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1. Surface wave pattern formation in a cylindrical container

   https://doi.org/10.1017/jfm.2021.97  

   Shao, X. ; Wilson, P. ; Saylor, J.R. ; Bostwick, J.B. ( May 2021 , Journal of Fluid Mechanics)

   Surface waves are excited by mechanical vibration of a cylindrical container having an air/water interface pinned at the rim, and the dynamics of pattern formation is analysed from both an experimental and theoretical perspective. The wave conforms to the geometry of the container and its spatial structure is described by the mode number pair ( $n,\ell$ ) that is identified by long exposure time white light imaging. A laser light system is used to detect the surface wave frequency, which exhibits either a (i) harmonic response for low driving amplitude edge waves or (ii) sub-harmonic response for driving amplitude above the Faraday wave threshold. The first 50 resonant modes are discovered. Control of the meniscus geometry is used to great effect. Specifically, when flat, edge waves are suppressed and only Faraday waves are observed. For a concave meniscus, edge waves are observed and, at higher amplitudes, Faraday waves appear as well, leading to complicated mode mixing. Theoretical predictions for the natural frequency of surface oscillations for an inviscid liquid in a cylindrical container with a pinned contact line are made using the Rayleigh–Ritz procedure and are in excellent agreement with experimental results. 

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2. Magma Oscillations in a Conduit‐Reservoir System, Application to Very Long Period (VLP) Seismicity at Basaltic Volcanoes: 1. Theory

   https://doi.org/10.1029/2019JB017437  

   Liang, Chao ; Karlstrom, Leif ; Dunham, Eric M. ( January 2020 , Journal of Geophysical Research: Solid Earth)

   Very long period (VLP, 2–100 s) seismic signals at basaltic volcanoes like Kilauea, Hawai‘i, and Mount Erebus, Antarctica are likely from resonant oscillations of magma within the shallow plumbing system. The system consists of conduits connected to cracks (dikes and sills) or reservoirs of other shapes. A quantitative understanding of wave propagation and resonance in a coupled conduit‐crack system is required to interpret observations. In this work, we idealize the system as an axisymmetric conduit coupled to a tabular crack, accounting for fluid inertia, compressibility, and viscosity, buoyancy, and crack wall elasticity. We perform time domain simulations and eigenmode analyses of the governing equations, linearized about a rest state. The fundamental mode or conduit‐reservoir mode reflects the balance of conduit magma inertia with buoyancy (and, for small cracks, crack wall elasticity). Magma oscillates in an effectively incompressible manner within the conduit, deflating and inflating the crack, which couples to the surrounding solid to produce observable surface displacements. For sufficiently low viscosity magmas, viscous effects are confined to boundary layers. Shorter period modes are primarily reverberating crack waves with negligible coupling to the conduit. Finally, we introduce an approximate reduced model for the conduit‐reservoir mode, which can also handle more general reservoir geometries (e.g., spherical chambers). The reduced model connects the observable VLP period and quality factor to two uniquely constrained parameters: the inviscid oscillation periodT₀and the viscous diffusion timeτ_visacross the conduit radius. Our models can be extended to study the seismic response of more complex magmatic systems.

    

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3. Rheological implications of embedded active matter in colloidal gels

   https://doi.org/10.1039/c9sm01496a  

   Szakasits, Megan E. ; Saud, Keara T. ; Mao, Xiaoming ; Solomon, Michael J. ( October 2019 , Soft Matter)

   null (Ed.)

   Colloidal gels represent an important class of soft matter, in which networks formed due to strong, short-range interactions display solid-like mechanical properties, such as a finite low-frequency elastic modulus. Here we examine the effect of embedded active colloids on the linear viscoelastic moduli of fractal cluster colloidal gels. We find that the autonomous, out-of-equilibrium dynamics of active colloids incorporated into the colloidal network decreases gel elasticity, in contrast to observed stiffening effects of myosin motors in actin networks. Fractal cluster gels are formed by the well-known mechanism of aggregating polystyrene colloids through addition of divalent electrolyte. Active Janus particles with a platinum hemisphere are created from the same polystyrene colloids and homogeneously embedded in the gels at dilute concentration at the time of aggregation. Upon addition of hydrogen peroxide – a fuel that drives the diffusiophoretic motion of the embedded Janus particles – the microdynamics and mechanical rheology change in proportion to the concentration of hydrogen peroxide and the number of active colloids. We propose a theoretical explanation of this effect in which the decrease in modulus is mediated by active motion-induced softening of the inter-particle attraction. Furthermore, we characterize the failure of the fluctuation–dissipation theorem in the active gels by identifying a discrepancy between the frequency-dependent macroscopic viscoelastic moduli and the values predicted by microrheology from measurement of the gel microdynamics. These findings support efforts to engineer gels for autonomous function by tuning the microscopic dynamics of embedded active particles. Such reconfigurable gels, with multi-state mechanical properties, could find application in materials such as paints and coatings, pharmaceuticals, self-healing materials, and soft robotics. 

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4. Energetic Electron Precipitation Driven by Electromagnetic Ion Cyclotron Waves from ELFIN’s Low Altitude Perspective

   https://doi.org/10.1007/s11214-023-00984-w  

   Angelopoulos, V. ; Zhang, X. -J. ; Artemyev, A. V. ; Mourenas, D. ; Tsai, E. ; Wilkins, C. ; Runov, A. ; Liu, J. ; Turner, D. L. ; Li, W. ; et al ( July 2023 , Space Science Reviews)

   We review comprehensive observations of electromagnetic ion cyclotron (EMIC) wave-driven energetic electron precipitation using data collected by the energetic electron detector on the Electron Losses and Fields InvestigatioN (ELFIN) mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000 keV electrons with good pitch-angle and energy resolution. EMIC wave-driven precipitation exhibits a distinct signature in energy-spectrograms of the precipitating-to-trapped flux ratio: peaks at >0.5 MeV which are abrupt (bursty) (lasting ∼17 s, or$\Delta L\sim 0.56$$\Delta L\sim 0.56$) with significant substructure (occasionally down to sub-second timescale). We attribute the bursty nature of the precipitation to the spatial extent and structuredness of the wave field at the equator. Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Case studies employing conjugate ground-based or equatorial observations of the EMIC waves reveal that the energy of moderate and strong precipitation at ELFIN approximately agrees with theoretical expectations for cyclotron resonant interactions in a cold plasma. Using multiple years of ELFIN data uniformly distributed in local time, we assemble a statistical database of ∼50 events of strong EMIC wave-driven precipitation. Most reside at$L\sim 5-7$$L\sim 5-7$at dusk, while a smaller subset exists at$L\sim 8-12$$L\sim 8-12$at post-midnight. The energies of the peak-precipitation ratio and of the half-peak precipitation ratio (our proxy for the minimum resonance energy) exhibit an$L$$L$-shell dependence in good agreement with theoretical estimates based on prior statistical observations of EMIC wave power spectra. The precipitation ratio’s spectral shape for the most intense events has an exponential falloff away from the peak (i.e., on either side of$\sim 1.45$$\sim 1.45$MeV). It too agrees well with quasi-linear diffusion theory based on prior statistics of wave spectra. It should be noted though that this diffusive treatment likely includes effects from nonlinear resonant interactions (especially at high energies) and nonresonant effects from sharp wave packet edges (at low energies). Sub-MeV electron precipitation observed concurrently with strong EMIC wave-driven >1 MeV precipitation has a spectral shape that is consistent with efficient pitch-angle scattering down to ∼ 200-300 keV by much less intense higher frequency EMIC waves at dusk (where such waves are most frequent). At ∼100 keV, whistler-mode chorus may be implicated in concurrent precipitation. These results confirm the critical role of EMIC waves in driving relativistic electron losses. Nonlinear effects may abound and require further investigation.

    

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5. Oscillations of a soft viscoelastic drop

   https://doi.org/10.1038/s41526-021-00169-1  

   Tamim, Saiful I. ; Bostwick, Joshua B. ( November 2021 , npj Microgravity)

   A soft viscoelastic drop has dynamics governed by the balance between surface tension, viscosity, and elasticity, with the material rheology often being frequency dependent, which are utilized in bioprinting technologies for tissue engineering and drop-deposition processes for splash suppression. We study the free and forced oscillations of a soft viscoelastic drop deriving (1) the dispersion relationship for free oscillations, and (2) the frequency response for forced oscillations, of a soft material with arbitrary rheology. We then restrict our analysis to the classical cases of a Kelvin–Voigt and Maxwell model, which are relevant to soft gels and polymer fluids, respectively. We compute the complex frequencies, which are characterized by an oscillation frequency and decay rate, as they depend upon the dimensionless elastocapillary and Deborah numbers and map the boundary between regions of underdamped and overdamped motions. We conclude by illustrating how our theoretical predictions for the frequency-response diagram could be used in conjunction with drop-oscillation experiments as a “drop vibration rheometer”, suggesting future experiments using either ultrasonic levitation or a microgravity environment.

    

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