Gustavo Castillo ● Profesor Asociado

Far from simple steric repulsion, i.e., volumetric repulsion, the dynamics of granular systems are driven by a menagerie of interactions: dissipative collisions, van der Waals forces, electrostatic Coulomb and polarization forces, viscous drag and, in the presence of even minute amounts of liquid, capillary bridges or ice coatings. Despite its importance and the development of many powerful experimental, numerical and theoretical tools, until now, a unified description of granular media is lacking, even for the simplest model situation of perfectly spherical, impenetrable and dissipative particles. One of the most important topics in granular media research is clustering, for both fundamental and applied reasons. Clustering produces large gradients, which makes usual gradient perturbations schemes more difficult, which even poses questions about the validity of continuum approaches for these systems. Clustering and coarsening are also relevant for many industrial applications, including grain and powder storage, transport and manipulation, in the food, mining and chemical industries, to mention a few. Electrostatically–induced granular clustering has emerged as a mechanism with fundamental and practical implications. The electrification of such systems occurs through tribocharging—the exchange of charge between contacting surfaces. Despite its importance, how insulators transfer such large amounts of charge during contact is not well-understood. How this can also occur for identical materials during contact is puzzling as well. Furthermore, the nature of the charge carrier is also not settled. Concerning applications, just recently electrostatically–induced granular clustering has been revealed as a possible enhancing mechanism for granular coarsening in a very important and unsolved issue: the formation of planetesimals, which can be considered as baby planets (from 1 km size it is expected that gravity should be the driving accretion force). Indeed, despite clear evidence, our current theoretical understanding is that rocky planets should not exist; a basic ingredient seems to be missing for explaining the clustering of grains in the sub- mm to cm range. We propose that electrification through tribocharging is the missing ingredient. Thus, the main objective of this proposal is to address how different pair-wise interactions and, in general, particle and collisional properties, lead to sustained cluster growth. We are developing two experimental systems to make concrete steps toward this goal. In the first, we are using a free-fall apparatus to observe collisions between sub-mm particles in vacuum and zero-gravity conditions. In the second, we are forging into the new territory of interactions between millimeter-scale particles or clusters with a controlled acoustic levitation setup. In order to understand the microphysics of grain growth in the sub-mm to cm range, our immediate objective is to characterize the sticking efficiencies and dominant forces—including the possibility of same-material tribocharging—in a variety of conditions. In the first experimental setup, we will focus on few-particle interactions and clustering. In the second, we will study controlled collisions between a few up to many-particle clusters. Working on these two experiments in tandem will enable us to characterize collisions over decades of data in cluster size and impact energy, and quantify same-material tribocharging. For both experiments we will use standard dielectric materials (as ZrO2:SiO2 composites) as a benchmark. Then, we will use analog meteorite materials (e.g. San Carlos Olivine) in both setups, and original meteorite grains (Allende meteorite) with the ultrasonic setup, where controlled collisions can be done for smaller amounts of material. The outcome of such experiments will be a phase portrait of collisional aggregation efficiency covering features ranging from particle and cluster size to particle interactions, particle composition and impact energy. One particular contribution we will focus on is the effect of tribocharging on the formation efficiency of larger clusters, which should be relevant toward our current understanding of asteroid and planetesimal formation.

Las ondas y las estructuras coherentes están presentes como entidades individuales en varios contextos físicos, astronómicos y geofísicos, y particularmente en los fluidos. Las situaciones realistas generalmente involucran a ambos, lo que lleva a procesos de interacción complejos que son difíciles de separar y desenredar. En esta propuesta nos enfocamos en estudiar experimentalmente cómo la presencia de estructuras coherentes, tales como vórtices o singularidades, afectan las propiedades de las ondas superficiales en un régimen turbulento. Para ello, construiremos dos montajes experimentales para poder estudiar de forma sistemática las propiedades estadísticas de las ondas cuando interactúan con las estructuras mencionadas. Ambos sistemas tienen la ventaja de que, ajustando los parámetros de forzamiento, podemos controlar la aparición e intensidad de las estructuras. Por lo tanto, un estudio sistemático de su influencia en turbulencia de ondas (WT) es sencillo. La naturaleza intermitente del campo de ondas, así como los mecanismos detrás de la ruptura del espectro de WT en presencia de estas estructuras son algunas de las preguntas que pretendemos responder. Para abordar estas preguntas, proponemos realizar mediciones espaciotemporales, como la fotografía de luz difusa (DLP) y la velocimetría de imagen de partículas (PIV). Los resultados que surjan de esta investigación podrían ser de gran importancia para una teoría que, si bien es válida en muchos sistemas, aún está incompleta. Las aplicaciones de los resultados a otros sistemas, como los flujos geofísicos, también podrían ser posibles y bastante relevantes para una amplia comunidad.

Los laboratorios de fabricación digital son espacios que cuentan con maquinaria y personal capacitado para facilitar el diseño y desarrollo de prototipos y para promover la innovación en productos, procesos y servicios. Se conciben como laboratorios que facilitan herramientas de fabricación avanzada y capacidades a la comunidad en general, pudiendo ser más enfocados a emprendedores, empresas e institutos de investigación. Una característica común es que sirven como plataforma para estimular el aprendizaje y la invención en la comunidad. Las máquinas y capacidades técnicas instaladas en estos laboratorios brindan la oportunidad de encontrar soluciones innovadoras a problemas comunes y ser incubadores de microemprendimientos que resuelvan problemas de forma innovadora y sustentable. El primer laboratorio de fabricación digital, junto con el concepto FabLab, aparece en el MIT (Massachussets Institute of Technology, Estados Unidos) en el año 2000. Actualmente, existe una red mundial de alrededor de 3000 FabLabs distribuidos en 5 continentes. En Chile se pueden encontrar 17 de estos laboratorios, la mayoría de ellos concentrados en la Región Metropolitana; 2 en la Región del Maule y ninguno en la Región de O’Higgins. La ausencia de un laboratorio regional está en concordancia con estadísticas del año 2016 que reportan apenas 118 m2 de espacios dedicados a innovación en la Región de O’Higgins frente a 27 936 m2 en la Región Metropolitana. En ese contexto, la Región de O’Higgins es la segunda región con menor superficie dedicada a innovación. La instalación de un laboratorio de fabricación digital en la Región de O’Higgins se identifica como una gran oportunidad para promover la innovación, brindando acceso a equipos y a capacitaciones sobre herramientas de fabricación avanzada a industrias y emprendedores regionales.

In this project, we plan to study the hydrodynamic wave-vortex interaction problem from an experimental point of view using different setups. The aim is to gain further understanding about the influence of vorticity on the propagation of waves and, to a lesser extend, to study how the vorticity field is modified by the presence of waves. Specifically, we plan to study the influence of a vortex field on a sloshing wave, to track the wave scattering, damping and dissipation. Then, we will study the influence of vorticity on wave-turbulence, in order to see how the wave statistics (wave spectrum, height distributions) and properties (dispersion relation, dissipation mechanisms) are affected by vorticity. Finally we will study how an array of vortices induced by a Kelvin-Helmholtz instability can generate surface waves and the back-reaction of the waves on the vortices. The proposed research is based on the collaboration efforts from the long- standing scientific relation between french and chilean experimental nonlinear laboratories: the Matter-out-of- equilibrium laboratory (LMFE) of the Physics Department from the Universidad de Chile and the Nonlinear Physics group from the Laboratoire de Physique Statistique de l''Ecole Normale Supérieure de Paris, France. The expected outcomes of this proposal are: i) to consolidate and expand our french collaboration network including new research labotarories (Laboratoire de Matière et Systèmes Complèxes, Paris, France and Laboratoire des Écoulements Géophysiques et Industriels, Université de Grenoble-Alpes, Grenoble, France), ii) to co-sign two (2) research publications in Q1 journals, iii) to train postdocs and graduate students in experimental acoustical and optical techniques to measure temporal or spatiotemporal surface wave deformations.

Waves are ubiquitous in nature. They are all around us in our daily lives, we find them in several contexts, in particular in fluids. They usually involve a complex variety of interaction processes, and di↵erent mechanisms. Of our particular inter- est is the case of waves at the interface between two fluids when they are perturbed. When strongly forced, the nonlinear interactions can produce a turbulent-like regime called wave turbulence. Theoretical, numerical and experimental studies have made a great deal of progress on this subject, and yet, there are several aspects that have not been properly addressed, namely the role of viscosity on the energy flux as it cascades through di↵erent scales or the physical origin of the intermittency phenomenon. In this proposal, we will consider the problem of capillary wave turbulence from an experimental and numerical point of view. One of the main complications to study surface wave turbulence is that, in the same system, there is involvement of di↵erent types of waves, such as the case of gravito-capillary wave turbulence. Thus, it becomes of foremost importance to study wave turbulence on the presence of only one type of waves. Thereby, in order to study pure capillary wave turbulence, gravity waves must be negligible. We propose to study a system of capillary surface waves at the interface of two immiscible and incompressible fluids, water and silicon oil, of almost equal densities and layer depths, thus preventing the action of gravity. By changing the kinematic viscosity ⌫, and density ⇢ of both fluids, it is possible to control the relation between injected and dissipated power, thus exploring several regimes in a simple and controlled way. We pose to implement the technique called Free-surface synthetic Schlieren that allows a reconstruction of the instantaneous surface topography. Velocity fields will be explored by using the standard Particle Image Velocimetry. We will make use of an already existing experimental setup which will be modified in order to accomplish these techniques adequately. With these measurements we will be able to compute the spectrum in frequency f and wavevector k, hence accessing to statistical and dynamical properties of capillary wave turbulence, such as intermittency, or the function of the injected power on the system. We also propose to use the open source solver GERRIS and make a systematic study on the role of viscosity on the cascading of the energy flux.

Position that involved teaching an undergraduate course to students from ENS along with research on an experimental study of wave turbulence.