Liquid Crystal Institute - Dr. Antal Jakli

Current Research

Study of structures of bent core smectic liquid crystals:
Sponsored by NSF Focus Research Group: “Ferroelectric Phenomena in soft matter system” DMS-0456221

Participants: University of Minnesota (Maria_Carme Calderer); Purdue University (Daniel Phillips, Patricia Bauman, Jie Shen); Kent State University (A. Jákli, O. Lavrentovich and E. Gratland)

The aim is to understand and analyze novel phenomena and structures in bent-core liquid crystals. We are studying the properties of fluid fibers of smectic banana liquid crystals.
Recently we have studied:

• Binary mixtures of bent-core liquid crystal molecules: one with a chiral cholesterol unit attached to one end of the bent core (B-Ch), the other one is an achiral molecule with two flour atoms (B-2F). B-Ch has a helical SmC* phase with P=30nC/cm2 polarization, whereas B-2F posses synclinic and anticlinic antiferroelectric SmCP phases with as high as 900nC/cm2 polarization. Mixtures with low B-Ch content show simultaneous ferroelectric Pc and antiferroelectric Pb polarizations. Mixtures of intermediate concentrations are found to be ferroelectric B7-type phases at high temperature ranges, and SmC* at lower temperatures. The B-Ch dominated mixtures have higher Pc than of the pure P-Ch indicating that Pb has the same sign as of Pc. Whereas in SmC* of rod-like chiral molecules simultaneous molecular chirality and director tilt lead to a macroscopic polarization, in bent-core liquid crystals the situation is much more complex. In addition to the polarization due to the bent shape (Pb), we may have a polarization due to simultaneous tilt and molecular chirality, (Pc). The studies not only revealed the existence of this polarization, but also showed that molecular chirality leads to an out-of layer polarization component, too.

• Carboxylate terminated bent-core liquid crystalline materials obtained from B.K. Sadashiva, Bangalore, India and K. Fodor-Csorba, Budapest, Hungary, which have shown novel and exciting new properties that bring new insights into the physics of banana liquid crystals. Some of these types of materials form a B7 phase that shows an unusual analogous electro-optical switching similar to electroclinic effect, but bistable. The second type of substances have orthogonal smectic (resembles SmA) phases either directly below the isotropic or the nematic phase. Some of the carbonate bent-core liquid crystals show a fast giant field –induced biaxiality with the variation of the birefringence up to 0.03. The optical switching between the different birefringent states is very fast (below a microsecond) and does not involve change of the optical axis. This giant field – induced birefringence offers a wide range of practical applications. Other members of the carbonate bent-core liquid crystals show an odd-even type alteration of the nanoscopic director organization as a function of the number of carbon atoms (n) in the terminal chains. The mesophases with odd number of carbon atoms in the terminal chains appear to have locally anticlinic FE structures and the polar order averages out in a range much larger than the layer spacing d. However, the mesophases with even number of carbon atoms in the terminal chains how normal antiferroelectric structures with a synclinic (racemic) or an anticlinic (chiral) local layer arrangement. These effects probably have entropic reasons: the end chain units prefer to be parallel because the out of layer fluctuations are allowed in this case.

• Structures of free standing bent-core fibers. It was investigated how the diameter of the fibers depend on the length and temperature of the fibers. We were able to stabilize these fibers under cooling to their crystalline phase. This enabled us to have a closer look on their 3D structures by Scanning Electron Microscopy at room temperature. After setting up the relevant terms in the free energy Based on these experiments we are working on a simplified model with the mathematicians participating in the project, in our lab we are working on a simplified physical model hat can explain the stability and main properties of the fibers. The general model will be worked out by the mathematicians participating in the project.

In part of this project we will also organize a workshop in June 2007 in the Liquid Crystal Institute to introduce liquid crystals to mathematics and theoretical physics students.

Fluid phases of bent-core molecules – Novel physics and applications
Sponsored by NSF DMR-0606160

S. Sprunt, J. Gleeson, and A. Jakli
Kent State University

Liquid crystals are familiar to the public as the basis of the multi-billion dollar flat panel display industry. Recently, there has been a surge of interest in a new class of liquid crystal molecules shaped more like a boomerang than the traditional thin “cigar” profile. Combined with liquid crystalline ordering, the new shape could lead to exciting new materials’ properties. We are studying new liquid crystalline states of matter in which individual “boomerang” molecules are grouped into microscopic clusters, which subsequently serve as units for building up novel orientationally ordered phases. We will perform experiments, not only in our labs at Kent State but also at the NSF-funded National High Field Magnet Laboratory at Florida State, designed to reveal and characterize these phases. Our research will also focus on a phenomenon called “flexoelectricity”, which seems to be enhanced by the “boomerang” shape three orders of magnitudes compared to rod-shape molecules. This could potentially be the basis of green micro-power generators. Imagine harvesting your legs’ energy during walking to charge your cell phone. Integral to our project are education components designed to foster 21st century skills. Highlights include technical training in the nexus of optical, mechanical, and electrical responsivity of soft matter, research practice based on small, timeline-driven teams, and learning (from a technical background) to manage intellectual property.

Energy conversion based on giant flexoelectric effect in bent-core nematic liquid crystals

J.T. Gleeson, A. Jákli, S.N. Sprunt
(ONR)

Goals
• Establish the fundamental limits of the response of giant flexoelectric materials based on bent-core nematic liquid crystals (BCNs)
• Determine the extent to which flexoelectricity dictates the structure of electrically induced instabilities
• Measure the “bare” flexoelectric coefficients through a combination of recording electrical response to direct mechanical flexure and absolute-intensity, dynamic light scattering measurements that probe the coupling of flexoelectricity to thermal fluctuations of the nematic optic axis
• Demonstrate the feasibility of energy conversion applications based on the novel electromechanical properties of BCNs exhibiting giant flexoelectricity

Unfunded Research

Electric field-induced spinning, translation and circling of solid particles in liquid crystals

Antal Jákli, Guangxun Liao, Mike Dorjgotov, Ivan Smalyukh, Oleg D. Lavrentovich
Chemical Physics Interdisciplinary Program and Liquid Crystal Institute, Kent state University, Kent, OH 44242, USA

Electric field induced motion (spinning, translation and circling) of spherical and cylindrical glass particles was studied in calamitic and bent-core liquid crystal films sandwiched between two glass plates with conductive inner surfaces. Under DC and low frequency AC fields, above a threshold Es~1-5V/?m (depending on material, phase, temperature and frequency) a symmetry breaking transition takes place and the particles start to spin around an axis perpendicular to the electric field (Quincke rotation). The Miesovicz viscosity ?a of various liquid crystal phases were deduced from the electric field dependence of the angular velocity of the spinning particles.
At fields Etr>Es the spinning particles begin to translate along the substrates normal to the electric fields. Such “electromigration” has also been observed by others, but only for the AC field and no detectable electrospinning. We propose that the electromigration is actually triggered by the electrospinning, which causes a lateral gradient of the electric field near the particles thus creating an electrostrictive force that sets the translation.
Finally we also demonstrated a novel collective circling motion of particles around the air-LC meniscus at air bubbles in homeotropic SmA cells. The linear speed of the revolving particles is independent of the radius of the air bubble, i.e., the angular velocity is inversely proportional to the radius. This effect does not exist in planar alignment or in other liquid crystal phases. The details of this motion and the underlying physical mechanism will be also discussed.

Piezoelectricity of cell membranes:
A possible mechanism for mechano-, and magneto-receptions in biology

A. Jákli, J. Harden, C. Notz, C. Bailey, D. Boetti, F. Wang, P. Westerman

We show that phospholipids, which are the main constituents of cell membranes are piezoelectric. This is done by periodically shearing and compressing films of hydrated L-?-Phosphatidylcholine, inducing tilt of the molecules with respect to the bilayer’s normal and produced electric current perpendicular to the tilt plane, corresponding to a polarization of about 300 nC/cm2 at 5 degrees of tilt. We also measured electric currents induced by less than a 100G alternating magnetic field in hydrated phospholipids doped with 0.5wt% of ferrofluid of magnetite (Fe3O4) nanoparticles. This suggests that rotation of magnetic particles found in migratory animals may induce local reorientation of the lipid molecules thus allowing navigation based on magnetic information. Very recently we started a project to show this effect in actual cell membranes.