Absence Of Shear Flow Research Articles

Flow states of two dimensional active gels driven by external shear.

Using a minimal hydrodynamic model, we theoretically and computationally study the Couette flow of active gels in straight and annular two-dimensional channels subject to an externally imposed shear. The gels are isotropic in the absence of externally- or activity-driven shear, but have nematic order that increases with shear rate. Using the finite element method, we determine the possible flow states for a range of activities and shear rates. Linear stability analysis of an unconfined gel in a straight channel shows that an externally imposed shear flow can stabilize an extensile fluid that would be unstable to spontaneous flow in the absence of the shear flow, and destabilize a contractile fluid that would be stable against spontaneous flow in the absence of shear flow. These results are in rough agreement with the stability boundaries between the base shear flow state and the nonlinear flow states that we find numerically for a confined active gel. For extensile fluids, we find three kinds of nonlinear flow states in the range of parameters we study: unidirectional flows, oscillatory flows, and dancing flows. To highlight the activity-driven spontaneous component of the nonlinear flows, we characterize these states by the average volumetric flow rate and the wall stress. For contractile fluids, we only find the linear shear flow and a nonlinear unidirectional flow in the range of parameters that we studied. For large magnitudes of the activity, the unidirectional contractile flow develops a boundary layer. Our analysis of annular channels shows how curvature of the streamlines in the base flow affects the transitions among flow states.

Discontinuous shear thickening (DST) transition with spherical iron particles coated by adsorbed brush polymer

We explore the rheology of very concentrated (0.55 < Φ < 0.67) suspensions of carbonyl iron particles coated by a small polymer. A strong discontinuous shear thickening (DST) is observed in a large range of volume fraction presenting some specific behaviors in comparison with other systems. In particular, the DST transition can appear suddenly without being preceded by shear thickening. The presence of a frictional network of particles is confirmed by a simultaneous measurement of the electric resistance of the suspension and of the rheological curve. Using the Wyart–Cates (W–C) model, we show that with increasing the volume fraction, the fraction of frictional contacts increases more quickly with the stress, contrary to the prediction of numerical simulations. The same behavior is observed in the presence of a magnetic field with a strong increase in the viscosity before the DST transition. We interpret this behavior by the interpenetration of the polymer layer under the effect of the shear stress—and of the magnetic stress—followed by the expulsion of the polymer out of the surfaces between two particles in contact. We point out that above the DST transition, we do not observe a jamming in the range of volume fraction, whereas it is predicted by the W–C model. The frictional contacts are created by a shear stress and not by a static stress, so in the absence of shear flow, the polymer can adsorb again on the surface and lubricate the frictional contacts. We thus predict an asymptotic non-zero shear rate reproducing the experimental behavior.

A discontinuous autoinhibitory module masks the A1 domain of von Willebrand factor

Essentials The mechanism for the auto-inhibition of von Willebrand factor (VWF) remains unclear. Hydrogen exchange of two VWF A1 fragments with disparate activities was measured and compared. Discontinuous residues flanking A1 form a structural module that blocks A1 binding to the platelet. Our results suggest a potentially unified model of VWF activation. Click to hear an ISTH Academy presentation on the domain architecture of VWF and activation by elongational flow by Dr Springer SUMMARY: Background How von Willebrand factor (VWF) senses and responds to shear flow remains unclear. In the absence of shear flow, VWF or its fragments can be induced to bind spontaneously to platelet GPIbα. Objectives To elucidate the auto-inhibition mechanism of VWF. Methods Hydrogen-deuterium exchange (HDX) of two recombinant VWF fragments expressed from baby hamster kidney cells were measured and compared. Results The shortA1 protein contains VWF residues 1261-1472 and binds GPIbα with a significantly higher affinity than the longA1 protein that contains VWF residues 1238-1472. Both proteins contain the VWF A1 domain (residues 1272-1458). Many residues in longA1, particularly those in the N- and C-terminal sequences flanking the A1 domain, and in helix α1, loops α1β2 and β3α2, demonstrated markedly reduced HDX compared with their counterparts in shortA1. The HDX-protected region in longA1 overlaps with the GPIbα-binding interface and is clustered with type 2B von Willebrand disease (VWD) mutations. Additional comparison with the HDX of denatured longA1 and ristocetin-bound longA1 indicates the N- and C-terminal sequences flanking the A1 domain form cooperatively an integrated autoinhibitory module (AIM) that interacts with the HDX-protected region. Binding of ristocetin to the C-terminal part of the AIM desorbs the AIM from A1 and enables longA1 binding to GPIbα. Conclusion The discontinuous AIM binds the A1 domain and prevents it from binding to GPIbα, which has significant implications for the pathogenesis of type 2B VWD and the shear-induced activation of VWF activity.

Pulsed back-and-forward cross-flow batch membrane emulsification with high productivity to obtain highly uniform and concentrate emulsions

Membrane emulsification of soybean oil in an aqueous solution of 2wt% Tween 80 was performed using pulsed back-and-forward flow. A tubular Shirasu porous glass (SPG) membrane with mean pore diameter of 3.1μm was used. Soybean oil was pressed through the membrane and the force that determines droplets detachment was exerted by the continuous phase tangentially agitated by inverting the flow direction back-and-forward within the membrane lumen. Alternatively, droplets were spontaneously detached from the pore opening in the absence of shear flow at the membrane surface.The main advantage of the pulsed back-and-forward cross-flow membrane emulsification is that it eliminates the shear stress outside of the membrane related to the recirculation of emulsion droplets along the pump and fitting circuit as it happens in conventional cross-flow membrane emulsification. Results obtained indicated that it was possible obtaining uniform emulsion droplets also when high concentration of dispersed phase was reached with high productivity comparing with the traditional cross-flow membrane operation. The method is suitable for the production of monodispersed particles containing bioactive molecule (that are shear sensitive) or viscous dispersed materials or large droplets.

Three-Dimensional Modeling of Active Migration of Living Cells in a Microchannel

Leukocytes and other living cells can deform and migrate actively toward the source of chemoattractant. This directed movement of cells, known as chemotaxis, plays a crucial role in both innate and adaptive immune response of the body, and cancer metastasis. The cell's active deformation and migration are driven by intracellular forces generated by polymerization/depolymerization of cytoskeletal filaments. To numerically study these processes, we have implemented the Dembo's reactive interpenetrating flow formalism and polymerization force model into our three-dimensional volume-of-fluid (VOF)-based computational algorithm for viscoelastic cell deformation. The resulting computational model takes into account passive mechanical properties of the cell, extracellular diffusion of chemoattractant molecules, intracellular diffusion of signaling molecules, intracellular active force generation, and physiological shear flow conditions. We have applied this algorithm to study chemotaxis of nonadherent leukocytes in a rectangular microchannel in the presence or absence of shear flow. The numerical data have been tested against the corresponding experimental data.

Comment on invalid JWKB unstable solutions of combined inertial-Rossby waves

In three recent papers (McKenzie, 2009, 2011a, b) on the combined theory of gravity inertial-Rossby waves on a β-plane it was shown that “local” JWKB solutions exhibit complex roots for the wave frequency if the speed of the low-frequency, long-wavelength westward propagating Rossby wave exceeded that of the higher frequency (KelvinPoincare) inertial wave speed. If the ratio of the former (Rossby) speed to the latter (inertial) speed is denoted by m (a function of β, f the Coriolis parameter and V the inertial speed) the condition for complex roots m> 1 translates into a region in the parameter space of the latitude of the β-plane and the rotational Mach/Froude number of the planet. Therefore it is tempting to associate these complex roots in this parameter space with instability of the system arising from the coalescence of modes (Akhiezer et al., 1967), or from the interaction between positive and negative energy waves (Cairns, 1971). However a note of caution was sounded in the latest paper of the series (McKenzie, 2011b) to the effect that JWKB solutions whose latitudinal wave number remained unconstrained, may not, in fact, provide an acceptable solution to the “global” problem which involves the imposition of latitudinal boundary conditions. Indeed Paldor (2010) showed that if mid-latitude JWKB type solutions are subject to, for example, rigid wall conditions in one hemisphere, the corresponding eigenvalues precluded the fulfillment of the “instability condition” m> 1, and therefore that the system was globally stable. Furthermore Paldor emphasized that in the well known case of an equatorial β-plane, the eigenvalues and eigenfunctions correspond to stable (real) values for the wave frequency. Finally, a global energy invariant for the system (Pedlosky, 1987; Paldor, 2010) demonstrates that, in the absence of shear flow or negative buoyancy, the system is stable. In summary the latitudinal dependence in the equations arising from the variation of the Coriolis force, which is an essential feature of the Rossby wave, together with appropriate boundary conditions gives rise to an eigenvalue problem instead of a purely local solution with its associated dispersion equations. This eigenvalue problem (whether the βplane is centred on mid-latitudes or at the equator) has no unstable solutions. Therefore it is clear that the claim that the system is unstable, based solely upon local JWBK solutions, made by me is invalid.

Multi-lamellar vesicle formation in a long-chain nonionic surfactant: C16E4/D2O system

The temperature dependent rheological and structural behavior of a long-chain C16E4 (tetraethylene glycol monohexadecyl ether) surfactant in D2O has been studied within the regime of low shear range. In the absence of shear flow, the system forms a lamellar liquid crystalline phase at relatively high temperatures. The present paper reports on the shear-induced multi-lamellar vesicle (MLV) formation in C16E4/D2O at 40wt.% of surfactant in the temperature range of 40–55°C. The transition from planar lamellar structure to multi-lamellar vesicles has been investigated by time-resolved experiments combining rheology and nuclear magnetic resonance (rheo-NMR), rheo small-angle neutron scattering (rheo-SANS) and rheometry. The typical transient viscosity behavior of MLV formation has been discovered at low shear rate value of 0.5s−1.

Shirasu porous glass (SPG) membrane emulsification in the absence of shear flow at the membrane surface: Influence of surfactant type and concentration, viscosities of dispersed and continuous phases, and transmembrane pressure

There has been an increasing interest in membrane emulsification using Shirasu porous glass (SPG) membranes due to the better control of the droplet size, lower energy consumption, and simplicity of design. In this study, SPG membrane emulsification in the absence of shear flow at the membrane surface was investigated in order to optimize the emulsification process and to gain deep insight into the droplet formation behavior. At transmembrane pressures slightly higher than the capillary pressure, a faster decrease in the interfacial tension resulted in the spontaneous formation of smaller droplets with a narrower droplet size distribution. In this case, the dispersed phase distorted by the tortuous membrane pores was spontaneously transformed into spherical droplets by interfacial tension. The drop diameter increased with a decrease in the ratio of the dispersed-phase viscosity to the continuous-phase viscosity. The droplet formation behavior was changed from the spontaneous droplet formation to the continuous outflow of the dispersed phase at a critical dispersed-phase velocity, which was determined by the capillary number: below the critical capillary number, uniformly sized droplets are spontaneously generated; above the critical capillary number, dispersed phase flowed out continuously, resulting in polydispersed larger droplet formation. The critical capillary number was independent of the equilibrium interfacial tension and the dispersed-phase viscosity.

Shear flow over a liquid drop adhering to a solid surface

The hydrostatic shape, transient deformation, and asymptotic shape of a small liquid drop with uniform surface tension adhering to a planar wall subject to an overpassing simple shear flow are studied under conditions of Stokes flow. The effects of gravity are considered to be negligible, and the contact line is assumed to have a stationary circular or elliptical shape. In the absence of shear flow, the drop assumes a hydrostatic shape with constant mean curvature. Families of hydrostatic shapes, parameterized by the drop volume and aspect ratio of the contact line, are computed using an iterative finite-difference method. The results illustrate the effect of the shape of the contact line on the distribution of the contact angle around the base, and are discussed with reference to contact-angle hysteresis and stability of stationary shapes. The transient deformation of a drop whose viscosity is equal to that of the ambient fluid, subject to a suddenly applied simple shear flow, is computed for a range of capillary numbers using a boundary-integral method that incorporates global parameterization of the interface and interfacial regriding at large deformations. Critical capillary numbers above which the drop exhibits continued deformation, or the contact angle increases beyond or decreases below the limits tolerated by contact angle hysteresis are established. It is shown that the geometry of the contact line plays an important role in the transient and asymptotic behaviour at long times, quantified in terms of the critical capillary numbers for continued elongation. Drops with elliptical contact lines are likely to dislodge or break off before drops with circular contact lines. The numerical results validate the assumptions of lubrication theory for flat drops, even in cases where the height of the drop is equal to one fifth the radius of the contact line.

Kinetics of crystallization in a shearing colloidal suspension.

We report the kinetics of crystallization as observed in simulations of a charged colloidal suspension under shear. We find that the imposed shear flow inhibits crystal nucleation. Crystallization was only observed for shear rates at or below \ensuremath{\gamma}\ifmmode \dot{}\else \.{}\fi{}${\mathrm{\ensuremath{\tau}}}_{0}$=0.07 (where ${\mathrm{\ensuremath{\tau}}}_{0}$ is the time required for a particle to diffuse the averge interparticle distance in a dilute suspension). Under the influence of these low shear rates the suspension crystallizes into a single shear-aligned crystal, as opposed to the polycrystalline structure found in the absence of shear flow. Crystal growth is found to be proportional to the effective supercooling, as measured from the melting temperature of the shearing crystal. We conclude that, by analogy with the case of crystallization at zero shear, the effective supercooling provides an operational measure of the ``thermodynamic'' stability of the shearing crystal. (c) 1995 The American Physical Society

Brownian dynamics simulation of a multi-subunit deformable particle in simple shear flow

The structure and dynamics of a multi-subunit deformable particle are investigated by Brownian dynamics simulations. Pairwise hydrodynamic interactions between spherical subunits are described by the Rotne–Prager diffusion tensor and non-hydrodynamic interactions by means of an attractive energy parameter, E. In the absence of shear flow, the average simulated particle structure ranges from ‘stringy’(E≲kT) to compact, with a transition region in between. Time-dependent numerical results are presented for the average subunit coordination number and the average particle perimeter area as a function of increasing shear rate. Particle elongation in the flow field leads to a reduction in the effective fractal dimension of the particle. The greatest sensitivity to shear rate occurs in the stringy–compact transition regime. Substantially different dynamic behaviour is obtained when the simulation is repeated without the inclusion of hydrodynamic interactions.

Correlation functions in non-Newtonian couette flow. A group theory and molecular dynamics approach

We develop new aspects of the statistical mechanics of non-Newtonian shear flow. The phenomena of non-Newtonian flow in the dense liquid and gaseous states are investigated by NEMD computer simulations applied to model monatomic fluids. We use mainly the PUT equations of motion for shear flow, but they are implemented in a new way. We show that in non-Newtonian shear flow there appear non-vanishing correlation functions, of the generic form, 〈vα(0)vβ(t)〉, and 〈Pαβ(0)Pγδ(t)〉 for the soft-sphere and Lennard-Jones fluids in two and three dimensions. These correlation functions are trivially zero in the absence of shear flow. They become highly structured with shear flow and generally have a finite negative value at t= 0. They can exhibit time-reversal asymmetry, especially at large shear rate due to the vorticity term in the strain rate tensor.

On photospheric and chromospheric penumbral waves

The full magnetoatmospheric wave equation is numerically solved for the particular penumbral model of Nye and Thomas (1974). For chromospheric running penumbral waves it is found that the maximum vertical velocity occurs at the base of the Hα region (z ≈ 300 km) much lower than previous WKB estimates have suggested. The maximum ‘vertical’ kinetic energy occurs at z ≈ - 130 km. Very significant horizontal velocities are also found for these waves, and, in the absence of shear flow it appears that previous estimates of photospheric vertical velocities of order 10−1 km s−1 could be substantial underestimates. For the photospheric events of Musman et al. (1976), a high vertical velocity maximum is found in the corona, and the modes appear highly dispersive for periods ≥ 220 s. The effects of a sinusoidal shear flow profile on running penumbral waves are examined, and it is found that the eigenvalues (horizontal wavenumber k) are changed little, but the eigenmodes become significantly distorted; the position of the vertical velocity peak rises compared to the zero flow case, and the velocity below that peak drops significantly. This effect may well cancel the increased estimates based on zero flow.