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1. Compressible viscoelasticity of cell membranes determined by gigahertz-frequency acoustic vibrations

   https://doi.org/10.1016/j.pacs.2023.100494  

   Yu, Kuai ; Jiang, Yiqi ; Chen, Yungao ; Hu, Xiaoyan ; Chang, Junlei ; Hartland, Gregory V. ; Wang, Guo Ping ( June 2023 , Photoacoustics)

   Membrane viscosity is an important property of cell biology, which determines cellular function, development and disease progression. Various experimental and computational methods have been developed to investigate the mechanics of cells. However, there have been no experimental measurements of the membrane viscosity at high-frequencies in live cells. High frequency measurements are important because they can probe viscoelastic effects. Here, we investigate the membrane viscosity at gigahertz-frequencies through the damping of the acoustic vibrations of gold nanoplates. The experiments are modeled using a continuum mechanics theory which reveals that the membranes display viscoelasticity, with an estimated relaxation time of ca. ps. We further demonstrate that membrane viscoelasticity can be used to differentiate a cancerous cell line (the human glioblastoma cells LN-18) from a normal cell line (the mouse brain microvascular endothelial cells bEnd.3). The viscosity of cancerous cells LN-18 is lower than that of healthy cells bEnd.3 by a factor of three. The results indicate promising applications of characterizing membrane viscoelasticity at gigahertz-frequency in cell diagnosis. 

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2. Attitude​ ​Control​ ​System​ ​Complexity​ ​Reduction​ ​via​ ​Tailored​ ​Viscoelastic​ ​Damping Co-Design

   Chendi Lin, Daniel R ( February 2018 , 2018 AAS Guidance & Control Conference)

   Intelligent structures utilize distributed actuation, such as piezoelectric strain actuators, to control flexible structure vibration and motion. A new type of intelligent structure has been introduced recently for precision spacecraft attitude control. It utilizes lead zirconate titanate (PZT) piezoelectric actuators bonded to solar arrays (SAs), and bends SAs to use inertial coupling for small-amplitude, highprecision attitude control and active damping. Integrated physical and control system design studies have been performed to investigate performance capabilities and to generate design insights for this new class of attitude control system. Both distributed- and lumped-parameter models have been developed for these design studies. While PZTs can operate at high frequency, relying on active damping alone to manage all vibration requires high-performance control hardware. In this article we investigate the potential value of introducing tailored distributed viscoelastic materials within SAs as a strategy to manage higher-frequency vibration passively, reducing spillover and complementing active control. A case study based on a pseudo-rigid body dynamic model (PRBDM) and linear viscoelasticity is presented. The tradeoffs between control system complexity, passive damping behavior, and overall dynamic performance are quantified. 

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3. “Measures of Dissipation in Viscoelastic Media” Extended: Toward Continuous Characterization Across Very Broad Geophysical Time Scales

   https://doi.org/10.1029/2019GL083529  

   Lau, Harriet C. P. ; Holtzman, Benjamin K. ( August 2019 , Geophysical Research Letters)

   We develop a conceptual/quantitative framework whereby measurements of Earth's viscoelasticity may be assessed across the broad range of geophysical processes, spanning seismic wave propagation, postseismic relaxation, glacial isostatic adjustment, and mantle convection. Doing so requires overcoming three challenges: (A) separating spatial variations from intrinsic frequency dependence in mechanical properties; (B) reconciling different conceptual and constitutive viscoelastic models used to interpret observations at different frequencies; and (C) improving understanding of linear and nonlinear transient deformation mechanisms and their extrapolation from laboratory to earth conditions. We focus on (B), first demonstrating how different mechanical models lead to incompatible viscosity estimates from observations. We propose the determination of the “complex viscosity”—a frequency‐dependent parameter complementary to other measures of dissipation (including frequency‐dependent moduli and attenuation)—from such observations to reveal a single underlying broadband mechanical model. The complex viscosity illuminates transient creep in the vicinity of the Maxwell time, where most ambiguity lies.

    

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4. The Role of Frequency and Impedance Contrasts in Bandgap Closing and Formation Patterns of Axially-Vibrating Phononic Crystals

   https://doi.org/10.1115/1.4063815  

   Al Ba’ba’a, Hasan B. ; Nouh, Mostafa ( March 2024 , Journal of Applied Mechanics)

   Bandgaps, or frequency ranges of forbidden wave propagation, are a hallmark of phononic crystals (PnCs). Unlike their lattice counterparts, PnCs taking the form of continuous structures exhibit an infinite number of bandgaps of varying location, bandwidth, and distribution along the frequency spectrum. While these bandgaps are commonly predicted from benchmark tools such as the Bloch-wave theory, the conditions that dictate the patterns associated with bandgap symmetry, attenuation, or even closing in multi-bandgap PnCs remain an enigma. In this work, we establish these patterns in one-dimensional rods undergoing longitudinal motion via a canonical transfer-matrix-based approach. In doing so, we connect the conditions governing bandgap formation and closing to their physical origins in the context of the Bragg condition (for infinite media) and natural resonances (for finite counterparts). The developed framework uniquely characterizes individual bandgaps within a larger dispersion spectrum regardless of their parity (i.e., odd versus even bandgaps) or location (low versus high-frequency), by exploiting dimensionless constants of the PnC unit cell which quantify the different contrasts between its constitutive layers. These developments are detailed for a bi-layered PnC and then generalized for a PnC of any number of layers by increasing the model complexity. We envision this mathematical development to be a future standard for the realization of hierarchically structured PnCs with prescribed and finely tailored bandgap profiles. 

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5. Leveraging Vibrations and Guided Waves in a Human Skull

   https://doi.org/10.1115/IMECE2021-71315  

   Kohtanen, Eetu ; Mazzotti, Matteo ; Ruzzene, Massimo ; Erturk, Alper ( November 2021 , Proceedings of the ASME 2021 International Mechanical Engineering Congress and Exposition)

   This work is centered on high-fidelity modeling, analysis, and rigorous experiments of vibrations and guided (Lamb) waves in a human skull in two connected tracks: (1) layered modeling of the cranial bone structure (with cortical tables and diploë) and its vibration-based elastic parameter identification (and validation); (2) transcranial leaky Lamb wave characterization experiments and radiation analyses using the identified elastic parameters in a layered semi analytical finite element framework, followed by time transient simulations that consider the inner porosity as is. In the first track, non-contact vibration experiments are conducted to extract the first handful of modal frequencies in the auditory frequency regime, along with the associated damping ratios and mode shapes, of dry cranial bone segments extracted from the parietal and frontal regions of a human skull. Numerical models of the bone segments are built with a novel image reconstruction scheme that employs microcomputed tomographic scans to build a layered bone geometry with separate homogenized domains for the cortical tables and the diploë. These numerical models and the experimental modal frequencies are then used in an iterative parameter identification scheme that yields the cortical and diploic isotropic elastic moduli of each domain, whereas the corresponding densities are estimated using the total experimental mass and layer mass ratios obtained from the scans. With the identified elastic parameters, the average error between experimental and numerical modal frequencies is less than 1.5% and the modal assurance criterion values for most modes are above 0.90. Furthermore, the extracted parameters are in the range of the results reported in the literature. In the second track, the focus is placed on the subject of leaky Lamb waves, which has received growing attention as a promising alternative to conventional ultrasound techniques for transcranial transmission, especially to access the brain periphery. Experiments are conducted on the same cranial bone segment set for leaky Lamb wave excitation and radiation characterization. The degassed skull bone segments are used in submersed experiments with an ultrasonic transducer and needle hydrophone setup for radiation pressure field scanning. Elastic parameters obtained from the first track are used in guided wave dispersion simulations, and the radiation angles are accurately predicted using the aforementioned layered model in the presence of fluid loading. The dominant radiation angles are shown to correspond to guided wave modes with low attenuation and a significant out-of-plane polarization. The experimental radiation spectra are finally compared against those obtained from time transient finite element simulations that leverage geometric models reconstructed from microcomputed tomographic scans. 

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