A csoport múlt évi munkája

Granular materials. — Grain alignment has been investigated in hopper flows using X-ray CT measurements.

The packing fraction, grain alignment, orientational order parameter, and flow field in a 3D hopper has been investigated (Fig. 1a) based on X-ray CT measurements. We analyzed subsequent clogged states for 6 materials (Fig. 1b-g) including elongated particles (pegs), lentils, and nearly spherical grains (peas). We have shown that for elongated particles the grains get ordered in the flowing parts of the silo. Similarly to the case of simple shear flows, the average orientation of the rods is not parallel to the streamlines but encloses a small angle with it. The order parameter increases as the grains travel downwards the silo and the local shear deformation grows (Fig. 1h). In most parts of the hopper, the orientational distribution of the grains did not reach the stationary orientational distribution observed for simple shear.

Modeling of soft materials. — The limit of validity of linear elasticity has been tested in athermal soft-sphere packings.

The shear response of soft solids can be described with linear elasticity, provided the applied deformation is slow and weak. However, both of these approximations break down when the material loses rigidity, such as in foams and emulsions near their jamming point. When deformations are applied too quickly, the material becomes stiffer. On the other hand, when deformations are too large, the material softens and eventually flows. Using computer simulations of athermal soft-sphere packings we identified characteristic strain and time scales that quantify the limit of validity of linear elasticity, and related these scales to changes in the microscopic contact network. Our findings indicate that the mechanical response of jammed solids are generically nonlinear and rate-dependent on experimentally accessible strain and time scales. (Fig. 1i).

Electric-field-induced patterns in nematic liquid crystals. – The effect of superimposed dc and ac applied voltages has been studied on two types of spatially periodic instabilities in nematic liquid crystals, flexoelectric domains (FD) and electroconvection (EC).

Determining the onset characteristics (threshold voltage and critical wave vector), we found that, unexpectedly, the superposition of driving with different time symmetries inhibits the pattern-forming mechanisms for FD and EC as well. As a consequence, the onset shifts to much higher voltages than the individual dc or ac thresholds. A dc-bias-induced reduction of the crossover frequency from the conductive to the dielectric EC regimes and a peculiar transition between two types of flexodomains with different wavelengths were detected. Independent impedance measurements have proved that the applied dc voltage component substantially affects both the electrical conductivity and its anisotropy. Taking into account the experimentally detected variations of these parameters in the linear stability analysis of the underlying nematohydrodynamic equations, a qualitative agreement with the experimental findings on the onset behavior of spatially periodic instabilities was obtained.

Figure 1. (a-h) The packing and orientation of anisometric grains has been determined by X-ray CT. (i) The limits of validity of linear elasticity in athermal soft-sphere packings.

Figure 2. Stability limit curves in the acdc voltage plane for the instabilities in the nematic liquid crystal 1OO8 at 5 Hz with typical pattern snapshots of 64 m× 64 mm. FD and FDSW are flexodomains, EC CR denotes conductive regime of electroconvection.

Carbon nanotube/epoxy composites. – The effect of temperature and filler concentration on the electrical parameters of a composite (carbon nanotubes dispersed in an epoxy matrix has been investigated).

We found that the electric and dielectric behavior of these composites follows Jonscher’s universal dielectric response. The frequency dependence could be interpreted by a fractal model. The fractal dimension evaluated from the impedance data are close to that obtained by neutron scattering. The critical exponents describing the concentration dependence of the conductivity and the dielectric constant obtained in the vicinity of the percolation threshold are in good agreement with the theoretical values. The temperature coefficient of the resistivity is typically negative, except for composites with nanotube concentration exceeding the percolation threshold, where at temperatures below the glass transition a positive temperature coefficient was detected.

Synthesis of mesogenic compounds. — A series of five-ring pyridine-based bent-core compounds has been synthesized, bearing different substituents at the peripheral phenyl rings (CH3O, Cl and NO2). Their mesomorphic behaviour has been investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray scattering, and then compared with the unsubstituted parent compound. The introduction of the methoxy groups at the peripheral phenyl rings of the bent core results in a non-mesomorphic compound, whereas the chloro- and nitro-substituted compounds form enantiotropic B1-like phases. Significant changes of the textures and transition temperatures of the mesophase have been observed under UV light indicating the possibility to design self-organized molecules suitable for UV indicators (see Figure 3.).

Figure 3.  Molecular structure of the synthesized pyridine-based bent-core compounds and demonstration of their sensitivity to UV light.

Liquid crystal composite materials. — Magnetic properties of a ferronematic, i.e., a nematic liquid crystal doped with magnetic nanoparticles in low volume concentration have been studied, with the focus on the ac magnetic susceptibility. A weak dc bias magnetic field (a few Oe) applied to the ferronematic in its isotropic phase increases the ac magnetic susceptibility considerably. Passage of the isotropic-to-nematic phase transition resets this enhancement irreversibly (unless the dc bias field is applied again in the isotropic phase). A phenomenological explanation has been proposed which associates the discovered effect with the aggregation of nanoparticles in the course of the isotropic-to-nematic phase transition and their disaggregation under the influence of a dc (bias) magnetic field.