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It is well known that two permanent magnets of fixed orientation will either always repel or attract one another regardless of the distance between them. However, if one magnet is rotated at sufficient speed, a stable position at a given equilibrium distance can exist for a second free magnet. The equilibrium is produced by magnetic forces alone, which are strong enough to maintain a levitating state under gravity. We show that a stable levitation can be obtained when the rotating magnet is tilted from the rotation axis, with no offset in its position. In this regime, the levitating magnet remains centered and its spinning rate remains negligible, while its magnetic moment precesses in synchronization with the driving magnet. We provide a physical explanation of the levitation through a model relying on static dipolar interactions between the two magnets and present experimental results which validate the proposed theory.

We report an instability of a slider slowly dragged at the surface of a granular bed in a quasistatic regime. The boat-shaped slider sits on the granular medium under its own weight and is free to translate vertically and to rotate around the pitch axis while a constant horizontal speed is imposed. For a wide range of parameters (mass, length, shape, velocity) a regular pattern of peaks and troughs spontaneously emerges as the slider travels forward. This instability is studied through experiments using a conveyor belt and by means of two-dimensional discrete elements method simulations. We show that the wavelength and amplitude of the pattern scale as the length of the slider. We also observe that the ripples disappear for low and high masses, indicating an optimal confining pressure. The effect of the shape, more specifically the inclination of the front spatula, is studied and found to drastically influence both the wavelength and the amplitude. Finally, we show that the mechanical details (friction, cohesion) of the contact point between the slider and the pulling device is critical and remains to be fully understood.

Dirt cones are meter-scale structures encountered at the surface of glaciers, which consist of ice cones covered by a thin layer of ashes, sand or gravel, and which form naturally from an initial patch of debris. In this article, we report field observations of cone formation in the French Alps, laboratory-scale experiments reproducing these structures in a controlled environment, and two-dimensional discrete-element-method-finite-element-method numerical simulations coupling the grains mechanics and thermal effects. We show that cone formation originates from the insulating properties of the granular layer, which reduces ice melting underneath as compared to bare ice melting. This differential ablation deforms the ice surface and induces a quasi static flow of grains that leads to a conic shape, as the thermal length become small compared to the structure size. The cone grows until it reaches a steady state in which the insulation provided by the dirt layer exactly compensates for the heat flux coming from the increased external surface of the structure. These results allowed us to identify the key physical mechanisms at play and to develop a model able to quantitatively reproduce the various field observations and experimental findings., Comment: 10 pages, 8 figures, and supplementary materials (6 pages, 5 figures)

A glacier table consists of a rock supported by a slender column of ice and form naturally on glaciers. We investigate the onset of their formation at a smaller scale in a controlled environment. Depending on the size and thermal conductivity of a cap, it can either form of a table standing on an ice foot, or sink into the ice block. A one-dimension conduction model shows that the differential ice melting is controlled by a competition between two effects: a geometrical amplification, and a heat flux reduction due to the higher temperature of the cap as compared to the ice. Our model captures the transition between the two regimes and identifies a dimensionless number which controls the onset of glacier tables formation., Comment: 8 pages, 8 figures; final version, title corrected

Asteroids and other Small Solar System Bodies (SSSBs) are of high general and scientific interest in many aspects. The origin, formation, and evolution of our Solar System (and other planetary systems) can be better understood by analysing the constitution and physical properties of small bodies in the Solar System. Currently, two space missions (Hayabusa2, OSIRIS-REx) have recently arrived at their respective targets and will bring a sample of the asteroids back to Earth. Other small body missions have also been selected by, or proposed to, space agencies. The threat posed to our planet by near-Earth objects (NEOs) is also considered at the international level, and this has prompted dedicated research on possible mitigation techniques. The DART mission, for example, will test the kinetic impact technique. Even ideas for industrial exploitation have risen during the last years. Lastly, the origin of water and life on Earth appears to be connected to asteroids. Hence, future space mission projects will undoubtedly target some asteroids or other SSSBs. In all these cases and research topics, specific knowledge of the structure and mechanical behaviour of the surface as well as the bulk of those celestial bodies is crucial. In contrast to large telluric planets and dwarf planets, a large proportion of such small bodies is believed to consist of gravitational aggregates ('rubble piles') with no -- or low -- internal cohesion, with varying macro-porosity and surface properties (from smooth regolith covered terrain, to very rough collection of boulders), and varying topography (craters, depressions, ridges) [...]., Comment: This is a pre-print version of an article to be published in AARv, and available online at https://rdcu.be/bHM9E

Synthetic schlieren is an digital image processing optical method relying on the variation of optical index to visualize the flow of a transparent fluid. In this article, we present a step-by step, easy-to-implement and affordable experimental realization of this technique. The method is applied to air convection caused by a warm surface. We show that the velocity of rising convection plumes can be linked to the temperature of the warm surface and propose a simple physical argument to explain this dependence. Moreover, using this method, one can reveal the tenuous convection plumes rising from ounce's hand, a phenomenon invisible to the naked eye. This spectacular result may help student realize the power of careful data acquisition combined with astute image processing techniques.

The acoustic radiation force exerted on a small sphere located at the focus of an ultrasonic beam is measured in a soft gel. It is proved to evolve quadratically with the local amplitude of the acoustic field. Strong oscillations of the local pressure are observed and attributed to an acoustic Fabry-P{\'e}rot effect between the ultrasonic emitter and the sphere. Taking this effect into account with a simple model, a quantitative link between the radiation force and the acoustic pressure is proposed and compared to theoretical predictions in the absence of dissipation. The discrepancy between experiment and theory suggests that dissipative effects should be taken into account for fully modeling the observations., Comment: Includes erratum to be published in APL

We demonstrated recently that polyelectrolytes with cationic moieties along the chain and a single anionic head are able to form physical hydrogels due to the reversible nature of the head-to-body ionic bond. Here we generate a variety of such polyelectrolytes with various cationic moieties and counterion combinations starting from a common polymeric platform. We show that the rheological properties (shear modulus, critical strain) of the final hydrogels can be modulated over three orders of magnitude depending on the cation/anion pair. Our data fit remarkably well within a scaling model involving a supramolecular head-to-tail single file between cross-links, akin to the behaviour of pine-processionary caterpillar. This model allows the quantitative measure of the amount of counterion condensation from standard rheology procedure., Comment: As submitted to Soft Matter

Fatigue refers to the changes in material properties caused by repeatedly applied loads. It has been widely studied for, e.g., construction materials, but much less has been done on soft materials. Here, we characterize the fatigue dynamics of a colloidal gel. Fatigue is induced by large amplitude oscillatory stress (LAOStress), and the local displacements of the gel are measured through high-frequency ultrasonic imaging. We show that fatigue eventually leads to rupture and fluidization. We evidence four successive steps associated with these dynamics: (i) the gel first remains solid, (ii) it then slides against the walls, (iii) the bulk of the sample becomes heterogeneous and displays solid-fluid coexistence, and (iv) it is finally fully fluidized. It is possible to homogeneously scale the duration of each step with respect to the stress oscillation amplitude $\sigma_0$. The data are compatible with both exponential and power-law scalings with $\sigma_0$, which hints at two possible interpretations in terms of delayed yielding in terms activated processes or of the Basquin law. Surprisingly, we find that the model parameters behave nonmonotonically as we change the oscillation frequency and/or the gel concentration., Comment: 13 pages, 7 figures, submitted to Soft Matter

We report experimental results on the dynamics of a granular packing submitted to high-intensity focused ultrasound. Acoustic radiation pressure is shown to remotely induce local rearrangements within a pile as well as global motion around the focal spot in an initially jammed system. We demonstrate that this fluidization process is intermittent for a range of acoustic pressures and hysteretic when the pressure is cycled. Such a first-order-like unjamming transition is reproduced in numerical simulations in which the acoustic pressure field is modeled by a localized external force. Further analysis of the simulated packings suggests that in the intermittent regime unjamming is not associated with any noticeable prior structural signature. A simple two-state model based on effective temperatures is proposed to account for these findings., Comment: 11 pages, 8 figures

Tuning the chain-end functionality of a short-chain cationic homopolymer, owing to the nature of the initiator used in the ATRP polymerisation step, can be used to mediate the formation of a gel of this poly(electrolyte) in water. While a neutral end group gives a solution of low viscosity, a highly homogeneous gel is obtained with a phosphonate anionic moiety, as characterized by rheometry and diffusion NMR. This novel type of supramolecular control over poly(electrolytic) gel formation could find potential use in a variety of applications in the field of electroactive materials.

Since its introduction, landscape genomics has developed quickly with the increasing availability of both molecular and topo-climatic data. The current challenges of the field mainly involve processing large numbers of models and disentangling selection from demography. Several methods address the latter, either by estimating a neutral model from population structure or by inferring simultaneously environmental and demographic effects. Here we present Sam$\beta$ada, an integrated approach to study signatures of local adaptation, providing rapid processing of whole genome data and enabling assessment of spatial association using molecular markers. Specifically, candidate loci to adaptation are identified by automatically assessing genome-environment associations. In complement, measuring the Local Indicators of Spatial Association (LISA) for these candidate loci allows to detect whether similar genotypes tend to gather in space, which constitutes a useful indication of the possible kinship relationship between individuals. In this paper, we also analyze SNP data from Ugandan cattle to detect signatures of local adaptation with Sam$\beta$ada, BayEnv, LFMM and an outlier method (FDIST approach in Arlequin) and compare their results. Sam$\beta$ada is an open source software for Windows, Linux and MacOS X available at \url{http://lasig.epfl.ch/sambada}, Comment: 1 figure in text, 1 figure in supplementary material The structure of the article was modified and some explanations were updated. The methods and results presented are the same as in the previous version

We report on the response of a yield stress material, namely a colloidal gel made of attractive carbon black particles, submitted to large amplitude oscillatory shear stress (LAOStress). At a constant stress amplitude well below its apparent yield stress, the gel displays fatigue and progressively turns from an elastic solid to a viscous fluid. The time-resolved analysis of the strain response, of the Fourier spectrum, and of Lissajous plots allows one to define two different timescales $\tau_w<\tau_f$ associated with the yielding and fluidization of the gel. Coupling rheology to ultrasonic imaging further leads to a local picture of the LAOStress response in which the gel first fails at the walls at $\tau_w$ and then undergoes a slow heterogeneous fluidization involving solid--fluid coexistence until the whole sample is fluid at $\tau_f$. Spatial heterogeneities are observed in both the gradient and vorticity directions and suggest a fragmentation of the initially solidlike gel into macroscopic domains eroded by the surrounding fluidized suspension., Comment: 21 pages, 11, figures, submitted to the Journal of Rheology

The stress-induced yielding scenario of colloidal gels is investigated under rough boundary conditions by means of rheometry coupled to local velocity measurements. Under an applied shear stress, the fluidization of gels made of attractive carbon black particles suspended in a mineral oil is shown to involve a previously unreported shear rate response $\dot \gamma (t)$ characterized by two well-defined and separated timescales $\tau_c$ and $\tau_f$. First $\dot \gamma(t)$ decreases as a weak power law strongly reminiscent of the primary creep observed in numerous crystalline and amorphous solids, coined the "Andrade creep." We show that the bulk deformation remains homogeneous at the micron scale, which demonstrates that if plastic events take place or if any shear transformation zone exists, such phenomena occur at a smaller scale. As a key result of this paper, the duration $\tau_c$ of this creep regime decreases as a power law of the viscous stress, defined as the difference between the applied stress and the yield stress with an exponent ranging between 2 and 3 depending on the gel concentration. The end of this first regime is marked by a jump of the shear rate by several orders of magnitude, while the gel slowly slides as a solid block experiencing strong wall slip at both walls, despite rough boundary conditions. Finally, a second sudden increase of the shear rate is concomitant to the full fluidization of the material which ends up being homogeneously sheared. The corresponding fluidization time $\tau_f$ robustly follows an exponential decay with the applied shear stress as already reported for smooth boundary conditions. Finally, we highlight a few features that are common to attractive colloidal gels and to solid materials by discussing our results in the framework of theoretical approaches of solid rupture (kinetic, fiber bundle, and transient network models)., Comment: Main: 18 pages, 15 figures - Supplemental: 5 pages, 6 figures - Accepted for publication in Soft Matter (Nov. 2013)

The Taylor-Couette flow of a dilute micellar system known to generate shear-induced structures is investigated through simultaneous rheometry and ultrasonic imaging. We show that flow instabilities must be taken into account since both the Reynolds number and the Weissenberg number may be large. Before nucleation of shear-induced structures, the flow can be inertially unstable, but once shear-induced structures are nucleated the kinematics of the flow become chaotic, in a pattern reminiscent of the inertio-elastic turbulence known in dilute polymer solutions. We outline a general framework for the interplay between flow instabilities and flow-induced structures., Comment: 4 pages, 4 figures, 3 supplemental movies and 1 supplemental figure

In this letter, we report results on the effect of temperature variations on a granular assembly through Molecular Dynamic simulations of a 2D granular column. Periodic dilation of the grains are shown to perfectly mimic such thermal cycling, and allows to rationalize the link between the compaction process, the local grains dynamics and finite size effects. Here we show that the individual grain properties, namely their roughness and elastic modulus define a minimal cycling amplitude of temperature \Delta Tc below which the dynamics is intermittent and spatially heterogeneous while confined into localized regions recently coined "hot spot" [Amon et al., Phys. Rev. Lett. 108, 135502 (2012)]. Above \Delta Tc, the whole column flows while the grains dynamics ranges continuously from cage-like at the bottom of the column to purely diffusive at the top. Our results provide a solid framework for the futur use of thermal cycling as an alternate driving method for soft glassy materials., Comment: 5 pages, 5 figures, submitted

We describe a technique coupling standard rheology and ultrasonic imaging with promising applications to characterization of soft materials under shear. Plane wave imaging using an ultrafast scanner allows to follow the local dynamics of fluids sheared between two concentric cylinders with frame rates as high as 10,000 images per second, while simultaneously monitoring the shear rate, shear stress, and viscosity as a function of time. The capacities of this "rheo-ultrasound" instrument are illustrated on two examples: (i) the classical case of the Taylor-Couette instability in a simple viscous fluid and (ii) the unstable shear-banded flow of a non-Newtonian wormlike micellar solution., Comment: 14 pages, 9 pages, submitted to the Review of Scientific Instruments

When submitted to the repeated passages of vehicles unpaved roads made of sand or gravel can develop a ripply pattern known as washboard or corrugated road. We propose a stability analysis based on experimental measurements of the force acting on a blade (or plow) dragged on a circular sand track and show that a linear model is sufficient to describe the instability near onset. The relation between the trajectory of the plow and the profile of the sand bed left after its passage is studied experimentally. The various terms in the expression of the lift force created by the flow of granular material on the plow are determined up to first order by imposing a sinusoidal trajectory to the blade on an initially flat sand bed, as well as by imposing a horizontal trajectory on an initially rippled sand bed. Our model recovers all the previously observed features of washboard road and accurately predicts the most unstable wavelength near onset as well as the critical velocity for the instability., Comment: 9 pages, 10 figures

We studied the drag and lift forces acting on an inclined plate while it is dragged on the surface of a granular media, both in experiment and numerical simulation. In particular, we investigated the influence of the horizontal velocity of the plate and its angle of attack. We show that a steady wedge of grains is moved in front of the plow and that the lift and drag forces are proportional to the weight of this wedge. These constants of proportionality vary with the angle of attack but not (or only weakly) on the velocity. We found a universal effective friction law which accounts for the dependence on all the above-mentioned parameters. The stress and velocity fields are calculated from the numerical simulations and show the existence of a shear band under the wedge and that the pressure is non-hydrostatic. The strongest gradients in stress and shear occur at the base of the plow where the dissipation rate is therefore highest., Comment: 8 pages, 10 figures, submitted to Phys. Rev. E

Various dispersions of attractive particles are known to aggregate into patterns of vorticity-aligned stripes when sheared in confined geometries. We report a thorough experimental investigation of such shear-induced vorticity alignment through direct visualization of carbon black gels in both simple plane shear and rotational shear cells. Control parameters such as the gap width, the strain rate, and the gel concentration are systematically varied. It is shown that in steady states the wavelength of the striped pattern depends linearly on the gap width $h$ while being insensitive to both the gel concentration $C$ and the shear rate $\gp$. The width of the vorticity-aligned flocs coincides with the gap width and is also independent of $C$ and $\gp$, which hints to a simple picture in terms of compressible cylindrical flocs. Finally, we show that there exists a critical shear rate $\gp_c$ above which structuration does not occur and that $\gp_c$ scales as $h^{-\alpha}$ with $\alpha=1.4\pm 0.1$ independently of $C$. This extensive data set should open the way to quantitative modelling of the vorticity alignment phenomenon in attractive colloidal systems., Comment: 10 pages, 14 figures, submitted to Soft Matter

Simultaneous rheological and velocity profile measurements are performed in a smooth Couette geometry on Laponite suspensions seeded with glass microspheres and undergoing the shear-induced solid-to-fluid (or yielding) transition. Under these slippery boundary conditions, a rich temporal behaviour is uncovered, in which shear localization is observed at short times, that rapidly gives way to a highly heterogeneous flow characterized by intermittent switching from plug-like flow to linear velocity profiles. Such a temporal behaviour is linked to the fragmentation of the initially solid sample into blocks separated by fluidized regions. These solid pieces get progressively eroded over time scales ranging from a few minutes to several hours depending on the applied shear rate $\dot{\gamma}$. The steady-state is characterized by a homogeneous flow with almost negligible wall slip. The characteristic time scale for erosion is shown to diverge below some critical shear rate $\dot{\gamma}^\star$ and to scale as $(\dot{\gamma}-\dot{\gamma}^\star)^{-n}$ with $n\simeq 2$ above $\dot{\gamma}^\star$. A tentative model for erosion is discussed together with open questions raised by the present results., Comment: 19 pages, 13 figures, submitted to Soft Matter

Granular surfaces subjected to forces due to rolling wheels develop ripples above a critical speed. The resulting pattern, known as "washboard" or "corrugated" road, is common on dry, unpaved roads. We investigated this phenomenon theoretically and experimentally, using laboratory-scale apparatus and beds of dry sand. A thick layer of sand on a circular track was forced by a rolling wheel on an arm whose weight and moment of inertia could be varied. We compared the ripples made by the rolling wheel to those made using a simple inclined plow blade. We investigated the dependence of the critical speed on various parameters, and describe a scaling argument which leads to a dimensionless ratio, analogous to the hydrodynamic Froude number, which controls the instability. This represents the crossover between conservative, dynamic forces and dissipative, static forces. Above onset, wheel-driven ripples move in the direction of motion of the wheel, but plow-driven ripples move in the reverse direction for a narrow range of Froude numbers., Comment: 11 pages, 12 figures, accepted for publication in Phys Rev E

Granular surfaces tend to develop lateral ripples under the action of surface forces exerted by rolling wheels, an effect known as washboard or corrugated road. We report the results of both laboratory experiments and soft-particle direct numerical simulations. Above a critical speed, the ripple pattern appears as small patches of traveling waves which eventually spread to the entire circumference. The ripples drift slowly in the driving direction. Interesting secondary dynamics of the saturated ripples were observed, as well as various ripple creation and destruction events. All of these effects are captured qualitatively by 2D soft particle simulations in which a disk rolls over a bed of poly-disperse particles in a periodic box. These simulations show that compaction and segregation are inessential to the ripple phenomenon. We also discuss a simplified scaling model which gives some insight into the mechanism of the instability., Comment: 5 pages 5 figures

The shape of a granular pile in a rotating drum is investigated. Using Discrete Elements Method (DEM) simulations we show that the "S shape" obtained for high rotation speed can be accounted for by the friction on the end plates. A theoretical model which accounts for the effect of the end plates is presented and the equation of the shape of the free surface is derived. The model reveals a dimensionless number which quantifies the influence of the end plates on the shape of the pile. Finally, the scaling laws of the system are discussed and numerical results support our conclusions.

The diffusion of a pulse of small grains in an horizontal rotating drum is studied through discrete elements methods simulations. We present a theoretical analysis of the diffusion process in a one-dimensional confined space in order to elucidate the effect of the confining end-plate of the drum. We then show that the diffusion is neither subdiffusive nor superdiffusive but normal. This is demonstrated by rescaling the concentration profiles obtained at various stages and by studying the time evolution of the mean squared deviation. Finally we study the self-diffusion of both large and small grains and we show that it is normal and that the diffusion coefficient is independent of the grain size.

We report experimental and numerical results on the growth of a super stable heap (SSH). Such a regime appears for flows in a thin channel and for high flow rate : the flow occurs atop a nearly static heap whose angle is stabilized by the flowing layer at its top and the side wall friction. The growth of the static heap is investigated in this paper. A theoretical analysis inspired by the BRCE formalism predicts the evolution of the growth process, which is confirmed by both experiments and numerical simulations. The model allows us to link the characteristic time of the growth to the exchange rate between the "moving" and "static" grains. We show that this rate is proportional to the height of the flowing layer even for thick flows. The study of upstream traveling waves sheds new light on the BCRE model.

Axial segregation of a binary mixture of grains in a rotating drum is studied using Molecular Dynamics (MD) simulations. A force scheme leading to a constant restitution coefficient is used and shows that axial segregation is possible between two species of grains made of identical material differing by size. Oscillatory motion of bands is investigated and the influence of the frictional properties elucidated. The mechanism of bands merging is explained using direct imaging of individual grains.