Non-normal tracer diffusion from stirring by swimming microorganisms

The experimentally observed non-Gaussian form of passive tracer distributions in media stirred by active swimmers (Leptos et al., Phys. Rev. Lett. 103, 198103 (2009)) are analyzed in terms of continuous time random walks. The walks are characterized by a trapping time distribution ??(??) with long time behaviour ??(??) ?? ?? ^?1??? and a step size distribution p(??x) ?? (??x)^?2??? . The experimentally observed behaviour that ??x ²?? ?? t is obtained for a one-parameter family of exponents with ?? = 2??. However, the distribution function for this case is non-Gaussian and shows exponential tails. The shape of the distributions agrees rather well with the experimental observations from Leptos et al. and allows for the determination of the exponents.      

Enhanced quantum searching via entanglement and partial diffusion

In this paper, we will define a quantum operator that performs the standard inversion about the mean only on a subspace of the system (Partial Diffusion Operator). This operator is used together with entanglement in a quantum search algorithm that runs in for searching an unstructured list of size N with M matches such that 1≤M≤N. We will show that the performance of the algorithm is more reliable than known fixed operators quantum search algorithms especially for multiple matches where we can get a solution after a single iteration with probability over 90% if the number of matches is approximately more than one-third of the search space. We will show that the algorithm will be able to handle the case where the number of matches M is unknown in advance in such that 1≤M≤N.     

Stirring by swimming bodies

We consider the stirring of an inviscid fluid caused by the locomotion of bodies through it. The swimmers are approximated by non-interacting cylinders or spheres moving steadily along straight lines. We find the displacement of fluid particles caused by the nearby passage of a swimmer as a function of an impact parameter. We use this to compute the effective diffusion coefficient from the random walk of a fluid particle under the influence of a distribution of swimming bodies. We compare with the results of simulations. For typical sizes, densities and swimming velocities of schools of krill, the effective diffusivity in this model is five times the thermal diffusivity. However, we estimate that viscosity increases this value by two orders of magnitude.     

Enhanced anomalous diffusion of sputtered atoms in nanosized pores

Plasma sputtering deposition of platinum catalysts in porous anodic aluminum oxide (AAO) templates is shown to generate an anomalous superdiffusion concentration profile. The growth of an overlayer between the hexagonal array of pores is shown to enhance the diffusion into the pores, leading to a diffusion coefficient having superlinear time dependence. The Pt clusters in the pores have a mean size of 10 nm and almost concentrate on the inner pore surface, and are present up to a depth of 6 μm.     

Enhanced diffusion due to active swimmers at a solid surface

We consider two systems of active swimmers moving close to a solid surface, one being a living population of wild-type E.?coli and the other being an assembly of self-propelled Au-Pt rods. In both situations, we have identified two different types of motion at the surface and evaluated the fraction of the population that displayed ballistic trajectories (active swimmers) with respect to those showing randomlike behavior. We studied the effect of this complex swimming activity on the diffusivity of passive tracers also present at the surface. We found that the tracer diffusivity is enhanced with respect to standard Brownian motion and increases linearly with the activity of the fluid, defined as the product of the fraction of active swimmers and their mean velocity. This result can be understood in terms of series of elementary encounters between the active swimmers and the tracers.     

On the validity of the Onsager reciprocal relations for simultaneous heat transfer and multi-component diffusion

In this work, the validity of the Onsager reciprocal relations (ORR) for the heat transfer and multi-component diffusion case in the presence of external forces is re-examined. It is shown that the ORR are necessarily fulfilled for this process even if the quasi-equilibrium postulate is applied. Moreover, it is shown that the resistance coefficients for this case are uniquely defined. It is believed that this work could contribute to the further understanding of the ORR.     

Enhanced diffusion due to neutron irradiation in Cu-Ni alloys at different temperatures

The review of the existing data on the reverse annealing of boron implants in silicon is given. The idea of the exchange reaction of boron substitutionals with silicon selfinterstitials as being responsible for the phenomenon is critically considered. Instead, the possible involvement of oxygen and oxygen-related secondary defects is proposed.     

Enhanced diffusion of a needle in a planar array of point obstacles

The transport of an infinitely thin, hard rod in a random, dense array of point obstacles is investigated by molecular dynamics simulations. Our model mimics the sterically hindered dynamics in dense needle liquids. Transport becomes increasingly fast at higher densities, and we observe a power-law divergence of the diffusion coefficient with exponent 0.8. This phenomenon is connected with a new divergent time scale, reflected in a zigzag motion of the needle, a two-step decay of the velocity-autocorrelation function, and a negative plateau in the non-Gaussian parameter. Finally, we provide a heuristic scaling argument for the new exponent.     

Hydrodynamic attraction of swimming microorganisms by surfaces

Cells swimming in confined environments are attracted by surfaces. We measure the steady-state distribution of smooth-swimming bacteria (Escherichia coli) between two glass plates. In agreement with earlier studies, we find a strong increase of the cell concentration at the boundaries. We demonstrate theoretically that hydrodynamic interactions of the swimming cells with solid surfaces lead to their reorientation in the direction parallel to the surfaces, as well as their attraction by the closest wall. A model is derived for the steady-state distribution of swimming cells, which compares favorably with our measurements. We exploit our data to estimate the flagellar propulsive force in swimming E. coli.     

Energy-transport by radiative diffusion

A new derivation of the general-relativistic Fourier equation is given for radiation transport by using the principle of conservation of momentum plus some rather simple assumptions. The Fourier equation at which I arrive is not the usual one but has an additional term. For this reason it leads to a hyperbolic equation for heat conduction, thus avoiding the paradox of infinite velocity of heat propagation, which is a consequence of the usual Fourier equation, as the latter one leads to a parabolic equation for heat conduction. The new Fourier equation is compared with the one that was given by Kranys by using ad hoc assumptions.     

Oscillatory flows induced by microorganisms swimming in two dimensions

We present the first time-resolved measurements of the oscillatory velocity field induced by swimming unicellular microorganisms. Confinement of the green alga C. reinhardtii in stabilized thin liquid films allows simultaneous tracking of cells and tracer particles. The measured velocity field reveals complex time-dependent flow structures, and scales inversely with distance. The instantaneous mechanical power generated by the cells is measured from the velocity fields and peaks at 15 fW. The dissipation per cycle is more than 4 times what steady swimming would require.     

End-to-end diffusion coefficients and distance distributions from fluorescence energy transfer measurements: Enhanced resolution by using multiple donors with different lifetimes

We describe a method to improve the resolution of donor-to-acceptor distance distributions in molecules which are flexing on the timescale of the fluorescence lifetime. We measured the timedependent donor decays of two donor (D)-acceptor (A) pairs, where the donor lifetimes were substantially different. The donors were an indole residue (5.7 ns) and a naphthalene residue (24.4 ns). The same dansyl acceptor was used for both D-A pairs. The donor decays are complex due to both a distribution of D-A distances and D-A diffusion. Using the donor decay data for each D-A pair alone, it is difficult to resolve both the distance distribution and the D-to-A diffusion coefficient. However, these values are unambiguously recovered from global analysis of the data from both D-A pairs. The increased resolution from the global analysis is apparently the result of the complementary information content of the data for each D-A pair. The shorter-lived indole donor provides more information on the time-zero distance distribution because there is less time for D-A diffusion, and the longer-lived naphthyl donor is quenched to a greater extent than indole due to the longer time for diffusion-enhanced energy transfer. Simulations were also used to demonstrate the increased resolution of global analysis with different lifetime donors to obtain distance distribution parameters in the presence of D-A diffusion.     

Modeling of breaststroke swimming

A mechanical system that models swimming using a pair of two-chain extremities is considered. The motion of the system under study is similar to swimming of a frog and some other animals, in which lower extremities play the main role. This type of motion is characteristic of competitive breaststroke swimming.     

Waveguide dye lasers prepared by diffusion

Waveguide dye lasers are prepared by diffusion of dye molecules into a commercial foil. Superradiance is achieved with Rhodamin 6 G, Rhodamin B and a mixture of Rhodamin 6 G and Cresylviolet.     

Grain boundary migration initiated by diffusion

A review is presented of work related to two new processes which arise in crystalline materials when impurities diffuse along grain boundaries from the surface of the material: diffusion-initiated grain boundary migration and recrystallization. We analyze the conditions under which DIGM occurs, the kinetics of the process, its driving forces, and also the changes in the grain fine structure and the near-grain regions, as well as the nucleation of new grains on the migrating boundaries, caused by an uncompensated impurity atom flux. We consider the mechanisms for DIGM. It is shown that not one of the mechanisms proposed to describe DIGM is capable of explaining all the experimentally observed properties of grain boundary migration under DIGM conditions. We note that changes in the grain boundary structure caused by the impurity atoms diffusing along it are due to diverse grain-boundary processes, which have important technological implications.V. D. Kuznetsov Physicotechnical Institute, Tomsk State University, Siberia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 34–57, May, 1992.     

Determination of diffusion coefficient in melts by the method of stationary diffusion source

This method eliminates some shortcomings encountered with other methods currently used for determination of the diffusion coefficients in melts. There are, with reasonable accuracy, fulfilled the starting and boundary conditions of a diffusion equation, and the diffusion process is not disturbed by melt flowing. This method considers a diffusion source at the boundary of a diffusion range and, when fulfilling certain conditions, both the solution of the diffustion equation and the evaluation of the diffusion coefficients is very simple. The required diffusion source, having a constant power, can be simulated by contacting the saturated gases of a substance, the diffusion of which is being investigated, with melt surface in which the diffusion takes place. The method takes into account application of radionuclides and this article brings about some criteria for application of that method. Simplicity is the main feature of the experimental arrangement.This work has been carried out in the scope of the State Research Task III-5-8 Equilibrium States of Metallurgical Systems.     

Enhanced Raman scattering provided by fullerene nanoclusters

Two new nonlinear optic effects are observed in crystalline solutions of fullerenes in toluene and carbon tetrachloride. Both are provided by a self-clustering of the solute molecules and are referred to fullerene-enhanced Raman scattering and solvent-enhanced luminescence. The effects are induced by the excitation of charge-transfer states of the fullerene nanoclusters, which makes the latter act as amplifiers of local electric fields. The text was submitted by the authors in English.