Spatiotemporal dynamics of multiple shear-banding events for viscoelastic micellar fluids in cone-plate shearing flows

We characterize the transient response of semi-dilute wormlike micellar solutions under an imposed steady shear flow in a cone-plate geometry. By combining conventional rheometry with 2-D Particle Image Velocimetry (PIV), we can simultaneously correlate the temporal stress response with time-resolved velocimetric measurements. By imposing a well defined shear history protocol, consisting of a stepped shear flow sweep, we explore both the linear and nonlinear responses of two surfactant solutions: cetylpiridinium chloride (CPyCl) and sodium salicylate (NaSal) mixtures at concentrations of [66:40] mM and [100:60] mM, respectively. The transient stress signal of the more dilute solution relaxes to its equilibrium value very fast and the corresponding velocity profiles remain linear, even in the strongly shear-thinning regime. The more concentrated solution also exhibits linear velocity profiles at small shear rates. At large enough shear rates, typically larger than the inverse of the relaxation time of the fluid, the flow field reorganizes giving rise to strongly shear-banded velocity profiles. These are composed of an odd number of shear bands with low-shear-rate bands adjacent to both gap boundaries. In the non-linear regime long transients (much longer than the relaxation time of the fluid) are observed in the transient stress response before the fluid reaches a final, fully-developed state. The temporal evolution in the shear stress can be correlated with the spatiotemporal dynamics of the multiple shear-banded structure measured using RheoPIV. In particular our experiments show the onset of elastic instabilities in the flow which are characterized by the presence of multiple shear bands that evolve and rearrange in time resulting in a slow increase in the average torque acting on the rotating fixture

Microfluidic bifurcating networks for power-law fluids

Bifurcating networks are widely found in nature and are often responsible for controlling fluids that exhibit complex rheological behaviour. Examples are the vascular branching network that drives blood throughout the human body, the oxygen respiratory system in the human lungs and the bifurcating formations of xylem responsible for the distribution of water and other nutrients in plants and trees. Here, we take advantage of the biomimetic principles obtained by studying these natural systems to design fluid distribution networks for use in lab-on-a-chip devices. The novel biomimetic design rule we have recently proposed allows us to generate bifurcating microfluidic networks of rectangular cross-section for use with power-law and Newtonian fluids [1]. The design is based on Murray’s law, which was originally derived for blood flow in the vascular system, using the principle of minimum work. Murray [2] considered Newtonian fluid flows to predict the optimum ratio between the diameters of the parent and daughter vessels in networks with circular cross-section to obtain a uniform wall-shear stress along the network. In our study, we have extended the relationship to consider the flow of power-law fluids in planar geometries (i.e. geometries of rectangular cross-section with constant depth) typical of lab-on-a-chip applications. Furthermore, the design rule has been generalised to consider a range of shear-stress distributions via a branching parameter, offering the ability to precisely control the shear-stress distribution and predict the flow resistance along the bifurcating network

Improving KantanMT training efficiency with fast align

In recent years, statistical machine translation (SMT) has been widely deployed in translators’ workflow with significant improvement of productivity. However, prior to invoking an SMT system to translate an unknown text, an SMT engine needs to be built. As such, building speed of the engine is essential for the translation workflow, i.e., the sooner an engine is built, the sooner it will be exploited. With the increase of the computational capabilities of recent technology the building time for an SMT engine has decreased substantially. For example, cloud-based SMT providers, such as KantanMT, can built high-quality, ready-to-use, custom SMT engines in less than a couple of days. To speed-up furthermore this process we look into optimizing the word alignment process that takes place during building the SMT engine. Namely, we substitute the word alignment tool used by KantanMT pipeline – Giza++ – with a more efficient one, i.e., fast_align. In this work we present the design and the implementation of the KantanMT pipeline that uses fast_align in place of Giza++. We also conduct a comparison between the two word alignment tools with industry data and report on our findings. Up to our knowledge, such extensive empirical evaluation of the two tools has not been done before

Customised bifurcating networks for mapping polymer dynamics in shear flows

Understanding the effect of varying shear stresses on individual polymer dynamics is important for applications such as polymer flooding, polymer induced drag reduction, or the design of DNA separation devices. In all cases, the individual polymer response to varying shear flows needs to be understood. A biomimetic design rule was recently proposed for bifurcating networks of rectangular channels of constant depth. These customised microfluidic geometries represent an elegant option to investigate, in a single device, multiple well-controlled shear stresses. Here, we present the first experimental realisation of such customised microfluidic networks, consisting of a series of rectangular microchannels with varying cross-sections, and we demonstrate their potential for testing polymer dynamics. We used microfluidic geometries optimised for both Newtonian and power-law fluids of constant or increasing average wall shear stress. The experimental model systems were tested using particle tracking velocimetry to confirm the theoretically predicted flow fields for shear-thinning xanthan gum solutions and a Newtonian fluid. Then, λ-DNA molecules were used as an example of shear sensitive polymers to test the effect of distinct shear stress distributions on their extension. By observing the conformation of individual molecules in consecutive channels, we demonstrate the effect of the varying imposed stresses. The results obtained are in good agreement with previous studies of λ-DNA extension under shear flow, validating the bifurcating network design. The customised microfluidic networks can thus be used as platforms for the investigation of individual polymer dynamics, in a large range of well-controlled local and cumulative shear stresses, using a single experiment

Ulrich bundles on ruled surfaces

In this short note, we study the existence problem for Ulrich bundles on polarized ruled surfaces, focusing our attention on the smallest possible rank. We show that existence of Ulrich line bundles occurs if and only if the coefficient αof the minimal section in the numerical class of the polarization equals one. For other polarizations, we prove the existence of rank two Ulrich bundles

Mucus Microrheology Measured on Human Bronchial Epithelium Culture

We describe an original method to measure mucus microrheology on human bronchial epithelium culture using optical tweezers. We probed rheology on the whole thickness of mucus above the epithelium and showed that mucus gradually varies in rheological response, from an elastic behavior close to the epithelium to a viscous one far away. Microrheology was also performed on mucus collected on the culture, on ex vivo mucus collected by bronchoscopy, and on another epithelium model. Differences are discussed and are related to mucus heterogeneity, adhesiveness, and collection method

Rheo-PIV of a shear-banding wormlike micellar solution under large amplitude oscillatory shear

We explore the behavior of a wormlike micellar solution under both steady and large amplitude oscillatory shear (LAOS) in a cone–plate geometry through simultaneous bulk rheometry and localized velocimetric measurements. First, particle image velocimetry is used to show that the shear-banded profiles observed in steady shear are in qualitative agreement with previous results for flow in the cone–plate geometry. Then under LAOS, we observe the onset of shear-banded flow in the fluid as it is progressively deformed into the non-linear regime—this onset closely coincides with the appearance of higher harmonics in the periodic stress signal measured by the rheometer. These harmonics are quantified using the higher-order elastic and viscous Chebyshev coefficients e [subscript n] and v [subscript n] , which are shown to grow as the banding behavior becomes more pronounced. The high resolution of the velocimetric imaging system enables spatiotemporal variations in the structure of the banded flow to be observed in great detail. Specifically, we observe that at large strain amplitudes (γ [subscript 0] ≥ 1), the fluid exhibits a three-banded velocity profile with a high shear rate band located in-between two lower shear rate bands adjacent to each wall. This band persists over the full cycle of the oscillation, resulting in no phase lag being observed between the appearance of the band and the driving strain amplitude. In addition to the kinematic measurements of shear banding, the methods used to prevent wall slip and edge irregularities are discussed in detail, and these methods are shown to have a measurable effect on the stability boundaries of the shear-banded flow.Spain. Ministerio de Educación y Ciencia (MEC) (Project FIS2010-21924-C02-02

Rheology of complex fluids. Stochastic switches in the galactose signalling network

Màster en BiofísicaPart I: Many biological fluids are complex fluids. Complex fluids present a particular mesoscopic structure which provides them viscoelastic effects, among others. These fluids can show viscous or solid behavior depending on the time-scale in which they are operating. These properties can have a major influence on the biological processes in which the fluids are involved. Rheological techniques can be applied in order to characterize these fluids. In the present work experiments using a cone-plane geometry rheometer will be done on a complex fluid with worm-like chain micellar structure, CPyCl-NaSal [100:60], in order to present a complete rheological characterization. In addition, a particular example of biorheology applied to blood samples will be presented. Part II: Bistability is observed in many biological processes and in particular in processes involved in cellular differentiation. It can arise from positive feedback transcription networks and can be found in a colony of cells by the coexistence of two populations with different stable concentration of a specific protein. In M. Acar, A. Becskei, A. van Oudenaarden, ¿Enhancement of cellular memory by reducing stochastic transitions¿, Letters to nature 435, 228 (2005), Yeast Saccharomyces cerevisiae cells were found to switch from one stable state to another one of the Galactose-signalling network. This network is mainly governed by a positive feedback loop and the switching rate depended on the state from which cells came. In order to describe such phenomena a deterministic model based on positive feedback loops is not sufficient. The aim of the present work is to understand from a theoretical point of view how these transitions are originated and predict the rate at which they jump between different stable states. For this purpose numerical simulations have been done based on Langevin equations with multiplicative noise and later compared to theoretical predictions derived from Fokker-Plank equations. In order to become familiar with the required numerical and theoretical techniques the Ginzburg-Landau model has been previously studied. Our results show that fluctuations in the maximal transcription rate can be responsible for this phenomenon. For the galactose network, our results predict that fluctuations in the levels of proteins Gal4p and Gal80 are crucial

'Esto de enseñar los poemas es una impudicia'

Publicat a Navarra hoy

Oscillatory pipe flow of wormlike micellar solutions

[eng] Wormlike micelles are viscoelastic fluids that present an intermediate behavior between solids and ordinary liquids since they are elastic at short time scales but flow easily at large time scales. In opposition to Newtonian fluids, which have constant viscosity, these fluids usually exhibit a non-Newtonian response with a rate-dependent shear viscosity. Wall-bounded oscillatory flows of Newtonian and complex fluids are found in many practical situations. Oscillatory pipe flows are especially important in physiology in connection with the circulatory and respiratory systems of human beings, as well as in industrial processes such as fluid pumping, secondary oil recovery or filtration, and in acoustics. Pulsating flows are of particular interest also in the rheological characterization of complex fluids. We analyze the laminar oscillatory flow of viscoelastic fluids using the Maxwell and Oldroyd-B models. We have shown that in wall-bounded oscillatory flows of viscoelastic fluids the two characteristic lengths of the Ferry waves, the damping length and wavelength, together with the characteristic separation of the walls, define all the flow properties for fluid models with a linear shear-stress equation in unidirectional flow. In wall-bounded settings there exists the possibility that shear waves generated at different locations superpose themselves before decaying so that the shear waves interfere, giving rise to a resonant flow at well defined frequencies of driving. The theoretical predictions obtained for the laminar velocity profiles are validated by carrying out time-resolved Particle Image Velocimetry (PIV) experiments in a vertical pipe at small driving amplitudes. The oscillatory pipe flow has been investigated in the whole range of experimentally accessible driving frequencies and amplitudes, and classified in three main flow regimes: laminar, vortical, and non-axisymmetric vortical. By ramping up and down the driving amplitude at constant frequency we have been able to characterize the transition from laminar to more complex flows, under controlled driving conditions. The first hydrodynamic instability occurs when the laminar base flow becomes unstable against the formation of axisymmetric toroidal vortices that appear distributed along the cylinder. The calculation of root-mean-square fluctuations in the vertical direction, of the vertical and radial components of the velocity (averaged in time or over the tube diameter) has allowed to determine the critical amplitude at which the instability sets in with high accuracy. In the vortical flow an abrupt increase of the fluctuations is observed, that accounts for the loss of the vertical translational symmetry and the formation of vortices in the flow. This transition exhibits hysteresis when the driving amplitude is ramped up and down, which makes us presume that the bifurcation from the laminar flow has a subcritical nature. A second hydrodynamic instability occurs when the vortical flow loses the axial symmetry. In this flow regime the vortices are heavily distorted and no longer axisymmetric. The velocity and vorticity maps of the vortical flow measured in a meridional plane of the tube appear periodic in time, on time scales comparable to the driving period. Interestingly, the vortex formation is favored in the acceleration phases of the piston oscillation. Besides, we have uncovered a spatio-temporal dynamics on long time scales (much larger than the relaxation time of the fluid) that substantially modifies the flow organization. This slow dynamics is more effective in the bottom half of the cylinder, specially next to the driving piston. A global inspection of the vortical flow along the tube length reveals that the instability takes place earlier in the bottom part of the tube, in the vicinity of the driving piston. At increasing the driving amplitude the boundary between laminar and vortical flow progressively raises towards the top regions. And above a critical driving amplitude the entire fluid flow is vortical. The mechanism triggering the hydrodynamic instability from the laminar to the axisymmetric vortical flow is not yet clear.[cat] L'objectiu d'aquesta Tesi és estudiar el flux oscil•latori vertical en fluids micel•lars. Els fluids micel•lars són fluids complexos amb propietats viscoelàstiques, de manera que mostren un comportament intermedi entre els sòlids i els líquids: són elàstics a escales de temps curtes però flueixen a escales de temps més llargues. En contraposició als fluids Newtonians, que tenen una viscositat constant, els fluids complexos mostren un comportament no-Newtonià, amb una viscositat que depèn del ritme de deformació. El fluxos oscil•latoris de fluids Newtonians o complexos en geometries confinades són especialment importants en fisiologia, en relació amb el sistema circulatori i respiratori d'éssers humans, i també en processos industrials com el bombejat de fluids, l'extracció de petroli, i en particular són interessants en la caracterització reològica de fluids complexos. Primer estudiem el flux oscil•latori des d'una perspectiva teòrica i analitzem el flux laminar de fluids viscoelàstics utilitzant els models de Maxwell i Oldroyd-B en un tub vertical. Hem mostrat que en fluxos confinats existeix la possibilitat que les ones de cisalla generades a les diferents parets se sobreposin abans d'esmorteir-se i que eventualment donin lloc a un fenomen de ressonància. Les prediccions teòriques obtingudes pel flux laminar són validades duent a terme experiments de Velocimetria d'Imatges de Partícules (PIV) en un tub vertical, per amplituds petites del forçament oscil•latori. Quan s'incrementa l'amplitud de l'oscil•lació el flux laminar evoluciona cap a fluxos que presenten una dependència espai-temporal més complexa. Fent rampes d'amplitud creixent a una freqüència fixada hem pogut caracteritzar experimentalment la transició del flux laminar a aquests fluxos més complexos, sota condicions de forçament ben controlades. La primera inestabilitat apareix quan el flux laminar esdevé inestable amb la corresponent formació d'anells de vorticitat apilats al llarg del tub. Es manifesta una segona inestabilitat per amplituds del forçament més grans, per la qual el flux vortical perd la simetria axial. En aquest nou règim els vòrtex estan fortament distorsionats i no són axisimètrics. Fent rampes d’amplitud creixent i decreixent hem observat que aquestes dues transicions presenten histèresi, i que per tant són de caràcter subcrític