Biomedical subjects

Patrick Ilg

Publications and source records attributed to Patrick Ilg.

7 recordsLinked to original sources

PubMed2006

Robustness of the periodic and chaotic orientational behavior of tumbling nematic liquid crystals.

The dynamical behavior of molecular alignment strongly affects physical properties of nematic liquid crystals. A theoretical description can be made by a nonlinear relaxation equation of the order parameter and leads to the prediction that rather complex even chaotic orientational behavior occur. Here the influence of fluctuating shear rates on the orientational dynamics especially on chaotic solutions is discussed. With the help of phase portraits and time evolution diagrams, we investigated the influence of different fluctuation strengths on the flow aligned, isotropic, and periodic solutions. To explore the effect of fluctuations on the chaotic behavior, we calculated the largest Lyapunov exponent for different fluctuation strengths. We found in all cases that small fluctuations of the shear rate do not affect the basic features of the dynamics of tumbling nematics. Furthermore, we present an amended potential modeling the isotropic to nematic transition and discuss the equivalence and difference to the commonly used Landau-de Gennes potential. In contrast to the Landau-de Gennes potential, our potential has the advantage to restrict the order parameter to physically admissible values. In the case of extensional flow, we show that the amended potential leads for increasing extensional rate to a better agreement with experimental results.

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PubMed2005

Anisotropic self-diffusion in ferrofluids studied via Brownian dynamics simulations.

The anisotropic self-diffusion in ferrofluids in the presence of a magnetic field is studied by Brownian dynamics simulations. It is found that the diffusion parallel to the magnetic field is hindered when compared to the case without field. Depending on the strength of dipolar interactions, the diffusion perpendicular to the applied field is either enhanced or hindered due to the field. The average diffusion coefficient is found to decrease with increasing concentration and dipolar interaction strength, but to be independent of the magnetic field strength for moderate dipolar interactions. Comparison to mean-field models show agreement for moderate dipolar interaction strengths but significant deviations in the regime of strong dipolar interactions.

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PubMed2005

Anisotropic diffusion in nematic liquid crystals and in ferrofluids.

A unified, mean-field kinetic theory approach to the anisotropic translational diffusion observed in liquid crystals and in ferrofluids is proposed. In the dilute regime, unified expressions for the parallel as well as for the perpendicular diffusion coefficient in terms of orientational order parameters are found that apply for liquid crystals as well as for ferrofluids. This result explains the common origin of the anisotropic diffusion found in liquid crystals and in ferrofluids. Differences between the two liquids appear in the semi-dilute regime, where the diffusion coefficients depend on the specific interaction potentials. Explicit expressions for the diffusion coefficients are worked out also in this regime within a mean-field approximation. Comparisons with previous theoretical and experimental results are performed, showing satisfactory agreement to the present results.

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PubMed2005

Anisotropy of the magnetoviscous effect in ferrofluids.

The anisotropy of the magnetoviscous effect in ferrofluids subjected to weak planar Couette flow is investigated by extensive molecular simulations. The field and concentration dependence of the viscosity coefficients are found to depend on the relative orientation of the magnetic field with respect to the flow geometry. Comparison with dynamical mean-field models shows satisfactory agreement for moderate interaction strengths. In the semidilute regime it is found that the anisotropy contains valuable information on particle interaction.

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PubMed2005

Magnetoviscosity of semidilute ferrofluids and the role of dipolar interactions: comparison of molecular simulations and dynamical mean-field theory.

Extensive molecular simulations on a model ferrofluid are performed in order to study magnetoviscous and viscoelastic phenomena in semidilute ferrofluids. Simulation results of the nonequilibrium magnetization, shear viscosity, and normal stress differences are presented. Rotational and configurational contributions to the shear viscosity are analyzed and their influence on the magnetoviscous effect is discussed. The simplified model of noninteracting magnetic dipoles describes the nonequilibrium magnetization and the rotational viscosity, but does not account for configurational viscosity contributions and normal stress differences. Improved mean-field models that overcome these limitations show good agreement with the simulation results for weak dipolar interactions where the models should apply. Comparisons to simulation results for various interaction strengths allow us to determine the range of validity of the mean-field models.

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PubMed2003

Dynamics of colloidal suspensions of ferromagnetic particles in plane Couette flow: comparison of approximate solutions with Brownian dynamics simulations.

The stationary and oscillatory properties of dilute ferromagnetic colloidal suspensions in plane Couette flow are studied. Analytical expressions for the off-equilibrium magnetization and the shear viscosity are obtained within the so-called effective field approximation. We also investigate the predictions of a different approximation based on the linearized moment expansion. Direct numerical simulation of the kinetic model are performed in order to test the range of validity of these approximations.

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PubMed2002

Magnetization dynamics, rheology, and an effective description of ferromagnetic units in dilute suspension.

The rheological properties of a dilute suspension of ellipsoidal ferromagnetic particles in the presence of a magnetic field are studied on the basis of a kinetic model, where the flow and magnetic external fields couple in qualitatively different ways to the orientational behavior of the suspension. In the uniaxial phase the stress tensor is found to be of the same form as in the Ericksen-Leslie theory for nematic liquid crystals in the steady state. Expressions for a complete set of viscosity coefficients in terms of orientational order parameters are worked out. In the low Péclet number regime, the viscosity coefficients are given as explicit functions of the magnetic field and a particle shape factor, where the shape factor may equally represent a nonspherical unit (agglomerate, chain) composed of spherical particles. Effects due to possible flow-induced breakup of units are not covered in this work. Further, by considering the magnetization as the only relevant variable, a magnetization equation within an effective field approach is derived from the kinetic equation and compared to existing magnetization equations. The alignment angle of the magnetization and the first and second normal stress coefficient are studied for the special case of plane Couette flow. The assumptions employed are tested against a Brownian dynamics simulation of the full kinetic model, and a few comparisons with experimental data are made.

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