We present an experimental study of the mechanical impulse propagation through a horizontal alignment of elastic spheres of progressively decreasing diameter phi(n): namely, a tapered chain. Experimentally, the diameters of spheres which interact via the Hertz potential are selected to keep as close as possible to an exponential decrease, phi(n+1) = (1-q)phi(n), where the experimental tapering factor is either q(1) approximately equal to 5.60% or q(2) approximately equal to 8.27%. In agreement with recent numerical results, an impulse initiated in a monodisperse chain (a chain of identical beads) propagates without shape changes and progressively transfers its energy and momentum to a propagating tail when it further travels in a tapered chain. As a result, the front pulse of this wave decreases in amplitude and accelerates. Both effects are satisfactorily described by the hard-sphere approximation, and basically, the shock mitigation is due to partial transmissions, from one bead to the next, of momentum and energy of the front pulse. In addition when small dissipation is included, better agreement with experiments is found. A close analysis of the loading part of the experimental pulses demonstrates that the front wave adopts a self-similar solution as it propagates in the tapered chain. Finally, our results corroborate the capability of these chains to thermalize propagating impulses and thereby act as shock absorbing devices.

Pressure-imposed rheometry is used to examine the influence of surface roughness on the rheology of immersed and dry frictional spheres in the dense regime. The quasi-static value of the effective friction coefficient is not significantly affected by particle roughness while the critical volume fraction at jamming decreases with increasing roughness. These values are found to be similar in immersed and dry conditions. Rescaling the volume fraction by the maximum volume fraction leads to collapses of rheological data on master curves. The asymptotic behaviors are examined close to the jamming transition. arXiv:1904.12633v1 [cond-mat.soft]

We investigate the dynamical response of a mass defect in a one-dimensional non-loaded horizontal chain of identical spheres which interact via the nonlinear Hertz potential. Our experiments show that the interaction of a solitary wave with a light intruder excites localized mode. In agreement with dimensional analysis, we find that the frequency of localized oscillations exceeds the incident wave frequency spectrum and nonlinearly depends on the size of the intruder and on the incident wave strength. The absence of tensile stress between grains allows some gaps to open, which in turn induce a significant enhancement of the oscillations amplitude. We performed numerical simulations that precisely describe our observations without any adjusting parameters. [3]. For example, the presence of an isotope in a perfect linear crystal is known to enhance optical waves absorption at given frequencies [4]. One-dimensional chains of beads interacting via the Hertz potential are systems suitable to observe nonlinear localization effects. A loaded chain of identical beads is dispersive, allowing small perturbations to propagate as linear or weakly nonlinear acoustic waves [5]. In contrast, when grains in a chain barely touch one another, the energy of an impulse only propagates as fully nonlinear solitary waves [5,6,7] resulting from the balance between dispersion and nonlinearity of the medium. Nesterenko early described this regime as a sonic vacuum limit [8]. Dissipative effects such as viscoelasticity or friction only attenuate and spread these solitary waves [7,9]. In contrast, any heterogeneity of the medium capable of unbalancing dispersion and nonlinearity results in breaking the solitary wave symmetry. For example, a narrow pulse propagating in a chain of beads with decreasing sizes develops a long tail which spreads in time the momentum transfer [10,11,12]. Designing powerfull impact protection systems takes advantage of these features [10,11,13]. Granular chains made of successions of heavy and light beads also proved valuable efficiency in energy absorption [14]. More recently, fully nonlinear waves with finite-width were observed in chains containing periodic mass defects or soft inclusions [15]. Such nonlinear dimer chains are expected to support additionnal optical modes and forbidden band gap when subjected to a static load [15].The elementary interaction of either lighter or heavier intruders with solitary waves in non-loaded monodisperse chains of beads has been investigated numerically [16]. When a solitary wave reaches a mass defect, energy is partially reflected into a backward traveling solitary wave and is partially transmitted to the intruder. A heavy impurity slowly translates, leading to a large transmitted solitary waves train in the forward direction [16], similarly to what was observed in stepped chains [5,8,12]. A light intruder oscillates and scatters forward and backward weak delayed solitary waves trains [16].In this letter, we investigate experimentally the interaction of a solitary wave wi...

We experimentally investigate the flow field in plane geometry around a slowly moving rigid finger in a dry, randomly packed granular medium. The finger enters the medium vertically from its free surface, in analogy with indentation tests on ductile materials. By developing a particle imaging velocimetry technique, we identify a localized flow around the finger, limited by two nearly symmetric shear bands that nucleate near the fingertip and reach the free surface of the granular compact. Evolution of the shear bands is discontinuous, exhibiting nucleation-relaxation processes as the finger moves downward. We present a simple model accounting for the shape of the shear bands at early stages of nucleation. We measure the force applied by the finger and the sources of dilation as well. A mechanism that relates local dilations to the total volume increase of the medium is proposed.

Pressure-and volume-imposed rheology is used to study suspensions of non-colloidal, rigid fibres in the concentrated regime for aspect ratios ranging from 3 to 15. The suspensions exhibit yield stresses. Subtracting these apparent yield stresses reveals a viscous scaling for both the shear and normal stresses. The variation in aspect ratio does not affect the friction coefficient (ratio of shear and normal stresses), but increasing the aspect ratio lowers the maximum volume fraction at which the suspension flows. Constitutive laws are proposed for the viscosities and the friction coefficient close to the jamming transition.

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Granular ensembles subjected to confinement forces can reach metastable states that often break down via formation of shear bands for sufficiently high deviatoric stress. In this article we investigate the flow induced in a granular ensemble that is perturbed by a vertically moving finger in a quasiplanar geometry. The flow exhibits spiral-like shear bands and evolves discontinuously in time, in concert with an oscillating penetration force. We characterize the nature of this nucleation-relaxation type process for loose to dense packing fractions. The nucleation dynamics is reasonably well described by a simple Mohr-Coulomb failure criterium in which the friction coefficient is a function of packing fraction. We contrast our findings with the recent work of Gravish et al. [Phys. Rev. Lett. 105, 128301 (2010)].

Development and evaluation of a real-time plant water stress sensor, based on the electrophysiological behavior of fruit-bearing woody plants is presented. Continuous electric potentials are measured in tree trunks for different irrigation schedules, inducing variable water stress conditions; results are discussed in relation to soil water content and micro-atmospheric evaporative demand, determined continuously by conventional sensors, correlating this information with tree electric potential measurements. Systematic and differentiable patterns of electric potentials for water-stressed and no-stressed trees in 2 fruit species are presented. Early detection and recovery dynamics of water stress conditions can also be monitored with these electrophysiology sensors, which enable continuous and non-destructive measurements for efficient irrigation scheduling throughout the year. The experiment is developed under controlled conditions, in Faraday cages located at a greenhouse area, both in Persea americana and Prunus domestica plants. Soil moisture evolution is controlled using capacitance sensors and solar radiation, temperature, relative humidity, wind intensity and direction are continuously registered with accurate weather sensors, in a micro-agrometeorological automatic station located at the experimental site. The electrophysiological sensor has two stainless steel electrodes (measuring/reference), inserted on the stem; a high precision Keithley 2701 digital multimeter is used to measure plant electrical signals; an algorithm written in MatLab(®), allows correlating the signal to environmental variables. An electric cyclic behavior is observed (circadian cycle) in the experimental plants. For non-irrigated plants, the electrical signal shows a time positive slope and then, a negative slope after restarting irrigation throughout a rather extended recovery process, before reaching a stable electrical signal with zero slope. Well-watered plants presented a continuous signal with daily maximum and a minimum EP of similar magnitude in time, with zero slope. This plant electrical behavior is proposed for the development of a sensor measuring real-time plant water status.