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.

Solid descending flow due to gravity is frequently applied to industrial processes. Several kinds of models have been proposed like the plug flow model, the potential flow model, the kinematic model, etc. In this study, a viscous flow model based on the Navier-Stokes equation has been developed and was compared with the potential flow and the kinematic models. Both two and three dimensional experimental apparatuses have been constructed to observe the velocity fields. The concept of solid viscosity was quoted to describe the friction between particles and the value was obtained from the experimental data. Slip boundary condition was used at the wall and the friction between particles and the wall or the dead zone was expressed by the Fanning equation. Navier-Stokes equation was applied to simulate the solid flow. Thegoodagreementbetweenthe observed and computed results was obtained in different scale apparatuses at different solid descending velocities. The computation results gave about 0.07 Pa's as the solid viscosity of sand particles with the diameter of 0.001-0.002m. The solid~~as countercurrent flow was also simulated by the Navier-Stokes equation with the same value of solid viscosity.

The properties of both untangled and entangled linear polymer melts under shear flow are studied by nonequilibrium molecular dynamics simulations. The results reveal that the dependence of shear viscosity η on shear rate γ, expressed by n ~ γ(-n), exhibits three distinct regimes. The first is the well-known Newtonian regime, namely, η independent of shear rate at small shear rates γ < τ0(-1) (where τ0 is the longest polymer relaxation time at equilibrium). In the non-Newtonian regime (γ > τ0(-1)) the shear dependence of viscosity exhibits a crossover at a critical shear rate γc dividing this regime into two different regimes, shear thinning regime I (ST-I) and II (ST-II), respectively. In the ST-I regime (τ0(-1) < γ < γc), the exponent n increases with increasing chain length N, while in the ST-II regime (γ > γc) a universal power law n ~ γ(-0.37) is found for considered chain lengths. Furthermore, the longer the polymer chain is, the smaller the shear viscosity for a given shear rate in the ST-II regime. The simulation also shows that a characteristic chain length, below which γc will be equal to τ0(-1), lies in the interval 30 < N < 50. For all considered chain lengths in the ST-II regime, we also find that the first and second normal stress differences N1 and N2 follow power laws of N1 ~ γ(2/3) and N2 ~ γ(0.82), respectively; the orientation resistance parameter mG follows the relation mG ~ γ(0.75) and the tumbling frequency ftb follows ftb ~ γ(0.75). These results imply that the effects of entanglement on the shear dependences of these properties may be negligible in the ST-II regime. These findings may shed some light on the nature of shear thinning in flexible linear polymer melts.

Hepatopoietin (HPO) is a novel human hepatotrophic growth factor, which specifically stimulates proliferation of cultured primary hepatocytes in vitro and liver regeneration after liver partial hepatectomy in vivo. Recently, the identification of the mitogenic effect of HPO on hepatoma cell lines and the existence of HPO-specific receptors indicate that HPO acts via its specific cell surface receptor. However, the molecular mechanism of HPO action is not fully elucidated. In this report, we examined the signal transduction events induced by HPO in hepatoma cell line (HepG2). Our results demonstrated that HPO induces phosphorylation of mitogenactivated protein kinase kinase and mitogen-activated protein kinase (MAPK) in a rapid and transient manner. HPO stimulates tyrosine phosphorylation of epidermal growth factor receptor (EGFR). Furthermore, we observed that both MAPK activation and the mitogenic effect of HPO on HepG2 cells were completely blocked by AG1478, a specific inhibitor of EGFR tyrosine kinase activity. However, the effects of HPO were not antagonized by an EGFR-blocking antibody, mAb528, which blocks the interaction between epidermal growth factor and EGFR, indicating that stimulation of tyrosine phosphorylation of EGFR by HPO was not mediated by epidermal growth factor. In contrast, genistein, a general tyrosine kinase inhibitor, significantly attenuated the tyrosine phosphorylation of EGFR in response to HPO. In conclusion, our results suggest that tyrosine phosphorylation of EGFR may play a critical role in MAPK activation and mitogenic stimulation by HPO. Hepatopoietin (HPO)1 is a novel human hepatotrophic growth factor, an orthologue of rat augmenter of liver regeneration or hepatic stimulator substance (1). In 1975, LaBrecque and Pesh (2) first reported that in the livers of weanling rats or partially hepatectomized rats, there existed a polypeptide, named hepatic stimulator substance, that could specifically stimulate DNA synthesis of hepatic cells. The existence of hepatic stimulator substance-related activities has been reported in other species including mice, cows, dogs, pigs, and humans (3). Hagiya et al. (4) cloned the cDNA of rat augmenter of liver regeneration, which is the same as rat hepatic stimulator substance. Subsequently, Giorda et al. (5) and Yang et al. (6) cloned the cDNA of human augmenter of liver regeneration or HPO by screening the cDNA library of human fetal liver. HPO encodes a novel protein with no sequence similarity to any known growth regulator. Interestingly, HPO is highly related to the yeast ERV (essential for respiration and viability) gene products. However, the functional relevance of HPO and ERV is currently unclear (7). Yang et al. (8,9) demonstrated that the recombinant human HPO stimulated proliferation of hepatocytes as well as hepatoma cells in vitro. HPO also promotes regeneration and recovery of damaged hepatocytes and rescues acute hepatic failure in vivo (8, 9). Thus, these observations support the contention that HPO is a hepatotrophic growth fa...

How polymers with different architectures respond to shear stress is a key issue to develop a fundamental understanding of their dynamical behaviors. We investigate the conformation, orientation, dynamics, and rheology of individual star polymers in a simple shear flow by multiparticle collision dynamics integrated with molecular dynamics simulations. Our studies reveal that star polymers present a linear transformation from tumbling to tank-treading-like motions as the number of arms increases. In the transformation region, the flow-induced deformation, orientation, frequency of motions, and rheological properties show universal scaling relationships against the reduced Weissenberg number, independent of the number and the length of arms. Further, we make a comprehensive comparison on the flow-induced behaviors between linear, ring, and star polymers. The results indicate that distinct from linear polymers, star and ring polymers present weaker deformation, orientation change, and shear thinning, either contributed by a dense center or without ends.

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We develop a self-consistent-field lattice model for block copolymers and propose a novel and general method to solve the self-consistent-field equations. The approach involves describing the polymer chains in a lattice and employing a two-stage relaxation procedure to evolve a system as rapidly as possible to a free-energy minimum. In order to test the validity of this approach, we use the method to study the microphases of rod-coil diblock copolymers. In addition to the lamellar and cylindrical morphologies, micellar, perforated lamellar, gyroid, and zigzag structures have been identified without any prior assumption of the microphase symmetry. Furthermore, this approach can also give the possible orientation of the rods in different structures.

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