46 research outputs found

    Hydrodynamics of Rod-Like Colloids and Vesicles

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    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

    Dynamical regimes and hydrodynamic lift of viscous vesicles under shear

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    The dynamics of two-dimensional viscous vesicles in shear flow, with different fluid viscosities ηin\eta_{\rm in} and ηout\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 ηin/ηout\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 ycmy_{\rm {cm}} of the vesicle center of mass from the wall is well described by an effective power law ycm2y_{\rm {cm}}^{-2} for intermediate distances 0.8Rpycm3Rp0.8 R_{\rm p} \lesssim y_{\rm {cm}} \lesssim 3 R_{\rm p} with vesicle radius RpR_{\rm p}. The boundary-integral calculation indicates that the lift force decays asymptotically as 1/[ycmln(ycm)]1/[y_{\rm {cm}}\ln(y_{\rm {cm}})] far from the wall.Comment: 13 pages, 13 figure

    A Necklace Model for Vesicles Simulations in 2D

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    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]

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

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    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

    Dynamics of Fluid Vesicles in Oscillatory Shear Flow

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    The dynamics of fluid vesicles in oscillatory shear flow was studied using differential equations of two variables: the Taylor deformation parameter and inclination angle θ\theta. In a steady shear flow with a low viscosity ηin\eta_{\rm {in}} of internal fluid, the vesicles exhibit steady tank-treading motion with a constant inclination angle θ0\theta_0. In the oscillatory flow with a low shear frequency, θ\theta oscillates between ±θ0\pm \theta_0 or around θ0\theta_0 for zero or finite mean shear rate γ˙m\dot\gamma_{\rm m}, respectively. As shear frequency fγf_{\gamma} increases, the vesicle oscillation becomes delayed with respect to the shear oscillation, and the oscillation amplitude decreases. At high fγf_{\gamma} with γ˙m=0\dot\gamma_{\rm m}=0, another limit-cycle oscillation between θ0π\theta_0-\pi and θ0-\theta_0 is found to appear. In the steady flow, θ\theta periodically rotates (tumbling) at high ηin\eta_{\rm {in}}, and θ\theta and the vesicle shape oscillate (swinging) at middle ηin\eta_{\rm {in}} and high shear rate. In the oscillatory flow, the coexistence of two or more limit-cycle oscillations can occur for low fγf_{\gamma} in these phases. For the vesicle with a fixed shape, the angle θ\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

    Prävalenz sexueller Gewalterfahrungen in der Kindheit in einem gynäkologischen Kollektiv

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    Within recent years, there is an increasing amount of studies confirming that childhood sexual abuse (CSA) experiences are a risk factor for numerous somatic and psychosocial long-termconsequences. For obstetrics, an increased probability of obstetric risk factors was found like teenage pregnancy, drug abuse and insufficient prenatal care as well as a variety of complications during pregnancy and delivery – in particular pre-term delivery. With a prevalence of 20%, CSA is a clinically relevant factor in the general female population. However, there is a lack of data on the general prevalence of CSA in an obstetrical study group which could make it possible to estimate the real relevance of CSA in pre- and postnatal care. In the present study, prevalences of 11.5% to 14.6% were found depending on the underlying definition of CSA. Comparable prevalences were also found in the few existing studies on obstetric collectives. The Aachen results are below the prevalences of CSA in different study groups with known sequels such as chronic pelvic pain or psychiatric disorders (median 34% to 41%). In addition, prevalences of obstetric study groups are below those of the general female population. Prevalences of CSA vary for example with the underlying definition and the method of collecting data like questionnaire or interview. The reason for the relatively low prevalence in obstetric collectives may be explained by an increased number of women who decide against establishing their own family after an experience of CSA, because they may have no confidence in their abilities as a mother or may have difficulties to maintain longterm relationships. According to our results, one can assume that each 7th to 10th woman who presents for prenatal care has experienced CSA. Because CSA may have influences on pregnancy and delivery, victims of CSA require specific pre- and postnatal care like an increased incorporation into clinical decisions, prevention of triggering flash backs as well as sensitive treatment with respect and confidence. Therefore, the victim's requirements should be integrated into the existing gynecological attendance. Ideally, perinatal caregivers should ask for experiences of CSA in the first consultation beside other risk factors for pregnancy like high blood pressure or drug abuse. A gynecological attendance which is adapted to the particular needs of victims of CSA is a key contribution to the reduction of long-term sequels of CSA which has been insufficiently used in the past

    Prävalenz sexueller Gewalterfahrungen in der Kindheit in einem gynäkologischen Kollektiv

    No full text
    Within recent years, there is an increasing amount of studies confirming that childhood sexual abuse (CSA) experiences are a risk factor for numerous somatic and psychosocial long-termconsequences. For obstetrics, an increased probability of obstetric risk factors was found like teenage pregnancy, drug abuse and insufficient prenatal care as well as a variety of complications during pregnancy and delivery – in particular pre-term delivery. With a prevalence of 20%, CSA is a clinically relevant factor in the general female population. However, there is a lack of data on the general prevalence of CSA in an obstetrical study group which could make it possible to estimate the real relevance of CSA in pre- and postnatal care. In the present study, prevalences of 11.5% to 14.6% were found depending on the underlying definition of CSA. Comparable prevalences were also found in the few existing studies on obstetric collectives. The Aachen results are below the prevalences of CSA in different study groups with known sequels such as chronic pelvic pain or psychiatric disorders (median 34% to 41%). In addition, prevalences of obstetric study groups are below those of the general female population. Prevalences of CSA vary for example with the underlying definition and the method of collecting data like questionnaire or interview. The reason for the relatively low prevalence in obstetric collectives may be explained by an increased number of women who decide against establishing their own family after an experience of CSA, because they may have no confidence in their abilities as a mother or may have difficulties to maintain longterm relationships. According to our results, one can assume that each 7th to 10th woman who presents for prenatal care has experienced CSA. Because CSA may have influences on pregnancy and delivery, victims of CSA require specific pre- and postnatal care like an increased incorporation into clinical decisions, prevention of triggering flash backs as well as sensitive treatment with respect and confidence. Therefore, the victim's requirements should be integrated into the existing gynecological attendance. Ideally, perinatal caregivers should ask for experiences of CSA in the first consultation beside other risk factors for pregnancy like high blood pressure or drug abuse. A gynecological attendance which is adapted to the particular needs of victims of CSA is a key contribution to the reduction of long-term sequels of CSA which has been insufficiently used in the past
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