Facultad de Ciencias

We describe the stochastic rotational dynamics and magnetization curves
of a suspension of interacting magnetic colloidal particles of a ferrofluid
under an external magnetic field. The orientation of the magnetic dipole
moments of these particles is considered as a fluctuating variable, so within
the framework of mesoscopic irreversible thermodynamics, we derive an
equivalent Fokker–Planck equation (FPe), which gives us the orientational
distribution function (Odf) of the magnetic dipole moments of these particles.
The numerical solution of the FPe allows us to observe the temporal
evolution of the orientation of the magnetic dipoles to the equilibrium limit,
once an external magnetic field is applied to the ferrofluid. We observe that
the dipoles orient better in the direction of the field when the particles are
bigger or the temperature of the solvent is decreased. Decreasing the average
distance between the magnetic particles does not give us an important
improvement in their orientation with respect to the external magnetic field.
Finally, with the solution of the FPe and the partition function, we determine
an average magnetic dipole moment of the particles and, changing
the magnetic field applied to the ferrofluid, we obtain a set of magnetization
curves which fits to experimental results by tuning the interactions
among the particles. These results are useful for different applications of
magnetizable composite particles.

In the present work, we develop molecular dynamics (MD) simulations in the NPT (isobaric–isothermic) ensemble to
analyse the effect of an external electrostatic field over a cubic methane hydrate crystallite. The amplitude of the field is in
the range 0.5–3.0 V/nm. For the simulations, we used the SPC/E rigid water model and a single-site model for methane at a
temperature of 248K and a pressure of 20 bar. When the external electrostatic field is applied, the water dipoles are oriented
in such a way that the methane molecules can diffuse far away from the water cages, hence the clathrate dissociation takes
place. This last phenomenon was observed for intensities above 1.5 V/nm. Taking the final configuration of each run as
input, we develop a new set of MD simulations, and we observe that the stable clathrate is not recovered immediately when
the external electrostatic field is turned off due to limitations in the simulation time.

Molecular dynamic simulations in the NPT ensemble were performed to analyse the structural properties of methane hydrate
by using rigid water models of three, four and five sites (SPC/E, TIP4P and TIP5P). An exploratory study was carried out
concerning the capabilities of these water models in the near region of the phase transition line (pressure–temperature). This
task was achieved by performing simulations at a constant pressure of 20 atm, varying the temperature from 200 to 300 K.
The radial distribution function as well as the coordination number for carbon and oxygen sites were obtained for the
above-mentioned temperatures: both of them allow us to observe the stable–unstable phase transition of the methane
hydrate. As a result, we can mention that the structural properties estimated with the three sites water model show an
excellent agreement with the experimental results, specifically around the phase transition line. In addition, the coordination
number becomes a useful criterion to demonstrate the dissociation of the clathrates. On the other hand, the potential of mean
force between two methane molecules in water is shown in the same vicinity, evidencing the energetic cost to keep the
clathrate in a stable thermodynamic state.

We show the existence of precursors of crystalline and amorphous phases preceding the appearance of freezing and glass transitions in colloidal systems. The formation of crystalline clusters or amorphous local arrangements can be controlled by tuning the parameters of a proposed continuous potential and by changing the packing conditions.

We describe the entropy of liquids in the context of kinetic theory of dense gases. In the equilibrium regime the statistical entropy has an explicit dependence of the pair correlation function. In order to test the entropy functional, we use the Morse potential to reproduce experimental pair correlation functions of liquid sodium, using the molecular dynamics technique. With this information, we can compare the theoretical entropy with experimental thermodynamic data.

Metabolic transformations have been reported as involved in neoplasms survival. This suggests a role of metabolic pathways as potential cancer pharmacological targets. Modulating tumor's energy production pathways may become a substantial research area for cancer treatment. The significant role of metabolic deregulation as inducing transcriptional instabilities and consequently whole-system failure, is thus of foremost importance.

This work is concerned with the description of the time evolution for the entropy functional in a dense fluid out of equilibrium. We consider the fluid as a collection of N particles interacting via the superposition of pairwise additive short range intermolecular potentials. Such an interaction is assumed to be central and the particles do not have internal structure. To reach this goal we use the maximum entropy formalism in which the Gibbs entropy is maximized under the restrictions imposed by the normalization of the N-particle distribution function and the knowledge of the one and two-particle distribution functions. This procedure leads to an expression for the entropy functional in terms of such distribution functions. On the other hand, the evolution equations allow for the construction of a balance equation for the entropy functional in the system. An analysis of the entropy flux and entropy source is given for a particular expression in the Lagrange multipliers coming from the maximization procedure. Such an analysis shows the connection of the entropy production with the conversion rate of kinetic and potential energy between particles. Interesting conclusions about the temperature are reached in this case.

Using the scheme of mesoscopic non-equilibrium thermodynamics, we construct the one- and two-particle Fokker–Planck equations for a system of interacting Brownian particles. By means of these equations we derive the corresponding balance equations. We obtain expressions for the heat flux and the pressure tensor which enable one to describe the kinetic and potential energy interchange of the particles with the heat bath. Through the momentum balance, we analyze in particular the diffusion regime to obtain the collective diffusion coefficient in terms of the hydrodynamic and the effective forces acting on the Brownian particles.