Dynamical regimes and hydrodynamic lift of viscous vesicles under shear

The dynamics of two-dimensional viscous vesicles in shear flow, with different fluid viscosities $\eta_{\rm in}$ and $\eta_{\rm out}$ inside and outside, respectively, is studied using mesoscale simulation techniques. Besides the well-known tank-treading and tumbling motions, an oscillatory swinging motion is observed in the simulations for large shear rate. The existence of this swinging motion requires the excitation of higher-order undulation modes (beyond elliptical deformations) in two dimensions. Keller-Skalak theory is extended to deformable two-dimensional vesicles, such that a dynamical phase diagram can be predicted for the reduced shear rate and the viscosity contrast $\eta_{\rm in}/\eta_{\rm out}$. The simulation results are found to be in good agreement with the theoretical predictions, when thermal fluctuations are incorporated in the theory. Moreover, the hydrodynamic lift force, acting on vesicles under shear close to a wall, is determined from simulations for various viscosity contrasts. For comparison, the lift force is calculated numerically in the absence of thermal fluctuations using the boundary-integral method for equal inside and outside viscosities. Both methods show that the dependence of the lift force on the distance $y_{\rm {cm}}$ of the vesicle center of mass from the wall is well described by an effective power law $y_{\rm {cm}}^{-2}$ for intermediate distances $0.8 R_{\rm p} \lesssim y_{\rm {cm}} \lesssim 3 R_{\rm p}$ with vesicle radius $R_{\rm p}$. The boundary-integral calculation indicates that the lift force decays asymptotically as $1/[y_{\rm {cm}}\ln(y_{\rm {cm}})]$ far from the wall.Comment: 13 pages, 13 figure

Hydrodynamics of Rod-Like Colloids and Vesicles

We investigate the dynamics of rod-like colloids and vesicles by means of computer simulations. These two systems are examples of the rich dynamics in "soft-matter" systems, which is characterized by large relaxation times. Therefore, dynamical behavior in soft-matter systems is easily accessable experimentally, and soft materials are driven into non-equilibrium states, already by weak external fields. Both systems have in common that they serve as model systems for transport phenomena in cell biology. We focus on the influence of hydrodynamic interactions. This is realized by the use of a mesoscale hydrodynamics simulation technique called the "Multi Particle Collision Dynamics" (MPC) method, which takes the solvent into account explicitly. We calculate self-diffusion constants of rod-like colloids in the isotropic and nematic phases. Rod diffusion is strongly influenced by steric and hydrodynamic interactions between rods. Due to the anisotropy of the nematic phase also diffusion is anisotropic in such systems. We find that hydrodynamic effects lead to an increased diffusion. Moreover, our simulations show that the diffusion anisotropy of the nematic phase depends on the rod aspect ratio. Our simulation results are compared to experimental measurements of our cooperation partners (group J. K. G. Dhont, FZ-Jülich) who measured diffusion constants of rod-like fd-viruses suspensions. Our observations of the hydrodynamic enhancement and the anisotropy of rod self-diffusion are in good agreement with the experiments. A small amount of spherical tracer colloids is added to the rod suspensions described above, and tracer-sphere diffusion constants are determined. They also exhibit a strong diffusion anisotropy in the nematic phase. The effect of the rod network on tracer-sphere diffusion can be divided into a steric and hydrodynamic contribution. Our results are in good agreement with theoretical predictions which incorporate hydrodynamic effects. An important quantity for the calculation of the theoretical diffusion constants is the hydrodynamic screening length, which is difficult to measure in experiments, but can be directly calculated in simulations. Due to the high concentration of rods, the typically long-ranged hydrodynamic interactions, which depend inversely proportional on the distance between colloids, are screened such that they decay exponentially. We have developed a method which allows us to calculate hydrodynamic screening lengths from the equilibrium fluctuations of solvent shear waves. With this method, we are also able to determine anisotropic screening lengths in nematic systems. We show that hydrodynamic screening lengths are of the order of typical distances between neighboring rods. The calculated screening lengths are able to explain tracer-sphere diffusion constants quantitatively. Far more complex than rod suspensions are vesicles, as they have an internal dynamics. We study vesicles in shear flow in a two-dimensional model system which shows a variety of interesting dynamical phenomena. Depending on the viscosity ratio, i.e. the ratio between the inner and the outer viscosity of the vesicle, they can either ``tumble'', ``swing'' or show ``tank-treading''. In the tumbling regime, the vesicle orientation permanently rotates, in the swinging regime the vesicle exhibits temporally periodical changes in shape and orientation and in the tank-treading regime both shape and orientation are constant, whereas the membrane rotates around the enclosed volume. For the first time, a transition from tank-treading to swinging with increasing viscosity contrast could be shown in computer simulations. Our simulations are in good agreement with a phenomenological theoretical description. Close to walls, tumbling is strongly suppressed. Furthermore, the vesicle is repelled from the wall. The origin of this repulsion is the hydrodynamical lift force. We find that the lift force decays inversely proportional to the squared wall distance and that it decays with increasing viscosity contrast. The lift force is of relevance for the motion of blood cells in blood flow

A Necklace Model for Vesicles Simulations in 2D

International audienceThe aim of this paper is to propose a new numerical model to simulate 2D vesicles interacting with a newtonian fluid. The inextensible membrane is modeled by a chain of circular rigid particles which are maintained in cohesion by using two different type of forces. First, a spring force is imposed between neighboring particles in the chain. Second, in order to model the bending of the membrane, each triplet of successive particles is submitted to an angular force. Numerical simulations of vesicles in shear flow have been run using Finite Element Method and the FreeFem++[1] software. Exploring different ratios of inner and outer viscosities, we recover the well known "Tank-Treading" and "Tumbling" motions predicted by theory and experiments. Moreover, for the first time, 2D simulations of the "Vacillating-Breathing" regime predicted by theory in [2] and observed experimentally in [3] are done without special ingredient like for example thermal fluctuations used in [4]

Dynamics of Fluid Vesicles in Oscillatory Shear Flow

The dynamics of fluid vesicles in oscillatory shear flow was studied using differential equations of two variables: the Taylor deformation parameter and inclination angle $\theta$. In a steady shear flow with a low viscosity $\eta_{\rm {in}}$ of internal fluid, the vesicles exhibit steady tank-treading motion with a constant inclination angle $\theta_0$. In the oscillatory flow with a low shear frequency, $\theta$ oscillates between $\pm \theta_0$ or around $\theta_0$ for zero or finite mean shear rate $\dot\gamma_{\rm m}$, respectively. As shear frequency $f_{\gamma}$ increases, the vesicle oscillation becomes delayed with respect to the shear oscillation, and the oscillation amplitude decreases. At high $f_{\gamma}$ with $\dot\gamma_{\rm m}=0$, another limit-cycle oscillation between $\theta_0-\pi$ and $-\theta_0$ is found to appear. In the steady flow, $\theta$ periodically rotates (tumbling) at high $\eta_{\rm {in}}$, and $\theta$ and the vesicle shape oscillate (swinging) at middle $\eta_{\rm {in}}$ and high shear rate. In the oscillatory flow, the coexistence of two or more limit-cycle oscillations can occur for low $f_{\gamma}$ in these phases. For the vesicle with a fixed shape, the angle $\theta$ rotates back to the original position after an oscillation period. However, it is found that a preferred angle can be induced by small thermal fluctuations.Comment: 11 pages, 13 figure

<i>µ</i>-XRF Studies on the Colour Brilliance in Ancient Wool Carpets

Many handmade ancient and recent oriental wool carpets show outstanding brilliance and persistence of colour that is not achieved by common industrial dyeing procedures. Anthropologists have suggested the influence of wool fermentation prior to dyeing as key technique to achieve the high dyeing quality. By means ofμ-XRF elemental mapping of mordant metals we corroborate this view and show a deep and homogenous penetration of colourants into fermented wool fibres. Furthermore we are able to apply this technique and prove that the fermentation process for ancient specimens cannot be investigated by standard methods due to the lack of intact cuticle layers. This finding suggests a broad range of further investigations that will contribute to a deeper understanding of the development of traditional dyeing techniques. Spectroscopic studies add information on the oxidation states of the metal ions within the respective mordant-dye-complexes and suggest a partial charge transfer as basis for a significant colour change when Fe mordants are used.</jats:p

X-ray microscopy reveals the outstanding craftsmanship of Siberian Iron Age textile dyers

The excellent craftsmanship of ancient Oriental and Central Asian textile dyers is already demonstrated in the remarkable brilliance and fastness of the colours of the so-called Pazyryk carpet, the by far oldest pile carpet found to date. This specimen resembles the advanced craftsmanship of Iron Age Central Asian textile production. We have employed synchrotron-based µ-XRF imaging to detect the distribution of metal organic pigments within individual fibres of the Pazyryk carpet (about 2500 years old) and compare the results to wool fibres, which we prepared according to traditional Anatolian dyeing recipes. We observe congruent pigment distribution within specimens from the Pazyryk carpet and natural wool fibres that we have fermented prior to dyeing. Therefore, we conclude that the superior fermentation technique has been utilized about 2000 years earlier than known so far

Activation and effector functions of human NK cells after interaction with Leishmania

Hintergrund und Fragestellung: Die einzelligen Leishmania-Parasiten, die Erreger der teils tödlich verlaufenden Leishmaniose werden über den Stich von Sandmücken auf viele unter-schiedliche Säugetiere, wie auch den Menschen, übertragen. Durch die fortschreitende Aus-breitung des Vektors und verstärkte Zuwanderung aus Endemiegebieten hat die Leishmaniose auch in Deutschland in den letzten Jahren immer mehr an Bedeutung gewonnen. Leishmanien werden nach der Übertragung passiv über Phagozytose in unterschiedliche Zellen, vor allem Immunzellen, aufgenommen und vermehren sich intrazellulär. Eine effektive Immunantwort gegen Leishmanien ist daher mit der Produktion von leishmanizid wirkenden, reaktiven Sauerstoff- und Stickstoffspezies (z. B. Stickstoffmonoxid [NO]) durch die infizierten Zellen selbst verbunden. Die NO-Produktion wird vor allem durch das Zytokin Interferon (IFN)-gamma induziert, das von T-Lymphozyten und Natürlichen Killer (NK)-Zellen produziert wird. Im Mausmodell der Leishmaniose konnte nachgewiesen werden, dass IFN-gamma von NK-Zellen bei einer Leishmanien-Infektion einen protektiven Effekt hat. Auch beim Menschen gibt es Hinweise darauf, dass NK-Zellen an der Immunabwehr von Leishmanien beteiligt sind. Allerdings sind die zugrundeliegenden Mechanismen der NK-Zell-Aktivierung im menschlichen Körper teils noch unklar und die bisher erzielten Ergebnisse kontrovers. In der vorliegenden Arbeit wurde daher untersucht, ob aus dem Blut isolierte humane NK-Zellen in vitro in einem Zeitraum von 20 h durch Leishmanien aktiviert werden können und welche Faktoren hierfür notwendig sind.Ergebnisse: Die Kokultur von PBMCs mit promastigoten Stadien unterschiedlicher Leishma-nia-Arten führte zu einer gesteigerten Expression des Aktivierungsmarkers CD69 auf NK-Zellen. Die Stärke der Expression war spezies- und dosisabhängig, zeigte jedoch keinen Zusammenhang zum Organtropismus der jeweiligen Parasiten-Stämme. Während T- oder B-Lymphozyten keine Rolle spielten, wurden Leishmania-infizierte Monozyten als Induktoren der NK-Zell-Aktivierung identifiziert. Die Steigerung der CD69-Expression war dabei nicht abhängig von einer bestimmten Monozytensubpopulation. Sie konnte sowohl durch einen löslichen Faktor, der von Leishmania-infizierten Monozyten produziert wurde, als auch durch direkten Kontakt zwischen Monozyten und NK-Zellen ausgelöst werden. Während es in dieser Arbeit nicht möglich war, den löslichen Faktor zu identifizieren, konnte das auf Monozyten präsentierte, membrangebundene IL-18 für die kontaktabhängige Steigerung der CD69-Expression verantwortlich gemacht werden. Es wurden keine Hinweise auf eine direkte Akti-vierung der NK-Zellen durch Leishmanien gefunden. Trotz der CD69-Steigerung auf NK-Zellen wurde weder die Produktion von IFN-gamma noch eine NK-Zell-Zytotoxizität unter den genannten Bedingungen ausgelöst. Unter zusätzlicher Stimulation mit dem Zytokin IL-12 kam es jedoch in IL-18-abhängiger Weise zum Ablauf der genannten Effektormechanismen. Die Anwesenheit von neutrophilen Granulozyten oder unreifen, aus Monozyten generierten dendritischen Zellen (Mo-DCs) in PBMC-Leishmania-Kokulturen hatte allerdings keinen Einfluss auf die Effektorfunktionen der NK-Zellen. Die zelluläre Quelle des für die Aktivierung benötigten IL-12 in einer in vivo-Situation ist daher bisher nicht geklärt. Vielversprechende Kandidaten sind, wie bereits im Mausmodel gezeigt, reife DCs.Neben den indirekten Effekten der Leishmanien auf NK-Zellen konnte auch eine direkte, kon-taktabhängige Interaktion gemessen werden, die zur verminderten Expression des Oberflä-chenmoleküls CD56 auf den NK-Zellen führte. Welche Auswirkungen diese Reaktion auf die allgemeine Funktionalität der NK-Zellen hat, bedarf weiterer Untersuchungen.Schlussfolgerungen: Die vorliegenden Daten zeigen, dass NK-Zellen nicht Leishmania-infizierter Menschen in vitro in sehr kurzer Zeit durch unterschiedliche Leishmania spp. aktiviert werden können und dass hierfür spezifische Signale weiterer Zelltypen, wie z. B. über Monozyten präsentiertes IL-18, notwendig sind. Für die Auslösung NK-Zell-spezifischer Ef-fektorantworten sind jedoch weitere Stimuli wie IL-12 notwendig. Obwohl die Quelle für IL-12 in einer in vivo-Situation noch unklar ist, ist es wahrscheinlich, dass NK-Zellen auch in dieser Situation an der frühen Abwehr einer Leishmania-Infektion beteiligt sind.Background and Hypothesis: Protozoan Leishmania parasites, which are the causative agents of potentially lethal leishmaniasis, are transmitted by the bite of sandflies to a variety of mammals, including humans. Due to progressive spreading of the vector and considerable immigration from endemic countries over the last years, leishmaniasis has also gained en-hanced medical significance in Germany. After transmission, Leishmania parasites are pas-sively taken up via phagocytosis by different cell types, mainly immune cells, and replicate intracellularly. Thus, an effective immune response against Leishmania is associated with the host cell production of leishmanicidal reactive oxygen and nitrogen species (e. g. nitrogen monoxide [NO]). The NO production is mainly induced by the cytokine interferon (IFN)-gamma, which is produced by T-lymphocytes and NK cells. In a murine model of leishmaniasis, it was shown that NK cell-derived IFN-gamma plays a protective role in Leishmania infections. Similarly, in the human system there are some evidences that NK cells contribute to the immune response against Leishmania. However, the underlying mechanisms of NK cell activation in the human context are not entirely known and the data are somewhat controversial, so far. Therefore, this study intended to analyse whether human blood-derived NK cells can be activated by Leishmania in vitro (within a period of 20 hours), and to identify the inducers or stimuli of NK cell activation in this context.Results: The coculture of PBMCs with different Leishmania spp. led to an increased expres-sion of the activation marker CD69 on NK cells. This reaction was species- and dose-dependent but independent of the organotropism of the different parasite strains. Leishmania-infected monocytes were identified as triggers of NK cell activation, whilst neither T- nor B-lymphocytes exhibited any stimulatory effect on NK cells. However, CD69 upregulation revealed as independent on the monocyte subtype. CD69 expression was triggered by both, a soluble factor being produced by Leishmania-infected monocytes, and by direct contact of monocytes and NK cells. In the current study it was not possible to identify the soluble factor but to elucidate monocyte-derived, surface-bound IL-18 as trigger of contact-dependent CD69 induction. Overall, there was no evidence of direct contact-dependent NK cell activation by Leishmania stages themselves or of CD69-induced production of IFN-gamma or NK cell cytotoxicity. For the latter reactions an additional stimulation with the cytokine IL-12 is obviously necessary, which is effective only in the presence of IL-18. However, the addition of neutrophil granulocytes or immature monocyte-derived dendritic cells to PBMC-Leishmania coculture, had no effect on the NK cell effector function. Therefore, the cellular source of the IL-12 needed for activation in an in vivo situation is still unclear. Similar to the mouse model, promising candidates for this function are mature DCs.Besides indirect effects of Leishmania on NK cells, a direct interaction between these cells led to a contact-dependent reduction of surface CD56 expression on NK cells. The consequences of this change on NK cell function will be analysed in future projects.Conclusion: The current study shows that NK cells originating from non-infected humans are activated in vitro by different Leishmania spp. within a very short time span, and that these reactions depend on specific signals of other cell types, such as monocyte-derived surface-expressed IL-18. In addition, to trigger NK cell effector functions, IL-12 is needed as additional stimulus. Although the actual source of IL-12 in an in vivo situation remains unclear, it appears likely that NK cells contribute to the early immune defence in Leishmania-infections