Structural States of Filamentary Microgravity Dusty Plasma

TL;DR

This paper explores the filamentary structures in microgravity dusty plasma, revealing their liquid crystal-like properties and phase transitions influenced by pressure, supported by experimental data and molecular dynamics simulations.

Contribution

It demonstrates that filamentary dusty plasma exhibits liquid crystal behaviors and phase transitions, supported by experimental observations and simulations, advancing understanding of plasma structural states.

Findings

Filamentary dusty plasma shows liquid crystal-like properties.

Structural transitions occur with changing pressure, from weakly crystalline to liquid crystal states.

Filaments exhibit alignment suggestive of smectic phases.

Abstract

This study investigates the filamentary structural states of microgravity dusty plasma using data from the Plasmakristall-4 (PK-4) facility on board the International Space Station. The dust particles in the PK-4 discharge are observed to form field-aligned filaments and nested (layered) structures in response to changes in the plasma conditions, neutral gas pressure, and externally applied electric field. This work explores the possibility that these filamentary dusty plasmas exhibit properties of liquid crystals. The structural characteristics of the dust clouds are studied for nine sets of pressure-current conditions using pair correlation functions calculated for particles (i) within individual filaments, (ii) within the central plane of the dust cloud, and (iii) within successive planes of the cloud (the 3D cloud). It is observed that, at low pressure ($\approx$30 Pa), the entire cloud is in a weakly crystalline state with similar coupling of particles within filaments and among neighboring filaments. At high pressure ($\approx$70 Pa), the order within filaments improves (enhanced crystalline behavior), while the degree of freedom of filaments to move with respect to each other increases (enhanced liquid behavior). Since neutral gas pressure in dusty plasma acts as inverse temperature, we argue that the structural changes observed with increasing pressure are analogous to a transition to a nematic liquid crystal state. It is further observed that the filaments exhibit alignment in nested surfaces for several pressure-current conditions, suggesting the possibility of a smectic liquid crystal state. These results are confirmed by molecular dynamics simulations of the dust and ions using the DRIAD (Dynamic Response of Ions And Dust) code.