Biomedical subjects

M P Lettinga

Publications and source records attributed to M P Lettinga.

7 recordsLinked to original sources

PubMed2006

Multiple shear-banding transitions in a supramolecular polymer solution.

We report on the nonlinear rheology of a reversible supramolecular polymer based on hydrogen bonding. The coupling between the flow-induced chain alignment and breakage and recombination of bonds between monomers leads to a very unusual flow behavior. Measured velocity profiles indicate three different shear-banding regimes upon increasing shear rate, each with different characteristics. While the first of these regimes has features of a mechanical instability, the second shear-banding regime is related to a shear-induced phase separation and the appearance of birefringent textures. The shear-induced phase itself becomes unstable at very high shear rates, giving rise to a third banding regime.

Biophysics↗

PubMed2006

Vorticity banding in rodlike virus suspensions.

Vorticity banding under steady shear flow is observed in a suspension of semiflexible colloidal rods (fd virus particles) within a part of the paranematic-nematic biphasic region. Banding occurs uniformly throughout the cell gap within a shear-rate interval (.gamma-, .gamma+) , which depends on the fd concentration. For shear rates below the lower-border shear rate .gamma- only shear elongation of inhomogeneities, which are formed due to paranematic-nematic phase separation, is observed. Within a small region just above the upper-border shear rate .gamma+ , banding occurs heterogeneously. An essential difference in the kinetics of vorticity banding is observed, depending on the morphology of inhomogeneities formed during the initial stages of the paranematic-nematic phase separation. Particle tracking and polarization experiments indicate that the vorticity bands are in a weak rolling flow, superimposed on the applied shear flow. We propose a mechanism for the origin of the banding instability and the transient stability of the banded states. This mechanism is related to the normal stresses generated by inhomogeneities formed due to the underlying paranematic-nematic phase transition.

Journal Article↗

PubMed2005

Crystallization kinetics of colloidal spheres under stationary shear flow.

A systematic experimental study of dispersions of charged colloidal spheres is presented on the effect of steady shear flow on nucleation and crystal growth rates. In addition, the nonequilibrium phase diagram as it relates to the melting line is measured. Shear flow is found to strongly affect induction times, crystal growth rates, and the location of the melting line. The main findings are that (1) the crystal growth rate for a given concentration exhibits a maximum as a function of the shear rate; (2) contrary to the monotonic increase in the growth rate with increasing concentration in the absence of flow, a maximum of the crystal growth rate as a function of concentration is observed for sheared systems; and (3) the induction time for a given concentration exhibits a maximum as a function of the shear rate. These findings are partly explained on a qualitative level.

Journal Article↗

PubMed2005

Diffusion of spheres in crowded suspensions of rods.

Translational tracer diffusion of spherical macromolecules in crowded suspensions of rodlike colloids is investigated. Experiments are done using several kinds of spherical tracers in fd-virus suspensions. A wide range of size ratios L/2a of the length L of the rods and the diameter 2a of the tracer sphere is covered by combining several experimental methods: fluorescence correlation spectroscopy for small tracer spheres, dynamic light scattering for intermediate sized spheres, and video microscopy for large spheres. Fluorescence correlation spectroscopy is shown to measure long-time diffusion only for relatively small tracer spheres. Scaling of diffusion coefficients with a/xi, predicted for static networks, is not found for our dynamical network of rods (with xi the mesh size of the network). Self-diffusion of tracer spheres in the dynamical network of freely suspended rods is thus fundamentally different as compared to cross-linked networks. A theory is developed for the rod-concentration dependence of the translational diffusion coefficient at low rod concentrations for freely suspended rods. The proposed theory is based on a variational solution of the appropriate Smoluchowski equation without hydrodynamic interactions. The theory can, in principle, be further developed to describe diffusion through dynamical networks at higher rod concentrations with the inclusion of hydrodynamic interactions. Quantitative agreement with the experiments is found for large tracer spheres, and qualitative agreement for smaller spheres. This is probably due to the increasing importance of hydrodynamic interactions as compared to direct interactions as the size of the tracer sphere decreases.

Journal Article↗

PubMed2004

Microstructural response of a near-critical colloid-polymer mixture to shear flow.

Long-ranged microstructural order of a near-critical colloid-polymer mixture under stationary and oscillatory shear flow is studied by means of time-resolved small-angle light scattering. The distance from the critical point, the shear rate, and the frequency of oscillation are systematically varied. Unexpected shear-induced distortions of critical microstructural order is observed in directions perpendicular to the flow direction, more so on closer approach of the gas-liquid critical point. The measured distortion of long-ranged order can be quantitatively understood on the basis of an approximate solution of the Smoluchowski equation, provided that a nonanalytical distortion of short-ranged microstructural order is assumed. These short-ranged microstructural changes, induced by the flow, account for the observed distortion perpendicular to the flow direction, and are responsible for the shear-induced shift of the location of the gas-liquid critical point. The importance of short-ranged correlations renders the behavior of critical systems under shear flow nonuniversal. The origin of the nonanalytic dependence of short-ranged distortions on the shear rate remains unclear. Including these short-ranged microstructural distortions, we find a good agreement between theory and experiment for both stationary and oscillatory shear flow.

Journal Article↗

PubMed2004

Rotational dynamics of colloidal spheres probed with fluorescence recovery after photobleaching.

We report a polarized fluorescence recovery after photobleaching (pFRAP) method to measure the rotational dynamics of fluorescent colloids over a wide dynamic range. The method is based on the polarization anisotropy in the fluorescence intensity, generated by bleaching of fluorescently labeled particles with an intense pulse of linearly polarized laser light. The rotational mobilities of the fluorescent particles can be extracted from the relaxation kinetics of the postbleach fluorescence polarization anisotropy. Our pFRAP setup has access to correlation times over a range of time scales from tens of microseconds to tens of seconds, and is highly sensitive, so very low concentrations of labeled particles can be probed. We present a detailed description of the theoretical background of pFRAP. The performance of the equipment is demonstrated for fluorescent colloidal silica spheres, dispersed in pure solvents as well as in fd-virus suspensions.

Biomechanical Phenomena↗

PubMed2001

Rotational tracer diffusion in binary colloidal sphere mixtures.

We demonstrate that tracer/host size asymmetry and electrostatic interactions strongly affect rotational self-diffusion in binary mixtures of charged colloidal tracer and host spheres. Tracer diffusion coefficients, measured with time-resolved phosphorescence anisotropy, are compared with calculations of rotational diffusion including two- and three-particle hydrodynamic interactions. We also show that the inverse dependence of the rotational diffusion coefficient on the suspension viscosity is approached only at large size ratios.

Journal Article↗