Diatoms are non-motile, unicellular phytoplankton that have the ability to form colonies in the form of chains. Depending upon the species of diatoms and the linking structures that hold the cells together, these chains can be quite stiff or very flexible. Recently, the bending rigidities of some species of diatom chains have been quantified. In an effort to understand the role of flexibility in nutrient uptake and aggregate formation, we begin by developing a three-dimensional model of the coupled elastic -hydrodynamic system of a diatom chain moving in an incompressible fluid. We find that simple beam theory does a good job of describing diatom chain deformation in a parabolic flow when its ends are tethered, but does not tell the whole story of chain deformations when they are subjected to compressive stresses in shear. While motivated by the fluid dynamics of diatom chains, our computational model of semiflexible fibres illustrates features that apply widely to other systems. The use of an adaptive immersed boundary framework allows us to capture complicated buckling and recovery dynamics of long, semiflexible fibres in shear.

Choanoflagellates, eukaryotes that are important predators on bacteria in aquatic ecosystems, are closely related to animals and are used as a model system to study the evolution of animals from protozoan ancestors. The choanoflagellate Salpingoeca rosetta has a complex life cycle with different morphotypes, some unicellular and some multicellular. Here we use computational fluid dynamics to study the hydrodynamics of swimming and feeding by different unicellular stages of S. rosetta : a swimming cell with a collar of prey-capturing microvilli surrounding a single flagellum, a thecate cell attached to a surface and a dispersal-stage cell with a slender body, long flagellum and short collar. We show that a longer flagellum increases swimming speed, longer microvilli reduce speed and cell shape only affects speed when the collar is very short. The flux of prey-carrying water into the collar capture zone is greater for swimming than sessile cells, but this advantage decreases with collar size. Stalk length has little effect on flux for sessile cells. We show that ignoring the collar, as earlier models have done, overestimates flux and greatly overestimates the benefit to feeding performance of swimming versus being attached, and of a longer stalk for attached cells.

No abstract

No abstract

The application of Mg as a potential hydrogen storage material has been hindered due to the slow absorption rate of hydrogen in Mg films. Herein, we explore the hydrogenation process theoretically using DFT calculations, and compare the energy barriers for hydrogen diffusion through Pd nanoparticle/Mg film interfaces and their variations, i.e. Pd(H)/Mg(O).Decomposing the mechanism into basic steps, we show that Pd undergoes a strain-induced crystallographic phase transformation near the interface, and indicate that hydrogen saturation of Pd nanoparticles enhances their efficiency as nanoportals. Using energetic arguments, we explain why hydrogen diffusion is practically prohibited through native Mg oxide, and seriously suppressed through existing hydride domains. We experimentally investigate hydrogen flux through the nanoportals in Pd nanoparticle-decorated Mg films by PCT hydrogenation measurements, and demonstrate the effect of the surface oxide layer and film thickness on hydrogenation kinetics. We theoretically calculate an r~t 1/3 relationship for the radial growth of hemispherical hydride domains, and confirm this relationship by AFM. The diffusion constant of hydrogen in Mg films is estimated as D H film ≈8×10 -18 m 2 s -1 , based on TEM characterization. Our unique nanoportal configuration allows direct measurement of hydride domain sizes, thus forming a model system for the experimental investigation of the hydrogenation process in any material.