7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Dynamical Phase Transitions in Active Complex PlasmaRajaraman Ganesh1, 2 , Soumen De Karmakar3
1Institute for Plasma Research, Gandhinagar, 2 Homi Bhabha National Institute, Mumbai, 3 Instituteof Theoretical Physics IV, University of Stuttgart
e-mail (speaker): ganesh@ipr.res.in

Micron sized dust particles immersed in a lowtemperature plasma environment is known as ComplexPlasma, where the background plasma shields thenegatively charged dust particles. Depending on theionization and the background neutral density, Complex
Plasmas are known to exhibit liquid, solid, and hexatic
phases [1] that are typically observed at thermodynamic
equilibrium in two dimensions. Complex Plasmas arealso known to exhibit several non-equilibrium phases,
such as lane formation [2], depending on the external
drives that are, in general, incorporated at the largest
scales of the system. Nearly spherical, micron sized dust
particles in Complex Plasmas are recently being started
to be engineered as self-propelled particles, which ispossible by breaking the rotational symmetry. Forinstance, one hemisphere of the dielectric dust particles
are being coated with a metal, such as gold, thereby
producing a local thermal gradient in the presence of anilluminating Laser [3]. Such self-propelled particles are
known as Janus particles [3]. Janus particles coupledwith appropriate driving mechanism, for instancethermophoresis [3], makes the particles self-driven orself-propelled at the shortest scales, unlike globallydriven Complex Plasmas. Self-propelled particles in a
low temperature plasma background is known as Active
Complex Plasmas [4, 5], which is an emerging field ofresearch. Few early investigations [4, 6, 7] showpromising collective behavior in Active Complex Plasmathat are not known to be observed in conventional driven
Complex Plasmas. Large scale numerical studies [8] of
the inherently non-equilibrium inertial system of selfpropelled particles are found to be highly diffusivecompared to their passive counterpart, that is Complex
Plasma. Numerical investigations suggest that the nonequilibrium system is found to exhibit equilibrium-like
homogeneous phases to coexistence of phase separated
state of high density dynamical clusters and low density
phase, known as motility induced phase separation(MIPS) [9], with variation of the softness of the Janus
particles or the background charge density. Low density
Janus particles have been recently found to exhibit
various intriguing dynamic phases, such as MIPS [9] andflocking [8], and transition between the non-equilibriumphases has been reported [8, 10] with continuous
variation of the system parameters. Inertia of the Janus
particles, that is controlled by the background neutral
density, is found to substantially alter the spatio-temporal

dynamics of the system[8, 11]. Several of thesenumerical results will be explored, along withexperimental possibilities of the observed results.
References[1] E. V. Vasilieva, O. F. Petrov and M. M. Vasiliev,
Scientific Reports 11, 523 (2021)[2] K. R. Sütterlin et al., Phys. Rev. Lett. 102, 085003(2009).
[3] C. Bechinger et al., Rev. Mod. Phys. 88, 045006(2016).
[4] V. Nosenko et al., Phys. Rev. Res. 2, 033226 (2020).
[5] S. De Karmakar and R. Ganesh, Phys. Rev. E 101,032121 (2020).
[6] K. Arkar et al., Molecules 26, 561 (2021).
[7] V. Nosenko, Physics of Plasmas 29, 123701 (2022).
[8] S. De Karmakar, Ph.D. Thesis, HBNI (2023).
[9] S. De Karmakar and R. Ganesh, Soft Matter, 18,7301-7308 (2022).
[10] S. De Karmakar and R. Ganesh, Phys. Rev. E 106,044607 (2022).
[11] S. De Karmakar, A. Chugh, and R. Ganesh,
Scientific Reports 12, 22563 (2022).