Rheology of extremely shear thickening polymer dispersions^a) (passively viscosity switching fluids)

Abstract

SynopsisThe flow properties of concentrated polymer dispersions which exhibit extreme shear thickening have been investigated in steady and transient shearing flows using various types of rheometers. A phenomenological characterization based on few fluid parameters is proposed and pertinent test modes for their determination are discussed. The dispersions consist of monodisperse, electrostatically stabilized, solid spherical particles (diameter <0.5 /Lm) dispersed in glycols. They show a reversible steplike viscosity transition of nearly three powers of ten when an apparent critical shear rate 'Ye is exceeded. This jump from a low to a high viscosity state shows typical features of a shear-rate-induced phase transition, including hysteresis and the existence of a metastable low viscosity state above re. The viscosity step can be traced continuously by using stress controlled rheometers. Pronounced fluctuations of the rate of deformation with time are observed at the critical shear rate. The apparent critical shear rate depends on the rheometer geometry. Its change with the gap width in Couette flow and with the radius of circular dies can be interpreted by taking into account a true critical shear rate Yo and a wall slip velocity V S ' Periodic switching to the high viscosity state in oscillatory shear only occurs if both a critical rate amplitude and a critical shear amplitude are reached during the cycle. Applications of these dispersions for simple speed controlling or selectively damping mechanical elements are briefly introduced.

Viscoelastic properties of concentrated shear-thickening dispersions

“…Finally we can compare our results with those obtained by Laun (20). He also found Journal qfCotloid and Interface Science, Vol.…”

“…Most of his work was concerned with the steady shear or linear viscoelastic properties. In his most recent paper (20), however, he also deals with the nonlinear viscoelastic properties of concentrated shearthickening dispersions.…”

Fluid dynamics of dilatant fluids

“…In the present model, such an aspect of hardening is represented by the fact that the relaxational time scale for the state change is not constant but proportional to the shear rate (7). Then, the state variable φ reaches a steady value φ * in a time scale where the strain changes by the amount r. Consequently, the medium with φ M 1 can deform only up to a certain strain around r by a hard impact.…”

“…There are several peculiar features in the shear thickening of the dilatant fluid: (i) the thickening is so severe and instantaneous that it might be used even to make a body armor to stop a bullet [6], (ii) the relaxation after removal of the external stress occurs within a few seconds, that is quick but not as instantaneous as in the thickening process, (iii) the medium in the thickened state behaves like a rigid material allowing little elastic deformation as long as it is under stress, (iv) the viscosity shows hysteresis upon changing the shear rate [7], (v) noisy fluctuations have been observed in the response to an external shear stress in the thickening regime [7,8].…”

The rheology of shear thickening fluids with various ceramic particle additives

“…Fischer et al [24] demonstrated the potential for integrating STFs into structures exposed to dynamic flexural deformation by controlling their vibrational response. Laun et al [25] and Helber et al [26] investigated the damping behavior of STFs and suggested they could be used in damping and mounting of industrial machinery. STFs have also been studied in the applications of smart structures.…”