This study examines the structure and stability of filamentary dusty plasmas using data
from the Plasmakristall-4 (PK-4) facility on board the International Space Station. Under the
action of a polarity-switched DC electric field, the dust particles in the PK-4 discharge have
been found to organize into field-aligned extended filaments, which has been compared to the
filamentary state in electrorheological (ER) fluids. Here we discuss how, in addition to an
ER-type structural transition, the PK-4 dusty plasmas exhibit structural states reminiscent of
those observed in liquid crystals (LCs) with rod-shaped molecules. We find that dust particles
within the filaments are strongly coupled in a crystalline-like structure, while the coupling of
particles across filaments is liquid-like. In addition to a common orientation along a director
axis (nematic behavior), the dust filaments also appear to align in large-scale nested structures,
or shells (smectic behavior). Finally, these filaments are found to further arrange in hexagonal
patterns within the plane orthogonal to the director axis, suggesting the possibility for smectic-B
and smectic-C structural states. As the observed ER and LC features of the filamentary
dusty plasma states are sensitive to variations in the PK-4 discharge conditions, we argue that
these dusty plasmas can provide a controlled analogous system for the study of fundamental
phenomena in soft matter, such as the origins of pattern formation and universality of phase
transitions.
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Rods in a lyotropic chromonic liquid crystal: emergence of chirality, symmetry-breaking alignment, and caged angular diffusion
In lyotropic chromonic liquid crystals (LCLCs), twist distortion of the nematic director costs much less energy than splay or bend distortion. This feature leads to novel mirror-symmetry breaking director configurations when the LCLCs are confined by interfaces or contain suspended particles. Spherical colloids in an aligned LCLC nematic phase, for example, induce chiral director perturbations (“twisted tails”). The asymmetry of rod-like particles in an aligned LCLC offer a richer set of possibilities due to their aspect ratio ( α ) and mean orientation angle (〈 θ 〉) between their long axis and the uniform far-field director. Here we report on the director configuration, equilibrium orientation, and angular diffusion of rod-like particles with planar anchoring suspended in an aligned LCLC. Video microscopy reveals, counterintuitively, that two-thirds of the rods have an angled equilibrium orientation (〈 θ 〉 ≠ 0) that decreases with increasing α , while only one-third of the rods are aligned (〈 θ 〉 = 0). Polarized optical video-microscopy and Landau–de Gennes numerical modeling demonstrate that the angled and aligned rods are accompanied by distinct chiral director configurations. Angled rods have a longitudinal mirror plane (LMP) parallel to their long axis and approximately parallel to the substrate walls. Aligned rods have a transverse and longitudinal mirror plane (TLMP), where the transverse mirror plane is perpendicular to the rod's long axis. Effectively, the small twist elastic constant of LCLCs promotes chiral director configurations that modify the natural tendency of rods to orient along the far-field director. Additional diffusion experiments confirm that rods are angularly confined with strength that depends on α .
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- Award ID(s):
- 2003659
- NSF-PAR ID:
- 10330637
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 18
- Issue:
- 3
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 487 to 495
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1SM01209F
