The behavior of a magnetic filament in flow under the influence of an external magnetic field.

We present an extensive numerical study of the behaviour of a filament made of ferromagnetic colloidal particles subjected to the simultaneous action of a fluid flow and a stationary external magnetic field perpendicular to the flow lines. We found that in the presence of a shear flow, the tumbling motion observed at zero field is strongly inhibited when the external magnetic field is applied. The field is able to stabilise the filament with a well defined degree of alignment that depends on the balance between hydrodynamic and magnetic torques. In addition, for a Poiseuille flow, it has been found that the initial position has a long lasting influence on the behaviour of the magnetic filament when the external field is applied.

Flexible magnetic filaments under the influence of external magnetic fields in the limit of infinite dilution.

In the present work we use Langevin dynamics computer simulations to understand how the presence of a constant external magnetic field modifies the conformational phase diagram of magnetic filaments in the limit of infinite dilution. We have considered the filaments immersed in either a good (non-sticky filaments) or a poor (Stockmayer polymers) solvent. It has been found that in the presence of an applied field, filaments turn out to be much more susceptible to parameters such as temperature and solvent conditions. Filaments owe this increased susceptibility to the fact that the external magnetic field tends to level the free energy landscape as compared to the zero-field case. The field induces equalization in the free energy of competing conformational states that were separated by large energy differences in the zero-field limit. In this new scenario multistability arises, and manifests itself in the existence of broad regions in the phase diagram where two or more equilibrium configurations coexist. The existence of multistability greatly enhances the possibility of tuning the properties of the filament.

Collective thermodiffusion of colloidal suspensions.

The thermophoretic behavior of concentrated colloidal suspensions can be understood as the sum of single particle and collective effects. Here, we present a simulation model to investigate the particularities of the collective thermodiffusive effects in concentrated uncharged solutions, where the influence of different colloid-colloid interactions is analyzed. The concentration dependence found in our simulations qualitatively agrees with experimental results. Colloids with repulsive interactions are found to accumulate more effectively than the solvent in the warm areas, such that the corresponding Soret coefficients are negative and decrease with increasing concentration. The accumulation of colloids in the cold regions is facilitated by attraction, such that colloids with attractive interactions have larger values of the Soret coefficient. A thermodynamic argument that explains our results from equilibrium quantities is discussed as well.

Thermophoresis of colloids by mesoscale simulations.

The motion of a colloid induced by a temperature gradient is simulated by means of multiparticle collision dynamics, a mesoscale simulation technique. Two algorithms to quantify the thermophoretic behavior are employed and contrasted. The validity of the methods is verified as a function of the temperature gradient, system size, and algorithm parameters. The variation of the solvent-colloid interaction from attractive to purely repulsive interestingly results in the change of the colloid behavior from thermophobic to thermophilic.

Temperature inhomogeneities simulated with multiparticle-collision dynamics.

The mesoscopic simulation technique known as multiparticle collision dynamics is presented as a very appropriate method to simulate complex systems in the presence of temperature inhomogeneities. Three different methods to impose the temperature gradient are compared and characterized in the parameter landscape. Two methods include the interaction of the system with confining walls. The third method considers open boundary conditions by imposing energy fluxes. The transport of energy characterizing the thermal diffusivity is also investigated. The dependence of this transport coefficient on the method parameters and the accuracy of existing analytical theories is discussed.

The role of vacancies and local distortions in the design of new phase-change materials.

Phase-change materials are of tremendous technological importance ranging from optical data storage to electronic memories. Despite this interest, many fundamental properties of phase-change materials, such as the role of vacancies, remain poorly understood. 'GeSbTe'-based phase-change materials contain vacancy concentrations around 10% in their metastable crystalline structure. By using density-functional theory, the origin of these vacancies has been clarified and we show that the most stable crystalline phases with rocksalt-like structures are characterized by large vacancy concentrations and local distortions. The ease by which vacancies are formed is explained by the need to annihilate energetically unfavourable antibonding Ge-Te and Sb-Te interactions in the highest occupied bands. Understanding how the interplay between vacancies and local distortions lowers the total energy helps to design novel phase-change materials as evidenced by new experimental data.