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1. Benchmarking surface tension measurement method using two oscillation modes in levitated liquid metals

   https://doi.org/10.1038/s41526-021-00137-9  

   Brosius, Nevin ; Ward, Kevin ; Wilson, Evan ; Karpinsky, Zachary ; SanSoucie, Michael ; Ishikawa, Takehiko ; Matsumoto, Satoshi ; Narayanan, Ranga ( December 2021 , npj Microgravity)

   null (Ed.)

   Abstract The Faraday forcing method in levitated liquid droplets has recently been introduced as a method for measuring surface tension using resonance. By subjecting an electrostatically levitated liquid metal droplet to a continuous, oscillatory, electric field, at a frequency nearing that of the droplet’s first principal mode of oscillation (known as mode 2), the method was previously shown to determine surface tension of materials that would be particularly difficult to process by other means, e.g., liquid metals and alloys. It also offers distinct advantages in future work involving high viscosity samples because of the continuous forcing approach. This work presents (1) a benchmarking experimental method to measure surface tension by excitation of the second principal mode of oscillation (known as mode 3) in a levitated liquid droplet and (2) a more rigorous quantification of droplet excitation using a projection method. Surface tension measurements compare favorably to literature values for Zirconium, Inconel 625, and Rhodium, using both modes 2 and 3. Thus, this new method serves as a credible, self-consistent benchmarking technique for the measurement of surface tension. 

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2. Acoustic Levitation-Assisted Contactless Printing of Microdroplets for Biomedical Applications

   https://doi.org/10.1115/1.4062971  

   Tang, Tengteng ; Joralmon, Dylan ; Anyigbo, Tochukwu ; Li, Xiangjia ( January 2024 , Journal of Manufacturing Science and Engineering)

   Abstract The cell is a microcapsule system wherein biological materials are encapsulated by a thin membrane, which provides valuable information on the metabolism, morphology, development, and signal transduction pathways of the studied cell. The cell-inspired microdroplet has the characteristics of efficient nanoscale substance transportation, self-organization, and morphological adaptation. However, it is extremely difficult to manufacture such systems. Mostly vesicles such as liposomes, polymersomes, and microcapsules are first produced by a high-pressure homogenizer and microfluidizer as an emulsion and then encapsulated microcapsules by the drop or emulsion method. Currently, acoustic levitation opens entirely new possibilities for creating bioinspired microdroplets because of its ability to suspend tiny droplets in an antigravity and noncontact manner. Herein, we propose contactless printing of single-core or multi-core cell-inspired microdroplets via acoustic levitation. First, the oscillation mode and microscopic morphology of the droplets under different ultrasonic vibration frequencies are shown by simulation, and the curing characteristics of the shell structure under different ultraviolet illumination conditions are quantitatively measured. The feasibility of manufacturing multi-core microdroplets and manufacturing submillimeter-scale particles based on oil trapping is extensively studied. To explore the morphological adaptability of microdroplets, ferromagnetic Fe3O4 nanoparticles are used to give magnetic-responsive properties to cells, and the microscopic deformation and motion in microfluidic channels under the magnetic field are characterized. Finally, the proposed printing method proves the versatility of in-space contactless printing of complex 3D beam structures and provides a powerful platform for developing biomedical devices and microrobots and studying morphogenesis and synthetic biological systems. 

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3. A Comprehensive Perturbative Formalism for Phase Mixing in Perturbed Disks. II. Phase Spirals in an Inhomogeneous Disk Galaxy with a Nonresponsive Dark Matter Halo

   https://doi.org/10.3847/1538-4357/acd641  

   Banik, Uddipan ; van den Bosch, Frank C. ; Weinberg, Martin D. ( July 2023 , The Astrophysical Journal)

   We develop a linear perturbative formalism to compute the response of an inhomogeneous stellar disk embedded in a nonresponsive dark matter (DM) halo to various perturbations like bars, spiral arms, and encounters with satellite galaxies. Without self-gravity to reinforce it, the response of a Fourier mode phase mixes away due to an intrinsic spread in the vertical (Ω_z), radial (Ω_r), and azimuthal (Ω_ϕ) frequencies, triggering local phase-space spirals. The detailed galactic potential dictates the shape of phase spirals: phase mixing occurs more slowly and thus phase spirals are more loosely wound in the outer disk and in the presence of an ambient DM halo. Collisional diffusion due to scattering of stars by structures like giant molecular clouds causes superexponential damping of the phase spiral amplitude. Thez–v_zphase spiral is one-armed (two-armed) for vertically antisymmetric (symmetric) bending (breathing) modes. Only transient perturbations with timescales (τ_P) comparable to the vertical oscillation period (τ_z∼ 1/Ω_z) can trigger vertical phase spirals. Each (n,l,m) mode of the response to impulsive (τ_P<τ= 1/(nΩ_z+lΩ_r+mΩ_ϕ)) perturbations is power-law (∼τ_P/τ) suppressed, but that to adiabatic (τ_P>τ) perturbations is exponentially weak ($\sim \exp \left[-{\left({\tau }_{\mathrm{P}}/\tau \right)}^{\alpha }\right]$) except for resonant (τ→ ∞ ) modes. Slower (τ_P>τ_z) perturbations, e.g., distant encounters with satellite galaxies, induce stronger bending modes. Sagittarius (Sgr) dominates the solar neighborhood response of the Milky Way (MW) disk to satellite encounters. Thus, if the Gaia phase spiral was triggered by a MW satellite, Sgr is the leading contender. However, the survival of the phase spiral against collisional damping necessitates an impact ∼0.6–0.7 Gyr ago.

    

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4. L Versus Time Structures of Dayside Magnetic Pulsations Detected by the European Quasi‐Meridional Magnetometer Array

   https://doi.org/10.1029/2019JA026796  

   Takahashi, Kazue ; Heilig, Balázs ( August 2019 , Journal of Geophysical Research: Space Physics)

   We studied the spatiotemporal structure of ground magnetic pulsations on the dayside by displaying magnetic field perturbations detected by the European quasi‐Meridional Magnetometer Array (EMMA) as 2‐D images in the magneticLvalue versus time space, called EMMAgrams. We generated EMMAgrams from observations made on 15 August 2015, including a previously studied pulsation event associated with an interplanetary shock. In addition to signatures of field line resonance driven by a cavity mode oscillation, we found poleward propagating structures withL‐independent periods in the Pc2 band. The Pc2 structures are attributed to periodic magnetohydrodynamic pulses (upstream waves) originating from the ion foreshock and propagating in the magnetosphere along the path proposed by Tamao. Ringing of local field lines atL‐dependent periods (transient pulsations) is also clearly detected as dispersive poleward propagating structures not only immediately after the shock impact but also during time periods of less obvious external disturbances. A transient pulsation decays after a few wave periods, and cross‐spectral analysis of transient pulsations detected at two stations with a small latitudinal separation indicates elevation of the cross phase in a band delimited by the field line resonance frequencies at the stations. Successive excitation of transient pulsations by variations of the solar wind dynamic pressure appears to contribute significantly to formation of similar cross‐phase peaks that are widely used in magnetoseismic studies.

    

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5. Pattern formation in Faraday instability—experimental validation of theoretical models

   https://doi.org/10.1098/rsta.2022.0081  

   Dinesh, B. ; Livesay, J. ; Ignatius, I. B. ; Narayanan, R. ( April 2023 , Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   n/a (Ed.)

   Two types of resonance-derived interfacial instability are reviewed with a focus on recent work detailing the effect of side walls on interfacial mode discretization. The first type of resonance is the mechanical Faraday instability, and the second is electrostatic Faraday instability. Both types of resonance are discussed for the case of single-frequency forcing. In the case of mechanical Faraday instability, inviscid theory can forecast the modal forms that one might expect when viscosity is taken into account. Experiments show very favourable validation with theory for both modal forms and onset conditions. Lowering of gravity is predicted to shift smaller wavelengths or choppier modes to lower frequencies. This is also validated by experiments. Electrostatic resonant instability is shown to lead to a pillaring mode that occurs at low wavenumbers, which is akin to Rayleigh Taylor instability. As in the case of mechanical resonance, experiments show favourable validation with theoretical predictions of patterns. A stark difference between the two forms of resonance is the observation of a gradual rise in the negative detuning instability in the case of mechanical Faraday and a very sharp one in the case of electrostatic resonance.

   This article is part of the theme issue ‘New trends in pattern formation and nonlinear dynamics of extended systems’.

    

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