Volume 28, Issue 5

Polytetrafluoroethylene (PTFE) micropowders were used to prepare PTFE-oil organogel. Carbonyl iron particles were dispersed in this organogel to fabricate magnetorheological fluids. The main aim of this paper is to investigate the influences on field-induced chains by non-magnetic microstructures inside these organogel-based MRFs. The field-induced anisotropy of MRFs between first normal stress differences to shear stresses and dynamic modulus reveal that organogel widen the non-Newtonian elastic range of MRFs as long as the non-magnetic particles content is appropriate. Both theoretical and experimental results of static and dynamic normal forces indicate that normal forces are influenced by magnetic and non-magnetic microstructures comprehensively. The dynamic normal forces are lower than static ones under low magnetic field strength while the phenomenon becomes opposite in the presence of high strength magnetic field. Moreover, a 3ITT test of normal forces exhibits a significant time-dependent behavior of the normal forces. The investigations of recovery ratio demonstrate that the non-magnetic microstructures help field-induced chains to recover at relatively low magnetic field strength but hinder this structural recovery at high magnetic field strength. Finally, a new investigation method on amplitude-dependent normal forces is introduced showing a four-region behavior as a function of strain amplitude, which reflects the internal microscopic evolution of MRFs and could be a proper way to study the influences on field-induced chains by non-magnetic particles.

In the last decades, one of the most critical issues concerning the control on the processing, structure and properties of nanocomposites is related to the dispersion of nanofiller in the polymer matrix and internal interactions resulting in percolation. In this study, we investigate the rheological behavior in oscillatory and steady shear flow of poly(lactic) acid based nanocomposites incorporating 0 – 12 wt% graphene nanoplates (GNP) and multi-walled carbon nanotubes (OH-MWCNT). The effect of the filler contents and aspect ratio on the viscosity and viscoelastic response is evaluated. Three rheological techniques are used for estimation of rheological percolation threshold. Due to different aspect ratio and state of dispersion of GNP and MWCNTs the percolation threshold differs significantly for both compositions ϕ p ≤ 1.5 wt% for MWCNT/PLA and ϕ p ≤ 5 wt% for GNP/PLA. The larger the aspect ratio of nanofiller, the lower is the rheological percolation threshold. The visualized structure by TEM analysis confirms the rheological predictions for both type composites. The index of flow was estimated by the power law slope of the flow curves and a better dispersion was assumed for MWCNTs in comparison with GNPs due to the surface modification. Based on the rheological percolation threshold and the flow index, nanocomposites were classified in three groups: Newtonian, percolated composites and elastic solids. Both characteristics are used to select the printing parameters for the three groups of nanocomposites, suitable for fused deposition modeling (FDM).

An approach of Nelson and Ewoldt [1] to create a viscosity model of the Herschel-Bulkley type in order to use only parameters with the potential of containing fluid information has been extended to be applied to drilling fluids using current industry standard procedures. The commonly used Herschel-Bulkley consistency parameter k is found inadequate in describing fluid properties properly as it has a unit dependent on n . Hence, the model is not optimum for digitalisation. The Herschel-Bulkley model is re-written and base its parameters directly on the yield stress and the additional or surplus shear stress at a pre-determined shear rate relevant for the flow situation to be considered. This approach is also applicable for Power-Law models.

In this research, a fully developed steady flow of a third-grade fluid in a pipe under an externally applied magnetic field with convection on wall is investigated. The governing equations including momentum and energy in the form of partial differential equations are reduced to ordinary differential equations which are solved numerically by using a finite element method (FEM) as part of the FlexPDE software package. For validity, the results are compared with the 4 th order Runge-Kutta method. The effect of different physical parameters such as the non-Newtonian parameter, the Biot number, the Hartmann number, the Eckert number on the dimensionless velocity profiles, the dimensionless velocity gradient profiles, the dimensionless temperature profiles, and the dimensionless gradient temperature profiles have been discussed. It is concluded that by increasing the non-Newtonian parameter and Hartman number the dimensionless velocity, the velocity gradient, the temperature and temperature gradient profiles reduce and thus the heat transfer of fluid flow, the shear stress and the skin friction on the pipe wall decrease. Increasing the Biot number caused a decrease of the temperature and a more uniform dimensionless temperature profile of the fluid within the pipe. Besides, with a decrease of the Prandtl number, the dimensionless temperature decreases inside the pipe. In fact, the dimensionless temperature profile becomes flat. For this reason, the dimensionless temperature gradient decreases on the pipe wall which causes the reduction of the heat transfer rate on the pipe wall. Further, by increasing the Eckert number, the dimensionless temperature of the fluid within the pipe and the heat transfer from the fluid to the pipe wall increases. Applying the FlexPDE software for solving governing equations numerically seems to lead to appropriate and reasonable results.