Synopsis The solidification of waxy components during the cool down of waxy crude oils in pipelines may provide complex yield stress fluid behavior with time-dependent characteristics, which has a critical impact for predicting flow restart after pipeline shut-in. Here, from a previous set of data at a local scale with the help of Magnetic Resonance Imaging and a new full set of data for various flow and temperature histories, we give a general picture of the rheological behavior of waxy crude oils. The tests include start flow tests at different velocities or creep tests at different stress levels, abrupt changes of velocity level, steady flow, after cooling under static or flowing conditions. We show that when the fluid has been cooled at rest it forms a structure that irreversibly collapses during the startup flow. Under these conditions, the evolution of the apparent viscosity mainly depends on the deformation undergone by the fluid for low or moderate deformation and starts to significantly depend on the shear rate for larger values. Even the (apparent) flow curve of statically cooled waxy crude oils was observed to be dependent on the flow history, more specifically on the maximum shear rate experienced by the material. After being sufficiently sheared, i.e., achieving an equilibrium state, the rheological behavior is that of a simple liquid for shear rates lower than the maximum historical one. A model is proposed to represent those trends experimentally observed. In contrast with most previous works in that field, the model is built without any a priori assumption based on classical behavior of a class of fluids. Finally, it is shown that this model
In this paper, the flow of blood through a curved vessel having stenosis and aneurysm is investigated. To evaluate the impact of stenosis and aneurysm in a curved channel, the curvilinear coordinates are used to formulate a suitable geometry. The flow and heat transfer are investigated in the presence of nanoparticles that play a significant role in blood flows through arteries and they are gaining popularity in hematological treatment. The dynamical behavior of blood flow is modeled by using Eyring-Powell fluid model and the coupled partial differential equations are formulated to study the blood rheology. The flow, and heat and mass transfer equations are numerically solved by using finite difference scheme. The effect of some significant parameters on blood flow through a curved channel with stenosis and aneurysm is discussed and displayed in graphs. The pattern of blood flow is also depicted through geometrical patterns.
Cluster growth process during the gelation is attractive from both scientific and biomedical point of views. Until now, there have been many attempts from theoretical and experimental approaches. However, the comparison of the experimental results with the theoretical prediction is ambiguous, because it is impossible to quench the gelation reaction and directly estimate the connectivity in experiments. In this study, we fabricated the states near the critical points using the Tetra-PEG gel system with off-stoichiometrically mixing. We estimated the connectivity directly using the NMR measurement. Using this system, we performed the DLS and viscosity measurements to discuss the cluster growth. The cluster size increase with approaching the gelation critical point showing the characteristic scaling relationship. The scaling exponent depended on the polymer concentration at preparation. At the high concentration region, the exponent agrees with the prediction of the percolation theory. While it downwardly deviates from the prediction in low concentration, which suggests the lattice approximation is failed below the overlapping concentration.
In a previous study (Sakai A., et al., ACS Cent. Sci., 4, 477 (2018)), a spherical microgel of gelatin prepared inside a lipid droplet was reported to have a higher surface elasticity than the bulk gel. In this study, we investigate the role of contact or lack of contact between gelatin and the lipid membrane as well as the micrometric confinement to isolate the dominant cause of this higher elasticity of microgels. For our experiment, we prepared a concave microgel of gelatin with two surfaces, with one surface in contact with the lipid membrane and the other without being in contact with the membrane. Next, we measured the elasticities of both the surfaces by using micropipette aspiration. Although the elasticity of the surface not in contact with the lipid membrane was slightly lower than that of the surface in contact with the membrane, the elasticity value was much higher than that for the bulk gel. Further, it was found that the droplet confinement without lipids did not decrease the elasticity of gelatin microgels. These results demonstrate that the dominant factor responsible for the higher elasticity of gelatin microgels is micrometric confinement and not their contact with the lipid membrane.
Poly(vinyl alcohol) (PVA) hydrogels were prepared by the repeated freezing and thawing cycles, and creep behavior of the gels was examined. In the gel preparation, the salt K2CO3 was introduced to obtain the clear gel structure. By the creep we also prepared the condensed PVA gels with unidirectional structure. The mechanical anisotropy was observed for the unidirectional gel.
Broadband dielectric spectrum measurements up to 50 GHz have been conducted for aqueous solution of 2-n-butoxyethanol (2BE) over a wide concentration range to investigate the state of molecular association of 2BE in aqueous solution. All the obtained dielectric relaxation spectra can be well decomposed into multiple Debye-type relaxation functions. The number of necessary relaxation modes clearly depended on the concentration of 2BE and was reasonably correlated with the molecular association state change of 2BE in aqueous solution. The concentration dependence of hydration number per 2BE molecule slightly decreased above a critical concentration for molecular association formation like micelles due to intermolecular hydrogen bonds formation between 2BE molecules.
We have compared the experimental results on solvent squeeze from cylindrical poly(vinyl alcohol) (PVA) gels with the model presented in this paper. The model says that a characteristic time for solvent squeeze, which is also a retardation time of the creep of gels, is proportional to the diameter squared. A simplified version of the model, which is valid when the creep strain is small enough, indicates that retardation time is independent of the applied stress and the height of gels approaches to a constant value in a single-exponential manner. Experiments on PVA gels say that the change in height occurs almost in a single-exponential manner. The retardation time was independent of the applied stress at small stresses but became longer in the high stress region, as in the case of the presented model. The diameter dependence of the retardation time for the PVA gels was weaker than the model prediction. As a whole, the simplified version of the model is useful to explain the squeezing behavior of the PVA gels.
Concentrated colloidal suspensions of monodispersed silica particles exhibited shear-thickening when a highly viscous suspending media was prepared using ethylene glycol. Although the shear rate at which shear-thickening occurred decreased in accordance with increases in the suspending media viscosity, the shear stress at the onset of shear-thickening behavior occurred was defined regardless of the suspending media viscosity. In addition, based on the dependence of the critical shear stress on the square reciprocal of the particle size, it was found that shear-thickening behavior can be explained by the diffusion-friction-dominant model.
The effect of suspending medium on the rheological behavior of monodispersed colloidal suspensions was examined to identify the characteristic rheological behavior of concentrated colloidal suspensions. Colloidal suspensions of monodispersed silica particles form colloidal crystals and exhibit shear-thinning flows. With the change of the suspending medium from an aqueous medium to a polyethylene-glycol di-acrylate monomer, both the viscosity of the suspensions and the degree of shear-thinning decreased, and the suspensions exhibited shear-thickening behavior under high shear rates. Additionally, increasing the particle size, critical shear-rate and critical shear-stress at the onset of shear-thickening decreased. Finally, the research found that shear-thickening becomes more noticeable as the increase in the ethylene-glycol chain lengths of the monomer and the particle size.
This study examines the simplest relevant molecular model of a polymeric liquid in large-amplitude oscillatory shear (LAOS) flow: the suspension of rigid dumbbells in a Newtonian solvent. For such suspensions, the viscoelastic response of the polymeric liquid depends exclusively on the dynamics of dumbbell orientation. We have previously derived explicit analytical expressions for the shear rate amplitude and frequency dependences of the first and third harmonics of the alternating shear stress response in LAOS. Higher harmonics sculpt the shear stress, distorting it from its sinusoidal shape. In this work, we derive the polymer contribution to the shear stress response up to and including the next higher, fifth harmonic. For this, we use the fourth order term in the expansion of the orientation distribution to calculate the shear stress response. Our analysis employs the general method of Bird and [J Armstrong Chem Phys, 56, 3680 (1972)]. Our expression is the only one to have been derived from a molecular theory for a fifth harmonic. Our paper thus provides the first glimpse of the molecular origins of a shear stress harmonic higher than the third.
We developed a high-sensitive apparatus for oscillatory flow birefringence measurement in a co-cylindrical geometry. The apparatus was comprised of a conventional rheometer, an Argon ion laser, and a lock-in amplifier. The laser was irradiated to samples under sinusoidal strains, and transmitted light intensities were analyzed using the lock-in amplifier to evaluate flow birefringence. TTL signals, which were transformed from sinusoidal strains using a comparator, were used as reference signals for lock-on detection at low frequencies. First, phase shifts between the true strain and TTL reference signal generated by the comparator were calibrated. Next, reliability of the apparatus was assessed using a wormlike micellar solution, whose flow birefringence behavior are well known. Finally, sensitivity of the birefringence measurement was checked using a low birefringent cellulose nanofiber (CNF) dispersion. Flow birefringence behavior of the CNF dispersion became measurable thanks to a long optical pass length of the geometry. The apparatus will be a strong tool to reveal dynamics of low birefringent solutions.
We investigated the elongational flows of the weakly entangled linear polymer melt using a coarse- grained molecular dynamics simulation. We extended the uniform extensional flow (UEF) method developed by Nicholson and Rutledge (D. A.Nicholson and G. C. Rutledge, J. Chem. Phys., 145 244903 (2016)) for application to Langevin dynamics. We succeeded in observing the elongational viscosity of the weakly entangled linear polymer melt from the equilibrium state to the steady state using the extended UEF method, whereas the conventional rectangular parallelepiped shape technique for extensional flows has failed to do so for over 20 years.
The effects of residual solvent on the glass transition temperature (Tg) of poly(methyl methacrylate) (PMMA) were studied. A small amount of solvent in a film obtained by solution casting was found to remain even after vacuum evaporation for 30 h and compression-molding at 200 °C. The amount of residual solvent was less than 0.5 %, as revealed by gas chromatography. Therefore, this phenomenon is different from the plasticizing effect. A shift of the Tg to a lower temperature was attributed to strong ion-dipole interactions between the solvent molecules and the carbonyl group in PMMA molecules. Such interactions suppressed the associated states of PMMA, which greatly affected chain mobility.