Negative mobility, the paradoxical response of inertial tracers in steady laminar flows


We study the mobility and the diffusion coefficient of an inertial tracer advected by a two-dimensional incompressible laminar flow, under the joint influence of external force and thermal noise. Extensive numerical simulations shows that the force-velocity curve of the tracer displays a complex behavior, including negative differential and absolute mobility in certain regimes. These effects rely upon a subtle interplay between inertia, external force and nonlinearity that contribute to keep the tracer in particular regions of phase space with a velocity opposite to the force. The relevance of this coupling is revisited in the framework of nonequilibrium response theory via a generalized Einstein relation to our system.

Ref.  A.Sarracino, F.Cecconi, A.Puglisi, A.Vulpiani, PRL 117, 174501 (2016).