38441030
Light-driven dynamic Archimedes spirals and periodic oscillatory patterns of topological solitons in anisotropic soft matter.
Oscillatory and excitable systems commonly exhibit formation of dynamic non-equilibrium patterns. For example, rotating spiral patterns are observed in biological, chemical, and physical systems ranging from organization of slime mold cells to Belousov-Zhabotinsky reactions, and to crystal growth from nuclei with screw dislocations. Here we describe spontaneous formation of spiral waves and a large variety of other dynamic patterns in anisotropic soft matter driven by low-intensity light. The unstructured ambient or microscope light illumination of thin liquid crystal films in contact with a self-assembled azobenzene monolayer causes spontaneous formation, rich spatial organization, and dynamics of twisted domains and topological solitons accompanied by the dynamic patterning of azobenzene group orientations within the monolayer. Linearly polarized incident light interacts with the twisted liquid crystalline domains, mimicking their dynamics and yielding patterns in the polarization state of transmitted light, which can be transformed to similar dynamic patterns in its intensity and interference color. This shows that the delicate light-soft-matter interaction can yield complex self-patterning of both. We uncover underpinning physical mechanisms and discuss potential uses.
92643177
Oscillations and Traveling Waves in Chemical Systems
The Mathematical Aspects of Temporal Oscillations in Reacting Systems Experimental and Mechanistic Characterization of Bromate-Ion-Driven Chemical Oscillations and Traveling Waves in Closed Systems A Quantitative Account of Oscillations, Bistability, and Traveling Waves in the Belousov- Zhabotinskii Reaction Mathematical Analysis of the Oregonator Model of the Belousov-Zhabotinskii Reaction Chemical Oscillators Based on Iodate Ion and Hydrogen Peroxide Numerical Techniques for Modeling and Analysis of Oscillating Reactions From Bistability to Sustained Oscillations in Homogeneous Chemical Systems in Flow Reactor Mode Halogen Based Oscillators in a Flow Reactor Periodically Perturbed Chemical Systems The Structure and Variety of Chemical Waves Propagating Reaction-Diffusion Fronts Organizing Centers for Chemical Waves in Two and Three Dimensions Gas Evolution Oscillators Isothermal Oscillations and Relaxation Ignitions in Gas-Phase Reactions: The Oxidations of Carbon Monoxide and Hydrogen Thermokinetic Oscillations in Homogeneous Gas Phase Oxidations Oscillating Chemical Reactions as an Example of the Development of a Subfield of Science Appendix: A Periodic Reaction and its Mechanism References Index.
4423035
Spatiotemporal evolution of ventricular fibrillation
Sudden cardiac death is the leading cause of death in the industrialized world, with the majority of such tragedies being due to ventricular fibrillation. Ventricular fibrillation is a frenzied and irregular disturbance of the heart rhythm that quickly renders the heart incapable of sustaining life. Rotors, electrophysiological structures that emit rotating spiral waves, occur in several systems that all share with the heart the functional properties of excitability and refractoriness. These re-entrant waves, seen in numerical solutions of simplified models of cardiac tissue, may occur during ventricular tachycardias,. It has been difficult to detect such forms of re-entry in fibrillating mammalian ventricles. Here we show that, in isolated perfused dog hearts, high spatial and temporal resolution mapping of optical transmembrane potentials can easily detect transiently erupting rotors during the early phase of ventricular fibrillation. This activity is characterized by a relatively high spatiotemporal cross-correlation. During this early fibrillatory interval, frequent wavefront collisions and wavebreak generation are also dominant features. Interestingly, this spatiotemporal pattern undergoes an evolution to a less highly spatially correlated mechanism that lacks the epicardial manifestations of rotors despite continued myocardial perfusion.
8955833
Effect of inhomogeneities on spiral wave dynamics in the Belousov-Zhabotinsky reaction.
We examine the effects of controlled, slowly varying spatial inhomogeneities on spiral wave dynamics in the light sensitive Belousov-Zhabotinsky chemical reaction-diffusion system. We measure the speed of the grain boundary that separates two spirals, the speed of the lower frequency spiral being swept away by the grain boundary, and the speed of the slow drift of the highest frequency spiral. The grain boundary speeds are shown to be related to the frequency of rotation and wave number of the spiral pattern, as predicted from analysis of the complex Ginzburg-Landau equation [M. Hendrey, Phys. Rev. Lett.10.1103/PhysRevLett.82.859 82, 859 (1999); M. Hendrey,, Phys. Rev. E10.1103/PhysRevE.61.4943 61, 4943 (2000)].
97079835
Meandering spiral waves in a bubble-free Belousov–Zhabotinsky reaction with pyrogallol
A thin layer of the Belousov–Zhabotinsky reaction with pyrogallol (PG) supports meandering spiral waves with outward-petal trajectories. Increasing [PG] or lowering [H2SO4] leads to an enlargement of the outer diameter of the tip trajectory, as well as an increment of the number of petals. Far from the spiral tip, the wavelength and the wave period also increase. However, the wave speed decreases, when [PG] is elevated or [H2SO4] is reduced. This reaction is suitable for further investigations of three-dimensional spiral dynamics with optical tomography because of slowly propagating waves, no bubbles, no induction time, and no inhibitory effects of oxygen.
11293105
Combined excitatory and inhibitory coupling in a 1-D array of Belousov-Zhabotinsky droplets.
We study the dynamical behavior of one-dimensional arrays of ∼100 μm diameter aqueous droplets containing the oscillatory Belousov-Zhabotinsky (BZ) reaction, separated by narrow gaps of a fluorinated oil. In this closed system, the malonic acid concentration decreases as the reaction proceeds. Starting with a low initial malonic acid concentration, we observe a series of attractors as a function of time in the following order: anti-phase attractors; in-phase attractors, which evolve into traveling waves; and mixed modes that contain either regions of in-phase droplets separated by anti-phase oscillators, or in-phase oscillators combined with non-oscillatory droplets. Most of the observations are consistent with numerical chemical models of the BZ reaction in which components that participate in the excitatory (bromine dioxide and bromous acid) and inhibitory (bromine) pathways diffuse between the droplets. The models are used to quantitatively assess the inter-drop coupling strength as a function of drop separation, drop size and malonic acid concentration. To experimentally establish the mechanism of excitatory coupling between the BZ droplets, we verify the transport through the fluorinated oil of chlorine dioxide and several weak acids, including malonic acid.
121065262
Spirals in liquid crystals in a rotating magnetic field
Abstract We have discovered spiral formation in nematic liquid crystal solitons under the influence of a rotating magnetic field. The shape of the spirals is well modeled by a rotating spiral of Archimedes, except in the immediate vicinity of the cores. The rotation of the driving magnetic field breaks the symmetry between counter rotating spirals, leading to differing wavelengths and spiral rotation rates. As the magnetic field rotation rate is decreased below a critical value, the wavelength diverges while the spiral rotation rate drops to zero. This divergence is related to a structural transition occurring within the solitons themselves. The spirals can be understood by examining the topology and symmetries of the solitons.
120283647
Light-induced spontaneous pattern formation in nematic liquid crystal cells
Optically induced instabilities were observed in nematic liquid crystals sandwiched between a photosensitive layer and a reference plate. The instabilities occurred when the light entered the cell from the reference plate and its polarization was parallel to the director alignment at the entrance face. Two kinds of patterns were detected, a static and a dynamic one. The underlying mechanism of pattern formation is the coupling between the director orientation on the photosensitive plate and the polarization direction of the light on that plate.
13088836
Solitons and pattern formation in liquid crystals in a rotating magnetic field.
We have discovered novel nonlinear dissipative dynamic patterns in nematic liquid crystals under the influence of a continuously rotating magnetic field. We present a state diagram of the soliton structures as a function of field and rotation rate and discuss physical models describing their growths and propagation and transitions.
27299150
Structural transition and motion of domain walls in liquid crystals under a rotating magnetic field.
A sharp transition from static to propagating domain walls is observed in a homeotropic nematic layer driven by a rotating magnetic field. We show experimentally that this transition is a manifestation of the transition of wall structure from Ising type to Bloch type. We find that the initial curvature of the wall plays an important role in this transition.
11175916
Spiral waves in liquid crystal.
We report experimental investigation and theoretical interpretation of the formation of sprial waves in a nematic liquid crystal subjected to a rotating magnetic field and a high frequency electrical field.
1974474
Living Crystals of Light-Activated Colloidal Surfers
Light On Clusters From schools of fish to bacterial colonies, large-scale phenomena—such as swarming or pattern formation—are ubiquitous. In such systems, there is a continuing question as to the relative importance of "intelligence" (biology) of the agents versus purely physical effects. Working with synthetic colloids, Palacci et al. (p. 936) show that self-organized clustering can be switched on and off to form crystals that dissolve when the light source is turned off. The particles consist of a hematite cube partly encapsulated in a polymeric sphere that is able to catalyze chemical reactions when exposed to light. The self-assembly behavior results from a combination of propelling forces, osmotic effects, and coupling between colloidal and tracer particles. Light-catalyzed reactions drive the far-from-equilibrium crystallization of colloids that couple to tracer particles. Spontaneous formation of colonies of bacteria or flocks of birds are examples of self-organization in active living matter. Here, we demonstrate a form of self-organization from nonequilibrium driving forces in a suspension of synthetic photoactivated colloidal particles. They lead to two-dimensional "living crystals," which form, break, explode, and re-form elsewhere. The dynamic assembly results from a competition between self-propulsion of particles and an attractive interaction induced respectively by osmotic and phoretic effects and activated by light. We measured a transition from normal to giant-number fluctuations. Our experiments are quantitatively described by simple numerical simulations. We show that the existence of the living crystals is intrinsically related to the out-of-equilibrium collisions of the self-propelled particles.
740749
Defect annihilation and proliferation in active nematics.
Liquid crystals inevitably possess topological defect excitations generated through boundary conditions, through applied fields, or in quenches to the ordered phase. In equilibrium, pairs of defects coarsen and annihilate as the uniform ground state is approached. Here we show that defects in active liquid crystals exhibit profoundly different behavior, depending on the degree of activity and its contractile or extensile character. While contractile systems enhance the annihilation dynamics of passive systems, extensile systems act to drive defects apart so that they swarm around in the manner of topologically well-characterized self-propelled particles. We develop a simple analytical model for the defect dynamics which reproduces the key features of both the numerical solutions and recent experiments on microtubule-kinesin assemblies.
4405648
Spontaneous motion in hierarchically assembled active matter
With remarkable precision and reproducibility, cells orchestrate the cooperative action of thousands of nanometre-sized molecular motors to carry out mechanical tasks at much larger length scales, such as cell motility, division and replication. Besides their biological importance, such inherently non-equilibrium processes suggest approaches for developing biomimetic active materials from microscopic components that consume energy to generate continuous motion. Being actively driven, these materials are not constrained by the laws of equilibrium statistical mechanics and can thus exhibit sought-after properties such as autonomous motility, internally generated flows and self-organized beating. Here, starting from extensile microtubule bundles, we hierarchically assemble far-from-equilibrium analogues of conventional polymer gels, liquid crystals and emulsions. At high enough concentration, the microtubules form a percolating active network characterized by internally driven chaotic flows, hydrodynamic instabilities, enhanced transport and fluid mixing. When confined to emulsion droplets, three-dimensional networks spontaneously adsorb onto the droplet surfaces to produce highly active two-dimensional nematic liquid crystals whose streaming flows are controlled by internally generated fractures and self-healing, as well as unbinding and annihilation of oppositely charged disclination defects. The resulting active emulsions exhibit unexpected properties, such as autonomous motility, which are not observed in their passive analogues. Taken together, these observations exemplify how assemblages of animate microscopic objects exhibit collective biomimetic properties that are very different from those found in materials assembled from inanimate building blocks, challenging us to develop a theoretical framework that would allow for a systematic engineering of their far-from-equilibrium material properties.
14698581
Giant volume change of active gels under continuous flow.
While living systems have developed highly efficient ways to convert chemical energy (e.g., ATP hydrolysis) to mechanical motion (e.g., movement of muscle), it remains a challenge to build muscle-like biomimetic systems to generate mechanical force directly from chemical reactions. Here we show that a continuous flow of reactant solution leads to by far the largest volume change to date in autonomous active gels driven by the Belousov-Zhabotinsky reaction. These results demonstrate that microfluidics offers a useful and facile experimental approach to optimize the conditions (e.g., fabrication methods, counterions, flow rates, concentrations of reagents) for chemomechanical transduction in active materials. This work thus provides much needed insights and methods for the development of chemomechanically active systems based on combining soft materials and microfluidic systems.
8063098
Large-area optoelastic manipulation of colloidal particles in liquid crystals using photoresponsive molecular surface monolayers
Noncontact optical trapping and manipulation of micrometer- and nanometer-sized particles are typically achieved by use of forces and torques exerted by tightly focused high-intensity laser beams. Although they were instrumental for many scientific breakthroughs, these approaches find few technological applications mainly because of the small-area manipulation capabilities, the need for using high laser powers, limited application to anisotropic fluids and low-refractive-index particles, as well as complexity of implementation. To overcome these limitations, recent research efforts have been directed toward extending the scope of noncontact optical control through the use of optically-guided electrokinetic forces, vortex laser beams, plasmonics, and optofluidics. Here we demonstrate manipulation of colloidal particles and self-assembled structures in nematic liquid crystals by means of single-molecule-thick, light-controlled surface monolayers. Using polarized light of intensity from 1,000 to 100,000 times smaller than that in conventional optical tweezers, we rotate, translate, localize, and assemble spherical and complex-shaped particles of various sizes and compositions. By controlling boundary conditions through the monolayer, we manipulate the liquid crystal director field and the landscape of ensuing elastic forces exerted on colloids by the host medium. This permits the centimeter-scale, massively parallel manipulation of particles and complex colloidal structures that can be dynamically controlled by changing illumination or assembled into stationary stable configurations dictated by the “memorized” optoelastic potential landscape due to the last illumination pattern. We characterize the strength of optically guided elastic forces and discuss the potential uses of this noncontact manipulation in fabrication of novel optically- and electrically-tunable composites from liquid crystals and colloids.
8313090
Anchoring-mediated interaction of edge dislocations with bounding surfaces in confined cholesteric liquid crystals.
We employ the fluorescent confocal polarizing microscopy to image edge dislocations in cholesteric liquid crystals. Surface anchoring at the bounding plates determines the structure and behavior of defects. Two types of plates set in-plane director orientation but differ in the type of associated anchoring potentials. Plates with strong polar anchoring and nonzero azimuthal anchoring repel the dislocations, while plates with weak polar anchoring and no azimuthal anchoring allow the dislocations to escape through the boundary. To explain the results, we propose a coarse-grained model of cholesteric anchoring.
7288332
Healing of defects at the interface of nematic liquid crystals and structured Langmuir-Blodgett monolayers.
We use Langmuir-Blodgett molecular monolayers and nematic liquid crystals as model two- and three-dimensional orientationally ordered systems to study the stability and healing of topological defects at their contact interfaces. Integer-strength defects at the monolayer induce disclinations of similar strength in the nematic that, however, do not propagate deep into the bulk, but rather form single- or double-split arch-shaped loops pinned to the interface. This behavior is qualitatively independent of the far-field director orientation and involves either half-integer singular or twist-escaped unity-strength nonsingular nematic disclinations. These two defect configurations can be selected by varying sample preparation given their comparable free energy, consistently with direct probing by use of laser tweezers.
44833428
Optical generation of crystalline, quasicrystalline, and arbitrary arrays of torons in confined cholesteric liquid crystals for patterning of optical vortices in laser beams.
Condensed matter systems with topological defects in the ground states range from the Abrikosov phases in superconductors, to various blue phases and twist grain boundary phases in liquid crystals, and to phases of skyrmion lattices in chiral ferromagnets and Bose-Einstein condensates. In nematic and chiral nematic liquid crystals, which are true fluids with long-range orientational ordering of constituent molecules, point and line defects spontaneously occur as a result of symmetry-breaking phase transitions or due to flow, but they are unstable, hard to control, and typically annihilate with time. Here we describe the optical generation of two-dimensional crystalline, quasicrystalline, and arbitrary ensembles of particlelike structures manifesting both skyrmionlike and Hopf fibration features--dubbed "torons"--composed of looped double twist cylinders and point defects embedded in a uniform director field. In these two-dimensional lattices, we then introduce various dislocations, defects in positional ordering of the torons. We show that the periodic defect lattices with and without dislocation are light- and voltage-tunable reconfigurable two-dimensional diffraction gratings and can be used to generate various controlled phase singularities in the diffracted laser beams. The results of computer simulations of optical images, diffraction patterns, and phase distributions with optical vortices are in a good agreement with the corresponding experimental findings.
19124283
Mutually tangled colloidal knots and induced defect loops in nematic fields.
Colloidal dispersions in liquid crystals can serve as a soft-matter toolkit for the self-assembly of composite materials with pre-engineered properties and structures that are highly dependent on particle-induced topological defects. Here, we demonstrate that bulk and surface defects in nematic fluids can be patterned by tuning the topology of colloidal particles dispersed in them. In particular, by taking advantage of two-photon photopolymerization techniques to make knot-shaped microparticles, we show that the interplay of the topologies of the knotted particles, the nematic field and the induced defects leads to knotted, linked and other topologically non-trivial field configurations. These structures match theoretical predictions made on the basis of the minimization of the elastic free energy and satisfy topological constraints. Our approach may find uses in self-assembled topological superstructures of knotted particles linked by nematic fields, in topological scaffolds supporting the decoration of defect networks with nanoparticles, and in modelling other physical systems exhibiting topologically analogous phenomena.
10766536
Electrically driven multiaxis rotational dynamics of colloidal platelets in nematic liquid crystals.
We describe field-induced multiaxis rotations of colloids in a nematic liquid crystal. Anchoring of the nematic director to the colloidal platelet's surface and interplay of dielectric and elastic energies enable robust control over colloid orientation that cannot be achieved in isotropic liquids. Because of the anisotropy of the fluid and the platelike shape of particles, the colloids can be forced to rotate about four different rotational axes even for a fixed direction of the applied field. The time scale of these unexpected voltage-dependent dynamics varies over four orders of magnitude (10⁻²-10²  s) and promises a number of novel electro-optic, photonic, and display applications.
20093713
Electrically and optically tunable plasmonic guest-host liquid crystals with long-range ordered nanoparticles.
Practical guest-host devices in which dichroic dye molecules follow electrical switching of a liquid crystal host remain elusive for decades despite promising efficient displays and emergent applications such as smart windows. This is mainly because of poor stability, surface precipitation, and limited means for property engineering of the dyes. To overcome these challenges, we develop plasmonic metal nanoparticle analogues of dichroic guest-host liquid crystals. Nematic dispersions of aligned anisotropic gold nanoparticles are obtained by polymer passivation of their surfaces to impose weak tangential boundary conditions for orientation of anisotropic host molecules. Control of the ensuing surface interactions leads to long-range ordered colloidal dispersions, allowing for collective optical and electrical switching of rod- and platelet-like nanoparticles. This facile control of mesostructured plasmonic medium's optical properties in visible and infrared spectral ranges is of interest for many applications.
30781926
Optics of Liquid Crystal Displays
This tutorial covers an introduction to liquid crystal technology and principles of operation of various modes of liquid crystal displays as well as the development of birefringent optical thin film technologies (e.g., polarizers, compensators) for improving the viewing quality of these displays.
120291810
Excitability and pattern formation in a liquid crystal Fabry-Pérot interferometer
Abstract We demonstrate a nonlinear Fabry–Perot interferometer with a nematic liquid crystal to be an optical excitable system. The excitability is shown to arise from the competition of two Kerr type nonlinearities resulting from field-induced molecular reorientation and thermal absorption in the presence of laser radiation. We have observed all the essential features of excitability in this system when it is spatially extended, namely, propagating pulses in one-dimensional space and spiral waves in two dimensions. We have further studied patterns resulting from random noise excitations in this system.
205250617
Topography from topology: photoinduced surface features generated in liquid crystal polymer networks.
Films subsumed with topological defects are transformed into complex, topographical surface features with light irradiation of azobenzene-functionalized liquid crystal polymer networks (azo-LCNs). Using a specially designed optical setup and photoalignment materials, azo-LCN films containing either singular or multiple defects with strengths ranging from |½| to as much as |10| are examined. The local order of an azo-LCN material for a given defect strength dictates a complex, mechanical response observed as topographical surface features.