© Dr. N. H. Siboni

Non-monotonic rheology of a magnetic liquid crystal system in an external field

Utilizing molecular dynamics simulations, we report a non-monotonic dependence of the shear stress on the strength of an external magnetic field (H) in a liquid-crystalline mixture of magnetic and non-magnetic anisotropic particles. This non-monotonic behavior is in sharp contrast with the well-studied monotonic H-dependency of the shear stress in conventional ferro uids, where the shear stress increases with H until it reaches a saturation value. We relate the origin of this non-monotonicity to the competing effects of particle alignment along the shear-induced direction, on the one hand, and the magnetic feld direction on the other hand. To isolate the role of these competing effects, we consider a two-component mixture composed of particles with effectively identical steric interactions, where the orientations of a small fraction, i.e. the magnetic ones, are coupled to the external magnetic field. By increasing H from zero, the orientations of the magnetic particles show a Fréederickz-like transition and eventually start deviating from the shear-induced orientation, leading to an increase in shear stress. Upon further increase of H, a demixing of the magnetic particles, from the non-magnetic ones, occurs which leads to a drop in shear stress, hence creating a non-monotonic response to H. Unlike the equilibrium demixing phenomena reported in previous studies, the demixing observed here is neither due to size-polydispersity nor due to a wall-induced nematic transition. Based on a simplified Onsager analysis, we rather argue that it occurs solely due to packing entropy of particles with different shear- or magnetic-field-induced orientations.

FIG. 1. A snapshot of the simulation setup in absence of the shear and the external magnetic field: red, gray, and blue ellipsoidal particles represent magnetic, non-magnetic, and wall particles, respectively. The sample is in an isotropic state, the state where the walls are created. The sample is sheared by a relative motion of the walls along x-direction.
FIG. 2. Shear stress as a function of the magnetic field strength at three different temperatures. The temperature range covers both isotropic and nematic phases of the unperturbed system. The results show a non-monotonic dependence of the shear stress on the external magnetic field.

FIG. 3. Representative configurations associated with the four values of the field, which are indicated by bullets and the Roman numbers on the T = 1:5 curve in Fig. (2). The snapshots suggest not only orientational but also spatial distribution of the magnetic particles is affected by the magnetic field.

Dr. Nima Hamidi Siboni            

Institut für Theoretische Physik

Technische Universität Berlin

Hardenbergstr. 36

D-10623 Berlin

Phone +49 30 314 28850
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Http Dr. N. H. Siboni
Institute of Theoretical Physics
Technische Universität Berlin
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