Peristaltic pumping of thixotropic fluids is studied numerically in a flexible tube assuming that the fluid of interest is thixotropic and obeys Moore's rheological model. Assuming the flow to be laminar, axisymmetric and incompressible, the equations of motion are simplified using the long-wavelength approximation. It is shown that at high Reynolds numbers, the axial pressure gradient has the same functional form as the peristaltic wave with an amplitude which decays as the Reynolds number is increased. Using finite difference method (FDM) to solve the equations of motion, it is concluded that the peristaltic pumping of thixotropic fluids is governed by the ratio of the breakdown to the buildup parameters only, not their separate absolute values. An increase in the viscosity ratio (i.e., the ratio of zero-shear to infinite-shear viscosities) is found to decrease the axial velocity with no significant effect on the wall shear stress. But, an increase in the speed of the peristaltic wave is predicted to increase both the axial velocity and also the maximum wall shear stress.
In the present work, forced oscillations of a single, spherical, encapsulated gas bubble, immersed in an unbounded thixotropic liquid, is investigated numerically. Relying on the Moore's model to represent the thixotropicity of the liquid, and the Kelvin-Voigt model to represent the viscoelasticity of the enclosing shell, the modified integro-differential Rayleigh-Plesset equation is solved numerically using the Gauss-Laguerre Quadrature (GLQ) method. It is shown that the thixotropic behavior of the surrounding liquid plays a key role in the emergence of harmonics. It is also shown that the viscoelastic properties of the shell material and also its wall thickness can dramatically affect the rise of harmonics for encapsulated contrast bubbles used in medical sonography.
We propose a coarse-grained simulation model for segment interactions of entangled polymers in a primitive chain network (PCN) model. We express interactions between two segments in a PCN by a pairwise Gaussian soft-core potential. We show that our pairwise interaction model can improve several properties at small length or short time scales. The short range parts of the radial distribution functions by our model are qualitatively different from one of the original PCN model, which implies the short-range effective repulsive interaction. Such a short-range repulsive interaction affects several statistical quantities. We also show that some properties seem to be rather unphysical, which may be due to artifacts of the model. We discuss origins for this problem and possible ways to improve the model.
Nonlinear flow behavior and the corresponding fluorescent behavior were examined for a 3.0 wt% aqueous solution of hydrophobically modified ethoxylated urethane (HEUR; Mw = 1.1×105) having fluorescent pyrenyl (Py) groups at the chain ends. These end groups associated with each other to serve as cores of HEUR flower micelles, and these micelles were further connected into super-bridges to form a transient network. Correspondingly, single-Maxwellian relaxation reflecting the thermal reorganization of this network was observed in the linear viscoelastic regime. Under steady shear flow, the HEUR solution exhibited thinning of both viscosity η(γ) and first normal stress coefficient Ψ1(γ) at shear rates γ above the equilibrium relaxation frequency 1/τ. Fluorescent emission intensity IE(γ) from excimers formed by Py groups associated in a hydrophobic environment of the micellar cores and the intensity IM(γ) from dissociated Py groups (referred to as monomers) isolated in the aqueous phase were measured under flow simultaneously with η and Ψ1. The IE/IM ratio was found to decrease only slightly (by a factor of ∼4 %) on an increase of γ up to 6/τ well in the thinning regime. (For γ = 6/τ, η and Ψ1 decreased from respective zero-shear values by factors of ∼50 % and ∼75 %, respectively) Thus, the significant thinning at those γ was accompanied by a negligible change in the HEUR core structure that corresponds to just a slight shift of the association/dissociation balance of the Py groups to the dissociated state. From this result, the thinning is quite possibly attributed to flow-induced disruption of the network connectivity through conversion of the super-bridges into super-loops, both having the same core structure.
In a previous paper, we reported that a block co-polymer constructed with side chain crystalline monomer and monomer having solvent affinity (Side Chain Crystalline Block Co-Polymer : SCCBC) decreased the viscosity of polyethylene particle dispersion incredibly. The dispersion system also showed the Thermal Rheological (TR) phenomenon that was with increasing the temperature the viscosity increased to the almost original viscosity of the dispersion system without SCCBC and with decreasing temperature the viscosity decreased reversibly. In this study, we considered the effect of the sequence distribution of SCCBC, species of side chain crystalline monomer, and species of solvent. We found that with using random copolymer, the TR behavior decreased. By changing the length of the side chain we could change the dispersant effect and also could control the transition temperature of TR phenomenon. We also found a specific SCCBC structure which could apply to the Teflon particle dispersion system. Moreover, we found that the dispersant effect depended on the species of the solvent and the dependence could arrange by the interfacial tension between solvent and solvent affinity monomer.
Dynamic viscoelastic, dielectric and dynamic flow birefringence measurements were carried out in a highly viscoelastic system (DO3CH/C10), solutions of tris-3,7-dimethyloctyl-cis-1,3,5-cyclohexanetricarboxamide (DO3CH) in n-decane (C10), for a wide concentration (c) range to investigate structure and dynamics of supramolecular polymers formed in the system due to intermolecular hydrogen bonding. The DO3CH/C10 system showed pronounced viscoelastic behavior possessing two relaxation modes: a fast mode with a relaxation time (τ1) independent of c and strength G1 ∝ c2, and a slow mode with the other relaxation time (τ2) highly dependent of c and strength G2 ∝ c1∼1.3. In dielectric and dynamic flow birefringence behavior, singe relaxation modes were clearly observed, which corresponded to the slow relaxation mode with τ2 in viscoelastic behavior. These reveal that two kinds of supramolecular polymers are formed in the system, i.e. a rigid rodlike supramolecular polymer of ca. 45 nm length, which is formed by ∼100 DO3CH molecules via 3-fold hydrogen bonding of amide groups and bear a macro-electric dipole moment of ca. 1100 D, and a flexible supramolecuar polymer with little optical anisotropy and electric dipole moment, which is formed by almost randomly connected DO3CH molecules via hydrogen bonding bearing many defects responsible for a “phantom-crossing” mechanism at entanglement points.