Searcharxiv⌕ Search

arXiv subjects

G. Gonnella

Publications and source records attributed to G. Gonnella.

At least 37 records · Page 2Linked to original sources
arXiv2011

Switching and defect dynamics in multistable liquid crystal devices

We investigate the switching dynamics of multistable nematic liquid crystal devices. In particular we identify a remarkably simple 2-dimensional (2D) device which exploits hybrid alignment at the surfaces to yield a bistable response. We also consider a 3-dimensional (3D) tristable nematic device with patterned anchoring, recently implemented in practice, and discuss how the director and disclination patterns change during switching.

cond-mat.soft↗
arXiv2010

Phase separation of binary fluids with dynamic temperature

Phase separation of binary fluids quenched by contact with cold external walls is considered. Navier-Stokes, convection-diffusion, and energy equations are solved by lattice Boltzmann method coupled with finite-difference schemes. At high viscosity, different morphologies are observed by varying the thermal diffusivity. In the range of thermal diffusivities with domains growing parallel to the walls, temperature and phase separation fronts propagate towards the inner of the system with power-law behavior. At low viscosity hydrodynamics favors rounded shapes, and complex patterns with different lengthscales appear. Off-symmetrical systems behave similarly but with more ordered configurations.

cond-mat.soft↗
arXiv2009

Monte Carlo study of the phase transition in the Critical behavior of the Ising model with shear

The critical behavior of the Ising model with non-conserved dynamics and an external shear profile is analyzed by studying its dynamical evolution in the short time regime. Starting from high temperature disordered configurations (FDC), the critical temperature $T_c$ is determined when the order parameter, defined as the absolute value of the transversal spin profile, exhibits a power-law behavior with an exponent that is a combination of some of the critical exponents of the transition. For each value of the shear field magnitude, labeled as $\dotγ$, $T_c$ has been estimated and two stages have been found: 1) a growing stage at low values of $\dotγ$, where $T_c\sim\dotγ^ψ$ and $ψ=0.52(3)$; 2) a saturation regime at large $\dotγ$. The same values of $T_c(\dotγ)$ were found studying the dynamical evolution from the ground state configuration (GSC) with all spins pointing in the same direction. By combining the exponents of the corresponding power laws obtained from each initial configuration the set of critical exponents was calculated. These values, at large external field magnitude, define a new critical behavior different from that of the Ising model and of other driven lattice gases.

cond-mat.stat-mech↗
arXiv2009

Hybrid lattice Boltzmann model for binary fluid mixtures

A hybrid lattice Boltzmann method (LBM) for binary mixtures based on the free-energy approach is proposed. Non-ideal terms of the pressure tensor are included as a body force in the LBM kinetic equations, used to simulate the continuity and Navier-Stokes equations. The convection-diffusion equation is studied by finite difference methods. Differential operators are discretized in order to reduce the magnitude of spurious velocities. The algorithm has been shown to be stable and reproducing the correct equilibrium behavior in simple test configurations and to be Galilean invariant. Spurious velocities can be reduced of about an order of magnitude with respect to standard discretization procedure.

cond-mat.soft↗
arXiv2009

Fluctuation relations in non-equilibrium stationary states of Ising models

Fluctuation relations for the entropy production in non equilibrium stationary states of Ising models are investigated by Monte Carlo simulations. Systems in contact with heat baths at two different temperatures or subject to external driving will be studied. In the first case, by considering different kinetic rules and couplings with the baths, the behavior of the probability distributions of the heat exchanged in a time $τ$ with the thermostats, both in the disordered and in the low temperature phase, are discussed. The fluctuation relation is always verified in the large $τ$ limit and deviations from linear response theory are observed. Finite-$τ$ corrections are shown to obey a scaling behavior. In the other case the system is in contact with a single heat bath but work is done by shearing it. Also for this system the statistics collected for the mechanical work shows the validity of the fluctuation relation and preasymptotic corrections behave analogously to the case with two baths.

cond-mat.stat-mech↗
arXiv2008

Heat fluctuations in Ising models coupled with two different heat baths

Monte Carlo simulations of Ising models coupled to heat baths at two different temperatures are used to study a fluctuation relation for the heat exchanged between the two thermostats in a time $τ$. Different kinetics (single--spin--flip or spin--exchange Kawasaki dynamics), transition rates (Glauber or Metropolis), and couplings between the system and the thermostats have been considered. In every case the fluctuation relation is verified in the large $τ$ limit, both in the disordered and in the low temperature phase. Finite-$τ$ corrections are shown to obey a scaling behavior.

cond-mat.stat-mech↗
arXiv2005

Morphologies and flow patterns in quenching of lamellar systems with shear

We study the behavior of a fluid quenched from the disordered into the lamellar phase under the action of a shear flow. The dynamics of the system is described by Navier-Stokes and convection-diffusion equations with pressure tensor and chemical potential derived by the Brazovskii free-energy. Our simulations are based on a mixed numerical method with Lattice Boltzmann equation and finite difference scheme for Navier-Stokes and order parameter equations, respectively. We focus on cases where banded flows are observed with two different slopes for the component of velocity in the direction of the applied flow. Close to the walls the system reaches a lamellar order with very few defects and the slope of the horizontal velocity is higher than the imposed shear rate. In the middle of the system the local shear rate is lower than the imposed one and the system looks as a mixture of tilted lamellae, droplets and small elongated domains. We refer to this as to a region with a Shear Induced Structures (SIS) configuration. The local behavior of the stress shows that the system with the coexisting lamellar and SIS regions is in mechanical equilibrium. This phenomenon occurs, at fixed viscosity, for shear rates under a certain threshold; when the imposed shear rate is sufficiently large, lamellar order develops in the whole system. Effects of different viscosities have been also considered: The SIS region is observed only at low enough viscosity. We compare the above scenario with the usual one of shear banding. In particular, we do not find evidence for a plateau of the stress at varying imposed shear rates in the region with banded flow. We interpret our results as due to a tendency of the lamellar system to oppose to the presence of the applied flow.

cond-mat.soft↗
arXiv2005

Response of Autonomic Nervous System to Body Positions: Fourier and Wavelet Analysis

Two mathematical methods, the Fourier and wavelet transforms, were used to study the short term cardiovascular control system. Time series, picked from electrocardiogram and arterial blood pressure lasting 6 minutes, were analyzed in supine position (SUP), during the first (HD1), and the second parts (HD2) of $90^{\circ}$ head down tilt and during recovery (REC). The wavelet transform was performed using the Haar function of period $T=2^j$ ($% j=1$,2,$... $,6) to obtain wavelet coefficients. Power spectra components were analyzed within three bands, VLF (0.003-0.04), LF (0.04-0.15) and HF (0.15-0.4) with the frequency unit cycle/interval. Wavelet transform demonstrated a higher discrimination among all analyzed periods than the Fourier transform. For the Fourier analysis, the LF of R-R intervals and VLF of systolic blood pressure show more evident difference for different body positions. For the wavelet analysis, the systolic blood pressures show much more evident difference than the R-R intervals. This study suggests a difference in the response of the vessels and the heart to different body positions. The partial dissociation between VLF and LF results is a physiologically relevant finding of this work.

physics.bio-ph↗
arXiv2005

Scaling and hydrodynamic effects in lamellar ordering

We study the kinetics of domain growth of fluid mixtures quenched from a disordered to a lamellar phase. At low viscosities, in two dimensions, when hydrodynamic modes become important, dynamical scaling is verified in the form $C(\vec k, t) \sim L^α f[(k-k_M)L]$ where $C$ is the structure factor with maximum at $k_M$ and $L$ is a typical length changing from power law to logarithmic growth at late times. The presence of extended defects can explain the behavior of $L$. Three-dimensional simulations confirm that diffuse grain boundaries inhibit complete ordering of lamellae. Applied shear flow alleviates frustration and gives power-law growth at all times.

cond-mat.soft↗
arXiv2004

Finite difference lattice Boltzmann model with flux limiters for liquid-vapor systems

In this paper we apply a finite difference lattice Boltzmann model to study the phase separation in a two-dimensional liquid-vapor system. Spurious numerical effects in macroscopic equations are discussed and an appropriate numerical scheme involving flux limiter techniques is proposed to minimize them and guarantee a better numerical stability at very low viscosity. The phase separation kinetics is investigated and we find evidence of two different growth regimes depending on the value of the fluid viscosity as well as on the liquid-vapor ratio.

cond-mat.soft↗
arXiv2003

Phase separation of incompressible binary fluids with Lattice Boltzmann Methods

We introduce new versions of lattice Boltzmann methods (LBM) for incompressible binary mixtures where fluctuations of total density are inhibited. As a test for the improved algorithms we consider the problem of phase separation of two-dimensional binary mixtures quenched from a disordered state into the coexistence region. We find that the stability properties of LBM are greatly improved. The control of density fluctuations and the possibility of running longer simulations allow a more precise evaluation of the growth exponent.

cond-mat.soft↗
arXiv2003

Lattice Boltzmann Study of Velocity Behaviour in Binary Mixtures Under Shear

We apply lattice Boltzmann methods to study the relaxation of the velocity profile in binary fluids under shear during spinodal decomposition. In simple fluids, when a shear flow is applied on the boundaries of the system, the time required to obtain a triangular profile is inversely proportional to the viscosity and proportional to the square of the size of the system. We find that the same behaviour also occurs for binary mixtures, for any component ratio in the mixture and independently from the time when shear flow is switched on during phase separation.

physics.flu-dyn↗
arXiv2002

Phase separating binary fluids under oscillatory shear

We apply lattice Boltzmann methods to study the segregation of binary fluid mixtures under oscillatory shear flow in two dimensions. The algorithm allows to simulate systems whose dynamics is described by the Navier-Stokes and the convection-diffusion equations. The interplay between several time scales produces a rich and complex phenomenology. We investigate the effects of different oscillation frequencies and viscosities on the morphology of the phase separating domains. We find that at high frequencies the evolution is almost isotropic with growth exponents 2/3 and 1/3 in the inertial (low viscosity) and diffusive (high viscosity) regimes, respectively. When the period of the applied shear flow becomes of the same order of the relaxation time $T_R$ of the shear velocity profile, anisotropic effects are clearly observable. In correspondence with non-linear patterns for the velocity profiles, we find configurations where lamellar order close to the walls coexists with isotropic domains in the middle of the system. For particular values of frequency and viscosity it can also happen that the convective effects induced by the oscillations cause an interruption or a slowing of the segregation process, as found in some experiments. Finally, at very low frequencies, the morphology of domains is characterized by lamellar order everywhere in the system resembling what happens in the case with steady shear.

cond-mat.soft↗
arXiv2002

Computer simulations of domain growth in off-critical quenches of two-dimensional binary mixtures

The phase separation of two-dimensional binary mixtures has been studied through numerical Langevin simulations based on a Ginzburg-Landau free energy. We have considered not symmetric mixtures with and without imposed shear flow. In the sheared case our main results are as follows: (1) domains are distorted by the flow; (2) the structure factor has four peaks; (3) excess viscosity shows a peak whose position is independent of shear rate but its height decreases increasing shear rate.

cond-mat.soft↗
arXiv2002

The segregation of sheared binary fluids in the Bray-Humayun model

The phase separation process which follows a sudden quench inside the coexistence region is considered for a binary fluid subjected to an applied shear flow. This issue is studied in the framework of the convection-diffusion equation based on a Ginzburg-Landau free energy functional in the approximation scheme introduced by Bray and Humayun [{\it Phys.Rev.Lett.} {\bf 68}, 1559, (1992)]. After an early stage where domains form and shear effects become effective the system enters a scaling regime where the typical domains sizes $L_\parallel $, $L_\perp$ along the flow and perpendicular to it grow as $t^{5/4}$ and $t^{1/4}$. The structure factor is characterized by the existence of four peaks, similarly to previous theoretical and experimental observations, and by exponential tails at large wavevectors.

cond-mat.soft↗
arXiv2002

Ordering of the lamellar phase under a shear flow

The dynamics of a system quenched into a state with lamellar order and subject to an uniform shear flow is solved in the large-N limit. The description is based on the Brazovskii free-energy and the evolution follows a convection-diffusion equation. Lamellae order preferentially with the normal along the vorticity direction. Typical lengths grow as $γt^{5/4}$ (with logarithmic corrections) in the flow direction and logarithmically in the shear direction. Dynamical scaling holds in the two-dimensional case while it is violated in D=3.

cond-mat.soft↗
arXiv2002

Effects of an external drive on the fluctuation-dissipation relation of phase-ordering systems

The relation between the autocorrelation $C(t,t_w)$ and the integrated linear response function $χ(t,t_w)$ is studied in the context of the large-N model for phase-ordering systems subjected to a shear flow. In the high temperature phase $T>T_c$ a non-equilibrium stationary state is entered which is characterized by a non-trivial fluctuation-dissipation relation $χ(t-t_w)=\tilde χ(C(t-t_w))$. For quenches below $T_c$ the splitting of the order parameter field into two statistically independent components, responsible for the stationary $C^{st}(t-t_w)$ and aging $C^{ag}(t/t_w)$ part of the autocorrelation function, can be explicitly exhibited in close analogy with the undriven case. In the regime $t-t_w\ll t_w$ the same relation $χ(t-t_w)=\tilde χ(C^{st}(t-t_w))$ is found between the response and $C^{st}(t-t_w)$, as for $T>T_c$ . The aging part of $χ(t,t_w)$ is negligible for $t_w\to \infty$, as without drive, resulting in a flat $χ(C)$ in the aging regime $t-t_w\gg t_w$.

cond-mat.stat-mech↗