Stationary shapes of deformable particles moving at low Reynolds numbers

Lecture Notes of the Summer School ``Microswimmers -- From Single Particle Motion to Collective Behaviour'', organised by the DFG Priority Programme SPP 1726 (Forschungszentrum J{\"{u}}lich, 2015).Comment: Pages C7.1-16 of G. Gompper et al. (ed.), Microswimmers - From Single Particle Motion to Collective Behaviour, Lecture Notes of the DFG SPP 1726 Summer School 2015, Forschungszentrum J\"ulich GmbH, Schriften des Forschungszentrums J\"ulich, Reihe Key Technologies, Vol 110, ISBN 978-3-95806-083-

Transcriptional and Linkage Analyses Identify Loci that Mediate the Differential Macrophage Response to Inflammatory Stimuli and Infection

Macrophages display flexible activation states that range between pro-inflammatory (classical activation) and anti-inflammatory (alternative activation). These macrophage polarization states contribute to a variety of organismal phenotypes such as tissue remodeling and susceptibility to infectious and inflammatory diseases. Several macrophage- or immune-related genes have been shown to modulate infectious and inflammatory disease pathogenesis. However, the potential role that differences in macrophage activation phenotypes play in modulating differences in susceptibility to infectious and inflammatory disease is just emerging. We integrated transcriptional profiling and linkage analyses to determine the genetic basis for the differential murine macrophage response to inflammatory stimuli and to infection with the obligate intracellular parasite Toxoplasma gondii. We show that specific transcriptional programs, defined by distinct genomic loci, modulate macrophage activation phenotypes. In addition, we show that the difference between AJ and C57BL/6J macrophages in controlling Toxoplasma growth after stimulation with interferon gamma and tumor necrosis factor alpha mapped to chromosome 3, proximal to the Guanylate binding protein (Gbp) locus that is known to modulate the murine macrophage response to Toxoplasma. Using an shRNA-knockdown strategy, we show that the transcript levels of an RNA helicase, Ddx1, regulates strain differences in the amount of nitric oxide produced by macrophage after stimulation with interferon gamma and tumor necrosis factor. Our results provide a template for discovering candidate genes that modulate macrophage-mediated complex traits

Influence de la flexion sur la dynamique d’une capsule dans un écoulement élongationnel

International audienceEffects of bending resistance on the dynamics of an elastic capsule under axisym- metric elongationnal flow are studied numerically. The method is based on the coupling of finite elements for interfacial forces with boundary element method for hydrodynam- ics. The bending resistance stabilizes the wrinkle appearing for weak capillary numbers due to negative tensions.Les effets de la résistance à la flexion sur la dynamique d’une capsule élastique soumise à un écoulement élongationnel axisymétrique sont étudiés numériquement. La méthode est basée sur le couplage des éléments finis pour le calcul des forces interfa- ciales avec la méthode intégrales de frontières pour l’hydrodynamique. La résistance à la flexion stabilise les formes plissées qui apparaissent à faible nombre capillaire du fait d’une zone de tension négatives

Pearling instability of a cylindrical vesicle

International audienceA cylindrical vesicle under tension can undergo a pearling instability, characterized by the growth of a sinusoidal perturbation which evolves towards a collection of quasi-spherical bulbs connected by thin tethers, like pearls on a necklace. This is reminiscent of the well-known Rayleigh-Plateau instability, where surface tension drives the amplification of sinusoidal perturbations of a cylinder of fluid. We calculate the growth rate of perturbations for a cylindrical vesicle under tension, considering the effect of both inner and outer fluids, with different viscosities. We show that this situation differs strongly from the classical Rayleigh-Plateau case in the sense that, first, the tension must be above a critical value for the instability to develop and, second, even in the strong tension limit, the surface preservation constraint imposed by the presence of the membrane leads to a different asymptotic behaviour. The results differ from previous studies on pearling due to the consideration of variations of tension, which are shown to enhance the pearling instability growth rate, and lower the wavenumber of the fastest growing mode

Modeling of microorganism swimming in Stokes flow

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Shape transition and migration of 3D vesicles in a confined Poiseuille flow

International audienceVesicles are essential models to understand the behavior of closed soft particle under flow as red blood cells. Their incompressible membranes are made of a fluid lipid bilayer with a resistance to bending. Vesicles are characterized by their deflation that permits to the vesicle shape to exhibit an amazing variety of shape (parachute, bullet, peanut, croissant, and slipper) and different types of dynamical behavior (tank- treading, tumbling, and trembling) in a simple flow. Much has been made of vesicle dynamics in an unbounded Poiseuille flow or 2D confined geometry. Here, we numerically investigate the shape transition and migration of a 3D vesicle in a confined Poiseuille flow by means of boundary element method, in which a wall boundary is additionally implemented. The vesicle motion is determined by three dimensionless parameters: the reduced volume ν, the bending capillary number Ca and the confinement β, namely the ratio of the characteristic size of the vesicle to the radius of the capillary. The intricate interplay among the wall, flow curvature and membrane bending leads to an extension of the set of vesicle morphologies. Particular attention here is paid to determining transition conditions under which a vesicle changes its characteristic shape. Fig. 1a shows a transition example for a slipper-like shape vesicle (featured by an offset of the center of gravity, i.e., Yg≠0) that becomes a bullet-like shape (Yg =0) when the confinement is increased beyond a critical value. The confinement also has a significant effect on the membrane flow structure, as illustrated in Fig. 1b, where one vertex or two ones can take place on the vesicle, depending mainly on the value of β

Axisymmetric Boundary Element Method for vesicles in a capillary

International audienceThe problem of a vesicle transported by a fluid flow can present a large range of length scales. One example is the case of a vesicle producing a tether, and eventually pearls, in an elongational flow. Another case occurs when a lubrication film is formed, such as during the short range interaction between two vesicles. Such problems are still challenging for 3D simulations. On the other hand, a good understanding could be obtained by first considering the axisymmetric regime when such a regime exists. An axisymmetric model could then be used, without the criticisms that can be made of a 2D approach. We propose such a model, primarily interested in flows through narrow cylindrical capillaries. Two options are compared, with and without explicit representation of the capillary boundaries by a mesh. The numerical effort is characterized as a function of the vesicle’s initial shape, the flow magnitude and the confinement. The model is able to treat typical configurations of red blood cells flowing through very narrow pores with extremely thin lubrication films

Influence of surface viscosity on droplets in shear flow

International audienceThe behaviour of a single droplet in an immiscible external fluid, submitted to shear flow is investigated using numerical simulations. The surface of the droplet is modelled by a Boussinesq–Scriven constitutive law involving the interfacial viscosities and a constant surface tension. A numerical method using Loop subdivision surfaces to represent droplet interface is introduced. This method couples boundary element method for fluid flows and finite element method to take into account the stresses due to the surface dilational and shear viscosities and surface tension. Validation of the numerical scheme with respect to previous analytic and computational work is provided, with particular attention to the viscosity contrast and the shear and dilational viscosities characterized both by a Boussinesq number Bq. Then, influence of equal surface viscosities on steady-state characteristics of a droplet in shear flow are studied, considering both small and large deformations and for a large range of bulk viscosity contrast. We find that small deformation analysis is surprisingly predictive at moderate and high surface viscosities. Equal surface viscosities decrease the Taylor deformation parameter and tank-treading angle and also strongly modify the dynamics of the droplet: when the Boussinesq number (surface viscosity) is large relative to the capillary number (surface tension), the droplet displays damped oscillations prior to steady-state tank-treading, reminiscent from the behaviour at large viscosity contrast. In the limit of infinite capillary number Ca, such oscillations are permanent. The influence of surface viscosities on breakup is also investigated, and results show that the critical capillary number is increased. A diagram (Bq;Ca) of breakup is established with the same inner and outer bulk viscosities. Additionally, the separate roles of shear and dilational surface viscosity are also elucidated, extending results from small deformation analysis. Indeed, shear (dilational) surface viscosity increases (decreases) the stability of drops to breakup under shear flow. The steady-state deformation (Taylor parameter) varies nonlinearly with each Boussinesq number or a linear combination of both Boussinesq numbers. Finally, the study shows that for certain combinations of shear and dilational viscosities, drop deformation for a given capillary number is the same as in the case of a clean surface while the inclination angle varies

Shape transition of a vesicle in a narrow cappillary

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