Biomedical subjects

A G Hoekstra

Publications and source records attributed to A G Hoekstra.

6 recordsLinked to original sources

PubMed2006

Mesoscopic simulations of systolic flow in the human abdominal aorta.

The complex nature of blood flow in the human arterial system is still gaining more attention, as it has become clear that cardiovascular diseases localize in regions of complex geometry and complex flow fields. In this article, we demonstrate that the lattice Boltzmann method can serve as a mesoscopic computational hemodynamic solver. We argue that it may have benefits over the traditional Navier-Stokes techniques. The accuracy of the method is tested by studying time-dependent systolic flow in a 3D straight rigid tube at typical hemodynamic Reynolds and Womersley numbers as an unsteady flow benchmark. Simulation results of steady and unsteady flow in a model of the human aortic bifurcation reconstructed from magnetic resonance angiography, are presented as a typical hemodynamic application.

Aorta, Abdominal↗

PubMed2002

Influence of stagnant zones on transient and asymptotic dispersion in macroscopically homogeneous porous media.

The role of stagnant zones in hydrodynamic dispersion is studied for creeping flow through a fixed bed of spherical permeable particles, covering several orders of characteristic time and length scales associated with fluid transport. Numerical simulations employ a hierarchical model to cope with the different temporal and spatial scales, showing good agreement with our experimental results on diffusion-limited mass transfer, transient, and asymptotic longitudinal dispersion. These data demonstrate that intraparticle liquid holdup in macroscopically homogeneous porous media clearly dominates over contributions caused by the intrinsic flow field heterogeneity and boundary-layer mass transfer.

Journal Article↗

PubMed2001

Radiation forces in the discrete-dipole approximation.

The theory of the discrete-dipole approximation (DDA) for light scattering is extended to allow for the calculation of radiation forces on each dipole in the DDA model. Starting with the theory of Draine and Weingartner [Astrophys. J. 470, 551 (1996)] we derive an expression for the radiation force on each dipole. These expressions are reformulated into discrete convolutions, allowing for an efficient, O(N logN) evaluation of the forces. The total radiation pressure on the particle is obtained by summation of the individual forces. The theory is tested on spherical particles. The resulting accumulated radiation forces are compared with Mie theory. The accuracy is within the order of a few percent, i.e., comparable with that obtained for extinction cross sections calculated with the DDA.

Journal Article↗

PubMed1994

Lissajous-like patterns in scatter plots of calibration beads.

Flow cytometric measurements of light scattering of polystyrene calibration beads revealed remarkable Lissajous-like loops in two-parameter scatter plots. The existence of such loops is shown to be in qualitative agreement with Lorenz-Mie scattering theory of homogeneous spheres. The occurrence of these patterns reflects the extreme particle size dependency of perpendicular light scattering of homogeneous spheres. These effects may give an explanation for the frequently observed phenomenon that polystyrene spheres reveal a relatively large CV in the perpendicular light scattering signals, whereas the CV of the forward light scattering signal is small. We conclude that one should be careful to optimize the perpendicular light scattering channel by minimizing the CV, because there is no linear relationship between instrument alignment quality and the CV of the perpendicular light scattering signals of calibration beads.

Calibration↗

PubMed1991

Effect of aniosmotic media on the volume of the T-lymphocyte nucleus.

Time-resolved measurements of the nuclear volume response of human peripheral T-lymphocytes, under aniosmotic conditions, are presented. In the experiments slit scanning FlowCytometry methods were used. We propose an extension to the standard solid viscoelastic model to interpret the observed dynamical behavior of the nucleus. It is shown that experimental and theoretical evidence indicates a passive nuclear response merely induced by a mechanical link (i.e., the cytoskeleton) between the cell membrane and the nuclear envelope. Implications of this work to the field of cellular mechanics and cytoskeletal rheology are surveyed.

Cell Nucleus↗

PubMed1988

Osmotic response of lymphocytes measured by means of forward light scattering: theoretical considerations.

Recent data provide solid experimental evidence that increase in cell size of human lymphocytes, caused by osmotic stress, is not directly proportional to forward scattering intensity but rather is inversely proportional. Here, we provide arguments that this phenomenon can be quantitatively explained by means of a model based on a modified Rayleigh-Debye-Gans theory. As a consequence, the current view that forward scattering intensities can be used as a measure of gross cell size needs to be reconsidered. In addition, we postulate that structural changes of the cytoplasm, imposed by an osmotic stress, reflect corresponding changes in the nucleus.

Flow Cytometry↗