1,477 research outputs found
Stirring by swimmers in confined microenvironments
We consider the tracer diffusion that arises from the run-and-tumble
motion of low Reynolds number swimmers, such as bacteria. In unbounded dilute
suspensions, where the dipole swimmers move in uncorrelated runs of length
, an exact solution showed that is independent of .
Here we verify this result in numerical simulations for a particular model
swimmer, the spherical squirmer. We also note that in confined
microenvironments, such as microscopic droplets, microfluidic devices and
bacterial microzones in marine ecosystems, the size of the system can be
comparable to . We show that this effect alone reduces the value of
in comparison to its bulk value, and predict a scaling form for its
relative decrease.Comment: submitted to JSTA
Fluid mixing by curved trajectories of microswimmers
We consider the tracer diffusion that arises from the run-and-tumble
motion of low Reynolds number swimmers, such as bacteria. Assuming a dilute
suspension, where the bacteria move in uncorrelated runs of length ,
we obtain an exact expression for for dipolar swimmers in three
dimensions, hence explaining the surprising result that this is independent of
. We compare to the contribution to tracer diffusion from
entrainment.Comment: 5 pages, 2 figure
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Quantitative analysis of hypertrophic myocardium using diffusion tensor magnetic resonance imaging
Systemic hypertension is a causative factor in left ventricular hypertrophy (LVH). This study is motivated by the potential to reverse or manage the dysfunction associated with structural remodeling of the myocardium in this pathology. Using diffusion tensor magnetic resonance imaging, we present an analysis of myocardial fiber and laminar sheet orientation in ex vivo hypertrophic (6 SHR) and normal (5 WKY) rat hearts using the covariance of the diffusion tensor. First, an atlas of normal cardiac microstructure was formed using the WKY b0 images. Then, the SHR and WKY b0 hearts were registered to the atlas. The acquired deformation fields were applied to the SHR and WKY heart tensor fields followed by the preservation of principal direction (PPD) reorientation strategy. A mean tensor field was then formed from the registered WKY tensor images. Calculating the covariance of the registered tensor images about this mean for each heart, the hypertrophic myocardium exhibited significantly increased myocardial fiber derangement (p ¼ 0.017) with a mean dispersion of 38.7 deg, and an increased dispersion of the laminar sheet normal (p = 0.030) of 54.8 deg compared with 34.8 deg and 51.8 deg, respectively, in the normal hearts. Results demonstrate significantly altered myocardial fiber and laminar sheet structure in rats with hypertensive LVH
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