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An interesting aspect of complex plasma is its ability to self-organize into a variety of structural configurations and undergo transitions between these states. A striking phenomenon is the isotropic-to-string transition observed in electrorheological complex plasma under the influence of a symmetric ion wakefield. Such transitions have been investigated using the Plasma Kristall-4 (PK-4) microgravity laboratory on the International Space Station (ISS). Recent experiments and numerical simulations have shown that, under PK-4 relevant discharge conditions, the seemingly homogeneous DC discharge column is highly inhomogeneous, with large axial electric field oscillations associated with ionization waves occurring on microsecond time scales. A multi-scale numerical model of the dust-plasma interactions is employed to investigate the role of the electric field on the charge of individual dust grains, the ion wakefield, and the order of string-like structures. Results are compared to dust strings formed in similar conditions in the PK-4 experiment., Comment: 18 pages, 11 figures

Shear flow in a three-dimensional complex plasma was experimentally studied in microgravity conditions using Plasmakristall-4 (PK-4) instrument on board the International Space Station (ISS). The shear flow was created in an extended suspension of microparticles by applying the radiation pressure force of the manipulation-laser beam. Individual particle trajectories in the flow were analyzed and from these, using the Navier-Stokes equation, an upper estimate of the complex plasma's kinematic viscosity was calculated in the range of $0.2$--$6.7~{\rm mm^2/s}$. This estimate is much lower than previously reported in ground-based experiments with 3D complex plasmas. Possible reasons of this difference are discussed., Comment: 5 pages, 4 figures

Three-dimensional structure of complex (dusty) plasmas was investigated under long-term microgravity conditions in the International-Space-Station-based Plasmakristall-4 facility. The microparticle suspensions were confined in a polarity-switched dc discharge. The experimental results were compared to the results of the molecular dynamics simulations with the interparticle interaction potential represented as a superposition of isotropic Yukawa and anisotropic quadrupole terms. Both simulated and experimental data exhibited qualitatively similar structural features indicating the bulk liquid-like order with the inclusion of solid-like strings aligned with the axial electric field. Individual strings were identified and their size spectrum was calculated. The decay rate of the size spectrum was found to decrease with the enhancement of string-like structural features.

Inactivation of microorganisms on sensitive surfaces by cold atmospheric plasma (CAP) is one major application in the field of plasma medicine because it provides a simple and effective way to sterilize heat-sensitive materials. Therefore, one has to know whether plasma treatment affects the treated surfaces, and thus causes long-term surface modifications. In this contribution, the effect of cold atmospheric Surface Micro-Discharge (SMD) plasma on different materials and its sporicidal behavior was investigated. Hence, different material samples (stainless steel, different polymers and glass) were plasma-treated for 16 hours, simulating multiple plasma treatments using an SMD plasma device. Afterwards, the material samples were analyzed using surface analysis methods such as laser microscopy, contact angle measurements and X-ray photoelectron spectroscopy (XPS). Furthermore, the device was used to investigate the behavior of Bacillus atrophaeus endospores inoculated on material samples at different treatment times. The interaction results for plasma-treated endospores show, that a log reduction of the spore count between 4.3 and 6.2 can be achieved within 15 min of plasma treatment. Besides, the surface analysis revealed, that there were three different types of reactions the probed materials showed to plasma treatment, ranging from no changes to shifts of the materials' free surface energies and oxidation. As a consequence, it should be taken into account that even though cold atmospheric plasma treatment is a non-thermal method to inactivate microorganisms on heatsensitive materials, it still affects surface properties of the treated materials. Therefore, the focus of future work must be a further classification of plasma-caused material modifications., Comment: 14 pages, 15 figures

The charge of microparticles immersed in the dc discharge of the Plasmakristall-4 experimental facility has been estimated using the particle velocities from experiments performed on Earth and under microgravity conditions on the International Space Station. The theoretical model used for these estimates is based on the balance of the forces acting on a single particle in the discharge. The model takes into account the radial dependence of the discharge parameters and describes reasonably well the experimental measurements., Comment: 6 pages, 5 figures, published in Phys. Plasmas

We report on the observation of the self-excited dust density waves in the dc discharge complex plasma. The experiments were performed under microgravity conditions in the Plasmakristall-4 facility on board the International Space Station. In the experiment, the microparticle cloud was first trapped in an inductively coupled plasma, then released to drift for some seconds in a dc discharge with constant current. After that the discharge polarity was reversed. DC plasma containing a drifting microparticle cloud was found to be strongly non-uniform in terms of microparticle drift velocity and plasma emission in accord with [Zobnin et.al., Phys. Plasmas 25, 033702 (2018)]. In addition to that, non-uniformity in the self-excited wave pattern was observed: In the front edge of the microparticle cloud (defined as head), the waves had larger phase velocity than in the rear edge (defined as tail). Also, after the polarity reversal, the wave pattern exhibited several bifurcations: Between each of the two old wave crests, a new wave crest has formed. These bifurcations, however, occurred only in the head of the microparticle cloud. We show that spatial variations of electric field inside the drifting cloud play an important role in the formation of the wave pattern. Comparison of the theoretical estimations and measurements demonstrate the significant impact of the electric field on the phase velocity of the wave. The same theoretical approach applied to the instability growth rate, showed agreement between estimated and measured values., Comment: 7 pages, 4 figures

We study the frequency dependencies of the fermion and photon dressing functions in dynamical gap generation in graphene. We use a low energy effective QED-like description, but within this approximation, we include all frequency dependent effects including retardation. We obtain the critical coupling by calculating the gap using a non-perturbative Dyson-Schwinger approach. Compared to the results of our previous calculation [1] which used a Lindhard screening approximation instead of including a self-consistently calculated dynamical screening function, the critical coupling is substantially reduced., Comment: 13 pages, 5 figures, published version

We study the frequency dependencies in the renormalization of the fermion Greens function for the $\pi$-band electrons in graphene and their influence on the dynamical gap generation at sufficiently strong interaction. Adopting the effective QED-like description for the low-energy excitations within the Dirac-cone region we self consistently solve the fermion Dyson-Schwinger equation in various approximations for the photon propagator and the vertex function with special emphasis on frequency dependent Lindhard screening and retardation effects., Comment: 19 pages, 5 figures