Ferromagnetic microswimmers

We propose a model for a novel artificial low Reynolds number swimmer, based on the magnetic interactions of a pair of ferromagnetic particles: one with hard and the other with soft magnetic properties, connected by a linear spring. Using a computational model, we analyze the behavior of the system and demonstrate that for realistic values of the parameters involved, the swimmer is capable of self-propelling with average speeds of the order of hundreds of micrometers per second.     

Trypanosomes – versatile microswimmers

Evolution has generated a plethora of flagellate microswimmers. They populate all natural waters, from the deep sea to the ponds in our neighbourhood. But flagellates also thrive in the bodies of higher organisms, where they mostly remain undetected, but can also become pathogenic. Trypanosomes comprise a large group of mostly parasitic flagellates that cause many diseases, such as human sleeping sickness or the cattle plague nagana. We consider African trypanosomes as extremely versatile microswimmers, as they have to adapt to very diverse microenvironments. They swim efficiently in the blood of their mammalian hosts, but also in various tissue spaces and even in the human brain. Furthermore, in the transmitting tsetse fly, trypanosomes undergo characteristic morphological changes that are accompanied by amazing transitions between solitary and collective types of motion. In this review, we provide a basic introduction to trypanosome biology and then focus on the complex type of rotational movement that trypanosomes display. We relate their swimming performance to morphological parameters and the respective microenvironment, developing a contemporary view on the physics of trypanosome motility. The genetically programmed successions of life style-dependent motion patterns provide challenges and opportunities for interdisciplinary studies of microswimmers.     

Optimal strokes for axisymmetric microswimmers

We present a theory for low-Reynolds-number axisymmetric swimmers and a general strategy for the computation of strokes of maximal efficiency. An explicit equation characterizing optimal strokes is derived, and numerical strategies to obtain solutions are discussed. The merits of this approach are demonstrated by applying it to two concrete examples: the three linked spheres of Najafi and Golestanian and the pushmepullyou of Avron, Kenneth, and Oakmin.     

Motion and mixing for multiple ferromagnetic microswimmers

This paper concerns the interaction of several ferromagnetic microswimmers, their motion and the resulting fluid mixing. Each swimmer consists of two ferromagnetic beads joined by an elastic link, and is driven by an external, time-dependent magnetic field. The external field provides a torque on a swimmer and, together with the varying attraction between the magnetic beads, generates a time-irreversible motion leading to persistent swimming in a low Reynolds number environment. The aim of the present paper is to consider the interactions between several swimmers. A regime is considered in which identical swimmers move in the same overall direction, and their motion is synchronised because of driving by the external field. It is found that two swimmers tend to encircle one another while three undergo more complicated motion that may involve the braiding of swimmer trajectories. By means of approximations it is established that the interaction between pairs of swimmers gives circulatory motion which falls off with an inverse square law and is linked to their overall speed of motion through the fluid. As groups of two or more swimmers move through the fluid they process fluid, leaving behind a trail of fluid that has undergone mixing: this is investigated by following streak lines numerically.     

How walls affect the dynamics of self-phoretic microswimmers

We study the effect of a nearby planar wall on the propulsion of a spherical phoretic micro-swimmer driven by reactions on its surface. An asymmetric coverage of catalysts on its surface which absorb reactants and generate products gives rise to an anisotropic interfacial flow that propels the swimmer. We analyse the near-wall dynamics of such a self-phoretic swimmer as a function of the asymmetric catalytic coverage of the surface. By an analysis of the fundamental singularities of the flow and concentration or electrostatic potential gradients generated we are able to obtain and rationalise a phase diagram of behaviours as a function of the characteristics of the swimmer surface. We find a variety of possible behaviours, from “bound states” where the swimmer remains near the wall to “scattering” or repulsive trajectories in which the swimmer ends far from the wall. The formation of some of the bound states is a purely wall-phoretic effect and cannot be obtained by simply mapping a phoretic swimmer to a hydrodynamic one.     

Efficiency of surface-driven motion: nanoswimmers beat microswimmers

Surface interactions provide a class of mechanisms which can be employed for propulsion of micrometer and nanometer sized particles. We investigate the related efficiency of externally and self-propelled swimmers. A general scaling relation is derived showing that only swimmers whose size is comparable to, or smaller than, the interaction range can have appreciable efficiency. An upper bound for efficiency at maximum power is 1/2. Numerical calculations for the case of diffusiophoresis are found to be in good agreement with analytical expressions for the efficiency.     

Hot ice computer

We experimentally demonstrate that supersaturated solution of sodium acetate, commonly called ‘hot ice’, is a massively-parallel unconventional computer. In the hot ice computer data are represented by a spatial configuration of crystallization induction sites and physical obstacles immersed in the experimental container. Computation is implemented by propagation and interaction of growing crystals initiated at the data-sites. We discuss experimental prototypes of hot ice processors which compute planar Voronoi diagram, shortest collision-free paths and implement and and or logical gates.     

Hot gluon propagator

In order to match two complementary approaches to quark gluon plasma, namely the classical hamiltonian lattice gauge field simulation which uses the temporal axial gauge, and hot perturbative QCD which rather uses the Coulomb or the covariant gauge, we obtain a general expression for the hard thermal loop gluon propagator for a variety of non-background gauge fixing conditions. The Coulomb energy is shown to be independent of the gauge fixing condition.     

Hot droplet stability

Properties of heated droplets are studied on an example of the thermal explosion of noble gas atomic clusters, simulated by employing the classical molecular dynamics. Sharp change in the droplet stability at temperatures corresponding to the respective critical isentrope is shown on the basis of the analysis of the largest fragment size.     

Hot electrons in semiconductors

Journal of Russian Laser Research -     

Hot luminescence in polyacetylene

We propose that the unusual frequency dependent lineshapes observed in Raman scattering experiments on polyacetylene are due to hot luminescence in very long polyene chains. Employing a generalized equation of motion technique we find that a photoexcited polyene is unstable with respect to a variety of symmetry lowering distortions which vary as a function of the frequency of the exciting radiation. Radiative decay during this relaxation is observably strong in the inelastically scattered photon spectrum and would explain the dominant observed lineshape variations.     

热核四重碎裂研究

对单体热核四重碎裂的实验研究结果诸如碎片的质量、速度、关联角度、不同质量碎片的发射几率进行了报道.     

生物物理学的几个热点领域

物理学与生物科学的交叉由来已久,这不仅解决了自然界许多重大的理论问题,并且在高层次上开辟了新的技术领域,如生物信息学、纳米生物学和脑与认知科学等.文章对当今生物物理学的这几个热点领域进行了介绍.