Ricardo Paredes

Vertical Granular Transport in a Vibrated U-Tube

We present an experimental study of the collective motion of grains inside a U shaped tube undergoing vertical oscillations, and we develop a very simple quantitative model that captures relevant features of the observed behavior. The height difference between the granular columns grows exponentially with time when the system is shaken at sufficiently low frequencies. In vacuum the exponential growth is suppressed.

Nematic phase transitions in two-dimensional systems

Simulations of nematic-isotropic transition of liquid crystals in two dimensions are performed using an

Topological transition in a two-dimensional model of liquid crystal.

O(2)

Evidence for a topological transition in nematic-to-isotropic phase transition in two dimensions

vector model characterised by non linear nearest neighbour spin interaction governed by the fourth Legendre polynomial

Reverse buoyancy in a vibrated granular bed: Computer simulations.

P_4

Three-dimensional simulations of a vertically vibrated granular bed including interstitial air.

. The system is studied through standard Finite-Size Scaling and conformal rescaling of density profiles or correlation functions. The low temperature limit is discussed in the spin wave approximation and confirms the numerical results, while the value of the correlation function exponent at the deconfining transition seems controversial.

On the critical behaviour of two-dimensional liquid crystals

Simulations of nematic-isotropic transition of liquid crystals in two dimensions are performed using an O2 vector model characterized by nonlinear nearest neighbor spin interaction governed by the fourth Legendre polynomial P4. The system is studied through standard finite-size scaling and conformal rescaling of density profiles of correlation functions. A topological transition between a paramagnetic phase at high temperature and a critical phase at low temperature is observed. The low temperature limit is discussed in the spin wave approximation and confirms the numerical results.

Transiciones de fase topológicas en cristales líquidos

Abstract The nematic-to-isotropic orientational phase transition, or equivalently the RP 2 model, is considered in two dimensions and the question of the nature of the phase transition is addressed. Using powerful conformal techniques adapted to the investigation of critical properties of two-dimensional scale-invariant systems, we report strong evidences for a transition governed by topological defects analogous to the Berezinskii–Kosterlitz–Thouless transition in two-dimensional XY model.

Topological transition in a two-dimensional model of liquid crystal

AbstractWe have performed molecular dynamics simulations of an intruder in a vibrated granular bed including interstitial fluid effects to account for the phenomenon of reverse buoyancy. We show that our model is able to reproduce the overall behaviour observed by previous experimental works and is the first finite-elements simulation to show the sinking of intruders lighter than the granular bed. To further advance our comprehension of this phenomenon, we studied the motion of the intruders in a single vibration cycle with respect to the bottom of the granular column, finding a substantial qualitative difference for heavy and light intruders and we compare these results with experiments using fine-sized glass beads. We show that, though heavy intruders seem unaffected by the force due to the fluid, the effect on light intruders is remarkable.