Absence Of Shear Research Articles

The effect of antiapoptosis genes on clarification performance

Optimal bioreactor harvest time is typically determined based on maximizing product titer without compromising product quality. We suggest that ease of downstream purification should also be considered during harvest. In this view, we studied the effect of antiapoptosis genes on downstream performance. Our hypothesis was that more robust cells would exhibit less cell lysis and thus generate lower levels of cell debris and host-cell contaminants. We focused on the clarification unit operation, measuring postclarification turbidity and host-cell protein (HCP) concentration as a function of bioreactor harvest time/cell viability. In order to mimic primary clarification using disk-stack centrifugation, a scale-down model consisting of a rotating disk (to simulate shear in the inlet feed zone of the centrifuge) and a swinging-bucket lab centrifuge was used. Our data suggest that in the absence of shear during primary clarification (typical of depth filters), a 20-50% reduction in HCP levels and 50-65% lower postcentrifugation turbidity was observed for cells with antiapoptosis genes compared to control cells. However, on exposing the cells to shear levels typical in a disk-stack centrifuge, the reduction in HCP was 10-15% while no difference in postcentrifugation turbidity was observed. The maximum benefit of antiapoptosis genes is, therefore, realized using clarification options that involve low shear, <1 × 10(6) W/m(3) and minimal damage to the cells.

Shear Orientation in Nematic Carbon Nanotube Dispersions: A Combined NMR Investigation

Carbon nanotubes were dispersed in a sodium dodecylsulfate/decanol/water nematic fluid. The long-term stability of the dispersions is ensured by the small density gradients existing between nanotubes and the nematic fluid, and by its viscosity, as well. Presumably, surfactant or nematic micelles adsorb onto nanotubes and concur to stabilize them. A Rheo 2H NMR characterization was performed. It was supported by classical 2H quadrupole splitting and pulsed field gradient spin–echo NMR, allowing to ascertain the diffusive trends therein. The nematic fluid shows uniaxial spectral profiles and marked diffusion anisotropy. No such effects were observed in nanotube-containing nematic dispersions. In addition, the measured water self-diffusion values are substantially lower than the pure nematic fluid. In the absence of shear, dispersed nanotubes do not modify the quadrupole splitting amplitude, but affect the spectral profiles. The reasons for the observed behavior are briefly outlined. In the presence of shear...

The effect of velocity shear on dynamo action due to rotating convection

AbstractRecent numerical simulations of dynamo action resulting from rotating convection have revealed some serious problems in applying the standard picture of mean field electrodynamics at high values of the magnetic Reynolds number, and have thereby underlined the difficulties in large-scale magnetic field generation in this regime. Here we consider kinematic dynamo processes in a rotating convective layer of Boussinesq fluid with the additional influence of a large-scale horizontal velocity shear. Incorporating the shear flow enhances the dynamo growth rate and also leads to the generation of significant magnetic fields on large scales. By the technique of spectral filtering, we analyse the modes in the velocity that are principally responsible for dynamo action, and show that the magnetic field resulting from the full flow relies crucially on a range of scales in the velocity field. Filtering the flow to provide a true separation of scales between the shear and the convective flow also leads to dynamo action; however, the magnetic field in this case has a very different structure from that generated by the full velocity field. We also show that the nature of the dynamo action is broadly similar irrespective of whether the flow in the absence of shear can support dynamo action.

Gelation Behavior of Model Wax–Oil and Crude Oil Systems and Yield Stress Model Development

An experimental investigation of waxy oil gelation behavior is performed using the methods of rheometry, optical analysis, tensiometry, and microscopy. Thermal history, shear history, asphaltene content, and chemical additives influence the gelation process. Consequently, the final gel state is strongly correlated to the gelation conditions. Various model wax–oil systems are prepared, including 5 wt % microcrystalline wax or 5 wt % macrocrystalline wax in dodecane, as well as 5 wt % macrocrystalline wax in Primol 352. Real waxy crude oils UL-YS1 and Se-7-E06 are also investigated. A new yield stress model is developed on the basis of experimental results and a modified Eyring theory to provide gel strength predictions for petroleum transport pipelines. In the absence of shear during gelation, the yield stress can be correlated with the thermal history. However, for non-quiescent gelation conditions, the gel strength is also dependent upon the imposed shear history during the gelation process. The imposed shearing influences the growth process of a volume-spanning crystal network. Finally, pressure wave propagation calculations are performed following the Rønningsen convention, highlighting the impact of yield stress on startup lag times associated with wax gelation in field transport pipelines.

Laboratory experiments on internal wave interactions with a pycnocline

Laboratory experiments have been performed to investigate the interaction of internal waves with a pycnocline. An oscillating cylinder generated internal wave beams, which were observed using the synthetic schlieren technique. Internal waves incident on the pycnocline layer excited higher-frequency modes. In the absence of shear, a discrete spectrum of harmonic modes was generated due to nonlinear effects. These harmonic modes might play a role in the formation of internal solitary waves which have been observed in ocean pycnoclines. With shear, a continuous spectrum of excited modes was found.

Microfluidic System for Facilitated Quantification of Nanoparticle Accumulation to Cells Under Laminar Flow

The identification of novel, synthetic targeting ligands to endothelial receptors has led to the rapid development of targeted nanoparticles for drug, gene and imaging probe delivery. Central to development and optimization are effective models for assessing particle binding in vitro. Here, we developed a simple and cost effective method to quantitatively assess nanoparticle accumulation under physiologically-relevant laminar flow. We designed reversibly vacuum-sealed PDMS microfluidic chambers compatible with 35mm petri dishes, which deliver uniform or gradient shear stress. These chambers have sufficient surface area for facile cell collection for particle accumulation quantitation through FACS. We tested this model by synthesizing and flowing liposomes coated with APN (K (D)~300μM) and VCAM-1-targeting (K (D)~30μM) peptides over HUVEC. Particle binding significantly increased with ligand concentration (up to 6mol%) and decreased with excess PEG. While the accumulation of particles with the lower affinity ligand decreased with shear, accumulation of those with the higher affinity ligand was highest in a low shear environment (2.4dyne/cm(2)), as compared with greater shear or the absence of shear. We describe here a robust flow chamber model that is applied to optimize the properties of 100nm liposomes targeted to inflamed endothelium.

Dynamics and correlation length scales of a glass-forming liquid in quiescent and sheared conditions

We numerically study dynamics and correlation length scales of a colloidal liquid in both quiescent and sheared conditions to further understand the origin of slow dynamics and dynamic heterogeneity in glass-forming systems. The simulation is performed in a weakly frustrated two-dimensional liquid, where locally preferred order is allowed to develop with increasing density. The four-point density correlations and bond-orientation correlations, which have been frequently used to capture dynamic and static length scales ξ in a quiescent condition, can be readily extended to a system under steady shear in this case. In the absence of shear, we confirmed the previous findings that the dynamic slowing down accompanies the development of dynamic heterogeneity. The dynamic and static length scales increase with α-relaxation time τα as a power law , with μ > 0. In the presence of shear, both viscosity and τα have power-law dependences on shear rate in the marked shear-thinning regime. However, the dependence of correlation lengths cannot be described by power laws in the same regime. Furthermore, the relation between length scales and dynamics holds for not too strong shear where thermal fluctuations and external forces are both important in determining the properties of dense liquids. Thus, our results demonstrate a link between slow dynamics and structure in glass-forming liquids even under nonequilibrium conditions.

Dynamic Negative Compressibility of Few-Layer Graphene, h-BN, and MoS2

We report a novel mechanical response of few-layer graphene, h-BN, and MoS(2) to the simultaneous compression and shear by an atomic force microscope (AFM) tip. The response is characterized by the vertical expansion of these two-dimensional (2D) layered materials upon compression. Such effect is proportional to the applied load, leading to vertical strain values (opposite to the applied force) of up to 150%. The effect is null in the absence of shear, increases with tip velocity, and is anisotropic. It also has similar magnitudes in these solid lubricant materials (few-layer graphene, h-BN, and MoS(2)), but it is absent in single-layer graphene and in few-layer mica and Bi(2)Se(3). We propose a physical mechanism for the effect where the combined compressive and shear stresses from the tip induce dynamical wrinkling on the upper material layers, leading to the observed flake thickening. The new effect (and, therefore, the proposed wrinkling) is reversible in the three materials where it is observed.

Enhanced orientation of the water‐assisted injection‐molded ipp in the presence of nucleating agent

AbstractThe nucleated isotactic polypropylene (iPP) was molded by water‐assisted injection molding. The crystalline morphology and orientation distribution were studied. The results show that shear brought by melt filling and pressurized water penetration can separately induce the formation of oriented structures in skin region (i.e., the region near mold cavity wall) and the water channel region. For virgin iPP, slightly oriented lamellae appear exclusively in the above aforementioned regions. However, shish‐kebab structure occurs not only in skin and water channel region of the iPP containing moderate content of nucleator (0.2 wt%) but also in the whole region of the iPP containing a higher content of nucleator (1 wt%). It is well known that nucleator cannot directly induce the development of shish‐kebab in the absence of shear, thus the results indicate: shear flow actually distributes over a much broader range than expected; in shear field, nucleator is significantly helpful for the shear which is not sufficient to solely induce oriented structure to promote the formation of the oriented structure. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Scaling and intermittency in incoherent α-shear dynamo

We consider mean-field dynamo models with fluctuating α effect, both with and without large-scale shear. The α effect is chosen to be Gaussian white noise with zero mean and a given covariance. In the presence of shear, we show analytically that (in infinitely large domains) the mean-squared magnetic field shows exponential growth. The growth rate of the fastest growing mode is proportional to the shear rate. This result agrees with earlier numerical results of Yousef et al. and the recent analytical treatment by Heinemann, McWilliams & Schekochihin who use a method different from ours. In the absence of shear, an incoherent α2 dynamo may also be possible. We further show by explicit calculation of the growth rate of third- and fourth-order moments of the magnetic field that the probability density function of the mean magnetic field generated by this dynamo is non-Gaussian.

Turbulence in a Sheared, Salt-Fingering-Favorable Environment: Anisotropy and Effective Diffusivities

Abstract Direct numerical simulations (DNS) of a shear layer with salt-fingering-favorable stratification have been performed for different Richardson numbers Ri and density ratios Rρ. In the absence of shear (Ri = ∞), the primary instability is square planform salt fingering, alternating cells of rising and sinking fluid. In the presence of shear, salt fingering takes the form of salt sheets, planar regions of rising and sinking fluid, aligned parallel to the sheared flow. After the onset of secondary instability, the flow becomes turbulent. The continued influence of the primary instability distorts the late-stage structure and hence biases isotropic estimates of the turbulent kinetic energy dissipation rate ε. In contrast, thermal and saline gradients evolve to become more isotropic than velocity gradients at their dissipation scales. Thus, the standard observational methodology of estimating the turbulent kinetic energy dissipation rate ε from vertical profiles of microscale gradients and assuming isotropy can underestimate its true value by a factor of 2–3, whereas estimates of thermal and saline dissipation rates using this approach are relatively accurate. Likewise, estimates of Γ from vertical profiles overestimate the true Γ by roughly a factor of 2. Salt sheets are ineffective at transporting momentum. Thermal and saline effective diffusivities decrease with decreasing Ri, despite the added energy source provided by background shear. After the transition to turbulence, the thermal to saline flux ratio and the effective Schmidt number remain close to the values predicted by linear theory.

Shear Flow Induced Changes in Apolipoprotein C-II Conformation and Amyloid Fibril Formation

The misfolding and self-assembly of proteins into amyloid fibrils that occur in several debilitating diseases are affected by a variety of environmental factors, including mechanical factors associated with shear flow. We examined the effects of shear flow on amyloid fibril formation by human apolipoprotein C-II (apoC-II). Shear fields (150, 300, and 500 s(-1)) accelerated the rate of apoC-II fibril formation (1 mg/mL) approximately 5-10-fold. Fibrils produced at shear rates of 150 and 300 s(-1) were similar to the twisted ribbon fibrils formed in the absence of shear, while at 500 s(-1), tangled ropelike structures were observed. The mechanism of the shear-induced acceleration of amyloid fibril formation was investigated at low apoC-II concentrations (50 μg/mL) where fibril formation does not occur. Circular dichroism and tryptophan fluorescence indicated that shear induced an irreversible change in apoC-II secondary structure. Fluorescence resonance energy transfer experiments using the single tryptophan residue in apoC-II as the donor and covalently attached acceptors showed that shear flow increased the distance between the donor and acceptor molecules. Shear-induced higher-order oligomeric species were identified by sedimentation velocity experiments using fluorescence detection, while fibril seeding experiments showed that species formed during shear flow are on the fibril formation pathway. These studies suggest that physiological shear flow conditions and conditions experienced during protein manufacturing can exert significant effects on protein conformation, leading to protein misfolding, aggregation, and amyloid fibril formation.

Mesoscopic simulation on phase behavior of star-like block polyether solution in the absence and presence of shear

The phase behavior of a kind of star-like block polyether in aqueous solution is studied by MesoDyn simulation method in the absence and presence of shear. With the polyether concentration increasing, different mesoscale structures are found in the absence and presence of shear. (1) In the absence of shear, there are four different structures formed with the increasing polyether concentration. The first is micellar phase, including sphere-like micelle and wormlike micelle, the second is bicontinuous phase, the third is lamellar phase, and the fourth is reversed micellar phase, including reversed worm-like micelle and reversed hexagonal micelle. (2) In the presence of shear, there are three morphologies formed in all studied concentrations; they are hexagonal phase, lamellar phase and reverse hexagonal phase, which align in the flow direction. The detailed structures and forming mechanisms of different aggregates are given by analyzing simulation results.

Viscoelastic and conductivity properties of thermoreversible polyaniline–DNNSA gel in m-cresol

The rheological behavior of polyaniline (PANI)–dinonylnaphthalene sulfonic acid (DNNSA) in m-cresol is studied for different weight percent (w/v) of PANI–DNNSA 0.5. From rheological viewpoint the sample behaves like viscous fluid at low concentration (2 wt%) and gel at the concentration ≥8 wt%. The 4 wt% PANI–DNNSA 0.5 in m-cresol is at typical viscoelastic percolation region which is sol in the absence of shear but show invariant storage modulus with frequency at 30 °C. SEM picture indicates fibrillar network structure in the gel and the doped polyaniline remain as nanofiber at ≤2 wt% concentrations. The complete doping of PANI in all the systems is confirmed from UV–vis spectra. The dc conductivity of the gel increases with increasing the concentration of PANI–DNNSA 0.5 in m-cresol showing a jump at ∼4% concentration (percolation threshold). ac-Conductivity increases with increase in PANI–DNNSA 0.5 concentration studied here for each frequency. At lower frequency (<10 5 Hz) ac-conductivity increase slowly with frequency but at higher frequency (>10 5 Hz) the increase is large having a curve like behavior. The gel state shows an increase of module by ∼5 orders and also an increase of ∼3 orders in ac-conductivity at the same frequency. The impedance spectroscopy results suggest the formation of combined resistance and capacitance ( R– C) circuits in the gel and the increase of PANI–DNNSA 0.5 in the gel increases the capacitive feature more dominantly though the path becomes less resistive. The process also signifies the preparation of DNNSA doped PANI nanofiber in the gel medium.

From convective to stellar dynamos

AbstractConvectively driven dynamos with rotation generating magnetic fields on scales large compared with the scale of the turbulent eddies are being reviewed. It is argued that such fields can be understood as the result of an α effect. Simulations in Cartesian domains show that such large-scale magnetic fields saturate on a time scale compatible with the resistive one, suggesting that the magnitude of the α effect is here still constrained by approximate magnetic helicity conservation. It is argued that, in the absence of shear and/or any other known large-scale dynamo effects, these simulations prove the existence of turbulent α2-type dynamos. Finally, recent results are discussed in the context of solar and stellar dynamos.

Immobilized Chemokine Fields and Soluble Chemokine Gradients Cooperatively Shape Migration Patterns of Dendritic Cells

Chemokines orchestrate immune cell trafficking by eliciting either directed or random migration and by activating integrins in order to induce cell adhesion. Analyzing dendritic cell (DC) migration, we showed that these distinct cellular responses depended on the mode of chemokine presentation within tissues. The surface-immobilized form of the chemokine CCL21, the heparan sulfate-anchoring ligand of the CC-chemokine receptor 7 (CCR7), caused random movement of DCs that was confined to the chemokine-presenting surface because it triggered integrin-mediated adhesion. Upon direct contact with CCL21, DCs truncated the anchoring residues of CCL21, thereby releasing it from the solid phase. Soluble CCL21 functionally resembles the second CCR7 ligand, CCL19, which lacks anchoring residues and forms soluble gradients. Both soluble CCR7 ligands triggered chemotactic movement, but not surface adhesion. Adhesive random migration and directional steering cooperate to produce dynamic but spatially restricted locomotion patterns closely resembling the cellular dynamics observed in secondary lymphoid organs.

Shear induced ordering in branched living polymer solutions

We present a coarse-grained model for the dynamics of living polymer solutions which explicitly includes solvent hydrodynamics. We show that lamellar and columnar structures emerge when the solution is subjected to simple shear. In the absence of shear, the model predicts a fluid–gel transition as a function of polymer concentration. At a threshold concentration, the self-intermediate scattering function indicates Zimm-like dynamics at large wave vectors and diffusive dynamics at small wave vectors. The kinetics of scission and recombination clearly demonstrates the existence of mean field and diffusion controlled regimes for the dynamics of living polymers at low and high concentrations respectively.

High-pressure real-time 129Xe NMR: monitoring of surfactant conformation during the self-assembly of reverse micelles in supercritical carbon dioxide

High-pressure (129)Xe NMR has been used to monitor in real time the self-assembly of reverse water-in-supercritical carbon dioxide micelles in the absence of shear, without the need for sensitivity enhancement techniques such as hyperpolarization.

Transient growth in a two-fluid channel flow under normal electric field

A linear model of a layered channel flow of two perfectly dielectric viscous fluids in the presence of uniform normal electric field is built. The effect of the normal electric field on transient growth of small disturbances is studied at two values of Weber number. The numerical result shows that the electric field enhances the transient growth for both two-dimensional and three-dimensional disturbance cases. The contours of optimal energy growth are represented in the wave number plane. When the electric field is small, the optimal disturbance that corresponds to the peak value of optimal growth is two dimensional. It is governed by the lift-up mechanism and is little influenced by the electric field. However, when the electrical Euler number exceeds a critical value, the optimal disturbance is three dimensional with streamwise uniform wave number and is partially dominated by the electric field, and moreover, the spanwise wave number has a linear relationship with the electrical Euler number. The comparison of exponential growth and transient growth is performed. It is shown that exponential growth becomes profound and even predominant over transient growth when the electric field is sufficiently strong. In addition, the mechanism of transient growth is discussed and it is found that the existence of material interface may cause transient growth in the absence of shear in basic flow.

Dynamics of dense sheared granular flows. Part 1. Structure and diffusion

Shear flows of inelastic spheres in three dimensions in the volume fraction range 0.4–0.64 are analysed using event-driven simulations. Particle interactions are considered to be due to instantaneous binary collisions, and the collision model has a normal coefficient of restitution en (negative of the ratio of the post- and pre-collisional relative velocities of the particles along the line joining the centres) and a tangential coefficient of restitution et (negative of the ratio of post- and pre-collisional velocities perpendicular to the line joining the centres). Here, we have considered both et = +1 and et = en (rough particles) and et = −1 (smooth particles), and the normal coefficient of restitution en was varied in the range 0.6–0.98. Care was taken to avoid inelastic collapse and ensure there are no particle overlaps during the simulation. First, we studied the ordering in the system by examining the icosahedral order parameter Q6 in three dimensions and the planar order parameter q6 in the plane perpendicular to the gradient direction. It was found that for shear flows of sufficiently large size, the system continues to be in the random state, with Q6 and q6 close to 0, even for volume fractions between φ = 0.5 and φ = 0.6; in contrast, for a system of elastic particles in the absence of shear, the system orders (crystallizes) at φ = 0.49. This indicates that the shear flow prevents ordering in a system of sufficiently large size. In a shear flow of inelastic particles, the strain rate and the temperature are related through the energy balance equation, and all time scales can be non-dimensionalized by the inverse of the strain rate. Therefore, the dynamics of the system are determined only by the volume fraction and the coefficients of restitution. The variation of the collision frequency with volume fraction and coefficient of restitution was examined. It was found, by plotting the inverse of the collision frequency as a function of volume fraction, that the collision frequency at constant strain rate diverges at a volume fraction φad (volume fraction for arrested dynamics) which is lower than the random close-packing volume fraction 0.64 in the absence of shear. The volume fraction φad decreases as the coefficient of restitution is decreased from en = 1; φad has a minimum of about 0.585 for coefficient of restitution en in the range 0.6–0.8 for rough particles and is slightly larger for smooth particles. It is found that the dissipation rate and all components of the stress diverge proportional to the collision frequency in the close-packing limit. The qualitative behaviour of the increase in the stress and dissipation rate are well captured by results derived from kinetic theory, but the quantitative agreement is lacking even if the collision frequency obtained from simulations is used to calculate the pair correlation function used in the theory.