In the present work, laminar flow of an incompressible thixotropic fluid is investigated numerically above a fixed semi-infinite plate. Assuming that the fluid of interest obeys Harris' thixotropic rheological model, vorticity/stream function formulation will be used to reduce equations of motion. The equations so obtained are then solved numerically using the finite difference technique. It is shown that for the Harris model to represent true thixotropic fluids (in a Lagrangian sense) the thixotropic parameter appearing in this fluid model should be negative. Numerical results obtained at a typical location (say, in the middle of the plate) show that for the viscosity to decrease by an increase in the thixotropic parameter of Harris model, this fluid property should be negative. Interestingly, an increase in the velocity gradient near the wall combined with a decrease in the viscosity of the fluid is predicted to give rise to an increase in the wall shear stress regardless of the fluid being thixotropic.
For a well-entangled polyisoprene (PI; molecular weight = 260k) equilibrated under pressurized carbon dioxide (CO2) at 25 °C, linear viscoelastic and dielectric data, respectively, were measured with a stress-controlled rheometer and a dielectric bridge being equipped with respective high-pressure cells. The viscoelastic and dielectric data shifted to higher frequencies with increasing CO2 pressure, indicating that the pressurized CO2 dissolved into PI thereby accelerating the global motion of PI. For those data at various CO2 pressure, time-CO2 pressure superposition held well and a single master curve was obtained, and the horizontal/vertical shift factors were consistent for the viscoelastic and dielectric data. These results indicated that the dissolved CO2 behaved just as an ordinal solvent to accelerate the global motion of PI as in ordinary solutions. In fact, the dynamic tube dilation (DTD) relationship between the viscoelastic and dielectric data, known to be valid for ordinary solutions/bulk of linear PI, was found to work also the PI/CO2 system, which confirmed the simple solvent role of the pressurized CO2 for the global motion of PI. Thus, the knowledge for ordinary polymer solutions would work for processing of polymer/CO2 systems.
In the present work, hydrodynamic instability of viscoplastic fluids is investigated in azimuthal pressure-driven flow between two fixed concentric cylindersthe so-called Dean flow. To avoid stress discontinuity anywhere in the gap, a regularized Bingham model was used to represent viscoplastic fluids. To determine conditions for the onset of instability, an infinitesimal disturbance was introduced to the basic flow and its evolution in time was checked using a normal-mode linear stability analysis. An eigenvalue problem was obtained which was solved numerically using the pseudo-spectral collocation method. A plot of the Dean number versus the Bingham number showed that the yield stress has always a stabilizing effect on the Dean flow. That is to say that, the critical Dean number is increased the higher the Bingham number. Results obtained at three different gap spacing suggest that by an increase in the gap size the flow becomes progressively more stable, but, only if the Bingham number is sufficiently low. At high enough Bingham numbers, however, it is predicted that by an increase in the gap size, Dean flow may become less stable.
Cellulose-graft-polyacrylonitriles were prepared by using KMnO4/different acids redox system and the obtained samples were characterized by POM, SEM, 13C-NMR, XRD, TGA and their mechanical properties were investigated as a function of crystallinity degree (Cr%). In all cases, Cr decreased with increase of graft yield irrespective of acids species. Stress at break, strain at break and Young's modulus decreased with decreasing Cr%, which can be attributed to the increase of the amorphous region and the main chain rapture of cellulose. The mechanical properties of the samples prepared by strong acids became more poor than those for samples prepared by weak acids, indicating that more chain rupture occurred when strong acids are used.
Surfactant drag reduction is well known as the energy saving technology which is able to cause drastic reduction of pumping power during the fluid transportation. The combination of certain type of cationic surfactant with suitable counter ion is often chosen as the drag reduction agents. Power saving up to 70∼80% is available in straight pipe line system by using surfactant additives. Design problem, in particular, the scale up law in surfactant drag reduction system was solved by using the modified viscoelastic damping factor model along with the measurements of nonlinear viscoelastic properties of surfactant solution. Based on the prediction of the proposed model, it was demonstrated that the surfactant drag reduction technology is promising to construct the large scale energy saving system for air conditioning. Up to now, over 150 sites of building air conditioning had adopted the surfactant drag reduction in Japan, and the practical pumping energy reduction had been reported as 20∼50% compared with the case of no drag-reducing agent.
Within the linear stimulus-response framework, dielectric responses of materials can be described in a dual way in terms of the complex dielectric constant ε*(ω) and complex dielectric modulus M*(ω) (= 1/ε*(ω)), both being defined as functions of angular frequency w. In a phenomenological sense, ε*(ω) and M*(ω) are completely equivalent to each other. In this sense, a characteristic dielectric relaxation time t can be defined for either quantity. However, in many cases, t directly reflecting a slow molecular process(es) underlying the dielectric relaxation is defined for ε*(ω) but not for M*(ω), which is similar to the situation for the viscoelastic modulus and compliance, G*(ω) and J*(ω), the former often serving as the fundamental quantity detecting the slow molecular process. Focusing on some examples, this article discusses the duality in the representation of the dielectric responses and the superiority of ε*(ω) compared to M*(ω).