Site-percolation transition of run-and-tumble particles

We study percolation transition of run and tumble particles (RTPs) on a two dimensional square lattice. RTPs in these models run to the nearest neighbour along their internal orientation with unit rate, and to other nearest neighbours with rates $p$. In addition, they tumble to change their internal orientation with rate $\omega$. We show that for small tumble rates, RTP-clusters created by joining occupied nearest neighbours irrespective of their orientation form a phase separated state when the rate of positional diffusion $p$ crosses a threshold; with further increase of $p$ the clusters disintegrate and another transition to a mixed phase occurs. The critical exponents of this re-entrant site-percolation transition of RTPs vary continuously along the critical line in the $\omega$-$p$ plane, but a scaling function remains invariant. This function is identical to the corresponding universal scaling function of percolation transition observed in the Ising model. We also show that the critical exponents of the underlying motility induced phase separation transition are related to corresponding percolation-critical-exponents by constant multiplicative factors known from the correspondence of magnetic and percolation critical exponents of the Ising model.

Soumya K. Saha, Aikya Banerjee, P. K. Mohanty · 2024