Research
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Using a multi-orifice cylindrical silo, the average flow rates Q1 and QN through one and N orifices, respectively, were first measured for the discharge of water or dry grains and then for the mixture. As expected, QN = NQ1 for monophasic discharges. Nevertheless, for the mixture, QN < NQ1, and the effect becomes more notorious as N augments and when the grain size is decreased. A simplified continuum model of a flow through a dynamic porous medium, with hydrodynamic resistance that increases with N, is used to reproduce the experimental results. Additional analysis in a two-dimensional cell reveals interacting parabolic flow profiles of the immersed particles close to the orifices, with a reduction in the average velocities when N is increased, which helps to understand the non-additivity of the total flow rate.
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Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate
Cover letter in Physical Review Letters, Editors suggestions and featured in Physics We report on the collision-coalescence dynamics of drops in Leidenfrost state using liquids with different physicochemical properties. Drops of the same liquid deposited on a hot concave surface coalesce practically at contact, but when drops of different liquids collide, they can bounce several times before finally coalescing when the one that evaporates faster reaches a size similar to its capillary length. The bouncing dynamics is produced because the drops are not only in Leidenfrost state with the substrate, they also experience Leidenfrost effect between them at the moment of collision. This happens due to their different boiling temperatures, and therefore, the hotter drop works as a hot surface for the drop with lower boiling point, producing three contact zones of Leidenfrost state simultaneously. We called this the triple Leidenfrost effect. |
Craters produced by explosions in a granular medium
We report on an experimental investigation of craters generated by explosions at the surface of a model granular bed. Following the initial blast, a pressure wave propagates through the bed, producing high-speed ejecta of grains and ultimately a crater. We analyzed the crater morphology in the context of large-scale explosions and other cratering processes. The process was analyzed in the context of large-scale explosions, and the crater morphology was compared with those resulting from other cratering processes in the same energy range. |
Surface depression profile during the discharge of a siloWhen rough grains in loose packing conditions are discharged from a silo, a conical depression with a single slope is formed at the surface. We observed that the increase of volume fraction generates a more complex depression, characterized by two angles of discharge: one at the bottom similar to the angle of repose and a considerably larger upper angle. Our results show that the surface profile helps to identify by simple visual inspection the packing conditions of a granular bed, being useful to prevent undesirable collapses during silo discharge in industry. |
Leidenfrost phenomenon on conical surfaces
A liquid droplet deposited on a sufficiently hot surface levitates on its own vapor and evaporates slowly. Droplets with radius smaller than the liquid capillary length become spherical because surface tension prevails against gravity, whereas large drops are flattened by their own weight. The intermediate vapor layer avoids adhesive forces and suppresses friction, allowing the droplet to easily move on a horizontal flat plate. The above phenomenon is called Leidenfrost state. Since its discovery 300 years ago, researchers studied this phenomenon in flat surfaces. Here, for the first time, we confined the droplet using conical plates, and we found that large liquid volumes can be levitated under confinement during a long time, displaying a rich variety of surface patterns. |
Flow of magnetic repelling grains in a two-dimensional silo
During a typical silo discharge, the material flow rate is determined by the contact forces between the grains. Here, we report an original study concerning the discharge of a two-dimensional silo filled with repelling magnetic grains. This non-contact interaction leads to a different dynamics from the one observed with conventional granular materials. Although the flow rate dependence on the aperture size follows roughly the power-law with an exponent 3/2 found in non-repulsive systems, the density and velocity profiles during the discharge are totally different. New phenomena must be taken into account. This study is an example of flow of non-contacting particles through bottlenecks, as it occurs with cars and traffic, pedestrians in the street, etc. |
It is well known that craters observed in lunar and planetary surfaces were mostly produced by meteorite impacts. However, there are alternative processes, such as volcanic eruptions and explosions, that can also produce craters. For instance, a maar is a natural depression created by an underground steam explosion that occurs when magma comes into contact with shallow ground water. We study experimentally a similar scenario: the cratering process due to the explosion and collapse of a pressurized air cavity inside a sand bed. |
We study how the bouncing dynamics of a hollow ball on a vibrating plate is modified when it is partially filled with liquid or grains. Whereas empty and liquid-filled balls display a dominant chaotic dynamics, a ball with grains exhibits a rich variety of stationary states, determined by the grain size and filling volume.
A ball dropped over a solid surface bounces several times before a complete stop. The bouncing can be reduced by introducing a liquid into the ball; however, the first rebound remains largely unaffected by the fluid. Granular materials can also work as dampers. We investigated the rebound of a container partially filled with a given mass of grains mi. During the collision, the kinetic energy of the container is partially transferred to the grains, the rebound is damped, and the fast energy dissipation through inter-particle collisions and friction decreases the bouncing time dramatically.
Sculpting sandcastles grain by grain.
When dry sand is poured on a table, a sandpile is formed. If the process is repeated in a pool of water, a similar pile is obtained. But, what happend at the interface water-air?
We found that an intriguing phenomenon occurs: From that moment, the dry grains hit a wet sandsurface and start to accumulate at the top of the heap, the angle of repose reaches 90◦, and a vertical sand tower emerges [see figure]. This self-assembly mechanism (only observed in the funicular and capillary regimes) could theoretically last while the capillary rise of water is possible; however, the structure collapses before reaching this limit. The collapse occurs when the weight of the tower surpasses the cohesive stress at its base.
Granular projectiles vs Granular tarjets.
Meteorite impacts is the main theory to explain the origin of craters on planetary surfaces. In the lab, the simplest way to perform impact experiments is by releasing balls from a finite height into granular targets. A myriad of low-speed impact experiments performed during the last decades used unbreakable solid balls. However, real asteroids have a dominating granular structure (as revealed by their high porosity) and disintegrate after collision. So, this raises an interesting question: Are the crater morphologies produced by solid and granular projectiles similar and described by the same relationships? We study the crater morphology obtained by dropping sand balls on a sand bed. Simple and complex craters, similar to those observed in planets and moons were obtained (see figure).
Meteorite impacts is the main theory to explain the origin of craters on planetary surfaces. In the lab, the simplest way to perform impact experiments is by releasing balls from a finite height into granular targets. A myriad of low-speed impact experiments performed during the last decades used unbreakable solid balls. However, real asteroids have a dominating granular structure (as revealed by their high porosity) and disintegrate after collision. So, this raises an interesting question: Are the crater morphologies produced by solid and granular projectiles similar and described by the same relationships? We study the crater morphology obtained by dropping sand balls on a sand bed. Simple and complex craters, similar to those observed in planets and moons were obtained (see figure).
Cooperative dynamics of a projectile in a granular medium. An object moving in a fluid experiences a drag force that depends on its velocity, shape and the properties of the medium. From this simplest case to the motion of a flock of birds or a school of fish, the drag forces and the hydrodynamic interactions determine the full dynamics of the system. Similar drag forces appear when a single projectile impacts and moves through a granular medium, and this case is well studied in the literature. On the other hand, the case in which a group of intruders impact a granular material has never been considered. Here, we study the simultaneous penetration of several intruders in a very low-density granular medium. We find that the intruders move through it in a collective way, following a cooperative dynamics, whose complexity resembles flocking phenomena in living systems or the movement of reptiles in sand, wherein changes in drag are exploited to efficiently move or propel. |
Infinite penetration of a projectil in a granular medium.
La fuerza de arrastre que un medio granular opone a ser penetrado es razonablemente bien descrita por el modelo K-D [20]. Según este modelo, el arrastre es igual a la suma de un término inercial cuadrático en la velocidad v y un término lineal con la profundidad z. Sin embargo, en una columna granular, la presión ejercida al fondo del material crece con la profundidad hasta saturar a un valor constante (efecto Janssen). Si el proyectil alcanza dicha zona, la dependencia lineal en z no puede ser válida. Nosotros demostramos teórica y experimentalmente, que el modelo K-D es sólo un caso particular válido para pequeñas penetraciones. Propusimos un modelo general que reemplaza el término lineal en z por una dependencia tipo Janssen, revelando que un proyectil con una densidad mayor a un valor crítico penetra en el medio granular hasta alcanzar una velocidad terminal. Esta saturación en la fuerza de arrastre también se estudió con proyectiles impactados horizontalmente en una cama granular a una profundidad en la que la presión ha saturado, desacoplando el efecto de la gravedad.
La fuerza de arrastre que un medio granular opone a ser penetrado es razonablemente bien descrita por el modelo K-D [20]. Según este modelo, el arrastre es igual a la suma de un término inercial cuadrático en la velocidad v y un término lineal con la profundidad z. Sin embargo, en una columna granular, la presión ejercida al fondo del material crece con la profundidad hasta saturar a un valor constante (efecto Janssen). Si el proyectil alcanza dicha zona, la dependencia lineal en z no puede ser válida. Nosotros demostramos teórica y experimentalmente, que el modelo K-D es sólo un caso particular válido para pequeñas penetraciones. Propusimos un modelo general que reemplaza el término lineal en z por una dependencia tipo Janssen, revelando que un proyectil con una densidad mayor a un valor crítico penetra en el medio granular hasta alcanzar una velocidad terminal. Esta saturación en la fuerza de arrastre también se estudió con proyectiles impactados horizontalmente en una cama granular a una profundidad en la que la presión ha saturado, desacoplando el efecto de la gravedad.
Superheating in granular matter.
Un sólido supercaliente es un material que permanece en estado sólido por encima de su punto de fusión. De forma análoga, nosotros encontramos que una monocapa de esferas vibrada fuertemente persiste como un cristal durante algún tiempo, hasta que sin perturbación externa alguna, el sistema se evapora abruptamente[13]. Esta transición es generada por la disipación de la energía a través de colisiones y el confinamiento de los granos. El sistema presenta otros comportamientos análogos a fenómenos termodinámicos como coexistencia de fases y metaestabilidad. |