Abstract
An innovative experimental apparatus for the direct measurement of yield stress was conceived and realized. It is based on a torsion pendulum equipped with a magnetic dipole and a rotating cylinder immersed in the material to be investigated. The pendulum equilibrium state depends on the mechanical torque applied due to an external magnetic induction field, elastic reaction of the suspension wire, and shear yield stress. Experimental results are reported showing that the behavior of the pendulum rotation angle, in different equilibrium conditions, provides evidence of the yield stress presence and enables its evaluation by equilibrium equations. The dependence on time of the equilibrium approach was also studied, contributing to shed light on the relaxation effect in the transition from a fluid-like to solid-like behavior, as well as on the eventual thixotropic effects in non-Newtonian fluids. The validity of the proposed technique and related experimental apparatus was tested in aqueous Carbopol solutions, with different weight percentages. The linear procedure, combined with the effectiveness and reliability of the proposed experimental method, candidates it to be used for the study of peculiar behaviors of other yield stress complex fluid such as blood, crude waxy oils, ice slurries, and coating layer used in the food industry and also for fault sliding in geodynamics.
Highlights
The study of yield stress fluids and, in particular, the questions related to their transition from solid-like behavior in static condition to liquid-like behavior in viscoelastic kineticVincenzo Iannotti and Luca Lanotte contributed to this work.Generally, in experiments that probe the transition from liquid to solid, the dynamic yield stressτyd is measured according to the definition τ yd 1⁄4 limγ→̇ 0 τ γ, namely, it is the shear stress measured in steady-state shear flow in the limit where the deformation rate goes to zero, so that the extrapolation of the flow curves (τ vs γ ) towards the vanishing shear rate is used
Afterwards, the pendulum was left under the sole action of the elastic moment recall and the rotation angle has been acquired while the static equilibrium conditions were restored
Since the rotation starts at the lowest applicable current, the mechanical moment, due to the internal friction of the pendulum, is negligible to inhibit rotation, in agreement with what was observed about the free oscillations in the “Temporal trends to spontaneously re-establish conditions of static equilibrium starting from flow condition: measurement of dynamic yield stress” section
Summary
The study of yield stress fluids and, in particular, the questions related to their transition from solid-like behavior in static condition to liquid-like behavior in viscoelastic kinetic. The exact knowledge of the applied magnetic torque and the induced rotation of the equipment in the fluid allow the identification of the relaxation time from a nonequilibrium flow condition to a state of static equilibrium, as well as the measurement of both static and dynamic yield stress. Considering the curves of the elastic modulus G′ and viscous modulus G′′ as functions of shear stress at a fixed frequency of 1 Hz, the value of stress corresponding to the initial drop of G (5% of the difference from the plateau value) could be taken as static yield stress This point represents the onset of the nonlinear region where the fluid is not behaving anymore as an elastic solid and flows like a liquid (Mezger 2006; Shih et al 1999). Bμ cosθe−Kθe−M f Sd ð3Þ from which the yield stress value can be calculated by measuring the equilibrium angle θe, being known all the other parameters on the right side of the equation
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