Tamás Börzsönyi, Ágnes Buka, Nándor Éber, Katalin Fodor-CsorbaA, Antal Jákli, Isván Jánossy, Péter Salamon, Ellák Somfai, Balázs Szabó, Tibor Tóth-Katona, Anikó VajdaA
Granular flows. — The motion and rotation of individual particles as well as the average flow properties of granular samples under shear was determined using X-ray tomography and optical methods. We have shown, that particles in a sheared dry granular material consisting of elongated grains rotate with fluctuating rotational velocity. The ensemble average of the grain orientation forms a small angle with the streamlines. The average rotational velocity of many particles is angle dependent, it is the smallest when the particle is aligned in the preferred direction and largest perpendicular to the preferred alignment, similarly to the rotation of a single prolate ellipsoid placed in a sheared viscous liquid. A review article was published,
summarizing the properties of granular materials composed of shape-anisotropic grains, focusing on packing, dense flows, shaken systems an granular gases.
Liquid crystal composite materials. — We have demonstrated that in ferronematics (liquid crystals doped with magnetic nanoparticles), both the threshold of the magnetic Fréedericksz transition and the dielectric response to low magnetic fields (far below the Fréedericksz transition) depend not only on the volume concentration of the magnetic particles, but also on the size. According to the results, the larger is the particle, the bigger are the effects (a larger decrease of the threshold of the Fréedericksz transition and a more pronounced linear response to low magnetic fields). Furthermore, we have shown that the type and the shape
anisotropy of the particles also make impact on these processes and, we have also pointed out the possible importance of the initial pretilt.
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Figure 1. Shear induced ordering of elongated particles. Image (a) shows the experimental split bottom shear cell, while images (b) and (c) show the reconstructed particle positions and orientations from the tomographic images.
Figure 2. Snapshots and their 2d Fourier transforms for (a) oblique EC rolls and (b) flexodomains.
Electric field driven pattern formation. — Characteristics of electric field induced patterns, including their temporal variations within the driving period, have been explored in the nematic 1OO8 in the 10 mHz < f < 100 Hz frequency range. The neighbouring figure shows typical snapshots and the corresponding 2d Fourier transforms for the two basic mechanisms: (a) oblique rolls of electroconvection (EC) patterns and (b) parallel stripes of flexodomains (FDs), and demonstrates their morphological differences. Unlike FDs, EC patterns at low frequencies do not have a sharp threshold, indicating an imperfect bifurcation with increasing imperfection toward lower f. By measuring the threshold voltages and critical wave numbers of FDs the temperature dependence of the flexoelectric parameter |e1-e3| could be determined.
Properties of bent-core mesogens. — Banana shaped (bent-core) molecules form a special novel class of liquid crystals. Electric field induced optically isotropic state has been detected in planar samples of a bent-core mesogen synthesized in our laboratory. The phenomenon indicates a 90° rotation of the optical axis as shown in the figure which can be regarded as a clear proof for the field-induced biaxiality the 'banana' nematic phase of this compound. The effect is present in mixtures of bent-core and rod like nematics too.
Scale-invariant dynamics in expanding space.
— We established an exact relation between the statistical properties of scale invariant space-time trajectories of interacting particles
in uniformly expanding and in fixed geometries. This relation generalizes standard conformal transformations as the natural symmetry of self-affine growth processes. The mapping corresponds to a nonlinear time transformation which converges to a finite value for a large class of trajectories. This enables an exact analysis of asymptotic properties in expanding domains, which are often nontrivial and random due to amplification of initial fluctuations.
Figure 4. Expanding radial growth structure and the same structure on a fixed domain with periodic boundary conditions, illustrated for the case of coalescing random walks. The distribution of the rescaled radial structure at radius r is identical to the distribution of the fixed domain structure at height h(r).
Figure 3. Alignment of the bent-core molecule, its permanent dipole moment and the optic axis without (above) and with (below) an applied electric field.
Grants and international cooperation
OTKA NN Anisometric granular materials (T. Börzsönyi, 2013-2016)
EU M-ERA.NET FP7 Magnetically active anisotropic composite systems (T. Tóth-Katona, 2013-2016)
OTKA K Electro and photomechanical effects in organic soft materials (I. Jánossy, 2010-2014)
MTA Infrastructure Devel. Modern experimental tools, rheomicroscope (Á. Buka, T.
Börzsönyi)
MÖB-DAAD (Hungarian-German bilateral) From anisotropic liquids to anisotropic granular materials (T. Börzsönyi, 2012-2013)
NFÜ-TÉT_CN (Hungarian-Chinese bilateral) Nonlinear structures in mesogens (Á. Buka, 2013-2015)
MTA-SAS (Hungarian-Slovak bilateral) Nanoparticles in anisotropic soft matter (T.
Tóth-Katona, 2013-2015)
MTA-ASCR (Hungarian-Czech bilateral) Photosensitive mesogenic monomers (T. Tóth-Katona, 2013-2015)
MTA-INSA (Hungarian-Indian bilateral) Dynamics of soft condensed matter (N. Éber, 2013-2015)
Publications
Articles
1. Ali A, Ball R, Grosskinsky S, Somfai E: Interacting particle systems in time-dependent geometries. J. STAT. MECH.-THEORY E. 2013:(9) Paper P09006. 30 p. (2013)
2. Börzsönyi T, Stannarius R: Granular materials composed of shape-anisotropic grains.
SOFT MATTER 9:(31) pp. 7401-7418. (2013)
3. Charif AC, Diorio N, Fodor-Csorba K, Puskas JE, Jakli A: A piezoelectric thermoplastic elastomer containing a bent-core liquid crystal. RSC ADVANCES 3:(38) pp. 17446-17452. (2013)
4. Cvetinov M, Obadović D, Stojanović M, Lazar D, Vajda A, Éber N, Fodor-Csorba K, Ristić I: Mesophase behaviour of binary mixtures of bent-core and calamitic compounds.
LIQUID CRYSTALS 40:(11) pp. 1512-1519. (2013)
5. Elamain O, Hegde G, Fodor-Csorba K, Komitov L: Field-induced optically isotropic state in bent core nematic liquid crystals: unambiguous proof of field-induced optical
biaxiality. J. PHYS. D 46:(45) Paper 455101. 7 p. (2013)
6. Elamain O, Hegde G, Fodor-Csorba K, Komitov L: Optically isotropic state in bent core nematic mixtures with rod like molecules induced by direct current electric field.
APPL. PHYS. LETT. 103:(16) Paper 163501. 4 p. (2013)
7. Eremin A, Jákli A: Polar bent-shape liquid crystals - From molecular bend to layer splay and chirality. SOFT MATTER 9:(3) pp. 615-637. (2013)
8. Jákli A: Liquid crystals of the twenty-first century – nematic phase of bent-core molecules. LIQUID CRYST. REV. 1:(1) pp. 65-82. (2013)
9. Jákli A, Éber N: Soft materials for linear electromechanical energy conversion. CURR.
OPIN. CHEM. ENG. 2:(1) pp. 120-124. (2013)
10. Karakus M, Sajadi A, Salamon P: Inter-continental summer school of liquid crystal physics, mathematics, photonics, electro-optics, and optomechanics: I-CAMP'13's overview. LIQUID CRYST. TODAY 22:(3) pp. 64-69. (2013)
11. Kopċanský P, Tomašoviċová N, Tóth-Katona T, Éber N, Timko M, Závišová V, Majorošová J, Rajňak M, Jadzyn J, Chaud X: Increasing the magnetic sensitivity of liquid crystals by rod-like magnetic nanoparticles. MAGNETOHYDRODYNAMICS 49:(3-4) pp. 586-591. (2013)
12. Salamon P, Éber N, Krekhov A, Buka A: Flashing flexodomains and electroconvection rolls in a nematic liquid crystal. PHYS. REV. E 87:(3) Paper 032505. 10 p. (2013)
13. Tomasovicova N, Timko M, Mitroova Z, Koneracka M, Rajnak M, Éber N, Tóth-Katona T, Chaud X, Jadzyn J, Kopcansky P: Capacitance changes in ferronematic liquid crystals induced by low magnetic fields. PHYS. REV. E 87:(1) Paper 014501. 4 p. (2013)
Book chapter
14. Somfai E: Statistical mechanics of complex systems. In: Ball R, Kolokoltsov V, MacKay RS (eds.) Complexity Science: The Warwick Master's Course. Cambridge: Cambridge University Press, 2013. pp. 210-245. (London Mathematical Society Lecture Note Series) (ISBN:9781107640566)
Conference proceeding
15. Stannarius R, Wegner S, Szabó B, Börzsönyi T: Shear alignment and orientational order of macroscopic rodlike grains. AIP CONF. PROC. 1542: pp. 74-77. (2013)