A. Lamura — Page 3 | Searcharxiv

arXiv2001

Semi-Flexible Polymer in a Uniform Force Field in Two Dimensions

The conformational properties of a semi-flexible polymer chain, anchored at one end in a uniform force field, are studied in a simple two-dimensional model. Recursion relations are derived for the partition function and then iterated numerically. We calculate the angular fluctuations of the polymer about the direction of the force field and the average polymer configuration as functions of the bending rigidity, chain length, chain orientation at the anchoring point, and field strength.

cond-mat.soft↗

arXiv2001

Multi-particle-collision dynamics: Flow around a circular and a square cylinder

A particle-based model for mesoscopic fluid dynamics is used to simulate steady and unsteady flows around a circular and a square cylinder in a two-dimensional channel for a range of Reynolds number between 10 and 130. Numerical results for the recirculation length, the drag coefficient, and the Strouhal number are reported and compared with previous experimental measurements and computational fluid dynamics data. The good agreement demonstrates the potential of this method for the investigation of complex flows.

cond-mat.soft↗

arXiv2000

Phase-separation of binary fluids in shear flow: a numerical study

The phase-separation kinetics of binary fluids in shear flow is studied numerically in the framework of the continuum convection-diffusion equation based on a Ginzburg-Landau free energy. Simulations are carried out for different temperatures both in d=2 and in d=3. Our results confirm the qualitative picture put forward by the large-N limit equations studied in \cite{noi}. In particular, the structure factor is characterized by the presence of four peaks whose relative oscillations give rise to a periodic modulation of the behavior of the rheological indicators and of the average domains sizes. This peculiar pattern of the structure factor corresponds to the presence of domains with two characteristic thicknesses whose relative abundance changes with time.

cond-mat↗

arXiv2000

Lattice Boltzmann simulations of segregating binary fluid mixtures in shear flow

We apply lattice Boltzmann method to study the phase separation of a two-dimensional binary fluid mixture in shear flow. The algorithm can simulate systems described by the Navier-Stokes and convection-diffusion equations. We propose a new scheme for imposing the shear flow which has the advantage of preserving mass and momentum conservation on the boundary walls without introducing slip velocities. Our main results concern the presence of two typical lenght scales in the phase separation process, corresponding to domains with two different thicknesses. Our simulations at low viscosity confirm previous results only valid in the limit of infinite viscosity.

cond-mat↗

arXiv2000

Structure and rheology of binary mixtures in shear flow

Results are presented for the phase separation process of a binary mixture subject to an uniform shear flow quenched from a disordered to a homogeneous ordered phase. The kinetics of the process is described in the context of the time-dependent Ginzburg-Landau equation with an external velocity term. The large-N approximation is used to study the evolution of the model in the presence of a stationary flow and in the case of an oscillating shear. For stationary flow we show that the structure factor obeys a generalized dynamical scaling. The domains grow with different typical lengthscales $R_x$ and $R_\perp$ respectively in the flow direction and perpendicularly to it. In the scaling regime $R_\perp \sim t^{α_\perp}$ and $R_x \sim γt^{α_x}$ (with logarithmic corrections), $γ$ being the shear rate, with $α_x=5/4$ and $α_\perp =1/4$. The excess viscosity $Δη$ after reaching a maximum relaxes to zero as $γ^{-2}t^{-3/2}$. $Δη$ and other observables exhibit log-time periodic oscillations which can be interpreted as due to a growth mechanism where stretching and break-up of domains cyclically occur. In the case of an oscillating shear a cross-over phenomenon is observed: Initially the evolution is characterized by the same growth exponents as for a stationary flow. For longer times the phase separating structure cannot align with the oscillating drift and a different regime is entered with an isotropic growth and the same exponents of the case without shear.

cond-mat↗

arXiv1999

Phase-ordering dynamics of binary mixtures with field-dependent mobility in shear flow

The effect of shear flow on the phase-ordering dynamics of a binary mixture with field-dependent mobility is investigated. The problem is addressed in the context of the time-dependent Ginzburg-Landau equation with an external velocity term, studied in self-consistent approximation. Assuming a scaling ansatz for the structure factor, the asymptotic behavior of the observables in the scaling regime can be analytically calculated. All the observables show log-time periodic oscillations which we interpret as due to a cyclical mechanism of stretching and break-up of domains. These oscillations are dumped as consequence of the vanishing of the mobility in the bulk phase.

cond-mat↗

arXiv1999

Two-scale competition in phase separation with shear

The behavior of a phase separating binary mixture in uniform shear flow is investigated by numerical simulations and in a renormalization group (RG) approach. Results show the simultaneous existence of domains of two characteristic scales. Stretching and cooperative ruptures of the network produce a rich interplay where the recurrent prevalence of thick and thin domains determines log-time periodic oscillations. A power law growth $ R(t) \sim t^α$ of the average domain size, with $α=4/3$ and $α= 1/3$ in the flow and shear direction respectively, is shown to be obeyed.

cond-mat↗

arXiv1998

Spinodal decomposition of a binary mixture in an uniform shear flow

Results are presented for the phase separation process of a binary mixture subject to an uniform shear flow quenched from a disordered to a homogeneous ordered phase. The kinetics of the process is described in the context of the time-dependent Ginzburg-Landau equation with an external velocity term. The one-loop approximation is used to study the evolution of the model. We show that the structure factor obeys a generalized dynamical scaling. The domains grow with different typical lengthscales $R_x$ and $R_y$ respectively in the flow and in the shear directions. In the scaling regime $R_y \sim t^{α_y}$ and $R_x \sim t^{α_x}$, with $α_x=5/4$ and $α_y =1/4$. The excess viscosity $Δη$ after reaching a maximum relaxes to zero as $γ^{-2}t^{-3/2}$, $γ$ being the shear rate. $Δη$ and other observables exhibit log-time periodic oscillations which can be interpreted as due to a growth mechanism where stretching and break-up of domains cyclically occur.

cond-mat↗

arXiv1998

A Lattice Boltzmann Model of Ternary Fluid Mixtures

A lattice Boltzmann model is introduced which simulates oil-water-surfactant mixtures. The model is based on a Ginzburg-Landau free energy with two scalar order parameters. Diffusive and hydrodynamic transport is included. Results are presented showing how the surfactant diffuses to the oil-water interfaces thus lowering the surface tension and leading to spontaneous emulsification. The rate of emulsification depends on the viscosity of the ternary fluid.

cond-mat.soft↗