Browsing by Subject "drag"

Now showing 1 - 3 of 3

Cargo Transport by Cytoplasmic Dynein Can Center Embryonic Centrosomes
(PLOS One, 2013-07-01) Longoria, Rafael A.; Shubeita, George
To complete meiosis II in animal cells, the male DNA material needs to meet the female DNA material contained in the female pronucleus at the egg center, but it is not known how the male pronucleus, deposited by the sperm at the periphery of the cell, finds the cell center in large eggs. Pronucleus centering is an active process that appears to involve microtubules and molecular motors. For small and medium-sized cells, the force required to move the centrosome can arise from either microtubule pushing on the cortex, or cortically-attached dynein pulling on microtubules. However, in large cells, such as the fertilized Xenopus laevis embryo, where microtubules are too long to support pushing forces or they do not reach all boundaries before centrosome centering begins, a different force generating mechanism must exist. Here, we present a centrosome positioning model in which the cytosolic drag experienced by cargoes hauled by cytoplasmic dynein on the sperm aster microtubules can move the centrosome towards the cell’s center. We find that small, fast cargoes (diameter ~100 nm, cargo velocity ~2 µm/s) are sufficient to move the centrosome in the geometry of the Xenopus laevis embryo within the experimentally observed length and time scales.
The Evolution Of Sediment Acoustic Models
(2012-11) Chotiros, N. P.; Isakson, M. J.; Chotiros, Nicholas P.; Isakson, Marcia J.
Sediment acoustic models contain two connected components, the geo-physical description of the sediment and the model of acoustic processes. Geo-physical descriptors are used in the classification of sediments, and they are based on grain size, density and other physical descriptors. Acoustic sediment models were initially fluid approximations that were simple to implement. As the need for accuracy increased, the fluid model was extended to stratified fluid and visco-elastic models. The latter, with five frequency-independent parameters, appeared to be consistent with sediment acoustic data up to the 1980s. More recent experimental data have revealed discrepancies in the frequency-dependence of attenuation and sound speed, particularly in the case of sandy sediments, which cover a large fraction of the continental shelves. Broad-band acoustic measurements of wave speeds and attenuations are more consistent with a poro-elastic model, consisting of Biot's theory with extensions to account for the physics of granular media. Aside from terminology, there is a fundamental difference between viscoelastic and poro-elastic theories. The former is based on two types of waves, a compressional wave and a shear wave, while the latter has an additional compressional wave, often called the Biot wave. There are currently two approaches to the development of sediment acoustic models: (a) visco-elastic models with frequency dependent parameters that mimic the observed behavior, and (b) poro-elastic models that reflect the physical processes. It is shown that (a) would be a significant improvement over existing models, but (b) is the preferred solution.
The Influence of Secondary Flows on Drag Forces
(University of Texas at Austin, 1965-07) Dalton, C.; Masch, F.D.