase, the temperature of the oscillator T o equals the environment temperature T bath of the system, as required by equilibrium statistical mechanics. Significantly, when we repeat the measurements in an aging glass we find a higher temperature and T o > T bath .All experiments to date on colloidal glasses [3,4,5] have relied on active, driven measurements. We conduct our experiments instead in a quasi-static limit which has several advantages. First, the experiments are simpler because there is no need to characterize the complete time-dependent response of the system. Second, the lack of an ...