Anisotropic diffusion in square lattice potentials: giant enhancement and control

The unbiased thermal diffusion of an overdamped Brownian particle in a square lattice potential is considered in the presence of an externally applied ac driving. The resulting diffusion matrix exhibits two orthogonal eigenvectors with eigenvalues $D_1>D_2>0$, indicating anisotropic diffusion along a "fast" and a "slow principal axis". For sufficiently small temperatures, $D_1$ may become arbitrarily large and at the same time $D_2$ arbitrarily small. The principal diffusion axis can be made to point into (almost) any direction by varying either the driving amplitude or the coupling of the particle to the potential, without changing any other property of the system or the driving.Comment: 7 pages, 7 figure

Asymptotic safety in the dark

We explore the Renormalization Group flow of massive uncharged fermions -- a candidate for dark matter -- coupled to a scalar field through a Higgs portal. We find that fermionic fluctuations can lower the bound on the scalar mass that arises from vacuum stability. Further, we discuss that despite the perturbative nonrenormalizability of the model, it could be ultraviolet complete at an asymptotically safe fixed point. In our approximation, this simple model exhibits two mechanisms for asymptotic safety: a balance of fermionic and bosonic fluctuations generates a fixed point in the scalar self-interaction; asymptotic safety in the portal coupling is triggered through a balance of canonical scaling and quantum fluctuations. As a consequence of asymptotic safety in the dark sector, the low-energy value of the portal coupling could become a function of the dark fermion mass and the scalar mass, thereby reducing the viable parameter space of the model.Comment: 16 pages plus appendix; 5 figure

Exploiting lattice potentials for sorting chiral particles

Several ways are demonstrated of how periodic potentials can be exploited for sorting molecules or other small objects which only differ by their chirality. With the help of a static bias force, the two chiral partners can be made to move along orthogonal directions. Time-periodic external forces even lead to motion into exactly opposite directions.Comment: 4 pages, 4 figure

Particle sorting by a structured microfluidic ratchet device with tunable selectivity: Theory and Experiment

We theoretically predict and experimentally demonstrate that several different particle species can be separated from each other by means of a ratchet device, consisting of periodically arranged triangular (ratchet) shaped obstacles. We propose an explicit algorithm for suitably tailoring the externally applied, time-dependent voltage protocol so that one or several, arbitrarily selected particle species are forced to migrate oppositely to all the remaining species. As an example we present numerical simulations for a mixture of five species, labelled according to their increasing size, so that species 2 and 4 simultaneously move in one direction and species 1, 3, and 5 in the other. The selection of species to be separated from the others can be changed at any time by simply adapting the voltage protocol. This general theoretical concept to utilize one device for many different sorting tasks is experimentally confirmed for a mixture of three colloidal particle species

Espace naturel, techniques agraires et végétation ségétale en pays Kassena

Les Kassena, une ethnie sédentaire appartenant au groupe linguistique des Gurunsi, habitent dans une région au sud du Burkina Faso et au nord du Ghana. Leur économie est basée sur l’autosubsistance à partir de la culture du mil et l’élevage du bétail. Avec une saison des pluies de six mois et une pluviométrie de plus de 900 mm, la région offre des conditions favorables à la culture du mil. Cette région est caractérisée par une forte inégalité en ce qui concerne la dispersion de l’habitat. Certaines régions, comme les alentours de la montagne de Tiébélé qui, selon les récits de la tradition orale, est l’ancien centre des Kassena de l’est, connaissent une importante population allant jusqu’à 100 habitants par km/carré. D’autres régions, notamment la vallée du Nazinon, étaient plus ou moins inhabitées jusqu’à ces dernières années. Ceci est probablement dû aux maladies endémiques comme l’onchocercose. Pour les régions les plus peuplées, nous avons cherché à connaître les techniques spécifiques qui ont permis à la population de s’installer et de s’alimenter. Les Kassena maîtrisent un système de culture permanente sur des champs terrassés exigeant beaucoup d’entretien. Ces champs sont soumis à un contrôle social et religieux. De plus, les Kassena cultivent des champs de brousse qui se trouvent souvent dans la plaine et à grande distance des villages. Ces champs de brousse sont cultivés d’une façon plus extensive. Le présent exposé décrit les techniques de l’agriculture en respect des conditions de l’environnement. L’objectif est de mieux comprendre les stratégies économiques et culturelles des cultivateurs de cette région

Chiral particle separation by a non-chiral micro-lattice

We conceived a model experiment for a continuous separation strategy of chiral molecules (enantiomers) without the need of any chiral selector structure or derivatization agents: Micro-particles that only differ by their chirality are shown to migrate along different directions when driven by a steady fluid flow through a square lattice of cylindrical posts. In accordance with our numerical predictions, the transport directions of the enantiomers depend very sensitively on the orientation of the lattice relatively to the fluid flow

Weak disorder strongly improves the selective enhancement of diffusion in a tilted periodic potential

The diffusion of an overdamped Brownian particle in a tilted periodic potential is known to exhibit a pronounced enhancement over the free thermal diffusion within a small interval of tilt-values. Here we show that weak disorder in the form of small, time-independent deviations from a strictly spatially periodic potential may further boost this diffusion peak by orders of magnitude. Our general theoretical predictions are in excellent agreement with experimental observations.Comment: Phys. Rev. Lett., in pres

Accumulating Particles at the Boundaries of a Laminar Flow

The accumulation of small particles is analyzed in stationary flows through channels of variable width at small Reynolds number. The combined influence of pressure, viscous drag and thermal fluctuations is described by means of a Fokker-Planck equation for the particle density. It is shown that in the limit of vanishing particle size a uniform particle distribution is always approached in the long time limit. For extended spherical particles, conditions are specified that lead to inhomogeneous densities and consequently to particle accumulation and depletion. Hereby the boundary conditions for the particle density play a decisive role: The centers of spherical particles must keep the minimal distance of their radius from the fluid boundaries. The normal components of the forces acting on the sphere then may assume finite values which are diffusively transported into the bulk of the fluid.Comment: final version, accepted by Physica