Synopsis: The Opposite of Friction

Particles on a surface can have a negative mobility when subject to an oscillating driving force.
Synopsis figure
D. Speer et al., Phys. Rev. E (2012)

The motion of paired particles (dimers) on a surface appears in a wide variety of different contexts, such as the layer-by-layer growth of a semiconductor or the hopping of molecules in an optical lattice. In Physical Review E, David Speer at Bielefeld University, Germany, and colleagues report a nonintuitive effect based on their calculations of the motion of a dimer of two identical particles on an isotropic surface: in spite of the intrinsic symmetry of the system, a periodic external potential acting on the dimer can cause a spontaneous symmetry breaking. This is reflected in the net motion of the dimer in a specific direction, which depends on the initial conditions. The essential condition for this effect is a non-convex interaction potential between the dimer components, such as one where the potential changes from repulsive to attractive with distance.

Speer et al. show that if a dimer is also subjected to thermal noise, it can exhibit a divergent diffusion constant at low temperatures – a counterintuitive result, given that the diffusion coefficient of a particle on a surface usually vanishes as the temperature goes to zero. Furthermore, under the action of a constant force, the dimer can have a “negative mobility,” where it moves in the opposite direction to that of the applied force. According to the authors, this behavior could be observed in a nanofriction experiment, where the friction force may be acting not against, but along the pulling direction – a phenomenon bearing some resemblance to so-called superlubricity. – Hernan Rozenfeld


Features

More Features »

Announcements

More Announcements »

Subject Areas

Statistical Physics

Previous Synopsis

Next Synopsis

Fluid Dynamics

Fluid Invasion

Read More »

Related Articles

Synopsis: The Force that Clumps Your Breakfast Cereal
Fluid Dynamics

Synopsis: The Force that Clumps Your Breakfast Cereal

By measuring the forces that cause floating objects to drift toward each other, researchers hope to better understand the interactions that cause particles to self-assemble in fluids. Read More »

Viewpoint: Surfing on a Wave of Quantum Chaos
Statistical Physics

Viewpoint: Surfing on a Wave of Quantum Chaos

A model based on Brownian motion describes the tsunami-like propagation of chaotic behavior in a system of quantum particles. Read More »

Synopsis: A Heat Engine Made of a Single Ion Spin
Quantum Physics

Synopsis: A Heat Engine Made of a Single Ion Spin

By converting electron spin into ion motion, researchers build a simple heat engine out of a single calcium ion. Read More »

More Articles