Role of noise on directed motion of active particles on Ratchet

The directed motion of a collection of Brownian particles is known to happen when temporal and spatial symmetries are broken in the system. There are different ways to break these symmetries[1]. Using asymmetric potential called Ratchet is one way to break spatial symmetry and by externally applying unbiased drive to break temporal symmetries one can observe ratchet effects in the system.[2] However, there exists a special kind of particle called active particles[3] which are inherently non-equilibrium in nature because of their ability to convert local energy into work. These particles are known to break the time-reversal symmetries at a particle level.

A question that naturally arises is that, is direction motion in a collection of active particles is always guaranteed even in the absence of an unbiased external drive. Does the interplay between thermal fluctuations and active dynamics in a ratchet always assist directed motion or are there parameter zones where directed motion is suppressed? Our present work explores these questions by considering a system of underdamped active point particles on the ratchet. One of the interesting findings is that we discover a critical activity strength below which noise assists in generating directed motion whereas above a critical strength of activity directed motion is suppressed. It is also found that the peak in-directed motion shifts to lower activity strengths with an increase in background fluctuations or noise. Understanding this problem has several practical applications such as particle separation, and transport of ions through nano-pores, to name a few.

[1] P. Reimann, Brownian motors: noisy transport far from equilibrium, Physics Reports, 361 (2002) 57 – 265

[2] A. Chugh and R. Ganesh, Emergence of directed motion in a 2D system of Yukawa particles on 1D Ratchet, Physica A, 593 (2022) 126913

[3] S. De Karmakar, R. Ganesh, Phase transition and emergence of active temperature in an active Brownian system in underdamped background, Phys. Rev. E 101 (3) (2020) 032121