Dynamical heterogeneity in soft particle suspensions under shear

We present experimental measurements of dynamical heterogeneities in a dense system of microgel spheres, sheared at different rates and at different packing fractions in a microfluidic channel, and visualized with high speed digital video microscopy. A four-point dynamic susceptibility is deduced from video correlations, and is found to exhibit a peak that grows in height and shifts to longer times as the jamming transition is approached from two different directions. In particular, the time for particle-size root-mean square relative displacements is found to scale as $\tau^* \sim (\dot \gamma \Delta \phi^4)^{-1}$ where $\dot\gamma$ is the strain rate and $\Delta\phi=|\phi-\phi_c|$ is the distance from the random close packing volume fraction. The typical number of particles in a dynamical heterogeneity is deduced from the susceptibility peak height and found to scale as $n^* \sim (\dot \gamma \Delta \phi^4)^{-0.3}$. Exponent uncertainties are less than ten percent. We emphasize that the same power-law behavior is found at packing fractions above and below $\phi_c$. Thus, our results considerably extend a previous observation of $n^* \sim \dot\gamma^{-0.3}$ for granular heap flow at fixed packing below $\phi_c$. Furthermore, the implied result $n^*\sim (\tau^*)^{0.3}$ compares well with expectation from mode-coupling theory and with prior observations for driven granular systems

Microfluidic rheology of soft colloids above and below jamming

The rheology near jamming of a suspension of soft colloidal spheres is studied using a custom microfluidic rheometer that provides stress versus strain rate over many decades. We find non-Newtonian behavior below the jamming concentration and yield stress behavior above it. The data may be collapsed onto two branches with critical scaling exponents that agree with expectations based on Hertzian contacts and viscous drag. These results support the conclusion that jamming is similar to a critical phase transition, but with interaction-dependent exponents.Comment: 4 pages, experimen

Comparing Flow Thresholds and Dynamics for Oscillating and Inclined Granular Layers

The onset and dynamics of flow in shallow horizontally oscillating granular layers are studied as a function of the depth of the layer and imposed acceleration. Measurements of the flow velocity made from the top and side are presented in the frame of reference of the container. As is also found for avalanches of inclined layers, the thresholds for starting and stopping of flow are slightly different. The variation with depth of the starting acceleration Γstart for the oscillating layer is similar to the corresponding variation of the tangent of the starting angle tan(Θstart) for avalanches in the same container at low frequencies, but deviates as the frequency is increased. However, the threshold behavior depends significantly on the measurement protocol. Just above Γstart, the motion decays with time as the material reorganizes over a minute or so, causing the apparent threshold to increase. Furthermore, the rms velocity as a function of acceleration rises more sharply above the starting threshold if the first minute or so of excitation is discarded. Once excited, the rheology of the material is found to vary in time during the cycle in surprising ways. If the maximum inertial force (proportional to the container acceleration amplitude) is slightly higher than that required to produce flow, the flow velocity grows as soon as the inertial force exceeds zero in each cycle, but jamming occurs long before the inertial force returns to zero. At higher Γ, the motion is fluidlike over the entire cycle. However, the fraction of the cycle during which the layer is mobile is typically far higher than what one would predict from static considerations or the behavior of the inclined layer. Finally, we consider the flow profiles as a function of both the transverse distance across the cell at the free surface and also as a function of the vertical coordinate in the boundary layer near the sidewall. These profiles have time-dependent shapes and are therefore significantly different from profiles previously measured for avalanche flows

Fluctuations and transport in a stirred fluid with a mean gradient

The effective thermal diffusivity D* and the probability distribution of temperature fluctuations are measured in a stirred fluid across which a temperature gradient is maintained. A distinct mixing transition is observed for D* as a function of Reynolds number R. Above the transitions, the distribution is strongly non-Gaussian and approaches an exponential exp(-‖δT‖/βξ), where β is the local temperature gradient and ξ the correlation length

Measurement of photon sorting at microwave frequencies in a cavity array metasurface

PublishedJournal ArticleWe present experimental results demonstrating the spatial sorting of incoming radiation in two spectral ranges. A metasurface composed of a periodically patterned metal of subwavelength thickness with dielectric inclusions concentrates and localizes electromagnetic fields near the surface. Light of the separate spectral bands is channeled into different geometrically tuned cavities within each spatially repeating unit cell. Excitation of cavity modes facilitates this simultaneous spatial- and spectral-selective absorption. The measured reflection and field profiles are presented and the spectral and spatial selectivity are shown. A method to apply these concepts to split radiation into three spectral bands is also proposed.This work was supported in part by the AFOSR Bioenergy project (FA9550-10-1-0350), in part by the NSF Industry/University Cooperative Research Center for Metamaterials (IIP-1068028), and in part by the EPSRC, U.K. funding through the QUEST project (ref: EP/I034548/1)