Hydrodynamic fluctuations and instabilities in ordered suspensions of self-propelled particles

We construct the hydrodynamic equations for {\em suspensions} of self-propelled particles (SPPs) with spontaneous orientational order, and make a number of striking, testable predictions:(i) SPP suspensions with the symmetry of a true {\em nematic} are {\em always} absolutely unstable at long wavelengths.(ii) SPP suspensions with {\em polar}, i.e., head-tail {\em asymmetric}, order support novel propagating modes at long wavelengths, coupling orientation, flow, and concentration. (iii) In a wavenumber regime accessible only in low Reynolds number systems such as bacteria, polar-ordered suspensions are invariably convectively unstable.(iv) The variance in the number N of particles, divided by the mean , diverges as $^{2/3}$ in polar-ordered SPP suspensions.Comment: submitted to Phys Rev Let

Clocking the Lyme Spirochete

In order to clear the body of infecting spirochetes, phagocytic cells must be able to get hold of them. In real-time phase-contrast videomicroscopy we were able to measure the speed of Borrelia burgdorferi (Bb), the Lyme spirochete, moving back and forth across a platelet to which it was tethered. Its mean crossing speed was 1,636 µm/min (N = 28), maximum, 2800 µm/min (N = 3). This is the fastest speed recorded for a spirochete, and upward of two orders of magnitude above the speed of a human neutrophil, the fastest cell in the body. This alacrity and its interpretation, in an organism with bidirectional motor capacity, may well contribute to difficulties in spirochete clearance by the host

Effect of a rigid nonpolar solute on the splay, bend elastic constants and on rotational viscosity coefficient of 4-4^\prime -n-octyl-cyanobiphenyl

The effect of a rigid nonpolar non-mesogenic solute, ``biphenyl'' which is (C_{6}H_{5}-C_{6}H_{5}), on the splay and bend elastic constants and on the rotational viscosity coefficient of (4,4^{\prime})-n-octyl-cyano biphenyl (8CB) is reported. The experiments involve the measurement of voltage dependence of capacitance of a cell filled with the mixture. Anomalous behavior of both (K_{11}) and (\Delta \epsilon) near the (N-S_{A}) transition have been observed.Comment: RevTeX - 8 figures. Accepted to be published in Physical Review

Family-Based Model Checking with mCRL2

\u3cp\u3eFamily-based model checking targets the simultaneous verfication of multiple system variants, a technique to handle feature-based variability that is intrinsic to software product lines (SPLs). We present an approach for family-based verification based on the feature μ-calculus μL\u3csub\u3ef\u3c/sub\u3e, which combines modalities with feature expressions. This logic is interpreted over featured transition systems, a well-accepted model of SPLs, which allows one to reason over the collective behavior of a number of variants (a family of products). Via an embedding into the modal μ-calculus with data, underpinned by the general-purpose mCRL2 toolset, off-the-shelf tool support for μLf becomes readily available. We illustrate the feasibility of our approach on an SPL benchmark model and show the runtime improvement that family-based model checking with mCRL2 offers with respect to model checking the benchmark product-by-product.\u3c/p\u3

The Intersection of Interfacial Forces and Electrochemical Reactions

We review recent developments in experimental techniques that simultaneously combine measurements of the interaction forces or energies between two extended surfaces immersed in electrolyte solutions—primarily aqueous—with simultaneous monitoring of their (electro)chemical reactions and controlling the electrochemical surface potential of at least one of the surfaces. Combination of these complementary techniques allows for simultaneous real time monitoring of angstrom level changes in surface thickness and roughness, surface–surface interaction energies, and charge and mass transferred via electrochemical reactions, dissolution, and adsorption, and/or charging of electric double layers. These techniques employ the surface forces apparatus (SFA) combined with various “electrochemical attachments” for in situ measurements of various physical and (electro)chemical properties (e.g., cyclic voltammetry), optical imaging, and electric potentials and currents generated naturally during an interaction, as well as when electric fields (potential differences) are applied between the surfaces and/or solution—in some cases allowing for the chemical reaction equation to be unambiguously determined. We discuss how the physical interactions between two different surfaces when brought close to each other (<10 nm) can affect their chemistry, and suggest further extensions of these techniques to biological systems and simultaneous in situ spectroscopic measurements for chemical analysis

Quantitative Modal Transition Systems

International audienceThis extended abstract offers a brief survey presentation of the specification formalism of modal transition systems and its recent extensions to the quantitative setting of timed as well as stochastic systems. Some applications will also be briefly mentioned

Cell shape normalization of normal and haploinsufficient NF1-melanocytes by micro-structured substrate interaction

Cell shape is influenced by physical surface topography of the substrate to which cells are attached [23,8]. Most prominent is the polarization and elongation of cells along the direction of microfabricated grooves. Microstructured substrates are also known to affect intracellular processes such as apoptosis [8], gene expression [9] or protein production [6]. However, little is known about the mechanism of how surface topography exerts its effects, although several studies have focused on the role of cytoskeletal elements such as actin microfilaments [29], focal contacts [28],and microtubules [24] as all these structures are observed to align with topographic features such as grooves. Most of these experiments were performed with cultured fibroblasts where filament structures showed to align along microstructured lines. We examined cell shape alternations of human melanocyte cells cultured on micro-grooved polymeric substrates. These cells were derived from healthy persons and patients suffering on Neurofibromatosis 1 (NF1). NF1 is caused by a germline mutation in the NFI-gene a classical tumor suppressor gene. The majority of the NF1 mutations are null mutations resulting in reduction of the related protein product, neurofibromin, to about 50% [16,11]. Neurofibromin shows a Ras-GAP activity [14,21] and is involved in regulation of growthin vitro [1,20]. In addition, it is found to be associated with the cytoskeleton (Xu and Gutmann 1997) and NF1 deficient cells exhibit morphological changes, as demonstrated for Nfl-/- Schwann cells [20] or cells from Drosophila homozygous for null mutations of an NF1 homolog [27]. Neurofibromin reduction can also result in morphological changes in cultured human NF1 keratinocytes in which it is co-localized with intermediate filaments [22]. We demonstrated that the cellular consequence of haploinsufficienc