Anchoring-dependent bifurcation in nematic microflows within cylindrical capillaries

Capillary microflows of liquid crystal phases are central to material, biological and bio-inspired systems. Despite their fundamental and applied significance, a detailed understanding of the stationary behaviour of nematic liquid crystals (NLC-s) in cylindrical capillaries is still lacking. Here, using numerical simulations based on the continuum theory of Leslie, Ericksen and Parodi, we investigate stationary NLC flows within cylindrical capillaries possessing homeotropic (normal) and uniform planar anchoring conditions. By considering the material parameters of the flow-aligning NLC, 5CB, we report that instead of the expected, unique director field monotonically approaching the alignment angle over corresponding Ericksen numbers (dimensionless number capturing viscous v/s elastic effects), a second solution emerges below a threshold flow rate (or applied pressure gradient). We demonstrate that the onset of the second solution, a nematodynamic bifurcation yielding energetically degenerate director tilts at the threshold pressure gradient, can be controlled by the surface anchoring and the flow driving mechanism (pressure-driven or volume-driven). For homeotropic surface anchoring, this alternate director field orients against the alignment angle in the vicinity of the capillary center; while in the uniform planar case, the alternate director field extends throughout the capillary volume, leading to reduction of the flow speed with increasing pressure gradients. While the practical realization and utilization of such nematodynamic bifurcations still await systematic exploration, signatures of the emergent rheology have been reported previously within microfluidic environments, under both homeotropic (Sengupta et al., Phys. Rev. Lett. 110, 048303, 2013) and planar anchoring conditions (Sengupta, Int. J. Mol. Sci. 14, 22826, 2013).Comment: 27 pages, 12 figure

Zelluläre Bewegungsabläufe im mikrofluidischen Lebensraum

Trypanosomen sind einzellige Blutstromparasiten und Erreger der in Afrika weit verbreiteten Schlafkrankheit des Menschen, sowie der Nagana Seuche in landwirtschaftlichen Nutztieren. Die Krankheit wird durch den Biss der Tsetsefliege übertragen und endet ohne medizinische Behandlung mit dem Tod des jeweiligen Wirtes. Der Name ist der griechischen Sprache entlehnt (Trypanosoma = Bohrkörperchen) und beschreibt bildlich den unter dem Mikroskop sichtbaren Bewegungsablauf, welcher auf dem ersten Blick dem Verlauf einer Korkenzieher Spindel ähnelt. Bei genauerer Betrachtung wird jedoch deutlich, dass die tatsächliche Bewegung der Zellen nicht nur wesentlich komplexer als bisher beschrieben ist, sondern auch von Zelle zu Zelle sowie über die Zeit variiert. Des Weiteren gilt die Frage zu klären, warum sich Zellen, die im Blutstrom leben, überhaupt aktiv bewegen, sind Sie doch nicht annähernd schnell genug, um gegen den Strom zu schwimmen. In diesem Kontext konnten Prof. Dr. M. Engstler und Kollegen zeigen, dass nur solche Zellen im Blut des Wirtes überleben, die sich aktiv bewegen. Sie machen hierfür den hydrodynamischen Fluss verantwortlich, welcher durch die aktiven Schwimmbewegungen der Zelle erzeugt wird: Antikörper, welche an der Oberfläche von Trypanosomen gebunden haben, werden mit Fluss und somit entgegen der Schwimmrichtung zum Kopfende der Zellen getragen, wo diese aufgenommen und somit unschädlich gemacht werden können. Rückwärts schwimmende Mutanten hingegen, häufen diese Antikörper am Fußende der Zelle an, dort können diese nicht abgebaut werden und bewirken somit ihrer Aufgabe entsprechend den Abbau der Trypanosomen selbst. Um herauszufinden, ob der beschriebene Mechanismus hydrodynamisch getriebener Proteinsortierung ein einzigartiges Phänomen oder aber ein in der uns bekannten Natur weit verbreitetes Prinzip darstellt, müssen wir a priori den genauen Bewegungsablauf sowie dessen hydrodynamische Wirkung auf die Zellen verstehen.Das Ziel dieser Arbeit ist den Bewegungsablauf von Trypanosomen in ihrer mikrofluidischen Umgebung messbar zu machen und zu beschreiben, um auf diesem Wege zu klären, warum sich Trypanosomen auf welche Art und Weise bewegen. Um dieses Ziel zu erreichen, haben wir zunächst ein sehr lichtempfindliches Fluoreszenzmikroskop zur optischen Mikromanipulation lebender Zellen entwickelt, welches bei hoher raumzeitlicher Auflösung und hoher optischer Kraftwirkung minimale phototoxische Wirkung zeigt. In Kombination mit speziell angepassten mikrofluidischen Methoden, erlaubt dieses Instrument die Manipulation einzelner lebender Zellen, sowie die präzise Kontrolle über Strömungsbedingungen und die räumliche Umgebung im Umfeld der Zellen. Dieser Aufbau ermöglicht also nicht nur einzelne Zellen gezielt im dreidimensionalen Raum zu positionieren, sie zu bewegen und deren Kräfte zu messen, sondern auch das gezielte Markieren lebender Zellen mit Fluoreszenzmarkern im Mikrofluss, sowie die unmittelbare Beobachtung und Quantifizierung des gesamten Prozesses.In dieser Arbeit konnte zum ersten Mal gezeigt werden, dass es möglich ist, lebende Trypanosomen über einen Zeitraum von t 6 µm in Ihrem Bewegungsablauf nicht beeinflussen, bei kleineren Abständen jedoch eine Synchronisation der Zellbewegung zu beobachten ist.Zusammengefasst unterstreichen diese Ergebnisse das Potential der angewendeten Methodik, sowie die Wichtigkeit dieser Forschung für das Verständnis der Zellbewegung, nicht nur am Beispiel von Trypanosoma brucei brucei, sondern auch für Effekte wie hydrodynamische Synchronisation komplexer Bewegungsabläufe von Organismen im Bereich kleiner Reynolds Zahlen. Wie in dieser Arbeit gezeigt werden konnte, ermöglicht die Kombination aus optischer Mikromanipulation und mikrofluidischer Methodik präzise Kraftmessungen an lebenden Zellen unter genau kontrollierten Strömungsbedingungen und birgt ein großes Potential für zukünftige Forschungen im vielen Bereichen der Biophysik

Impact of Microscopic Motility on the Swimming Behavior of Parasites : Straighter Trypanosomes are More Directional

Microorganisms, particularly parasites, have developed sophisticated swimming mechanisms to cope with a varied range of environments. African Trypanosomes, causative agents of fatal illness in humans and animals, use an insect vector (the Tsetse fly) to infect mammals, involving many developmental changes in which cell motility is of prime importance. Our studies reveal that differences in cell body shape are correlated with a diverse range of cell behaviors contributing to the directional motion of the cell. Straighter cells swim more directionally while cells that exhibit little net displacement appear to be more bent. Initiation of cell division, beginning with the emergence of a second flagellum at the base, correlates to directional persistence. Cell trajectory and rapid body fluctuation correlation analysis uncovers two characteristic relaxation times: a short relaxation time due to strong body distortions in the range of 20 to 80 ms and a longer time associated with the persistence in average swimming direction in the order of 15 seconds. Different motility modes, possibly resulting from varying body stiffness, could be of consequence for host invasion during distinct infective stages

Optical trapping reveals propulsion forces, power generation and motility efficiency of the unicellular parasites Trypanosoma brucei brucei

Unicellular parasites have developed sophisticated swimming mechanisms to survive in a wide range of environments. Cell motility of African trypanosomes, parasites responsible for fatal illness in humans and animals, is crucial both in the insect vector and the mammalian host. Using millisecond-scale imaging in a microfluidics platform along with a custom made optical trap, we are able to confine single cells to study trypanosome motility. From the trapping characteristics of the cells, we determine the propulsion force generated by cells with a single flagellum as well as of dividing trypanosomes with two fully developed flagella. Estimates of the dissipative energy and the power generation of single cells obtained from the motility patterns of the trypanosomes within the optical trap indicate that specific motility characteristics, in addition to locomotion, may be required for antibody clearance. Introducing a steerable second optical trap we could further measure the force, which is generated at the flagellar tip. Differences in the cellular structure of the trypanosomes are correlated with the trapping and motility characteristics and in consequence with their propulsion force, dissipative energy and power generation

Surface anchoring mediates bifurcation in nematic microflows within cylindrical capillaries

Capillary microflows of liquid crystal phases are central to material, biological and bio-inspired systems. Despite their fundamental and applied significance, a detailed understanding of the stationary behavior of nematic liquid crystals (NLC-s) in cylindrical capillaries is still lacking. Here, using numerical simulations based on the continuum theory of Leslie, Ericksen, and Parodi, we investigate stationary NLC flows within cylindrical capillaries possessing homeotropic (normal) and uniform planar anchoring conditions. By considering the material parameters of the flow-aligning NLC, 5CB, we report that instead of the expected, unique director field monotonically approaching the alignment angle over corresponding Ericksen numbers (dimensionless number capturing viscous vs elastic effects), a second solution emerges at a threshold flow rate (or applied pressure gradient). We demonstrate that the onset of the second solution, a nematodynamic bifurcation yielding distinct director configurations at the threshold pressure gradient, can be controlled by the surface anchoring and the flow driving mechanism (pressure-driven or volume-driven). For homeotropic surface anchoring, this alternate director field orients against the alignment angle in the vicinity of the capillary center; while in the uniform planar case, the alternate director field extends throughout the capillary volume, leading to reduction of the flow speed with increasing pressure gradients. While the practical realization and utilization of such nematodynamic bifurcations still await systematic exploration, signatures of the emergent rheology have been reported by the authors previously within microfluidic environments, under both homeotropic and planar anchoring conditions

Flow-induced structural orientation and relaxation in blends from biobased polylactide and liquid-crystalline polymers

Bio-based composite materials with enhanced properties have the potential to replace fossil-based polymers. Polylactic-acid (PLA) is a promising candidate for self-reinforced composites due to its tunable melting range [1]. However, for enhanced mechanical performance also additional reinforcement via blending with thermotropic liquid-crystalline polymers (TLCP) can be beneficial [2,3]. The effects of flow on the micro- and mesoscale structure need to be well correlated with chemical composition, capillary number concepts, and processing conditions.In this study, we investigate for pure and blended TLCP-PLA the effects of shear deformation and rate on the morphology by combined Rheo-Optics and Rheo-Scattering. Polarised Optical Microscopy (POM) in addition to a shear rheometer and shear-cell setups shows that the threaded morphology on a mesoscale breaks and orients along the flow direction under continuous flow, but reforms during relaxation. The molecular orientation during and after shear, respectively, is probed via a combination of rotational shear rheometry with synchrotron wide-angle X-ray diffraction analysis (WAXD). The acquired orientation as determined by the orientation factor along the azimuthal intensity distribution is found to be directly related to the shear strain, but hardly to the shear rate, and also vanishes within few minutes with flow cessation and relaxation. The results provide detailed insights into the structure evolution of the individual polymer phases as a function of flow profiles as well as material parameters

African Trypanosomes as model system for functional analyses of microbial motility

The locomotion of microorganisms in a microscopic world, where cells move through a fluid environment without using inertial forces, is a fascinating phenomenon in life science. Nature offers clever and inspiring strategies for self-propelling in an environment of no inertia. The flagellate African trypanosome, which causes African sleeping sickness, moves with help of a flagellum, which is firmly attached to its cell body. The beating flagellum leads to a strong distortion of the cell body and therefore to a swimming agitation of trypanosomes. We have found that trypanosomes use a hydrodynamic mechanism to defend against host’s immune attacks. Owing to continuous and directional swimming, host-derived antibodies attached to surface glycoproteins of the cell are dragged to the posterior cell pole, where they are rapidly internalized and destroyed. In the following we present new methodology and techniques to quantify the movements of proteins and the motility of cells. Moreover trypanosome motility schemes and their influence on cellular lifestyle and survival strategies are characterized

Flow-induced crystallization of i-PP studied by RheoNMR and RheoSAXS

We present a study on quiescent and flow-induced crystallization of isotactic polypropylene (i-PP) using novel RheoNMR and RheoSAXS techniques. The RheoNMR set-up [1-3] is based on a strain-controlled TA ARES with an integrated TD-NMR unit (30 MHz) which provides in-situ information on microscopic molecular dynamics of polymer chains on a nanometer and micro-/ millisecond scale. With RheoSAXS [4,5] on the other hand, we gain insight into evolving structures during crystallization on a nano-/ micrometer scale. In the current set-up [6] a Thermo HAAKE MARS II was combined with the strong and brilliant x-ray source (8.4 keV, 1021 photons/[s mm2 mrad2 0.1% BW]) at the German Electron Synchrotron (DESY, PETRA III, P10) to ensure a high 2D-SAXS image quality and real time monitoring of polypropylene crystallization. A short time steady-shear protocol was employed to study flow-induced crystallization, varying temperatures and flow conditions, respectively. By the use of combined techniques we were able to determine correlations of flow behavior with the evolving crystalline structures and to investigate critical flow conditions for the formation of oriented crystallites