Efrén Andablo-Reyes

Dynamic rheology of sphere- and rod-based magnetorheological fluids

The effect of particle shape in the small amplitude oscillatory shear behavior of magnetorheological (MR) fluids is investigated from zero magnetic field strengths up to 800 kA/m. Two types of MR fluids are studied: the first system is prepared with spherical particles and a second system is prepared with rodlike particles. Both types of particles are fabricated following practically the same precipitation technique and have the same intrinsic magnetic and crystallographic properties. Furthermore, the distribution of sphere diameters is very similar to that of rod thicknesses. Rod-based MR fluids show an enhanced MR performance under oscillatory shear in the viscoelastic linear regime. A lower magnetic field strength is needed for the structuration of the colloid and, once saturation is fully achieved, a larger storage modulus is observed. Existing sphere- and rod-based models usually underestimate experimental results regarding the magnetic field strength and particle volume fraction dependences of both storage modulus and yield stress. A simple model is proposed here to explain the behavior of microrod-based MR fluids at low, medium and saturating magnetic fields in the viscoelastic linear regime in terms of magnetic interaction forces between particles. These results are further completed with rheomicroscopic and dynamic yield stress observations.

Squeeze flow magnetorheology

This paper is concerned with an investigation of the rheological performance of magnetorheological fluids under squeeze flow. Preliminary results on Newtonian fluids are first compared to Stefan’s equation. Then, unidirectional monotonic compression tests are carried out in the presence of uniaxial external magnetic fields at slow compression rates under constant volume operation. Results are compared to Bingham plastic, biviscous, and single chain micromechanical squeeze flow models. Measurements using combined deformation modes (compression+small-strain oscillatory shear) suggest a compression-induced shear strengthen effect up to strains of ∼0.5. Particle-level dynamic simulations are in qualitatively good agreement with experimental observations.

Controlling friction using magnetic nanofluids

We study the thin-film rheological performance of superparamagnetic nanofluids in full film hydrodynamic regime under the presence of external magnetic field gradients. Ferrohydrodynamic Reynolds equation is numerically solved and compared to results obtained from tribo-rheological experiments in model face seals. A theoretical and experimental friction master curve is proposed that satisfactorily captures the lubrication behavior including hydrodynamics and magnetostatics in the problem without any fitting parameter.

On the nonparallelism effect in thin film plate–plate rheometry

The effect of nonparallelism in a plate–plate torsional geometry is investigated for confined Newtonian fluids by directly solving Reynolds equation. A nondimensionalization is proposed, and theoretical results are compared to triborheological experiments by Kavehpour and McKinley [Tribol. Lett. 17, 327–335 (2004)] in the form of a frictional Stribeck curve.

Stress relaxation in the nonequilibrium state of a polymer melt

The influence of entanglement density on the constraint renewal time is studied experimentally in transitory nonequilibrium polymer melts. The entanglement density, as quantified by the rubber elasticity, increases as the linear polymer melt transforms into the equilibrium state. The relaxation modulus obtained from linear step-strain deformations, performed at different points during the equilibration, shows an increase in constraint renewal time as the entanglement density increases. The normalized relaxation modulus curves collapse onto a single curve by rescaling the time axis with a factor G′¯(t)0.9 (where G′¯(t) is the normalized instantaneous modulus). These findings suggest that, though the relaxation time increases with the increasing number of entanglements, the mechanism responsible for stress relaxation, after application of step-strain, is similar to that in a fully entangled melt.

A study on the stability of the stress response of nonequilibrium ultrahigh molecular weight polyethylene melts during oscillatory shear flow

Dynamic shear flow and especially large-amplitude oscillatory shear have been a subject of interest to investigate limitations of theoretical approximations working under the assumption of simple shear. These studies have helped in understanding the kinematics involved in phenomena such as stick-slip and shear banding, among others. The nonequilibrium polymer melt, which transforms with time into the equilibrium state, provides a unique opportunity to investigate the influence of the thermodynamic melt state on the validity of simple shear approximations. Here, we show that during oscillatory deformation, the nonlinear rheological response of ultrahigh molecular weight polyethylene melts, having number-average molecular weight greater than one million g/mol, is strongly dependent on the entangled state of the same polymer. At sufficiently large strain amplitude, the stress response of the material departs from a periodic sinusoidal signal, with maximum stress decaying with consecutive cycles of deformatio...