Effect of friction in a toy model of granular compaction

We proposed a toy model of granular compaction which includes some resistance due to granular arches. In this model, the solid/solid friction of contacting grains is a key parameter and a slipping threshold Wc is defined. Realistic compaction behaviors have been obtained. Two regimes separated by a critical point Wc* of the slipping threshold have been emphasized : (i) a slow compaction with lots of paralyzed regions, and (ii) an inverse logarithmic dynamics with a power law scaling of grain mobility. Below the critical point Wc*, the physical properties of this frozen system become independent of Wc. Above the critical point Wc*, i.e. for low friction values, the packing properties behave as described by the classical Janssen theory for silos

Application of Minimal Subtraction Renormalization to Crossover Behavior near the 3^3He Liquid-Vapor Critical Point

Parametric expressions are used to calculate the isothermal susceptibility, specific heat, order parameter, and correlation length along the critical isochore and coexistence curve from the asymptotic region to crossover region. These expressions are based on the minimal-subtraction renormalization scheme within the $\phi^4$ model. Using two adjustable parameters in these expressions, we fit the theory globally to recently obtained experimental measurements of isothermal susceptibility and specific heat along the critical isochore and coexistence curve, and early measurements of coexistence curve and light scattering intensity along the critical isochore of $^3$He near its liquid-vapor critical point. The theory provides good agreement with these experimental measurements within the reduced temperature range $|t| \le 2\times 10^{-2}$

Review: Entropy maximization in the force network ensemble for granular solids. Authors: Brian P. Tighe, Adrianne R. T. van Eerd, and Thijs J. H. Vlugt. Phys. Rev. Lett, 100, 238001 (2008) : recommended with commentary

An ongoing debate in the soft condensed matter community concerns the distribution of contact forces, P(f), for jammed disordered solids. A jammed system has a non-zero yield stress, or alternatively, non-zero elastic coefficients. In pioneering work, Liu et al.[1] combined theory and experiments to yield the first results for P(f), which was an exponential: P(f) α exp(−f). The experiments have become a classic in the field, and are referred to as carbon-paper experiments for the use of carbon paper at the boundaries to measure the particle-wall forces

Novel Convective Instabilities Of Low- To Moderate-Prandtl Number Fluids

Superfluid mixtures have Prandtl numbers tunable between those of liquid metals and water: 0:04 ! Pr ! 2. Moreover, superfluid mixtures convect as regular fluids, i.e. classical Rayleigh-B'enard convection. With this unique Pr range and a variable aspect ratio (\Gamma) experiment we survey convective instabilities in the largely unexplored space 0:12 ! Pr ! 1:4 and 2 ! \Gamma ! 95. Among the novel behaviour found in the survey are: Instability competition greatly increases the complexity of convective states, but a heat-pulse method allows state selection. And, as \Gamma becomes large (? 44), the onset of convection changes from stationary to time-dependent. As \Gamma increases, oscillations arbitrarily close to the onset of convection arise, then give way to large-amplitude irregular fluctuations. INTRODUCTION The dynamics of Rayleigh-B'enard convection (RBC) rolls---convection in a horizontal layer of fluid subject to a destabilizing density gradient---have been one of the most v..

Landing on the Moon Cratering from a Jet

This project characterizes crater formation in a granular material by a jet of gas impinging on a granular material, such as a retro-rocket landing on the moon. We have constructed a 2D model of a planetary surface, which consists of a thin, clear box partially filled with granular materials (sand, lunar and Mars simulants...). A metal pipe connected to a tank of nitrogen gas via a solenoid valve is inserted into the top of the box to model the rocket. The results are recorded using high-speed video. We process these images and videos in order to test existing models and develop new ones for describing crater formation. A similar set-up has been used by Metzger et al.* We find that the long-time shape of the crater is consistent with a predicted catenary shape (Brandenburg). The depth and width of the crater both evolve logarithmically in time, suggesting an analogy to a description in terms of an activated process: dD/dt = A exp(-aD) (D is the crater depth, a and A constants). This model provides a useful context to understand the role of the jet speed, as characterized by the pressure used to drive the flow. The box width also plays an important role in setting the width of the crater. *P. T. Metzger et al. Journal of Aerospace Engineering (2009)ORBITE

Jamming transition in granular systems.

Recent simulations have predicted that near jamming for collections of spherical particles, there will be a discontinuous increase in the mean contact number Z at a critical volume fraction ϕc. Above ϕc, Z and the pressure P are predicted to increase as power laws in ϕ-ϕc. In experiments using photoelastic disks we corroborate a rapid increase in Z at ϕc and power-law behavior above ϕc for Z and P. Specifically we find a power-law increase as a function of ϕ-ϕc for Z-Zc with an exponent β around 0.5, and for P with an exponent ψ around 1.1. These exponents are in good agreement with simulations. We also find reasonable agreement with a recent mean-field theory for frictionless particles

Statistical properties of granular materials near jamming

This paper describes a series of experiments that probe the behavior of dense granular materials. We first establish a broad context for these studies that identifies several key properties: spatial inhomogeneity for forces represented by force networks and force chains, dilation, temporal fluctuations, and the general idea of jamming. Most of the experiments described here involve the use of photoelastic particles, and we give a discussion of some of the basic features of photoelasticity and its application to granular experiments. We then discuss experiments that probe first isotropic jamming, which occurs for a packing fraction of φ = φJ ≃ 0.84, and then shear jamming, which occurs for φ's less than φJ for frictional particles (and at least in some circumstances, for frictionless particles). Shear jamming involves force networks and stresses that are inherently anisotropic. They are not contained in the LiuNagel jamming scenario, which has been extensively studied in the context of frictionless sytems. In a third set of experiments we explore the idea that slow cyclic shear can provide an activation mechanism which is manifested in slow relaxation that appears to be consistent with a force ensemble picture. The last set of experiments involves impacts of a heavy intruder on a granular bed consisting of photoelastic particles. The impactor, whose speed is well below sonic, generates propagating force pulses along a more slowly evolving force network. This mechanism is sufficient to account for the stopping force that acts on the intruder