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Photoelastic techniques are used to make both qualitative and quantitative measurements of the forces within idealized granular materials. The method is based on placing a birefringent granular material between a pair of polarizing filters, so that each region of the material rotates the polarization of light according to the amount of local stress. In this review paper, we summarize the past work using the technique, describe the optics underlying the technique, and illustrate how it can be used to quantitatively determine the vector contact forces between particles in a 2D granular system. We provide a description of software resources available to perform this task, as well as key techniques and resources for building an experimental apparatus. [ABSTRACT FROM AUTHOR]

An introduction to articles published within the issue is presented on topics including imaging granular particles with electromagnetic waves, the advantages of X-ray tomography, and the measurement of particle displacements and velocities.

We investigate the dynamics of an insoluble surfactant on the surface of a thin viscous fluid spreading inward to fill a surfactant-free region. During the initial stages of surfactant self-healing, Marangoni forces drive an axisymmetric ridge inward to coalesce into a growing central distension; this is unlike outward spreading, in which the ridge only decays. In later dynamics, the distension slowly decays and the surfactant concentration equilibrates. We present results from experiments in which we simultaneously measure the surfactant concentration (using fluorescently tagged lipids) and the fluid height profile (via laser profilometry). We compare the results to simulations of a mathematical model using parameters from our experiments. For surfactant concentrations close to but below the critical monolayer concentration, we observe agreement between the height profiles in the numerical simulations and the experiment, but disagreement in the surfactant distribution. In experiments at lower concentrations, the surfactant spreading and formation of a Marangoni ridge are no longer present, and a persistent lipid-free region remains. This observation, which is not captured by the simulations, has undesirable implications for applications where uniform coverage is advantageous. Finally, we probe the generality of the effect, and find that distensions of similar size are produced independent of initial fluid thickness, size of initial clean region, and surfactant type. [ABSTRACT FROM AUTHOR]

For spatio-temporal chaos observed in numerical simulations of the complex Ginzburg–Landau equation (CGL) and in experiments on inclined-layer convection (ILC) we report numerical and experimental data on the statistics of defects and of defect loops. These loops consist of defect trajectories in space–time that are connected to each other through the pairwise annihilation or creation of the associated defects. While most such loops are small and contain only a few defects, the loop distribution functions decay only slowly with the quantities associated with the loop size, consistent with power-law behavior. For the CGL, two of the three power-law exponents are found to agree, within our computational precision, with those from previous investigations of a simple lattice model. In certain parameter regimes of the CGL and ILC, our results for the single-defect statistics show significant deviations from the previously reported findings that the defect dynamics are consistent with those of random walkers that are created with fixed probability and annihilated through random collisions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report experiments on defect-tracking in the state of undulation chaos observed in thermal convection of an inclined fluid layer. We characterize the ensemble of defect trajectories according to their velocities, relative positions, diffusion, and gain and loss rates. In particular, the defects exhibit incidents of rapid transverse motion which result in power law distributions for a number of quantitative measures. We examine connections between this behavior and Lévy flights and anomalous diffusion. In addition, we describe time-reversal and system size invariance for defect creation and annihilation rates. [ABSTRACT FROM AUTHOR]

A correction to the article "Focus on imaging methods in granular physics," by Axelle Amon and colleagues published in the July 10, 2017 online issue is presented.

The article offers information on student blogging on physics. It mentions that blogging offers several benefits to students from various major and backgrounds which include earning skills in exploring the web, assessing resources for quality, and taking into consideration what applications may offer good link to their topic. Moreover, it mentions that with the use of blogging, students can participate more in estimating quantities of their subject matter.

The article introduces a series of articles which presents the winning entries in the Gallery of Nonlinear Images competition organized by the American Physical Society in its March 2009 meeting in Pittsburgh, Pennsylvania and was sponsored by the APS Topical Group on Statistical and Nonlinear Physics.

The article offers information on a physics research carried out by James G. Puckett and Karen E. Daniels of the North Carolina State University, Raleigh, North Carolina. Under it, they generated ensembles of two-dimensional granular configurations. It was seen that dense granular materials like glasses, exhibit slowing dynamics as they approach the jamming transition from a liquid like state.

The article examines the effects of varying substrate mobility from a liquid to solid. According to the author, the Marangoni effect in a surfactant could modified the contact line dynamics since it has the ability to create a surface tension gradient that generates pattern-forming instabilities. The instability of the surfactant-laden drop influenced by both the substrate fluidity and the surfactant condition was explored. Results of the research was further discussed by the author.

Studies the process through which a meteor comes to rest and forms a crater. Visualization of the forces generated in the impact bed through the use of photoelastic particles; Usability of the bidisperse mixture in suppressing crystallization; Snapshots of force chains during a meteor collision.