Polymers In Shear Flow Research Articles

Adhesion of endothelial cells and endothelial progenitor cells on peptide-linked polymers in shear flow.

The initial adhesion of human umbilical vein endothelial cells (HUVECs), cord blood endothelial colony-forming cells (ECFCs), and human blood outgrowth endothelial cells (HBOECs) was studied under radial flow conditions. The surface of a variable shear-rate device was either coated with polymer films or covered by synthetic fibers. Spin-coating was applied to produce smooth polymer films, while fibrous scaffolds were generated by electrospinning. The polymer was composed of hexyl methacrylate, methyl methacrylate, poly(ethylene glycol) methacrylate (PEGMA), and CGRGDS peptide. The peptide was incorporated into the polymer system by coupling to an acrylate-PEG-N-hydroxysuccinimide comonomer. A shear-rate-dependent increase of the attached cells with time was observed with all cell types. The adhesion of ECs increased on RGD-linked polymer surfaces compared to polymers without adhesive peptides. The number of attached ECFCs and HBOECs are significantly higher than that of HUVECs within the entire shear-rate range and surfaces examined, especially on RGD-linked polymers at low shear rates. Their superior adhesion ability of endothelial progenitor cells under flow conditions suggests they are a promising source for in vivo seeding of vascular grafts and shows the potential to be used for self-endothelialized implants.

Tumbling and tank-treading dynamics of individual ring polymers in shear flow

The dynamics of individual ring polymers in a shear flow is studied by a hybrid mesoscale simulation approach. New insight into the dynamics of ring polymers is provided: ring polymers exhibit both tumbling and tank-treading motions. A novel angle autocorrelation function is proposed to analyze the tank-treading motion that usually coexists with tumbling and may be applied to other highly deformable soft objects such as vesicles. The shear dependence of the average gyration tensor, the orientation angle, the tumbling and the tank-treading frequencies is determined quantitatively. The simulations also reveal that the existence of the tank-treading motion apparently increases the intrinsic viscosity of ring polymers under shear flow.

S054011 MPCDを用いた複雑流体の数値シミュレーション

The behavior of star polymer in shear flow was investigated using Multi-Particle Collision Dynamics (MPCD),which is particle-based mesoscale hydrodynamic algorithm. MPCD is a suitable tool for the simulation of complex fluids because it can express both hydrodynamic interaction and Brownian motion, which are not negligible in the simulation of complex fluids. The present study considered the behavior of a star polymer under flow. A star polymer has many brunches interacting each other. In the present simulation a star polymer chain is consist of beads and elastic spring. The simulation was carried out by changing the number of brunches. The numerical results showed that the deformation behavior and conformation of star polymers depended on the number of branches.

Giant Nonmonotonic Stretching Response of a Self-Associating Polymer in Shear Flow

Self-associating polymers are ubiquitous in synthetic and biological systems. Here, we use a combination of simulation and theory to show that these polymers exhibit a counterintuitive strong nonmonotonic stretching response in shear flow. Furthermore, we demonstrate that this behavior can be tuned by controlling the barrier for dissociation of the bonds and develop a quantitative and predictive theory based on conformational transitions to explain the observed behavior. Our results can be important in understanding previous experimental and theoretical observations and further aid in the development of novel smart materials.

Theory of Tethered Polymers in Shear Flow: The Strong Stretching Limit

In the present article, we use simulation and theory to understand the case of flexible and semiflexible tethered polymers in the limit of high shear flows and consequently near-full extension of the chains. We adopt a random-walk picture along the axis of the polymer, which in combination with a statistical mechanical description of perturbations along the chain contour allows us to fully characterize the geometry of the polymer chain without any fit parameters. Incorporation of hydrodynamic interactions is also investigated, with the hydrodynamic lift force inducing changes in the chain equilibrium structure near the tether point that could have implications for adsorption and desorption kinetics.

Conformational and rheological properties of semiflexible polymers in shear flow

A theoretical description is provided for the nonequilibrium conformational and dynamical properties of a polymer in shear flow. Using a mean-field semiflexible chain model, which accounts for hydrodynamic interactions within the preaveraging approximation, analytical expressions are derived for the dependence of the deformation, orientation, and relaxation times on polymer persistence length and shear rate. Moreover, the rheological properties of a dilute polymer solution are discussed. The model yields shear thinning at large Weissenberg numbers. The analytical results are compared with fluorescence microscopy measurements of individual DNA molecules, which yield qualitative and partial quantitative agreement.

Biaxial phases of bent-core liquid crystal polymers in shear flows

We develop a kinetic theory of the biaxial phases for the bent-core or V-shaped liquid crystal polymer solution (VLCPS). The Brownian motion of the biaxial molecules, biaxial molecular excluded interaction, and the rotary convection are modeled explicitly. Using the hydrodynamic theory, we study the flow-driven orientational dynamics and the corresponding rheological response in selected regimes of plane shear flows, strength of intermolecular interaction, and the bent-angle of the molecule. We identify three steady state biaxial phases and three periodic motions at various regimes of the shear rate and strength of the intermolecular potential. Phase transition sequences among the orientational phases and motions are documented with respect to the strength of the intermolecular potential at fixed bent angles. The effect of the molecular configuration (or the bent- angle) of the V-shaped molecule on mesoscopic phases and their transition sequences is investigated as well.

Dynamics and rheology of biaxial liquid crystal polymers in shear flows

We study nematodynamics of a mesoscopic system consisting of sheared biaxial liquid crystalline polymers using a hydrodynamical kinetic theory, in which the biaxial liquid crystal polymer is modeled as a rigid, biaxial, ellipsoidal molecule immersed in viscous solvent. The governing Smoluchowski equation in the model is solved in selected regions of the material parameter space and a range of accessible shear rates using a Wigner–Galerkin spectral method. In addition to the truly biaxial flow-aligning steady states, log-rolling steady states, and out-of-plane steady states, we report the presence of two new time-periodic motions, chaotic motion and associated phase transitions in a range of shear rates and selected material parameters. Rheological signatures of the sheared mesoscopic system are identified with predominant shear thinning in all phases and alternating signs between the normal stress differences in steady vs time-dependent motions.

Nonlinear Response of Grafted Semiflexible Polymers in Shear Flow

Using Brownian-hydrodynamic and lattice-Boltzmann simulations, we study the nonlinear response of grafted semiflexible polymers to shear flow as a function of shear rate, grafting density, and chain stiffness. Simulation results for brush height and flow stagnation layer height agree well with a mean-field theory that incorporates the interplay of hydrodynamic screening and drag-induced polymer deformation. Our predictions for the stagnation height show excellent agreement with recent experiments on the nonlinear hydrodynamic drag of DNA-grafted colloids held in a laser trap.

Dynamics and Instabilities of Collapsed Polymers in Shear Flow

The dynamics of collapsed polymers in shear flow is very different from that of sheared polymers in good or Θ-solvent solvents, because cohesive attraction between monomers opposes hydrodynamic drag forces. Using hydrodynamic simulations and scaling arguments, we show that a polymeric globule displays a well-defined stretching transition at a threshold shear rate γ*. Below this critical shear rate, the chain remains in a slightly deformed but compact state, while for shear rates larger than γ* the globule undergoes periodic unfolding−refolding cycles. The mechanism for this shear-induced instability is associated with the presence of thermally excited polymeric protrusions and thus very different from the classical scenario of hydrodynamic instabilities at the interface between two viscous liquids. In fact, it turns out that the critical shear rate depends sensitively on temperature and on the microscopic cutoff length scale, i.e., the monomer size. Our results demonstrate that proper inclusion of hydro...

Finite-dimensional description of the long-term dynamics for the 2D Doi-Hess model for liquid crystalline polymers in shear flow

The existence of inertial manifolds for a Smoluchowski equation arising in the 2D Doi-Hess model for liquid crystalline polymers subjected to a shear flow is investigated. The presence of a non-variational drift term dramatically complicates the long-term dynamics from the variational gradient case, in which it is solely characterized by the steady states. Several transformations are used in order to transform the equation into a form suitable for application of the standard theory of inertial manifolds. A nonlinear and nonlocal transformation developed in Inertial manifolds for a Smoluchowski equation on a circle and Inertial manifolds for a Smoluchowski equation on the unit sphere,, to appear, is used to eliminate the first-order derivative from the micro-micro interaction term. A traveling wave transformation eliminates the first-order derivative from the non-variational term, transforming the equation into a nonautonomous one for which the theory of nonautonomous inertial manifolds applies.

Hydrodynamic screening of star polymers in shear flow

The mutual effects of the conformations of a star polymer in simple shear flow and the deformation of the solvent flow field are investigated by a hybrid mesoscale simulation technique. We characterize the flow field near the star polymer as a function of its functionality (arm number) f . A strong screening of the imposed flow is found inside the star polymer, which increases with increasing f . To elucidate the importance of hydrodynamic screening, we compare results for hydrodynamic and random solvents. The dependence of the polymer orientation angle on the Weissenberg number shows a power law behavior with super-universal exponent --independent of hydrodynamic and excluded-volume interactions. In contrast, the polymer rotation frequency changes qualitatively when hydrodynamic interactions are switched on.

Semiflexible Polymers in Shear Flow

The dynamics of semiflexible polymers under the influence of shear flow is studied analytically. Power laws are derived for various conformational and dynamical quantities which are in agreement with experimental findings. In particular, the tumbling motion is analyzed and expressions are provided for the probability distributions of the orientation angles and the tumbling time. The calculations explain the similarities in the behavior of flexible and semiflexible polymers as well as free-draining and nondraining systems.

Dynamics of relaxation of entangled polymers in shear flow

The application of shear flow to entangled polymer melts can strongly modify its rheological and physicochemical behaviors, giving rise to an acceleration of several chemical processes such as diffusion-controlled reactions. In the present work, we investigate the modification of conformational and diffusive properties of an entangled polymer in shear flow by numerical methods. The flow affects both the conformational and diffusive properties of the system, giving rise to a quasinematic ordering of the macromolecules which take prolate spheroid shape with the main axis aligned to the shear direction. The shear flow is found to accelerate the overall diffusion of the chains in all directions at times longer than the polymer relaxation time. The polymer chains display a quite peculiar displacement behavior in direction parallel to the flow. At the same conditions, the linear relation between the diffusion constant in direction perpendicular to the flow and the inverse of the relaxation time, usually adopted in equilibrium regimes, is shown to hold even in the presence of flow.

Multilayer Markov Chains with Applications to Polymers in Shear Flow

We present an analysis of multilayer Markov chains and apply the results to a model of a tethered polymer chain in shear flow. We find that the stationary probability measure in the direction of the flow is nonmonotonic, and has several maxima and minima for sufficiently high shear rates. This is in agreement with the experimental observation of cyclic dynamics for such polymer systems. Estimates for the stationary variance and expectation value were obtained and showed to be in accordance with our numerical results.

Star Polymers in Shear Flow

Linear and star polymers in solution are studied in the presence of shear flow. The solvent is described by a particle-based mesoscopic simulation technique, which accounts for hydrodynamic interactions. The scaling properties of the average gyration tensor, the orientation angle, and the rotation frequency are investigated for various arm lengths and arm numbers. With increasing functionality f, star polymers exhibit a crossover in their flow properties from those of linear polymers to a novel behavior, which resembles the tank-treading motion of elastic capsules.

Hydrodynamic effects in driven soft matter.

Recent theoretical works exploring the hydrodynamics of soft material in non-equilibrium situations are reviewed. We discuss the role of hydrodynamic interactions for three different systems: (i) the deformation and orientation of sedimenting semiflexible polymers, (ii) the propulsion and force-rectification with a nano-machine realized by a rotating elastic rod, and (iii) the deformation of a brush made of grafted semiflexible polymers in shear flow. In all these examples deformable polymers are subject to various hydrodynamic flows and hydrodynamic interactions. Perfect stiff nano-cylinders are known to show no orientational effects as they sediment through a viscous fluid, but it is the coupling between elasticity and hydrodynamic torques that leads to an orientation perpendicular to the direction of sedimentation. Likewise, a rotating stiff rod does not lead to a net propulsion in the Stokes limit, but if bending is allowed an effective thrust develops whose strength and direction is independent of the sense of rotation and thus acts as a rectification device. Lastly, surface-anchored polymers are deformed by shear flows, which modifies the effective hydrodynamic boundary condition in a non-linear fashion. All these results are obtained with hydrodynamic Brownian dynamics simulation techniques, as appropriate for dilute systems. Scaling analyses are presented when possible. The common theme is the interaction between elasticity of soft matter and hydrodynamics, which can lead to qualitatively new effects.

Numerical study of polymer tumbling in linear shear flows

We investigate numerically the dynamics of a single polymer in a linear shear flow. The effects of thermal fluctuations and randomly fluctuating velocity gradients are both analyzed. Angular, elongation and tumbling time statistics are measured numerically. We perform analytical calculations and numerical simulations for a linear single-dumbbell polymer model comparing the results with previous theoretical and experimental studies. For thermally driven polymers the balance between relaxation and thermal fluctuations plays a fundamental role, whereas for random velocity gradients the ratio between the intensity of the random part and the mean shear is the most relevant quantity. In the low-noise limit, many universal aspects of the motion of a polymer in a shear flow can be understood in this simplified framework.

Molecular dynamics study of tethered polymers in shear flow

Single macromolecules can now be isolated and characterized experimentally using techniques such as optical tweezers and videomicroscopy. An interesting and important single-molecule problem is that of the dynamics of a polymer chain tethered to a solid surface and subjected to a shear flow. An experimental study of such a system was reported by Doyle et al. (Phys. Rev. Lett. 84, 4769 (2000)), and their results showed a surprising recirculating motion of the DNA chain. We explore this problem using molecular dynamics computer simulations with explicit hydrodynamic interactions. The dynamical properties of a Freely Jointed Chain (FJC) with Finitely Extensible Nonlinear Elastic (FENE) links are examined in similar conditions (i.e., confined between two surfaces and in the presence of a Poiseuille flow). We see the remarkable cyclic polymer motion observed experimentally, and we show that a simple cross-correlation function can be used to measure the corresponding period of motion. We also propose a new empirical equation relating the magnitude of the shear flow to the amount of chain deformation, an equation that appears to apply for both weak and strong flows. Finally, we report on packing effects near the molecularly flat wall, an associated chain-sticking phenomenon, and the impact of the chain hydrodynamic drag on the local fluid flow.

Characteristic Periodic Motion of Polymers in Shear Flow

The motion of both free and tethered polymer molecules as well as rigid Brownian rods in unbound shear flow is found to be characterized by a clear periodicity or tumbling frequency. Periodicity is shown using a combination of single molecule DNA experiments and computer simulations. In all cases, we develop scaling laws for this behavior and demonstrate that the frequency of characteristic periodic motion scales sublinearly with flow rate.