Cavitation-induced force transition in confined viscous liquids under traction

We perform traction experiments on simple liquids highly confined between parallel plates. At small separation rates, we observe a simple response corresponding to a convergent Poiseuille flow. Dramatic changes in the force response occur at high separation rates, with the appearance of a force plateau followed by an abrupt drop. By direct observation in the course of the experiment, we show that cavitation accounts for these features which are reminiscent of the utmost complex behavior of adhesive films under traction. Surprisingly enough, this is observed here in purely viscous fluids.Comment: Submitted to Physical Review Letters on May 31, 2002. Related informations on http://www.crpp.u-bordeaux.fr/tack.htm

Criterion for purely elastic Taylor-Couette instability in the flows of shear-banding fluids

In the past twenty years, shear-banding flows have been probed by various techniques, such as rheometry, velocimetry and flow birefringence. In micellar solutions, many of the data collected exhibit unexplained spatio-temporal fluctuations. Recently, it has been suggested that those fluctuations originate from a purely elastic instability of the flow. In cylindrical Couette geometry, the instability is reminiscent of the Taylor-like instability observed in viscoelastic polymer solutions. In this letter, we describe how the criterion for purely elastic Taylor-Couette instability should be adapted to shear-banding flows. We derive three categories of shear-banding flows with curved streamlines, depending on their stability.Comment: 6 pages, 3 figure

Potential "ways of thinking" about the shear-banding phenomenon

Shear-banding is a curious but ubiquitous phenomenon occurring in soft matter. The phenomenological similarities between the shear-banding transition and phase transitions has pushed some researchers to adopt a 'thermodynamical' approach, in opposition to the more classical 'mechanical' approach to fluid flows. In this heuristic review, we describe why the apparent dichotomy between those approaches has slowly faded away over the years. To support our discussion, we give an overview of different interpretations of a single equation, the diffusive Johnson-Segalman (dJS) equation, in the context of shear-banding. We restrict ourselves to dJS, but we show that the equation can be written in various equivalent forms usually associated with opposite approaches. We first review briefly the origin of the dJS model and its initial rheological interpretation in the context of shear-banding. Then we describe the analogy between dJS and reaction-diffusion equations. In the case of anisotropic diffusion, we show how the dJS governing equations for steady shear flow are analogous to the equations of the dynamics of a particle in a quartic potential. Going beyond the existing literature, we then draw on the Lagrangian formalism to describe how the boundary conditions can have a key impact on the banding state. Finally, we reinterpret the dJS equation again and we show that a rigorous effective free energy can be constructed, in the spirit of early thermodynamic interpretations or in terms of more recent approaches exploiting the language of irreversible thermodynamics.Comment: 14 pages, 6 figures, tutorial revie

On-sky observations with an achromatic hybrid phase knife coronagraph in the visible

CONTEXT: The four-quadrant phase mask stellar coronagraph, introduced by D. Rouan et al., is capable of achieving very high dynamical range imaging and was studied in the context of the direct detection of extra-solar planets. Achromatic four-quadrant phase mask is currently being developed for broadband IR applications. AIMS: We report on laboratory and on-sky tests of a prototype coronagraph in the visible. This prototype, the achromatic hybrid phase knife coronagraph, was derived from the four-quadrant phase mask principle. METHODS: The instrumental setup implementing the coronagraph itself was designed to record the pre- and post-coronagraphic images simultaneously so that an efficient real-time image selection procedure can be performed. We describe the coronagraph and the associated tools that enable robust and repeatable observations. We present an algorithm of image selection that has been tested against the real on-sky data of the binary star HD80081 (* 38 Lyn). RESULTS Although the observing conditions were poor, the efficiency of the proposed method is proven. From this experiment, we derive procedures that can apply to future focal instruments associating adaptive optics and coronagraphy, targeting high dynamic range imaging in astronomy, such as detecting extra-solar planets

Vlasov moment flows and geodesics on the Jacobi group

By using the moment algebra of the Vlasov kinetic equation, we characterize the integrable Bloch-Iserles system on symmetric matrices (arXiv:math-ph/0512093) as a geodesic flow on the Jacobi group. We analyze the corresponding Lie-Poisson structure by presenting a momentum map, which both untangles the bracket structure and produces particle-type solutions that are inherited from the Vlasov-like interpretation. Moreover, we show how the Vlasov moments associated to Bloch-Iserles dynamics correspond to particular subgroup inclusions into a group central extension (first discovered in arXiv:math/0410100), which in turn underlies Vlasov kinetic theory. In the most general case of Bloch-Iserles dynamics, a generalization of the Jacobi group also emerges naturally.Comment: 45 page

Towards the Equation of State of Classical SU(2) Lattice Gauge Theory

We determine numerically the full complex Lyapunov spectrum of SU(2) Yang-Mills fields on a 3-dimensional lattice from the classical chaotic dynamics. The equation of state, S(E), is determined from the Kolmogorov-Sinai entropy extrapolated to the large size limit.Comment: 12 pages, 8 PS figures, LaTe

Coherent states for the hydrogen atom

We construct wave packets for the hydrogen atom labelled by the classical action-angle variables with the following properties. i) The time evolution is exactly given by classical evolution of the angle variables. (The angle variable corresponding to the position on the orbit is now non-compact and we do not get exactly the same state after one period. However the gross features do not change. In particular the wave packet remains peaked around the labels.) ii) Resolution of identity using this overcomplete set involves exactly the classical phase space measure. iii) Semi-classical limit is related to Bohr-Sommerfield quantization. iv) They are almost minimum uncertainty wave packets in position and momentum.Comment: 9 pages, 2 figures, minor change in language and journal reference adde

‘What do we do, doctor?’ Transitions of identity and responsibility: a narrative analysis

Transitioning from student to doctor is notoriously challenging. Newly qualified doctors feel required to make decisions before owning their new identity. It is essential to understand how responsibility relates to identity formation to improve transitions for doctors and patients. This multiphase ethnographic study explores realities of transition through anticipatory, lived and reflective stages. We utilised Labov’s narrative framework (Labov in J Narrat Life Hist 7(1–4):395–415, 1997) to conduct in-depth analysis of complex relationships between changes in responsibility and development of professional identity. Our objective was to understand how these concepts interact. Newly qualified doctors acclimatise to their role requirements through participatory experience, perceived as a series of challenges, told as stories of adventure or quest. Rules of interaction within clinical teams were complex, context dependent and rarely explicit. Students, newly qualified and supervising doctors felt tensions around whether responsibility should be grasped or conferred. Perceived clinical necessity was a common determinant of responsibility rather than planned learning. Identity formation was chronologically mismatched to accepting responsibility. We provide a rich illumination of the complex relationship between responsibility and identity pre, during, and post-transition to qualified doctor: the two are inherently intertwined, each generating the other through successful actions in practice. This suggests successful transition requires a supported period of identity reconciliation during which responsibility may feel burdensome. During this, there is a fine line between too much and too little responsibility: seemingly innocuous assumptions can have a significant impact. More effort is needed to facilitate behaviours that delegate authority to the transitioning learner whilst maintaining true oversight

Clinical assessors' working conceptualisations of undergraduate consultation skills: a framework analysis of how assessors make expert judgements in practice.

Undergraduate clinical assessors make expert, multifaceted judgements of consultation skills in concert with medical school OSCE grading rubrics. Assessors are not cognitive machines: their judgements are made in the light of prior experience and social interactions with students. It is important to understand assessors' working conceptualisations of consultation skills and whether they could be used to develop assessment tools for undergraduate assessment. To identify any working conceptualisations that assessors use while assessing undergraduate medical students' consultation skills and develop assessment tools based on assessors' working conceptualisations and natural language for undergraduate consultation skills. In semi-structured interviews, 12 experienced assessors from a UK medical school populated a blank assessment scale with personally meaningful descriptors while describing how they made judgements of students' consultation skills (at exit standard). A two-step iterative thematic framework analysis was performed drawing on constructionism and interactionism. Five domains were found within working conceptualisations of consultation skills: Application of knowledge; Manner with patients; Getting it done; Safety; and Overall impression. Three mechanisms of judgement about student behaviour were identified: observations, inferences and feelings. Assessment tools drawing on participants' conceptualisations and natural language were generated, including 'grade descriptors' for common conceptualisations in each domain by mechanism of judgement and matched to grading rubrics of Fail, Borderline, Pass, Very good. Utilising working conceptualisations to develop assessment tools is feasible and potentially useful. Work is needed to test impact on assessment quality

Resorption of Natural Calcium Carbonate by Avian Osteoclasts In Vitro

Osteoclasts isolated from the endosteum of 2.5 to 3-week chick tibia were cultured on glass coverslips or natural CaC03 (Tridacna) wafers for 2 and 4 days. The cells were exposed to the pH-dependent dye, acridine orange, and fluorescence was measured by a light microscope photometer. Fluorescence intensity values were higher in cells adherent to Tridacna wafers than in those incubated on glass after 2 and 4 days of culture (three and two-fold, respectively). Moreover, osteoclasts on Tridacna wafers were more flattened and were found to produce resorption pits. Acid production by osteoclasts cultured on Tridacna wafers was stimulated with 10-8 M parathyroid hormone and inhibited with 10-7 M acetazolamide or 10-7 M hydroxybenezoyl thiophene sulfonamide, as shown by changes in intensity of acridine orange fluorescence after 30, 60 and 120 minutes of treatment. These results indicate that osteoclasts cultured on natural CaC03 wafers mimic the behavior of osteoclasts cultured on other substrates. Further, the capacity to acidify was enhanced in cells cultured on CaC03 wafers. These results indicate that natural CaC03 Tridacna wafers provide a suitable substrate for osteoclasts in culture and demonstrate that carbonic anhydrase plays a role in carbonated substrate resorption