Soliton ratchets

The mechanism underlying the soliton ratchet, both in absence and in presence of noise, is investigated. We show the existence of an asymmetric internal mode on the soliton profile which couples, trough the damping in the system, to the soliton translational mode. Effective soliton transport is achieved when the internal mode and the external force are phase locked. We use as working model a generalized double sine-Gordon equation. The phenomenon is expected to be valid for generic soliton systems.Comment: 4 pages, 4 figure

Force and Motion Generation of Molecular Motors: A Generic Description

We review the properties of biological motor proteins which move along linear filaments that are polar and periodic. The physics of the operation of such motors can be described by simple stochastic models which are coupled to a chemical reaction. We analyze the essential features of force and motion generation and discuss the general properties of single motors in the framework of two-state models. Systems which contain large numbers of motors such as muscles and flagella motivate the study of many interacting motors within the framework of simple models. In this case, collective effects can lead to new types of behaviors such as dynamic instabilities of the steady states and oscillatory motion.Comment: 29 pages, 9 figure

Disorder Induced Diffusive Transport In Ratchets

The effects of quenched disorder on the overdamped motion of a driven particle on a periodic, asymmetric potential is studied. While for the unperturbed potential the transport is due to a regular drift, the quenched disorder induces a significant additional chaotic ``diffusive'' motion. The spatio-temporal evolution of the statistical ensemble is well described by a Gaussian distribution, implying a chaotic transport in the presence of quenched disorder.Comment: 10 pages, 4 EPS figures; submitted to Phys. Rev. Letter

Brownian motion exhibiting absolute negative mobility

We consider a single Brownian particle in a spatially symmetric, periodic system far from thermal equilibrium. This setup can be readily realized experimentally. Upon application of an external static force F, the average particle velocity is negative for F>0 and positive for F<0 (absolute negative mobility).Comment: 4 pages, 3 figures, to be published in PR

Force-Free Motion of a Mercury Drop Alternatively Submitted to Shifted Asymmetric Potentials

We present a macroscopic experimental realization of force-free motion consisting in a mercury drop experiencing alternatively in time two locally asymmetric and periodic potentials which are spatially shifted. A system of electrodes creates the potentials and the force applied to the drop is of electrocapillary nature. We study the macroscopic velocity of the drop as a function of the times during which it experiences each potential and investigate different regimes of macroscopic velocity. Adjusting the different times allows some of the drops to move whereas others of different local velocities have a zero macroscopic velocity. This system thus acts as a filter. We also study the case of stochastic modulation of the potentials. These results compare well with theoretical predictions and experimentally validate a model which has been proposed for motor protein assemblies.Nous présentons une réalisation expérimentale de mouvement sans force consistant à appliquer successivement à une goutte de mercure macroscopique deux potentiels localement asymétriques et périodiques, décalés dans l'espace. Chaque potentiel est créé par un système d'électrodes et la force appliquée à la goutte est d'origine électrocapillaire. Nous étudions la vitesse macroscopique de la goutte en fonction des temps d'adressage de chaque potentiel et mettons en évidence différents régimes de vitesse. Un ajustement des différents temps permet à certaines gouttes de se déplacer à vitesse finie alors que qu'autres, ayant des vitesses locales différentes dans chaque potentiel, ont une vitesse macroscopique nulle. Ce système joue donc le rôle d'un filtre. Nous étudions également le cas où les systèmes d'électrodes sont adressés de façon stochastique. Les résultats confirment les prédictions héoriques et valident expérimentalement un modèle qui a été proposé pour expliquer les phénomènes mis en jeu dans le mouvement des moteurs moléculaires

Systematic study of the settling kinetics in an aggregating colloidal suspension

We present a study of the settling of a strongly-aggregating colloidal suspension. Using well-controlled samples, the different settling behaviours have been systematically investigated according to the values of the volume fraction $\Phi$ of the suspension and of the inclination angle of the cell. In a vertical cell, three velocity regimes are observed. To describe the first and final regimes, we propose a simple 1D model that takes into account the microscopic structure of the gel constituted by a packing of fractal aggregates. The second regime coincides with the opening of fractures within the gel and its description would require a more complex model. When the cell is inclined at an angle $\alpha$ to the vertical, new settling regimes can emerge and an enhancement of the settling velocity can be observed as in usual macroscopic suspensions. Using both our 1D model and arguments similar to the ones used for macroscopic suspensions (PNK model), we propose a description of the influence of inclination that is in good agreement with the experimental data. We also present the various experimental settling behaviours we have observed in a $\Phi$ vs. $\alpha$ diagram that covers the whole range of experimental conditions

Settling in aggregating colloidal suspension: Analysis of cell inclination influence

We present the study of the settling of a colloidal gel in a cell tilted at an angle $\alpha$. In suspensions of non-colloidal particles, the inclination of the sedimentation cell leads to a large enhancement of the settling velocity known as "Boycott effect" well described by the PNK model. We show that the specific mechanical properties of a colloidal gel can give way to kinetics of settling very different from usual suspensions which can no longer be described using a simple PNK model. In particular, we study the influence of the very voluminous sediment generally formed in such systems. We thus propose a complete description of the kinetics of a settling colloidal gel according to its volume fraction and to the cell inclination angle. In the experimental section, the predicted settling regimes are compared to experimental results obtained with calcium carbonate suspensions. The agreement between theoretical expressions and experiments is good which shows that our results are relevant to most experimental studies of tenuous colloidal gels