This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or "nanofluids," was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band ͑Ϯ10% or less͒ about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are ͑small͒ systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. ͓J. Appl. Phys. 81, 6692 ͑1997͔͒, was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Neutron scattering experiments have revealed a fascinating interplay between the hole doping, the spin fluctuations, and the superconductivity of the cuprate superconductors. Recently, electrochemical techniques have been used to produce large single crystals of La 2 CuO 4+ y , which has mobile oxygen dopants. Staging behavior of the excess oxygen has been demonstrated, and the low-energy spin fluctuations in stage 6 La 2 CuO 4+ y have been measured. The spin fluctuations are incommensurate with the lattice and have spatial, energy, and temperature dependencies very much like those in La 2− x Sr x CuO 4 , with similar high transition temperature. This establishes the universality of the incommensurate spin fluctuations among the La 2 CuO 4 -based superconductors.

We study phase separation in a deeply quenched colloid-polymer mixture in microgravity on the International Space Station using small-angle light scattering and direct imaging. We observe a clear crossover from early-stage spinodal decomposition to late-stage, interfacial-tension-driven coarsening. Data acquired over 5 orders of magnitude in time show more than 3 orders of magnitude increase in domain size, following nearly the same evolution as that in binary liquid mixtures. The late-stage growth approaches the expected linear growth rate quite slowly.

The low-temperature magnetic excitations of the two-dimensional spin-5/2 square-lattice Heisenberg antiferromagnet Rb2MnF4 have been probed using pulsed inelastic neutron scattering. In addition to dominant sharp peaks identified with one-magnon excitations, a relatively weak continuum scattering is also observed at higher energies. This is attributed to neutron scattering by pairs of magnons and the observed intensities are consistent with predictions of spin wave theory.

Colloidal silica gels are shown to stiffen with time, as demonstrated by both dynamic light scattering and bulk rheological measurements. Their elastic moduli increase as a power law with time, independent of particle volume fraction; however, static light scattering indicates that there are no large-scale structural changes. We propose that increases in local elasticity arising from bonding between neighboring colloidal particles can account for the strengthening of the network, while preserving network structure. DOI: 10.1103/PhysRevLett.95.048302 PACS numbers: 82.70.Gg, 61.43.Hv, 62.20.Dc, 82.70.Dd Gels are dilute connected networks which are capable of supporting applied stresses; they are commonly used to control the rheological properties of complex materials. Such networks can be formed by the aggregation of colloidal particles, which occurs when an attractive interaction is induced between them. Network elasticity is highly sensitive to the connectivity and arrangement of particles in the constituent aggregates. Colloidal gels are out-ofequilibrium systems; as a result, these networks frequently display time-dependent properties, due to network restructuring. This kind of aging is modeled for generic nonequilibrium systems as an evolution toward lower energy states, as the system explores a complex energy landscape [1,2]. However, network elasticity is also dependent on the interactions between particles; therefore, time-dependent interactions may also lead to changes in network properties. For colloidal gels, it is generally assumed that interparticle attractions, such as those described by Derujaguin-LandauVerwey-Overbeek [3,4] or Asakura-Oosawa potentials [5], determine local bond elasticity, thus, in principle, limiting their strength [6,7]. In the absence of a steric stabilization layer, particle interactions can also arise from physical bonds, as a result of covalent bonding or polymer entanglements. Within this framework, time evolution of network properties can be linked to the interparticle potential, for example, through the transport of particles from secondary to primary minima [8]. Aging can also be a consequence of time-dependent physical bonding; for example, the sintering of aggregated polystyrene particles is thought to drive local shrinkage that deforms the network [9]. The strength of the local interparticle bonds will have a direct impact on the network elasticity; however, these critical effects have never been explored.In this Letter, we present measurements of the elasticity of colloidal silica gels. Surprisingly, we find that, upon gelation, their storage moduli G 0 increase as a power law in time, G 0 t 0:4 , independent of initial volume fraction . Moreover, the time evolution of the network persists long after gelation occurs, for the duration of the measurement. As a consequence, their elasticity maintains the same volume fraction dependence, G 0 3:6 , independent of time. We postulate that the stiffening of the network is a result of chemical reactions at the junctions be...

We show that the dynamics of large fractal colloid aggregates are well described by a combination of translational and rotational diffusion and internal elastic fluctuations, allowing both the aggregate size and internal elasticity to be determined by dynamic light scattering. The comparison of results obtained in microgravity and on Earth demonstrates that cluster growth is limited by gravity-induced restructuring. In the absence of gravity, thermal fluctuations ultimately inhibit fractal growth and set the fundamental limitation to the lowest volume fraction which will gel.

We report a neutron-scattering study of the dynamic spin correlations in Rb 2 MnF 4 , a two-dimensional spin-5/2 antiferromagnet. By tuning an external magnetic field to the value for the spin-flop line, we reduce the effective spin anisotropy to essentially zero, thereby obtaining a nearly ideal two-dimensional isotropic antiferromagnet. From the shape of the quasielastic peak as a function of temperature, we demonstrate dynamic scaling for this system and find a value for the dynamical exponent z. We compare these results to theoretical predictions for the dynamic behavior of the two-dimensional Heisenberg model, in which deviations from z ϭ1 provide a measure of the corrections to scaling.Recently, interest in the two-dimensional ͑2D͒ squarelattice Heisenberg antiferromagnet has intensified due in large part to the discovery of high-temperature superconductivity in the doped lamellar cuprates and the subsequent realization of the near-ideal 2D Heisenberg nature of their parent compounds. 1 The nearest-neighbor Heisenberg model is defined as:where J is the nearest-neighbor coupling which is positive for an antiferromagnet. Classically, S i is a three component vector of magnitude ͱS(Sϩ1) representing the spin at site i,

Our novel device acoustophoretically transfers cells from culture media to electroporation media and then electroporates them using integrated electrodes.