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Nicolás Mujica

Publications and source records attributed to Nicolás Mujica.

15 recordsLinked to original sources
arXiv2026

Kinetics of coagulation phenomena from a granular matter perspective

Aggregation processes play a central role in systems ranging from aerosol coagulation and cloud formation to dust growth in protoplanetary disks and granular materials. These processes are traditionally described by Smoluchowski's coagulation equation, which provides a mean-field account of growth through binary collisions. However, incorporation of granular physics-dissipative interactions, spatial heterogeneity, and force transmission through contact networks-reveals important limitations of this framework. In this review, we show how such effects lead to the breakdown of mean-field assumptions and motivate a view of aggregation as a multi-scale process shaped by the interplay between interactions, structure, and collective dynamics. Phenomena such as segregation, jamming, and clogging further highlight the role of mechanical constraints and spatial organization in limiting or redirecting growth. By integrating insights from granular physics, aerosol science, and astrophysics, we outline a unified perspective on coagulation in non-equilibrium particulate systems. This paper is part of the thematic issue "Sand, silos and asteroids: clustering challenges in granular materials research".

cond-mat.soft↗
arXiv2026

Impact of Initial Charge Distributions on the Kinetics of Charged Particle Coagulation

We investigate the kinetics of particle aggregation within the framework of the Smoluchowski coagulation equation, extending it to account for electrostatic interactions among charged clusters. Using a stochastic Monte Carlo implementation, we examine how different charge distributions and net system charge affect cluster growth dynamics. Electrostatic interactions are incorporated directly into the classical Brownian collision kernel, yielding charge-dependent modifications of the collision rates that may either enhance or suppress aggregation depending on the signs and magnitudes of the interacting charges. Our simulations reveal distinct regimes of growth: at intermediate times, charge heterogeneity accelerates or delays aggregation depending on the initial underlying charge distribution, while at long times the system tends toward quasi--stationary states whose properties depend on the net charge. Comparisons between Gaussian and Cauchy--Lorentz initial charge statistics highlight the role of heavy-tailed distributions in promoting faster cluster growth. These findings contribute to a unified understanding of coagulation kinetics in charged particulate systems, with potential implications for aerosol and astrophysical coagulation processes, volcanic ash aggregation, and clustering in industrial fluidized granular beds.

cond-mat.soft↗
arXiv2022

Nonlinear acoustic characterization of heterogeneous plasticity in bent aluminium samples

Knowledge of the state of plastic deformation in metallic structures is vital to prevent failure. This is why non-destructive acoustic tests based on the measurement of first order elastic constants have been developed and intensively used. However, plastic deformations, which are usually heterogeneous in space, may be invisible to these methods if the variation of the elastic constants is too small. In recent years, digital image correlation techniques, based on measurements carried out at the surface of a sample, have been successfully used in conjunction with finite element modeling to gain information about plastic deformation in the sample interior. Acoustic waves can penetrate deep into a sample and offer the possibility of probing into the bulk of a plastically deformed material. Previously, we have demonstrated that nonlinear acoustic methods are far more sensitive to changes in dislocation density than linear ones. Here, we show that the nonlinear Second Harmonic Generation method (SHG) is sensitive enough to detect different zones of von Mises stress as well as effective plastic strain in centimeter-size aluminium pieces. This is achieved by way of ultrasonic measurements on a sample that has undergone a three-point bending test. Because of the relatively low stress and small deformations, the sample undergoes plastic deformation by dislocation proliferation. Thus, we conclude that the nonlinear parameter measured by SHG is also sensitive to dislocation density. Our experimental results agree with numerical results obtained by Finite Element Method (FEM) modeling. We also support the acoustic results by X-Ray Diffraction measurements (XRD). Although intrusive and less accurate, they also agree with the acoustic measurements and plastic deformations in finite element simulations.

cond-mat.mtrl-sci↗
arXiv2020

Faraday-waves contact-line shear gradient induces streaming flow and tracers' self-organization: from rotating rings to spiral galaxy-like patterns

In this letter we experimentally demonstrate self-organization of small tracers under the action of longitudinal Faraday waves in a narrow container. We observe a steady current formation dividing the interface in small cells given by the symmetries of the Faraday wave. These streaming currents are rotating in each cell and their circulation increases with wave amplitude. This streaming flow drives the tracers to form patterns, whose shapes depend on the Faraday wave's amplitude: from low to high amplitudes we find dispersed tracers, a narrow rotating ring and a spiral galaxy-like pattern. We first describe the main pattern features, and characterize the wave and tracers' motion. We then show experimentally that the main source of the streaming flow comes from the time and spatial dependent shear at the wall contact line, created by the Faraday wave itself. We end by presenting a 2D model that considers the minimal ingredients present in the Faraday experiment, namely the stationary circulation, the stretching component due to the oscillatory wave and a steady converging field, which combined produce the observed self-organized patterns.

physics.flu-dyn↗
arXiv2016

Streaming patterns in Faraday waves

Waves patterns in the Faraday instability have been studied for decades. Besides the rich dynamics that can be observed on the waves at the interface, Faraday waves hide beneath them an elusive range of flow patterns --or streaming patterns-- which have not been studied in detail until now. The streaming patterns are responsible for a net circulation in the flow which are reminiscent of convection cells. In this article, we analyse these streaming flows by conducting experiments in a Faraday-wave setup. To visualize the flows, tracers are used to generate both trajectory maps and to probe the streaming velocity field via Particle Image Velocimetry (PIV). We identify three types of patterns and experimentally show that identical Faraday waves can mask streaming patterns that are qualitatively very different. Next we propose a three-dimensional model that explains streaming flows in quasi-inviscid fluids. We show that the streaming inside the fluid arises from a complex coupling between the bulk and the boundary layers. This coupling can be taken into account by applying modified boundary conditions in a three-dimensional Navier-Stokes formulation for the streaming in the bulk. Numerical simulations based on this theoretical framework show good qualitative and quantitative agreement with experimental results. They also highlight the relevance of three-dimensional effects in the streaming patterns. Our simulations reveal that the variety of experimental patterns is deeply linked to the boundary condition at the top interface, which may be strongly affected by the presence of contaminants along the surface.

physics.flu-dyn↗
arXiv2015

In situ monitoring of plastic deformation using ultrasound

Ultrasound has long been used as a non-destructive tool to test for the brittle fracture of materials. Could it be used as a similar tool to test for ductile failure? This study reports results of local measurements of the speed of shear waves, $v_T$, in aluminum under standard testing conditions at two different locations on the same sample, as a function of stress. There is a clear change in $v_T$ at the Yield stress, consistent with a proliferation of dislocations. This measurement provides a quantitative, continuous relation between dislocation density and externally applied stress.

cond-mat.mtrl-sci↗
arXiv2015

Universality and criticality of a second-order granular solid-liquid-like phase transition

We experimentally study the critical properties of the non-equilibrium solid-liquid-like transition that takes place in vibrated granular matter. The critical dynamics is characterized by the coupling of the density field with the bond-orientational order parameter $Q_4$, which measures the degree of local crystallization. Two setups are compared, which present the transition at different critical accelerations as a a result of modifying the energy dissipation parameters. In both setups five independent critical exponents are measured, associated to different properties of $Q_4$: the correlation length, relaxation time, vanishing wavenumber limit (static susceptibility), the hydrodynamic regime of the pair correlation function, and the amplitude of the order parameter. The respective critical exponents agree in both setups and are given by $ν_{\perp} = 1$, $ν_{\parallel} = 2$, $γ= 1$, $η\approx 0.6 - 0.67$, and $β=1/2$, whereas the dynamical critical exponent is $z = ν_{\parallel}/ν_{\perp} = 2$. The agreement on five exponents is an exigent test for the universality of the transition. Thus, while dissipation is strictly necessary to form the crystal, the path the system undergoes towards the phase separation is part of a well defined universality class. In fact, the local order shows critical properties while density does not. Being the later conserved, the appropriate model that couples both is model C in the Hohenberg and Halperin classification. The measured exponents are in accord with the non-equilibrium extension to model C if we assume that $α$, the exponent associated in equilibrium to the specific heat divergence but with no counterpart in this non-equilibrium experiment, vanishes.

cond-mat.stat-mech↗
arXiv2014

Measurement of velocity field in parametrically excited solitary waves

Parametrically excited solitary waves emerge as localized structures in high-aspect-ratio free surfaces subject to vertical vibrations. Herein, we provide the first experimental characterization of the hydrodynamics of these waves using Particle Image Velocimetry. We show that the underlying velocity field of parametrically excited solitary waves is mainly composed by an oscillatory velocity field. Our results confirm the accuracy of Hamiltonian models with added dissipation in describing this field. Remarkably, our measurements also uncover the onset of a streaming velocity field which is shown to be as important as other crucial nonlinear terms in the current theory. The observed streaming pattern is particularly interesting due to the presence of oscillatory meniscii.

physics.flu-dyn↗
arXiv2014

Dynamics of a first order transition to an absorbing state

A granular system confined in a quasi two-dimensional box that is vertically vibrated can transit to an absorbing state in which all particles bounce vertically in phase with the box, with no horizontal motion. In principle, this state can be reached for any density lower than the one corresponding to one complete monolayer, which is then the critical density. Below this critical value, the transition to the absorbing state is of first order, with long metastable periods, followed by rapid transitions driven by homogeneous nucleation. Molecular dynamics simulations and experiments show that there is a dramatic increase on the metastable times far below the critical density; in practice, it is impossible to observe spontaneous transitions close to the critical density. This peculiar feature is a consequence of the non-equilibrium nature of this first order transition to the absorbing state. A Ginzburg-Landau model, with multiplicative noise, describes qualitatively the observed phenomena and explains the macroscopic size of the critical nuclei. The nuclei become of small size only close to a second critical point where the active phase becomes unstable via a saddle node bifurcation. It is only close to this second critical point that experiments and simulations can evidence spontaneous transitions to the absorbing state while the metastable times grow dramatically moving away from it.

cond-mat.stat-mech↗
arXiv2013

Capillary-like Fluctuations of a Solid-Liquid Interface in a Non-Cohesive Granular System

One of the most noticeable collective motion of non-cohesive granular matter is clustering under certain conditions. In particular, when a quasi-two-dimensional monolayer of mono-disperse non-cohesive particles is vertically vibrated, a solid-liquid-like transition occurs when the driving amplitude exceeds a critical value. Here, the physical mechanism underlying particle clustering relies on the strong interactions mediated by grain collisions, rather than on grain-grain cohesive forces. In average, the solid cluster resembles a drop, with a striking circular shape. We experimentally investigate the coarse-grained solid-liquid interface fluctuations, which are characterized through the static and dynamic correlation functions in the Fourier space. These fluctuations turn out to be well described by the capillary wave theory, which allows us to measure the solid-liquid interface surface tension and mobility once the granular "thermal" kinetic energy is determined. Despite the system is strongly out of equilibrium and that the granular temperature is not uniform, there is energy equipartition at the solid-liquid interface, for a relatively large range of angular wave-numbers. Furthermore, both surface tension and mobility are consistent with a simple order of magnitude estimation considering the characteristic energy, length and time scales, which is very similar to what can be done for atomic systems. Please find the supplementary material of this article here: arXiv:1304.2646

cond-mat.soft↗
arXiv2012

Phase Transition in a Out-of-equilibrium Monolayer of Dipolar Vibrated Grains

We report an experimental study on the transition between a disordered liquid-like state and a ordered solid-like one, in a collection of magnetically interacting macroscopic grains. A monolayer of magnetized particles is vibrated vertically at a moderate density. At high excitation a disordered, liquid-like state is observed. When the driving dimensionless acceleration $Γ$ is quasi-statically reduced, clusters of ordered grains grow below a critical value $Γ_{\rm c}$. These clusters have a well defined hexagonal and compact structure. If the driving is subsequently increased, these clusters remain stable up to a higher critical value $Γ_{\rm l}$. Thus, the solid-liquid transition exhibits an hysteresis cycle. However, the lower onset $Γ_c$ is not well defined as it depends strongly on the acceleration ramp speed and also on the magnetic interaction strength. Metastability is observed when the driving is rapidly quenched from high acceleration, $Γ> Γ_{\rm l}$, to a low final excitation $Γ_q$. After this quench, solid clusters nucleate after a time lag $τ_o$, either immediately ($τ_o = 0$) or after some time lag ($τ_o>0$) that can vary from seconds up to several hundreds of seconds. The immediate growth occurs below a particular acceleration value $Γ_s$ ($\lesssim Γ_c$). In all cases, for $t \geq τ_o$ solid cluster's temporal growth can be phenomenologically described by a stretched exponential law. Finally, by taking into account the finite size of our system and by using simple assumptions we propose an alternative tractable theoretical model that reproduces cluster's growth, but which analytical form is not a stretched exponential law.

cond-mat.stat-mech↗
arXiv2012

Fluctuations and Criticality of a Granular Solid-Liquid-like Phase Transition

We present an experimental study of density and order fluctuations in the vicinity of the solid-liquid-like transition that occurs in a vibrated quasi-two-dimensional granular system. The two-dimensional projected static and dynamic correlation functions are studied. We show that density fluctuations, characterized through the structure factor, increase in size and intensity as the transition is approached, but they do not change significantly at the transition itself. The dense, metastable clusters, which present square symmetry, also increase their local order in the vicinity of the transition. This is characterized through the bond-orientational order parameter $Q_4$, which in Fourier space obeys an Ornstein-Zernike behavior. Depending on filling density and vertical height, the transition can be of first or second order type. In the latter case, the associated correlation length $ξ_4$, relaxation time $τ_4$, zero $k$ limit of $Q_4$ fluctuations (static susceptibility), the pair correlation function of $Q_4$, and the amplitude of the order parameter obey critical power laws, with saturations due to finite size effects. Their respective critical exponents are $ν_{\bot} = 1$, $ν_{||} = 2$, $γ= 1$, $η=0.67$, and $β=1/2$, whereas the dynamical critical exponent $z = ν_{||}/ν_{\bot} = 2$. These results are consistent with model C of dynamical critical phenomena, valid for a non-conserved critical order parameter (bond-orientation order) coupled to a conserved field (density).

cond-mat.stat-mech↗
arXiv2012

Ultrasound as a probe of dislocation density in aluminum

Dislocations are at the heart of the plastic behavior of crystalline materials yet it is notoriously difficult to perform quantitative, non-intrusive, measurements of their single or collective properties. Dislocation density is a critical variable that determines dislocation mobility, strength and ductility. On the one hand, individual dislocations can be probed in detail with transmission electron microscopy. On the other hand, their collective properties must be simulated numerically. Here we show that ultrasound technology can be used to measure dislocation density. This development rests on theory---a generalization of the Granato-Lücke theory for the interaction of elastic waves with dislocations---and Resonant Ultrasound Spectroscopy (RUS) measurements. The chosen material is aluminum, to which different dislocation contents were induced through annealing and cold rolling processes. The dislocation densities obtained with RUS compare favorably with those inferred from X-ray diffraction, using the modified Williamson-Hall method.

cond-mat.mtrl-sci↗
arXiv2011

Shear Modulus of an Elastic Solid under External Pressure as a function of Temperature: The case of Helium

The energy of a dislocation loop in a continuum elastic solid under pressure is considered within the framework of classical mechanics. For a circular loop, this is a function with a maximum at pressures that are well within reach of experimental conditions for solid helium suggesting, in this case, that dislocation loops can be generated by a pressure-assisted thermally activated process. It is also pointed out that pinned dislocations segments can alter the shear response of solid helium, by an amount consistent with current measurements, without any unpinning.

cond-mat.other↗