Wave transmission across an interface in a complex plasma
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Abstract:
Complex plasmas consist of low-temperature plasmas with embedded
microparticles. The microparticles acquire electric charges of thousands
of electrons and strongly interact with each other. Due to their relatively large size and slow speed, their movement can be recorded
with digital cameras and traced from frame to frame. Here, we study
the propagation of waves across an interface formed between two subclouds of microparticles of different sizes. For this, we use data recorded under microgravity using the PK-3 Plus Laboratory on board the International Space Station, as well as Langevin dynamics simulations of
a complex plasma. Firstly, we study how self-excited waves are transmitted across the interface and demonstrate that a collision zone and
a merger zone form. Secondly, we study the propagation of a solitary
wave across an interface and demonstrate that at low pressures, reflection at the interface can be observed.Keywords:
Interface (matter)
Reflection
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In this study, we report a systematic study of the response of a charged microparticle confined in an optical trap and driven by electric fields. The particle is embedded in a polar fluid, hence, the role of ions and counterions forming a double layer around the electrodes and the particle surface itself has been taken into account. We analyze two different cases: (i) electrodes energized by a step‐wise voltage (DC mode) and (ii) electrodes driven by a sinusoidal voltage (AC mode). The experimental outcomes are analyzed in terms of a model that combines the electric response of the electrolytic cell and the motion of the trapped particle. In particular, for the DC mode we analyze the transient particle motion and correlate it with the electric current flowing in the cell. For the AC mode, the stochastic and deterministic motion of the trapped particle is analyzed either in the frequency domain (power spectral density, PSD) or in the time domain (autocorrelation function). Moreover, we will show how these different approaches (DC and AC modes) allow us, assuming predictable the applied electric field (here generated by plane parallel electrodes), to provide accurate estimation (3%) of the net charge carried by the microparticle. Vice versa, we also demonstrate how, once predetermined the charge, the trapped particle acts as a sensitive probe to reveal locally electric fields generated by arbitrary electrode geometries (in this work, wire‐tip geometry).
Particle (ecology)
Electrohydrodynamics
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Abstract Small solid metallic objects in relative motion to thermal plasmas are studied by numerical simulations. We analyze supersonic motions, where a distinctive ion wake is formed behind obstacles. At these plasma drift velocities, ions enter the wake predominantly due to deflections by the electric field in the sheath around the obstacle. By irradiating the back side of the object by ultraviolet (UV) light, we can induce also an enhanced photo-electron population there. The resulting charge distribution gives rise to a pronounced local potential and plasma density well behind the object. This potential variation has the form of a three-dimensional ion acoustic double layer, containing also an ion phase space vortex. The analysis is supported also by one-dimensional numerical simulations to illustrate the importance of boundary conditions, Dirichlet and von Neumann conditions in particular.
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We actively control interfacial phenomena by optically trapping the interface in phase separated colloid–polymer mixtures using the gradient forces of a strongly focussed laser beam parallel to the interface.
Interface (matter)
Tension (geology)
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Particle beam
Particle-in-cell
Axial symmetry
Magnetosphere particle motion
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Particles of micrometer size externally introduced in plasmas usually find their positions of levitation in the sheath, where the gravity force is compensated by the strong electric field. Here due to electrostatic interaction they form different structures, which are interesting objects for the investigation of strongly coupled systems and critical phenomena. Because of the low
damping (e.g. in comparison to colloidal suspension) it is possible to measure the dynamics up to the relevant highest frequency (e.g. Einstein frequency) at the most elementary level of single particle motion. The task of this work was to analyze the three dimensional structure, dynamical processes and the limit of the cooperative behavior in small crystals. In addition to the
study of the systems formed, the immersed particles themselves may be used for diagnostics of the environment: estimation of parameters or monitoring of the processes inside plasma. The laboratory experiments are performed in two radio-frequency (RF) reactors with
parallel plate electrodes, where the lower electrode is a so-called adaptive electrode. This electrode is segmented into 57 small pixels independently driven in DC (direct current) and/or RF voltage. When RF voltage is applied to one of these pixels, a bright localized glow, secondary
plasma ball, appears above. Three dimensional dust crystals with less than 100 particles are formed inside this plasma ball - the ideal conditions for the investigation of the transition from cluster systems to collective systems. The investigation of the particle interactions in crystals is performed with an optical diagnostic, which allows determination of all three particle coordinates simultaneously with time resolution of 0.04 sec.
The experimental results are:
1. The binary interaction among particles in addition to the repelling Coulomb force exhibits also an attractive part, which is experimentally determined for the first time.
2. Analysis of the dynamical evolution shows the tendency of the systems to approach the state with minimum energy by rearranging particles inside.
3. The measured 63 particles' crystal vibrations are in close agreement with vibrations of a drop with surface tension. This indicates that even a 63 particle crystal already exhibits properties normally associated with the cooperative regime.
The possibility to use levitated particles as a new powerful diagnostic of the sheath region is proposed. The existence of different equilibrium positions of microparticles suspended in an Oxygen discharge provides evidence of a structured electronegative sheath, a feature so far only mathematically and numerically investigated.
Ball lightning
Glow discharge
Electrostatics
Dielectrophoresis
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There has been growing interest in exerting the radiation forces to trap and cluster the randomly distributed cells in body fluid, the micro-particles in water, or the microorganism in the fluid-like culture medium. The standing wave is extensively utilized as a patterning tool, and micro-particles assemble at the nodes or antinodes. For the frequencies above megahertz, the distances among nodes or antinodes are below millimeters, too close for cluster separation or detection. We create a scenario where traveling waves dominate instead of standing waves by way of scattering dissipation, and particles assemble at a central location. We first provide a theoretical prediction model based on the translational addition theorem and the partial-wave expansion method. We evaluate the attenuation exerted by an external plane wave on a set of particles immersed in the host-fluid medium, and the bulk acoustic wave (BAW) device is used to verify the prediction. It is found that, for the high concentration medium with particles, the scattering dissipation is significant and the forces are directed towards a central space instead of the nodes or antinodes of the standing waves, leading to meaningful separation of micro-particles from their host-fluid medium.
Standing wave
Acoustic Radiation Force
Particle (ecology)
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In this article, a strategy to track microparticles and link their trajectories adapted to the study of the melting of a quasi two-dimensional complex plasma crystal induced by the mode-coupling instability is presented. Because of the three-dimensional nature of the microparticle motions and the inhomogeneities of the illuminating laser light sheet, the scattered light intensity can change significantly between two frames, making the detection of the microparticles and the linking of their trajectories quite challenging. Thanks to a two-pass noise removal process based on Gaussian blurring of the original frames using two different kernel widths, the signal-to-noise ratio was increased to a level that allowed a better intensity thresholding of different regions of the images and, therefore, the tracking of the poorly illuminated microparticles. Then, by predicting the positions of the microparticles based on their previous positions, long particle trajectories could be reconstructed, allowing accurate measurement of the evolution of the microparticle energies and the evolution of the monolayer properties.
Microparticle
Tracking (education)
Intensity
Particle (ecology)
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Precise bidirectional transport and size fractionation of microscopic colloidal particles is demonstratedvia square-wave modulation of a magnetic landscape.
Energy landscape
Particle (ecology)
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Wave propagation through packed spherical particles is characterized by two distinct mechanisms, depending on the frequency of the wave content. Coherent wave pulses occur when the Hertzian contact model can be used, i.e., for frequencies low enough so that the granules behave as rigid bodies. Above a certain frequency, a chaotic time signal is the result of diffusive energy transmission through the grain contacts. This so-called coda wave is important for applications tracking the microstructure of materials, e.g., in nondestructive testing. This work looks for parallels between the two approaches by investigating the transmission of short ultrasound pulses in the unit cell of a granular material: two spheres in contact. Laser measurements on two identical large steel spheres show that the energy transmission happens in discrete steps, due to guided surface waves. The measured and modeled energy distribution evolution are similar to the one predicted by the diffusive theory. However, we show that the Hertz contact law can be applied locally in the region of the contact to quantify the pulse transmission, despite the fact that the entire sphere no longer behaves as a rigid body. This approach allows for the design of a configurable non-linear waveguide.
Hertz
SIGNAL (programming language)
Coda
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We study wave propagation in a chain of spherical particles containing a local resonator. The resonant particles are made of an aluminum outer spherical shell and a steel inner mass connected by a polymeric plastic structure acting as a spring. We characterize the dynamic response of individual particles and the transmitted linear spectra of a chain of particles in contact. A wide band gap is observed both in theoretical and experimental results. We show the ability to tune the acoustic transmission by varying the contact interaction between particles. Higher driving amplitude leads to the generation of nonlinearities both in the response of a single particle and that of the whole chain. For a single resonant particle, we observe experimentally a resonant frequency downshift, which follows a complex nonlinear behavior. In the chain of particles, nonlinearity leads to the generation of nonlinear harmonics and the presence of localized modes inside the band gap.
Particle (ecology)
Chain (unit)
Spherical shell
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