Viscoelasticity of 2D Liquids Quantified in a Dusty Plasma Experiment

Abstract

The viscoelasticity of two-dimensional liquids is quantified in an experiment using a dusty plasma. An experimental method is demonstrated for measuring the wavenumber-dependent viscosity, η(k), which is a quantitative indicator of viscoelasticity. Using an expression generalized here to include friction, η(k) is computed from the transverse current autocorrelation function (TCAF), which is found by tracking random particle motion. The TCAF exhibits an oscillation that is a signature of elastic contributions to viscoelasticity. Simulations of a Yukawa liquid are consistent with the experiment.

Viscosity calculated in simulations of strongly coupled dusty plasmas with gas friction

“…20,44,45 The few experiments that have been reported for viscoelasticity of dusty plasma include a descriptive presentation 44 and a characterization using a correlation function of the microscopic motion of dust particles. 45 In our recent 2D experiment, 20 a single horizontal layer of electrically charged dust particles was levitated in a glowdischarge plasma.…”

“…In this paper, we carry out further simulations for two purposes: to validate the g(k) calculation method taking into account gas friction, as presented in Ref. 20, and to assess the accuracy of the Green-Kubo relation for dusty plasmas with a modest level of gas friction.…”

“…Gas represents <10% of the mass of dust in a 3D dusty plasma experiment at 400 mTorr, 19 while 2D experiments have even less gas, with a pressure <20 mTorr. 9,20,21 There is an ion drag force due to a steady flow of ions, arising from the same dc electric fields that provide levitation and confinement of dust particles. This ion drag force is parallel to the ion flow.…”

“…Finally, in some experiments, laser radiation pressure forces are used to accelerate dust particles, for example, to create macroscopic flows 12,16,21,22 or simply raise the kinetic temperature of the dust particles without causing a macroscopic flow. 9,20,23,24 This kind of laser heating method is one of several ways that experimenters can control C so that the cloud of dust particles behaves like a liquid or a solid. 13,[24][25][26][27] We assume that the Coulomb interaction amongst charged dust particles is the dominant mechanism for viscosity in laboratory dusty plasma experiments.…”

“…The kinetic temperature of the dust cloud was raised by laser heating. 20,24 Viewing from above, we recorded movies of particle motion, then calculated particle positions and tracked them to calculate their velocities. Based on the trajectories of particles, the wave-numberdependent viscosity g(k) of the 2D dusty plasma was quantified using an expression we derived that accounts for gas friction.…”