arXiv subjects

M. M. Telo da Gama

Publications and source records attributed to M. M. Telo da Gama.

62 records · Page 4Linked to original sources

arXiv2003

Nonequilibrium wetting transitions with short range forces

We analyze within mean-field theory as well as numerically a KPZ equation that describes nonequilibrium wetting. Both complete and critical wettitng transitions were found and characterized in detail. For one-dimensional substrates the critical wetting temperature is depressed by fluctuations. In addition, we have investigated a region in the space of parameters (temperature and chemical potential) where the wet and nonwet phases coexist. Finite-size scaling analysis of the interfacial detaching times indicates that the finite coexistence region survives in the thermodynamic limit. Within this region we have observed (stable or very long-lived) structures related to spatio-temporal intermittency in other systems. In the interfacial representation these structures exhibit perfect triangular (pyramidal) patterns in one (two dimensions), that are characterized by their slope and size distribution.

cond-mat.stat-mech↗

arXiv2002

Depinning and wetting in nonequilibrium systems

We present an extension of equilibrium wetting to nonequilibrium situations particularly suited to systems with anisotropic interactions. Both critical and complete wetting transitions were found and characterized. We have identified a region in the space of parameters (temperature and chemical potential) where the wet and non-wet phases coexist. Emphasis is made on the analogies and differences between equilibrium and nonequilibrium wetting.

cond-mat.stat-mech↗

arXiv2002

Orientational order in Deposits of Magnetic Particles

We present preliminary results for the orientational order in deposits of dipolar particles, on one dimensional substrates. The deposits are generated using a model where the incoming dipolar particle interacts with the other particles in the deposit via a dipole-dipole potential. The interdipolar vectors are restricted to lie on a square lattice although the dipole moments are free to rotate in three dimensions. The path of the incoming particle is generated through a Monte Carlo scheme controlled by an effective temperature T, the case of pure diffusion-limited deposition corresponding to T=infinity$. We calculate the ferromagnetic and nematic order parameters and the dipolar orientational probability density of the deposits, at various stages of growth and two effective temperatures. The dipolar angular correlations along the rows and columns of the lattice are also investigated. We find that the orientational order of the deposits depends strongly on the lattice structure, the stage of growth and the effective temperature.

cond-mat.stat-mech↗

arXiv2002

Colloidal interactions in two dimensional nematics

The interaction between two disks immersed in a 2D nematic is investigated (i) analitically using the tensor order parameter formalism for the nematic configuration around isolated disks and (ii) numerically using finite element methods with adaptive meshing to minimize the corresponding Landau-de Gennes free energy. For strong homeotropic anchoring, each disk generates a pair of defects with one-half topological charge responsible for the 2D quadrupolar interaction between the disks at large distances. At short distance, the position of the defects may change, leading to unexpected complex interactions with the quadrupolar repulsive interactions becoming attractive. This short range attraction in all directions is still anisotropic. As the distance between the disks decreases their preferred relative orientation with respect to the far-field nematic director changes from oblique to perpendicular.

cond-mat.soft↗

arXiv2002

Geometrically-controlled twist transitions in nematic cells

We study geometrically-controlled twist transitions of a nematic confined between a sinusoidal grating and a flat substrate. In these cells the transition to the twisted state is driven by surface effects. We have identified the mechanisms responsible for the transition analytically and used exact numerical calculations to study the range of surface parameters where the twist instability occurs. Close to these values the cell operates under minimal external fields or temperature variations.

cond-mat.soft↗

arXiv2002

Stochastic theory of non-equilibrium wetting

We study a Langevin equation describing non-equilibrium depinning and wetting transitions. Attention is focused on short-ranged attractive substrate-interface potentials. We confirm the existence of first order depinning transitions, in the temperature-chemical potential diagram, and a tricritical point beyond which the transition becomes a non-equilibrium complete wetting transition. The coexistence of pinned and depinned interfaces occurs over a finite area, in line with other non-equilibrium systems that exhibit first order transitions. In addition, we find two types of phase coexistence, one of which is characterized by spatio-temporal intermittency (STI). A finite size analysis of the depinning time is used to characterize the different coexisting regimes. Finally, a stationary distribution of characteristic triangles or facets was shown to be responsible for the structure of the STI phase.

cond-mat↗

arXiv2001

Colloidal dipolar interactions in 2D smectic C films

We use a two-dimensional (2D) elastic free energy to calculate the effective interaction between two circular disks immersed in smectic-$C$ films. For strong homeotropic anchoring, the distortion of the director field caused by the disks generates additional topological defects that induce an effective interaction between the disks. We use finite elements, with adaptive meshing, to minimize the 2D elastic free energy. The method is shown to be accurate and efficient for inhomogeneities on the length scales set by the disks and the defects, that differ by up to 3 orders of magnitude. We compute the effective interaction between two disk-defect pairs in a simple (linear) configuration. For large disk separations, $D$, the elastic free energy scales as $\sim D^{-2}$, confirming the dipolar character of the long-range effective interaction. For small $D$ the energy exhibits a pronounced minimum. The lowest energy corresponds to a symmetrical configuration of the disk-deffect pairs, with the inner defect at the mid-point between the disks. The disks are separated by a distance that is twice the distance of the outer defect from the nearest disk. The latter is identical to the equilibrium distance of a defect nucleated by an isolated disk.

cond-mat.soft↗

arXiv1999

Crossover Scales at the Critical Points of Fluids with Electrostatic Interactions

Criticality in a fluid of dielectric constant D that exhibits Ising-type behavior is studied as additional electrostatic (i.e., ionic) interactions are turned on. An exploratory perturbative calculation is performed for small ionicity as measured by the ratio of the electrostatic energy to the strength of the short-range nonionic (i.e., van der Waals) interactions in the uncharged fluid. With the aid of distinct transformations for the short-range and for the Coulombic interactions, an effective Hamiltonian with coefficients depending on the ionicity is derived at the Debye-Hueckel limiting-law level for a fully symmetric model. The crossover between classical (mean-field) and Ising behavior is then estimated using a Ginzburg criterion. This indicates that the reduced crossover temperature depends only weakly on the ionicity (and on the range of the nonionic potentials); however, the trends do correlate with the, much stronger, dependence observed experimentally.

cond-mat.soft↗