Dynamics And Structure Of Liquid Crystal Colloids

This book brings together the many concepts and discoveries in liquid crystal colloids contributed over the last twenty years and scattered across numerous articles and book chapters. It provides both a historical overview of the development of the field and a clear perspective on the future applications in photonics. The book covers all phenomena observed in liquid crystal colloids with an emphasis on experimental tools and applications of topology in condensed matter, as well as practical micro-photonics applications. It includes a number of spectacular manifestations of new topological phenomena not found or difficult to observe in other systems. Starting from the early works on nematic colloids, it explains the basics of topological defects in ordered media, charge and winding, and the elastic forces between colloidal particles in nematics. Following a detailed description of experimental methods, such as optical tweezing and particle tracking, the book eases the reader into the theoretical part, which deals with elastic deformation of nematic liquid crystals due to inclusions and surface alignment. This is discussed in the context of basic mean field Landau-de Gennes Q-tensor theory, with a brief explanation of the free-energy minimization numerical methods. There then follows an excursion into the topology of complex nematic colloidal structures, colloidal entanglement, knotting and linking. Nematic droplets, shells, handlebodies and chiral topological structures are addressed in separate chapters. The book concludes with an extensive chapter on the photonic properties of nematic dispersions, presenting the concept of integrated soft matter photonics and discussing the concepts of nematic and chiral nematic microlasers, surface-sensitive photonic devices and smectic microfibers. The text is complemented by a large bibliography, explanatory sketches and beautiful micrographs.

Liquid-crystalline phases are now known to be formed by an ever growing range of quite diverse materials, these include those of low molecular weight as well as the novel liquid-crystalline polymers, such phases can also be induced by the addition of a solvent to amphiphilic systems leading to lyotropic liquid crystals. Irrespective of the structure of the constituent molecules these numerous liquid-cl)'Stailine phases are characterised by their long range orientational order. In addition certain phases exhibit elements of long range positional order. Our understanding, both experimental and theoretical, at the molecular level of the static behaviour of these fascinating and important materials is now well advanced. In contrast the influence of the long range order; both orientational and positional, on the molecular dynamics in liquid Cl)'Stais is less well understood. In an attempt to address this situation a NATO Advanced Study Institute devoted to liquid ctystal dynamics was held at n Ciocco, Barga, Italy in September 1989. This brought together experimentalists and theoreticians concerned with the various dynamical processes occurring in all liquid crystals. The skills of the participants was impressively wide ranging; they spanned the experimental techniques used in the study of molecular dynamics, the nature of the systems investigated and the theoretical models employed to understand the results. While much was learnt it was also recognised that much more needed to be done in order to advance our understanding of molecular dynamics in liquid Cl)'Stais.

Proceedings of the NATO Advanced Study Institute on Properties of Colloidal Systems, Aberystwyth, Wales, U.K., September 10-23, 1989

This book presents a compilation of self-contained chapters covering a wide range of topics within the broad field of soft condensed matter. Each chapter starts with basic definitions to bring the reader up-to-date on the topic at hand, describing how to use fluid flows to generate soft materials of high value either for applications or for basic research. Coverage includes topics related to colloidal suspensions and soft materials and how they differ in behavior, along with a roadmap for researchers on how to use soft materials to study relevant physics questions related to geometrical frustration.

Dynamics of micrometer-scale active colloids in complex fluids is of great fundamental and application interest. In this dissertation, we study the dynamics of active colloids placed in an orientationally ordered liquid. We demonstrate that the liquid crystal (LC) environment allows extracting useful work from the random motion of microswimmers and ambient external field in the form of self-propulsion of colloids. We also show that the simplest microswimmer in a low Reynolds number regime in an LC is a sphere; in contrast with the complicated structures and modes necessary for swimming in isotropic fluids. We demonstrate that a water droplet containing a suspension of randomly swimming bacteria placed in a nematic LC environment propels itself unidirectionally. The droplets with a hyperbolic hedgehog (HH) point defect in their vicinity that are of polar symmetry propel themselves while the droplets with Saturn ring (SR) director configuration of quadrupolar symmetry do not propel. In HH droplets, the propulsion is driven by the activity of bacteria; the droplets with higher bacteria concentration move faster, while the droplets with no bacteria or immotile bacteria do not propel. The bacteria generate a chaotic flow inside the droplet that is transferred to the nematic through the interface. The polar director structure around the HH droplet rectifies this chaotic flow into the directional self-propulsion of the droplet. The mechanism of motion is rooted in the coupling of flow and the director orientation in the nematic. The flow that is transferred into the nematic faces different resistances from the director structure around the droplet while streaming toward the HH as compared to the opposite direction. Thus, the net displacement is in the direction with less resistance. We show that the responsiveness of the LC to external cues such as electric field and light provides the means to dynamically control the speed and trajectories of the active droplets. An in-plane electric field reorients the director through dielectric coupling and thus redirects the droplet. Also, a high electric field transforms the HH into a disclination ring and thus reduces the droplet speed. When the ring becomes an equatorial SR the droplet stops. A focused laser beam is used to control the droplet motility and polarity of motion. The laser locally melts the director around the droplet and reversibly transforms the SR to HH, thus controlling the droplet motility. The beam is focused on the desired side of the droplet and determines the direction of motion by creating HH there. LC environment can also be used to enable and control the transport of inanimate colloidal particles. Previously, liquid crystal-enabled electrophoresis of colloids has been explored in which the particle propels with a speed quadratically proportional to the applied electric field. In this work, we describe highly nonlinear electrophoresis in nematic in which the velocity is proportional to the fourth and sixth powers of the field. This is the highest nonlinear electrophoresis effect ever reported. The mechanism is attributed to the field-generated charges in LC which modify the space charge. Finally, the observation and recording of the dynamics of birefringence environments require complex microscopy techniques. We describe a novel microscopy based on a polychromatic polarizing module (PPM) to map the director orientation of LC with a high spatial and temporal resolution. The technique allows us to study the dynamics of active LCs.

During the last decade, various powerful experimental tools have been developed, such as small angle X-ray and neutron scattering, X-ray and neutron reflection from interfaces, neutron spin-echo spectroscopy and quasi-elastic multiple light scattering and large scale computer simulations. Due to the rapid progress brought about by these techniques, one witnesses a resurgence of interest in the physicochemical properties of colloids, surfactants and macromolecules in solution. Although these disciplines have a long history, they are at present rapidly transforming into a new, interdisciplinary research area generally known as complex liquids or soft condensed matter physics: names that reflect the considerable involvement of the chemical and condensed matter physicists. This book is based on lectures given at a NATO ASI held in the summer of 1991 and discusses these new developments, both in theory and experiment. It constitutes the most up-to-date and comprehensive summary of the entire field.

Covering all types of soft matter, this unique book provides an easily accessible introduction to both theory and applciations.