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1. Scaling of pulsed nanosecond capillary plasmas at different specific energy deposition

   https://doi.org/10.1088/1361-6595/abc413  

   Zhu, Yifei; Starikovskaia, Svetlana M; Babaeva, Natalia Yu; Kushner, Mark J (December 2020, Plasma Sources Science and Technology)

   Abstract Nano-second, capillary discharges (nCDs) are unique plasma sources in their ability to sustain high specific energy deposition ω dep approaching 10 eV/molecule in molecular gases. This high energy loading on short timescales produces both high plasma densities and high densities of molecular exited states. These high densities of electrons and excited states interact with each other during the early afterglow through electron collision quenching and associative ionization. In this paper we discuss results from a two-dimensional computational investigation of a nCD sustained in air at a pressure of 28.5 mbar and with a voltage amplitude 20 kV. Discharges were investigated for two circuit configurations—a floating low voltage electrode and with the low voltage electrode connected to ground through a ballast resistor. The first configuration produced a single ionization wave from the high to low voltage electrode. The second produced converging ionization waves beginning at both electrodes. With a decrease of the tube radius, the velocity of the ionization fronts decreased while the shape of the ionization wave changed from the electron density being distributed smoothly in the radial direction, to being hollow shaped where there is a higher electron density near the tube wall. For sufficiently small tubes, the near-wall maxima merge to have the higher density on the axis of the capillary tube. In the early afterglow, the temporal and radial behavior of the N 2 (C 3 Π u ) density is a sensitive function of ω dep due to electron collision quenching. These trends indicate that starting from ω dep ⩾ 0.3 eV/molecule, it is necessary to take into account interactions of electrons with electronically excited species during the discharge and early afterglow. 

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2. Tackling Multi-Physics Nano-Scale Phenomena in Capillary Force Lithography with Small Data by Hybrid Intelligence

   https://doi.org/10.3390/mi14111984  

   Chapagain, Ashish; Cho, In Ho (November 2023, Micromachines)

   The scientific community has been looking for novel approaches to develop nanostructures inspired by nature. However, due to the complicated processes involved, controlling the height of these nanostructures is challenging. Nanoscale capillary force lithography (CFL) is one way to use a photopolymer and alter its properties by exposing it to ultraviolet radiation. Nonetheless, the working mechanism of CFL is not fully understood due to a lack of enough information and first principles. One of these obscure behaviors is the sudden jump phenomenon—the sudden change in the height of the photopolymer depending on the UV exposure time and height of nano-grating (based on experimental data). This paper uses known physical principles alongside artificial intelligence to uncover the unknown physical principles responsible for the sudden jump phenomenon. The results showed promising results in identifying air diffusivity, dynamic viscosity, surface tension, and electric potential as the previously unknown physical principles that collectively explain the sudden jump phenomenon. 

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3. The elastic Rayleigh drop

   https://doi.org/10.1039/C9SM01753D  

   Tamim, S. I.; Bostwick, J. B. (November 2019, Soft Matter)

   Bioprinting technologies rely on the formation of soft gel drops for printing tissue scaffolds and the dynamics of these drops can affect the process. A model is developed to describe the oscillations of a spherical gel drop with finite shear modulus, whose interface is held by surface tension. The governing elastodynamic equations are derived and a solution is constructed using displacement potentials decomposed into a spherical harmonic basis. The resulting nonlinear characteristic equation depends upon two dimensionless numbers, elastocapillary and compressibility, and admits two types of solutions, (i) spheroidal (or shape change) modes and (ii) torsional (rotational) modes. The torsional modes are unaffected by capillarity, whereas the frequency of shape oscillations depend upon both the elastocapillary and compressibility numbers. Two asymptotic dispersion relationships are derived and the limiting cases of the inviscid Rayleigh drop and elastic globe are recovered. For a fixed polar wavenumber, there exists an infinity of radial modes that each transition from an elasticity wave to a capillary wave upon increasing the elastocapillary number. At the transition, there is a qualitative change in the deformation field and a set of recirculation vortices develop at the free surface. Two special modes that concern volume oscillations and translational motion are characterized. A new instability is documented that reflects the balance between surface tension and compressibility effects due to the elasticity of the drop. 

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4. The magnetic topology of the inverse Evershed flow

   https://doi.org/10.1051/0004-6361/202142585  

   Prasad, A; Ranganathan, M; Beck, C; Choudhary, D P; Hu, Q (June 2022, Astronomy & Astrophysics)

   Context. The inverse Evershed flow (IEF) is a mass motion towards sunspots at chromospheric heights. Aims. We combined high-resolution observations of NOAA 12418 from the Dunn Solar Telescope and vector magnetic field measurements from the Helioseismic and Magnetic Imager (HMI) to determine the driver of the IEF. Methods. We derived chromospheric line-of-sight (LOS) velocities from spectra of H α and Ca  II IR. The HMI data were used in a non-force-free magnetic field extrapolation to track closed field lines near the sunspot in the active region. We determined their length and height, located their inner and outer foot points, and derived flow velocities along them. Results. The magnetic field lines related to the IEF reach on average a height of 3 megameter (Mm) over a length of 13 Mm. The inner (outer) foot points are located at 1.2 (1.9) sunspot radii. The average field strength difference Δ B between inner and outer foot points is +400 G. The temperature difference Δ T is anti-correlated with Δ B with an average value of −100 K. The pressure difference Δ p is dominated by Δ B and is primarily positive with a driving force towards the inner foot points of 1.7 kPa on average. The velocities predicted from Δ p reproduce the LOS velocities of 2–10 km s −1 with a square-root dependence. Conclusions. We find that the IEF is driven along magnetic field lines connecting network elements with the outer penumbra by a gas pressure difference that results from a difference in field strength as predicted by the classical siphon flow scenario. 

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5. Siphon Break Phenomena Associated With Pipe Leakage Location

   https://doi.org/10.1115/1.4054654  

   Zanje, Sumit R.; Bian, Linlong; Verma, Vivek; Yin, Zeda; Leon, Arturo S. (November 2022, Journal of Fluids Engineering)

   Abstract This work investigates the siphon break phenomenon associated with pipe leakage location. The present study is divided into two parts: (1) an unsteady three-dimensional (3D) computational fluid dynamics (CFD) model is developed to simulate the pressure head (water level) and discharge in the simulated siphon using the volume-of-fluid (VOF) technique under no-leakage condition and (2) using the model developed in the first part we investigated the siphon break phenomenon associated with pipe leakage location. The calculated results of transient water level and discharge rate at the simulated siphon for the no-leakage condition were in good agreement with the experimental measurements. In addition, the velocity, pressure fields, and phase fractions in the siphon pipe were analyzed in depth. The methodology and findings presented show that leakage above the hydraulic grade line and close to the top inverted U section of the siphon pipe ultimately leads to the siphon breakage, which is not the case for a leakage below the hydraulic grade line. It is also concluded that if leakage is above the hydraulic grade line and the leakage position is far away from the upper horizontal section of the siphon pipe, it may not lead to the immediate siphon breakage as ingested air gets removed with siphoning water, allowing it further time to cause complete siphon breakage. 

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