205,411 research outputs found
Small asymmetric Brownian objects self-align in nanofluidic channels
No full textAlthough the self-alignment of asymmetric macro-sized objects of a few tens of microns in size have been studied extensively in experiments and theory, access to much smaller length scales is still hindered by technical challenges. We combine molecular dynamics and stochastic rotation dynamics techniques to investigate the self-orientation phenomenon at different length scales, ranging from the micron to the nano scale by progressively increasing the relative strength of diffusion over convection. To this end, we model an asymmetric dumbbell particle in Hele-Shaw flow and explore a wide range of Péclet numbers (Pe) and different particle shapes, as characterized by the size ratio of the two dumbbell spheres (R). By independently varying these two parameters we analyse the process of self-orientation and characterize the alignment of the dumbbell with the direction of the fluid flow. We identify three different regimes of strong, weak and no alignment and we map out a state diagram in Pe versus R plane. Based on these results, we estimate dimensional length scales and flow rates for which these findings would be applicable in experiments. Finally, we find that the characteristic reorientation time of the dumbbell is a monotonically decreasing function of the dumbbell anisotropy.Accepted Author ManuscriptComplex Fluid Processin
Photooxidation of Particulate Organic Matter, Carbon/oxygen Stoichiometry, and Related Photoreactions
No full textWe investigated photoredox transformations of oxygen, carbon, peroxides, and iron that accompany “photodissolution” of suspended marine particulate organic carbon (POC), a sunlight-induced process that transfers POC to the dissolved organic carbon (DOC) pool. During 18- to 24-hour photodissolution experiments with POC of varying composition, about 0.28 mol of O2 was consumed per mole POC photodissolved. Mean dissolved inorganic carbon (DIC) production was 6% of initial POC in suspended river delta sediments and 1% in algal membrane detritus. The mean O2 loss:DIC production ratio was − 1.3:1 in sediment suspensions, which slightly exceeds the typical range reported for DOC. The O2 loss:DIC production ratio was − 7.7:1 in suspensions of algal detritus, which implies significant oxygen incorporation into (oxygenation of) organic matter. Irradiated sediment suspensions rapidly achieved low, steady-state peroxide concentrations but rose more slowly with algal detritus. Elevated iron concentrations in the 0.7–8.0 μm particle size fraction after 24 h of irradiation are consistent with photoredox cycling of metals and/or with physical disintegration of organic-mineral aggregates driven by organic matter dissolution. These oxidation and oxygenation results differ from analogous reactions previously found for marine DOC, and estimates of DIC production in particle-rich environments will require incorporation of POC-specific information
Photodissolution and Other Photochemical Changes upon Irradiation of Algal Detritus
No full textSeveral recent lines of literature point toward strong photoreactivity of phytoplanktonic detritus. We examined effects of irradiation of algal membrane fragments in various stages of decay, with emphasis on transfer of materials from solid to dissolved phase (photodissolution). After simulated solar irradiation for 24 h, up to several tens of percent of particulate organic matter converted to photodissolved organic matter (PDOM). Prior microbial decay enhanced PDOM production. PDOM had initially high C:N ratios, which decreased with irradiation time. Dissolved organic nitrogen dominated nitrogen photodissolution, followed by minor photoammonification and negligible nitrite plus nitrate production. Chromophoric particulate organic matter bleached at visible wavelengths and underwent dissolution, creation, and bleaching at ultraviolet (UV) wavelengths, resulting in net loss of color in particulates and net gain of largely UV-absorbing PDOM that also exhibited humic-type fluorescence. Solid phase proteinaceous material became less accessible to proteases after microbial decay but regained this accessibility upon irradiation. Irradiation under anoxic conditions roughly halved production of PDOM, including chromophores and humic fluorophores. Oxygen enhancement of these reactions, along with production of peroxides, implies a strong role for photosensitization. Pigments, unsaturated lipids, and tryptophan emerged as likely sources of reactive oxygen species. Lipid peroxides appeared as a reactive intermediate product. If these reactions in the ocean scale to pigment loss as found in our experiments, at least 5-15% of particulate organic matter could undergo photodissolution before settling in some planktonic environments. This photodissolution would enhance remineralization by photic zone microbial communities and thus upper ocean elemental recycling
Semiclassical magnetotransport in strongly spin-orbit coupled Rashba two-dimensional electron systems
No full textSemiclassical magnetoelectric and magnetothermoelectric transport in strongly spin-orbit coupled Rashba two-dimensional electron systems is investigated. In the presence of a perpendicular classically weak magnetic field and short-range impurity scattering, we solve the linearized Boltzmann equation self-consistently. Using the solution, it is found that when Fermi energy EF locates below the band crossing point (BCP), the Hall coefficient is a nonmonotonic function of electron density ne and not inversely proportional to ne. While the magnetoresistance (MR) and Nernst coefficient vanish when EF locates above the BCP, non-zero MR and enhanced Nernst coefficient emerge when EF decreases below the BCP. Both of them are nonmonotonic functions of EF below the BCP. The different semiclassical magnetotransport behaviors between the two sides of the BCP can be helpful to experimental identifications of the band valley regime and topological change of Fermi surface in considered systems.National Natural Science Foundation of China [11274018]SCI(E)[email protected]
Preparation, properties, and applications of magnetic hematite microparticles
No full textHematite microparticles are becoming increasingly important components in the soft matter field. The remarkable combination of magnetic and photocatalytic properties that characterize them, coupled with the variety of uniform and monodisperse shapes that they can be synthesized in, makes them a one of a kind colloidal model system. Thanks to these properties, hematite microparticles have been recently applied in several important soft matter applications, spanning from novel colloidal building blocks for self-assembly to necessary tools to investigate and understand fundamental problems. In this review article we provide a detailed overview of the traditional methods available for the preparation of hematite microparticles of different shapes, devoting special attention on some of the most common hiccups that could hider a successful synthesis. We furthermore review the particles' most important physico-chemical properties and their most relevant applications in the soft matter field.ChemE/Advanced Soft Matte
Quantum Composites: A Review, and New Results for Models for Condensed Matter Nuclear Science
No full textA composite is made up of constituent particles; the center of mass dynamics is that of a single particle, and the composite can have many internal states and degrees of freedom. The notion of a quantum composite is foundational to atomic, molecular, nuclear and particle physics; in our view it is also foundational to condensed matter nuclear science. It comes as a surprise that there do not appear to be review papers that discuss quantum composites. Here we consider elementary particles models, which are used to model composites; the most widely used example is that of the Dirac phenomenology for protons and neutrons. Quantum composite models can be developed from many-particle models, in some cases simply by rewriting in terms of center of mass and
relative operators, and in other cases through a reduction or transformation. We have proposed models for anomalies in condensed matter nuclear science which rely heavily on the notion of a relativistic quantum composite. In the nonrelativistic case there is a clean separation of center of mass and internal degrees of freedom, so that any coupling between them must occur through external field interactions. The relativistic composite has a sizeable coupling between the center of mass motion and internal degrees of freedom, which we have proposed is responsible for the anomalies in condensed matter nuclear science. We have developed a new
model in which the center of mass dynamics is modeled as nonrelativistic, but the internal structure is kept relativistic; this kind of model is much better adapted to problems in condensed matter nuclear science. Our approach has been strongly criticized, since in a Poincaré invariant theory the center of mass motion separates from the internal degrees of freedom in free space. We are able to rotate out the strongest part of this coupling in free space, consistent with Poincaré invariance. However, in the lattice the problem is in general much more complicated, and more powerful tools are required to diagonalize this relativistic coupling. The spin-boson
type of models that we have considered previously for this are the simplest idealized models that can be diagonalized; they describe rich dynamics not present in the free-space version of the problem. Keywords: Center of mass; Foldy-Wouthuysen transformation; Phonon-nuclear coupling; Quantum composit
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