Last year's work

( 2016)

Granular materials – Flow, jamming and segregation phenomena in sheared systems. — Segregation is often observed in granular mixtures due to size or weight difference of the grains. It is well known that heavier or smaller grains migrate downwards in a granular shear flow. We have shown, that grains differing only in surface friction (size and weight is the same) also segregate when sheared, where smooth grains accumulate in the lower regions of the shear zone (Fig. 1). Moreover, when gravity is negligible to other (compressional) forces, the sample still segregates with the smooth grains leaving the shear zone.

Figure 1. A mixture of smooth and rough beads is sheared. Smooth beads migrate to the bottom of the shear zone due to kinetic sieving.

The flow of elongated grains has been also studied in experiments and numerical simulations. We have experimentally shown, that the flow rate of a hopper is slightly decreased and clogging probability is higher for rod like particles compared to spheres. In numerical simulations, the effective friction of an assembly of frictionless spherocylinders was a non-monotonic, but predominantly decreasing function of the particle aspect ratio.

Two distinct strain amplitudes have been identified in soft glassy materials, such as emulsions, foams, suspensions and pastes, close to the jamming transition. Numerical simulations of oscillatorily sheared soft sphere packings revealed that as the strain amplitude is increased, the initially elastic systems undergo a softening transition, at which both the elastic and loss moduli drop and reach a new plateau. Increasing the strain amplitude further causes yielding, marked by diffusive particle trajectories.

Figure 2. Representative particle trajectories in oscillatory shear. For increasing shear amplitude (left to right), the elliptic trajectory corresponding to linear elasticity becomes non-periodic, but still bounded (softening), then diffusive (yielding).

Liquid crystals. — Tunable optical gratings based on the flexoelectric effect were created in a bent-core nematic liquid crystal. The wavelength of the structure was controlled by the applied d.c. voltage, as demonstrated by polarizing microscopy and light diffraction techniques (Fig. 3): higher voltage yields shorter wavelength. Visibility of the diffraction orders depended on the polarization of the illuminating laser beam (Fig. 4). The dynamical response of the system to switching between voltage levels were also explored. The characteristics and the mechanisms of switching were found to be different, depending on whether the lower voltage level is below or above the threshold of pattern onset. In both cases, the response to increasing voltage levels was much slower than that to decreasing ones.

Figure 3. Microphotographs and diffraction patterns of flexodomain gratings at increasing applied voltages.

Figure 4. Diffraction geometries and the corresponding diffraction images at (a) extraordinary and (b) ordinary illumination.

Liquid crystal composite materials. — Structural transitions under the combined action of magnetic and electric fields have been monitored in 6CHBT and 6CB liquid crystal-based ferronematics, obtained by doping the nematics with spherical and rod-like magnetic nanoparticles in low (≤ 10-4) volume concentrations. Based on the experimental data, the type of the anchoring at the liquid crystal‒nanoparticle interface (rigid or soft), as well as the mutual orientation of the magnetic moment of the nanoparticles and the nematic director have been determined.

Studies of the effect discovered the previous year, namely the increase of the alternating current magnetic susceptibility of ferronematics, induced by a dc bias magnetic field applied in the isotropic phase, which vanishes irreversibly when the ferronematic is cooled down to the nematic phase, have been continued. With the optimization of the ferronematic composition, the range of the d.c. bias magnetic field to which the ferronematic is sensitive without saturation was increased by about two orders of magnitude. This finding paves a way to application possibilities such as low magnetic field sensors or basic logical elements for information storage.


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.