Helmholtz-Institut Erlangen-Nürnberg,
Research Centre Jülich

Welcome to my personal website. Here you can find an overview of my scientific research interests, as well as useful contact and curriculum information.

Research interests

My research is driven by a profound interest in out-of-equilibrium dynamical systems. Self organization processes particularly call my attention, as the development of patterns, segregation and other sorts of collective dynamics present in many-particle systems.

Through my Masters and PhD I've been mainly studying granular materials in different geometries and energy injection conditions, using simulations and developing continuum theories, as also collaborating with fellow experimentalists.

Collective dynamics in granular systems

Granular segregation

Granular segregation
Click for more images and videos.

Grains segregation is a great example of an everyday phenomena that has proven very challenging to understand on its fundamental aspects. It should come as no surprise that collections of grains segregate; we observe it in cereal boxes, sand, or even in the composition of normal soil. Careful analysis of this phenomenon leads to interesting, general questions about non-equilibrium phase transitions, critical phenomena and many-particle dynamics. In the top image you can see snapshots of how an initially mixed collection of spheres of different sizes or masses end up segregating when vertically agitated, in a particularly shallow container. Grains can mix or segregate depending on how the container is vibrated. The properties of the grains, and even the height of the container were seen to play a fundamental role.

Explosions in shallow vibrated geometries

Granular matter often shows remarkable, unexpected phenomena, as it couples continuum-like behaviours with individual particle dynamics. When and how many-particle systems can be considered as continuum media is an exceptionally complex question, and involves the revision and expansion of hydrodynamic equations and thermodynamic concepts. I have a special interest in systems that push the limit of continuum models. An example is shown on the top image, where a cluster of heavier particles is seen to be in a cycle of violent expansions and compression phases. Surprisingly, the source of this phenomenon can be traced to the dynamics of a single particle.

Low-frequency oscillations in vibrated systems

Granular oscillator

A column of grains is seen to oscillate in a pseudo-periodic way when the whole is setup is vertically vibrated. This is another notable example of a collective behaviour of a system involving just a few hundred particles. It turns out that these oscillations can be described with a fairly good accuracy by a simple harmonic oscillator, a nice example of how many-particle systems (here only 300 particles are involved!) can collectively behave as fundamental physical systems. We (A. Thornton, S. Luding and me) are currently studying the influence of these oscillations in the transitions to convective regimes present in wider systems, as well as expanding the model to consider energy balances and other geometries.

Transition to buoyancy-driven convection

Granular dynamics can often show striking similarities with regular fluids. Studying the nature and limit of these similarities allows us to see, among other things, where does classical continuum theories fail to describe grains, and what is the origin of these discrepancies. One particularly interesting case is the transition in agitated granular systems from the Leidenfrost state to a buoyancy driven convective state, where convection rolls are generated. Together with D. van der Meer, S. Luding and A. R. Thornton we studied this transition in detail, paying special attention to the precursor states and the influence of fluctuations on the transition.

Dense granular packings, structure and dynamics

When densely packed, collections of grains show remarkable, apparently amorphous structures that determine their dynamical response. The development of force-chains, localized paths of contacts through which forces are transmitted, is one of the fundamental characteristics of granular media. Interestingly, depending on the speed of compression, grains can either arrange in crystalline order or enter amorphous, glassy-like metastable states. Researchers have for decades tried to find order in these at first sight amorphous states, using many different geometrical parameters. Together with V. Ogarko and S. Luding we have proposed a method to characterize dense packings based on the distribution of the spaces in between particles, and with this deduced that particles do present a local preferred arrangement in glassy states of four co-planar particles.

Other interests

Aside from physics, I have a passion for literature and philosophy. Since 2011 I've been using Goodreads to keep a register of the books I read; in case you are interested, you can take a look here. Sometimes I write small reviews, be warned that some are in Spanish!

Last updated October 2015. Completely copyleft.

Nicolas Rivas

FIG. 1. (color online). The subject (green), his niece Amanda (red and flowers) and the dog of his sister, Junior (white with black dots).



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