arXiv subjects

Nicolas Mujica

Publications and source records attributed to Nicolas Mujica.

7 recordsLinked to original sources

arXiv2025

Microscopic model for a granular solid-liquid-like phase transition

Forced granular matter in confined geometries presents phase transitions and coexistence. Depending on the system and forcing parameters, liquid-vapor and liquid-solid co-existing states are possible. For the solid-liquid coexistence that is observed in quasi-two-dimensional vibrated systems, both first- and second-order transitions have been reported. Experiments show that particles in the solid cluster move collectively, synchronized with the cell's vibration, in a similar way to the collect-and-collide regime observed in granular dampers. Here, we present a model that proposes a microscopic origin of this granular phase transition and co-existence. Imposing synchronicity, we model the solid cluster as an effective particle of zero restitution coefficient. In addition, we use the mechanical equilibrium between the two phases, with an equation of state validated for hard spheres relating the horizontal velocities in each phase. Balancing energy input and dissipation per unit time we obtain a global power equation, which relates the characteristic vertical and horizontal velocities to the microscopic relevant parameters (geometric and dissipation coefficients) as well as to the vibration amplitude and solid cluster's size. The predictions of the model compare quite well with our experimental results and with the experimental and dynamic simulation results reported elsewhere.

cond-mat.soft↗

arXiv2019

Hyperuniform states generated by a critical friction field

Hyperuniform states are an efficient way to fill up space for disordered systems. In these states the particle distribution is disordered at the short scale but becomes increasingly uniform when looked at large scales. Hyperuniformity appears in several systems, in static or quasistatic regimes as well as close to transitions to absorbing states. Here, we show that a vibrated granular layer, at the critical point of the liquid-to-solid transition, displays dynamic hyperuniformity. Prior to the transition, patches of the solid phase form, with length scales and mean lifetimes that diverge critically at the transition point. When reducing the wavenumber, density fluctuations encounter increasingly more patches that block their propagation, resulting in a static structure factor that tends to zero for small wavenumbers at the critical point, which is a signature of hyperuniformity. A simple model demonstrates that this coupling of a density field to a highly fluctuating scalar friction field gives rise to dynamic hyperuniform states. Finally, we show that the structure factor detects better the emergence of hyperuniformity, compared to the particle number variance.

cond-mat.stat-mech↗

arXiv2016

Dynamics of noncohesive confined granular media

Despite the ubiquitousness and technological and scientific importance of granular matter, our understanding is still very poor compared to molecular fluids and solids. Until today, there is no unified description, which indeed seems unreachable. However, it has been proposed that important advances could be attained for noncohesive, hard-sphere like systems, by combining fluid dynamics with phase-field modeling through an appropriate order parameter (Aranson and Tsimring, 2006). Here, we present a review of the dynamics of confined granular matter, for which this systematic approach has proven its value. Motivated by the pioneering work of Olafsen and Urbach (1998), many experimental, theoretical and numerical studies of model confined granular systems have been realized, which have unveiled a very large variety of fundamental phenomena. In this review, we focus on few of these fundamental aspects, namely phase coexistence, effective surface tension, and a detailed description of the liquid state.

cond-mat.soft↗

arXiv2011

Sudden chain energy transfer events in vibrated granular media

In a mixture of two species of grains of equal size but different mass, placed in a vertically vibrated shallow box, there is spontaneous segregation. Once the system is at least partly segregated and clusters of the heavy particles have formed, there are sudden peaks of the horizontal kinetic energy of the heavy particles, that is otherwise small. Together with the energy peaks the clusters rapidly expand and the segregation is partially lost. The process repeats once segregation has taken place again. Depending on the experimental or numerical parameters, the energy bursts can occur either randomly or with some regularity in time. An explanation for these events is provided based on the existence of a fixed point for an isolated particle bouncing with only vertical motion. The horizontal energy peaks occur when the energy stored in the vertical motion is partly transferred into horizontal energy through a chain reaction of collisions between heavy particles. A necessary condition for the observed regularity of the events is that chain reactions involve most of the heavy particles.

cond-mat.soft↗

arXiv2008

Measuring Dislocation Density in Aluminum with Resonant Ultrasound Spectroscopy

Dislocations in a material will, when present in enough numbers, change the speed of propagation of elastic waves. Consequently, two material samples, differing only in dislocation density, will have different elastic constants, a quantity that can be measured using Resonant Ultrasound Spectroscopy. Measurements of this effect on aluminum samples are reported. They compare well with the predictions of the theory.

cond-mat.mtrl-sci↗

arXiv2004

Experimental observation and characterization of the magnetorotational instability

Differential rotation occurs in conducting flows in accretion disks and planetary cores. In such systems, the magnetorotational instability can arise from coupling Lorentz and centrifugal forces to cause large radial angular momentum fluxes. We present the first experimental observation of the magnetorotational instability. Our system consists of liquid sodium between differentially rotating spheres, with an imposed coaxial magnetic field. We characterize the observed patterns, dynamics and torque increases, and establish that this instability can occur from a hydrodynamic turbulent background.

physics.flu-dyn↗

arXiv1999

Collective motion and solid-liquid-type transitions in vibrated granular layers

From pressure and surface dilation measurements, we show that a solid-liquid-type transition occurs at low excitation frequencies in vertically vibrated granular layers. This transition precedes subharmonic bifurcations from flat surface to standing wave patterns, indicating that these waves are in fact associated with the fluid like behavior of the layer. In the limit of high excitation frequencies, we show that a new kind of subharmonic waves can be distinguished. These waves do not involve any lateral transfer of grains within the layer and correspond to excitations for which the layer slightly bends alternately in time and space. These bending waves have very low amplitude and we observe them in a vibrated two-dimensional layer of photoelastic particles.

cond-mat↗