Biomedical subjects

J K Dhont

Publications and source records attributed to J K Dhont.

3 recordsLinked to original sources

PubMed2001

Flow dichroism in critical colloidal fluids.

Due to long-range correlations and slow dynamics of concentration fluctuations in the vicinity of the gas-liquid critical point, shear flow is very effective in distorting the microstructure of near-critical fluids. The anisotropic nature of the shear-field renders the microstructure highly anisotropic, leading to dichroism. Experiments on the dichroic behavior can thus be used to test theoretical predictions on microstructural order under shear flow conditions. We performed both static and dynamic dichroism and turbidity measurements on a colloid-polymer mixture, existing of silica spheres (radius 51 nm) and polydimethylsiloxane polymer (molar weight 204 kg/mol). Sufficiently far away from the critical point, in the mean-field region, the experimental data are in good agreement with theory. Very close to the critical point, beyond mean field, for which no theory exists yet, an unexpected decrease of dichroism on approach of the critical point is observed. Moreover, we do not observe critical slowing down of shear-induced dichroism, right up to the critical point, in contrast to the turbidity.

Journal Article↗

PubMed2001

Superposition rheology.

The interpretation of superposition rheology data is still a matter of debate due to lack of understanding of viscoelastic superposition response on a microscopic level. So far, only phenomenological approaches have been described, which do not capture the shear induced microstructural deformation, which is responsible for the viscoelastic behavior to the superimposed flow. Experimentally there are indications that there is a fundamental difference between the viscoelastic response to an orthogonally and a parallel superimposed shear flow. We present theoretical predictions, based on microscopic considerations, for both orthogonal and parallel viscoelastic response functions for a colloidal system of attractive particles near their gas-liquid critical point. These predictions extend to values of the stationary shear rate where the system is nonlinearly perturbed, and are based on considerations on the colloidal particle level. The difference in response to orthogonal and parallel superimposed shear flow can be understood entirely in terms of microstructural distortion, where the anisotropy of the microstructure under shear flow conditions is essential. In accordance with experimental observations we find pronounced negative values for response functions in case of parallel superposition for an intermediate range of frequencies, provided that microstructure is nonlinearly perturbed by the stationary shear component. For the critical colloidal systems considered here, the Kramers-Kronig relations for the superimposed response functions are found to be valid. It is argued, however, that the Kramers-Kronig relations may be violated for systems where the stationary shear flow induces a considerable amount of new microstructure.

Journal Article↗

PubMed1999

A constitutive relation describing the shear-banding transition.

An additional contribution to the standard expression for the shear stress must be considered in order to describe shear banding. A possible extension of the standard constitutive relation is proposed. Its physical, purely hydrodynamic origin is discussed. The corresponding Navier-Stokes equation is analyzed for the two-plate geometry, where flow gradients are assumed to exist only in the direction perpendicular to the two plates. The linearized Navier-Stokes equation is shown to be very similar to the Cahn-Hilliard equation for spinodal decomposition, with a similar term that stabilizes rapid spatial variations. Only slowly varying flow gradients are unstable. Just as in the initial stage of spinodal decomposition there is a most rapidly growing wavelength in the initial stage of the shear-banding transition, leading to a predictable number of shear bands. A modified Maxwell equal area construction is derived, which dictates the stress and the shear rates in the bands under controlled shear conditions, and which shows that under controlled stress conditions no true shear bands can coexist. The kinetics of the shear-banding transition is studied numerically. For the two-plate geometry it is found that there exist multiple stationary states under controlled shear conditions, depending on the initial state of the flow profile. Shear banding occurs not only when the system is initially unstable, but can also be induced outside the unstable region when the amplitude of the initial perturbation is large enough. The shear-banding transition can thus proceed via "spinodal demixing" (from an unstable initial state) or via "condensation." Under controlled stress conditions no stationary state is found. Here, coupling with flow gradients extending in other directions, not perpendicular to the two plates, should probably be taken into account.

Journal Article↗