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1. Observations of Breaking Wave Dissipation and Their Relationship to Atmosphere‐Ocean Energy Transfer

   https://doi.org/10.1029/2024JC022130  

   Hogan, L.; Zappa, C_J; Cifuentes‐Lorenzen, A.; Edson, J_B; O’Donnell, J.; Ullman, D_S (June 2025, Journal of Geophysical Research: Oceans)

   Abstract Energy is transferred from the atmosphere to the ocean primarily through ocean surface waves, and the majority is dissipated locally in the near‐surface ocean. Observations of turbulent kinetic energy (TKE) in the upper ocean have shown dissipation rates exceeding law‐of‐the‐wall theory by an order of magnitude. The excess near‐surface ocean TKE dissipation rate is thought to be driven primarily by wave breaking, which limits wave growth and transfers energy from the surface wave field to the wave‐affected layer of the ocean. Here, the statistical properties of breaking wave dynamics in a coastal area are extracted from visible imagery and used to estimate TKE dissipation rates due to breaking waves. The statistical properties of whitecap dynamics are quantified with Λ(c), a distribution of total whitecap crest length per unit area as a function of crest speed, and used to compute energy dissipation by breaking waves, S_ds. S_dsapproximately balances elevated subsurface dissipation in young seas but accounts for only a fraction of subsurface dissipation in older seas. The wind energy input is estimated from wave spectra from polarimetric imagery and laser altimetry. S_dsbalances the wind energy input except under high winds. Λ(c)‐derived estimates of TKE dissipation rates by breaking waves compare well with the atmospheric deficit in TKE dissipation, a measure of energy input to the wave field (Cifuentes‐Lorenzen et al., 2024). These results tie the observed atmospheric dissipation deficit and enhancement in subsurface TKE dissipation to wave driven energy transport, constraining the TKE dissipation budget near the air‐sea interface. 

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2. Dynamic diagnosis of metamaterials through laser-induced vibrational signatures

   https://doi.org/10.1038/s41586-023-06652-x  

   Kai, Yun; Dhulipala, Somayajulu; Sun, Rachel; Lem, Jet; DeLima, Washington; Pezeril, Thomas; Portela, Carlos M. (November 2023, Nature)

   Kareh, Kristina (Ed.)

   Mechanical metamaterials at the microscale exhibit exotic static properties owing to their en- gineered building blocks, but their dynamic properties have remained significantly less explored. Their design principles can target frequency-dependent properties and resilience upon high-strain-rate deformation, making them versatile materials for applications in lightweight impact resistance, acoustic waveguiding, or vibration damping. However, accessing dynamic properties at small scales has remained a challenge due to low-throughput and destructive characterization, or lack of existing testing protocols. Here we demonstrate a high-throughput non-contact framework that employs MHz-wave propagation signatures within a metamaterial to nondestructively extract dynamic linear properties, omnidirectional elastic information, damping properties, and defect quantification. Using rod-like tessellations of microscopic metamaterials, we report up to 94% direction- and rate-dependent dynamic stiffening at strain rates approaching 10^2 s^{−1}, in addition to damping properties 3 times higher than their constituent materials. We also show that frequency shifts in the vibrational response allow for characterization of invisible defects within the metamaterials, and that selective probing allows for construction of experimental elastic surfaces, previously only possible computationally. Our work provides a route for accelerated data-driven discovery of materials and microdevices for dynamic applications such as protective structures, medical ultrasound, or vibration isolation. 

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3. Spectro‐Microscopic Characterization of Elastomers Subjected to Laser‐Induced Shock Waves

   https://doi.org/10.1002/mame.202100506  

   Huynh, Nha Uyen; Gamez, Carlos; Youssef, George (December 2021, Macromolecular Materials and Engineering)

   Abstract The core of this research is separated into three domains, the ultrahigh strain rate response of elastomeric polymers, laser‐induced shock waves , and terahertz time‐domain spectroscopy (THz‐TDS). Elastomers, e.g., polyurea, constitute an advance class of materials suitable for many applications, specifically in high impact loading scenarios, thus, a laser‐induced shock wave (LSW) experimental technique is used to investigate the mechanical response of shock‐loaded polyurea. LSW can submit samples to a strain rate exceeding 10⁶s⁻¹at low strains, enabling determination of material intrinsic failure modes. The large deformation induced during shock loading may alter the macromolecule structure, which can only be detected spectroscopically. Therefore, this research incorporated terahertz bulk spectroscopy to detect and report molecular conformational changes. Microscopy techniques were also used to elucidate changes in the microscale properties, morphology, and topography. The interpretation of the results explicated brittle failure in terms of partial and total spallation and, remarkably, ductile failure leading to plastic deformation, including plastic bulging and adiabatic shearing, not previously associated with LSW technique. Furthermore, spectral changes found in the terahertz regime substantiated the validity of terahertz spectroscopy in elucidating the underlying mechanism associated with the impact mitigating properties of dynamically loaded polyurea. 

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4. Ballistic and Blast-Relevant, High-Rate Material Properties of Physically and Chemically Crosslinked Hydrogels

   https://doi.org/10.1007/s11340-024-01043-3  

   Bremer-Sai, E C; Yang, J; McGhee, A; Franck, C (April 2024, Experimental Mechanics)

   Abstract BackgroundHydrogels are one of the most ubiquitous polymeric materials. Among them gelatin, agarose and polyacrylamide-based formulations have been effectively utilized in a variety of biomedical and defense-related applications including ultrasound-based therapies and soft tissue injury investigations stemming from ballistic and blast exposures. Interestingly, while in most cases accurate prediction of the mechanical response of these surrogate gels requires knowledge of the underlying finite deformation, high-strain rate material properties, it is these properties that have remained scarce in the literature. ObjectiveBuilding on our prior works using Inertial Microcavitation Rheometry (IMR), here we present a comprehensive list of the high-strain rate (> 10$$^3$$ ${}^3$1/s) mechanical properties of these three popular classes of hydrogel materials characterized via laser-based IMR, further showing that the choice in finite-deformation, rate-dependent constitutive model can be informed directly by the type of crosslinking mechanism and resultant network structure of the hydrogel, thus providing a chemophysical basis of the the choice of phenomenological constitutive model. MethodsWe analyze existing experimental gelatin IMR datasets and compare the results with prior data on polyacrylamide. ResultsWe show that a Neo-Hookean Kelvin-Voigt (NHKV) model can suitably simulate the high-rate material response of dynamic, physically crosslinked hydrogels like gelatin, while the introduction of a strain-stiffening parameter through the use of the quadratic Kelvin-Voigt (qKV) model was necessary to appropriately model chemically crosslinked hydrogels such as polyacrylamide due to the nature of the static,covalent bonds that comprise their structure. ConclusionsIn this brief we show that knowledge of the type of underlying polymer structure, including its bond mobility, can directly inform the appropriate finite deformation, time-dependent viscoelastic material model for commonly employed tissue surrogate hydrogels undergoing high strain rate loading within the ballistic and blast regimes. 

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5. 3-D PRINTING OF DIELECTRIC ELECTROACTIVE POLYMER ACTUATORS AND CHARACTERIZATION OF DIELECTRIC FLEXIBLE MATERIALS

   Gonzalez, David; Newell, Brittany; Garcia, Jose; Noble, Lucas; Mamer, Trevor (October 2018, ASME Conference on Smart Materials, Adaptive Structures, and Intelligent Systems (SMASIS))

   Dielectric electroactive polymers are materials capable of mechanically adjusting their volume in response to an electrical stimulus. However, currently these materials require multi-step manufacturing processes which are not additive. This paper presents a novel 3D printed flexible dielectric material and characterizes its use as a dielectric electroactive polymer (DEAP) actuator. The 3D printed material was characterized electrically and mechanically and its functionality as a dielectric electroactive polymer actuator was demonstrated. The flexible 3-D printed material demonstrated a high dielectric constant and ideal stress-strain performance in tensile testing making the 3-D printed material ideal for use as a DEAP actuator. The tensile stress- strain properties were measured on samples printed under three different conditions (three printing angles 0°, 45° and 90°). The results demonstrated the flexible material presents different responses depending on the printing angle. Based on these results, it was possible to determine that the active structure needs low pre-strain to perform a visible contractive displacement when voltage is applied to the electrodes. The actuator produced an area expansion of 5.48% in response to a 4.3 kV applied voltage, with an initial pre-strain of 63.21% applied to the dielectric material. 

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