Absolute negative particle mobility

Abstract

tions, adapted quantitatively to the microstructure geometry and the experimentally determined free diffusion and mobility of the microbeads (see supplementary information). The key feature of the resulting response curve (Fig. 1) is the negative slope that is symmetrical around the origin -a distinct and unambiguous signature of absolute negative mobility. We also measured the average bead velocity by tracking the beads using real-time video microscopy for different values of U DC (see supplementary information) and found that the measured experimental response was in good agreement with theory ( Fig. 1).To explain how absolute negative mobility occurs, we consider the small gaps in the microstructure as 'traps' -electrical field lines can pass through them but the beads cannot. For 0<U DC <U 0 , the alternating total voltages DŽU 0 +U DC yield a back-and-forth motion of the beads along the x direction. Whenever a bead succeeds in passing through a large gap, it is trapped by the adjacent small gap in the x direction ( Fig. 1, inset), unless it thermally diffuses sufficiently far in the y direction to proceed through another large gap. The smaller the voltage, the more time it has to do so and the farther it gets before being trapped. As |+U 0 +U DC |>|ǁU 0 +U DC |, the bead becomes trapped after moving forwards a short distance when +U 0 +U DC >0, whereas it travels backwards for a longer distance when ǁU 0 +U DC <0, resulting in absolute negative mobility.We find that this response by a particle is very sensitive to particle size, even to the extent of allowing particles of different sizes to be steered in opposite directions (our unpublished results). Absolute negative mobility therefore holds promise for bioanalytical applications, for example in the sorting of cells, organelles and biochemical compounds.

Artificial Brownian motors: Controlling transport on the nanoscale

“…This is surely the case of the bottom-up design and operation of synthetic Brownian molecular devices, extensively covered by Kay et al ͑2007͒. Another such topic is the question of so-called absolute negative mobility, which occurs via quantum tunneling events in quantum systems ͑Keay et Aguado and Platero, 1997; Grifoni and Hänggi, 1998; Platero and Aguado, 2004͒ and through nonequilibrium-driven diffusive dynamics in classical systems ͑Eichhorn et al, 2002a͑Eichhorn et al, , 2002bRos et al, 2005;Machura, Kostur, et al, 2007;Kostur et al, 2008͒. Our main focus was on noise-assisted directional transport, shuttling, and pumping of individual or collective particle, charge, or matter degrees of freedom. The concept, however, extends as well to the transport of other degrees of freedom such as energy ͑heat͒ and spin modes ͑see also Sec.…”

Observation of Negative Absolute Resistance in a Josephson Junction

“…[14] and subsequently realized experimentally in Refs. [11,15]. As a first application of NAR, the separation of different particle species has been realized in Ref.…”

“…On the theoretical side, a considerably larger literature is available, most notably on different types of semiconductors and semiconductor heterostructures [7]. In all those cases (except [11]) NAR is based on purely quantum mechanical effects which cannot be transferred into the realm of classical physics. For classical systems, a first theoretical demonstration of the effect was provided in the context of a spatially periodic and symmetric model system of interacting Brownian particles, sub- * Electronic address: kleiner@uni-tuebingen.de jected to multiplicative white noise [12].…”

“…Yet, nonlinear systems being driven far from equilibrium can indeed exhibit not only a negative differential resistance but also a NAR effect. Unambiguous and convincing experimental observations of NAR are still quite scarce, involving systems consisting of electrons in a sample of bulk GaAs [8], electrons in semiconductor heterostructures [9], electrons in low dimensional conductors [10], and charged Brownian particles in structured microfluidic devices [11]. Apart from the low dimensional conductors, the system was always driven out of equilibrium by means of an ac driving force and then its response to an externally applied static perturbation was studied.…”

Asymmetry between pushing and pulling for crawling cells

“…Negative mobility has been discussed previously in the context of individual [40][41][42][43] and interacting [44,45] Brownian motors. The regimes where velocity of the crawling cell increases with an opposing pulling force at the rear have been envisioned in [46] where negative mobility was attributed to the coupling between the velocity of retraction and the applied force v − (Q) [47].…”