Oscillatory Deformation Research Articles

Isolating the Poroelastic Response of the Groundwater System in InSAR Data From the Central Valley of California

AbstractWorking with 5 years (2015–2019) of high quality interferometric synthetic aperture radar (InSAR) data covering the Central Valley of California, we developed a new approach to isolate the poroelastic component in InSAR data by estimating and removing the hydrologic loading component. K‐means clustering was used to identify the InSAR deformation time series dominated by the loading response and those dominated by the poroelastic response. The former time series include seasonal oscillations of uplift and subsidence related to mass changes in snow and ice in the adjacent mountain range, while the latter include interannual deformation and seasonal oscillations related to changing head in the valley. The loading component accounts for, on average, 56 ± 12% of the deformation in the Central Valley. Without correcting for the loading response, the deformation due to the poroelastic response (and therefore any derived estimate of change in head) is underestimated by 50%–60% in areas of the valley.

Experimental study on the elastic-plastic dynamic response of shallow-buried corrugated steel-plain concrete composite structures under long-duration plane blast wave loading

To study the elastic-plastic dynamic response characteristics of shallow-buried corrugated steel-plain concrete (CSPC) composite structures under long-duration plane blast loads, a series of tests were conducted in a large confined blast-resistant test facility using the plane charge explosion technique (PCET), and the dynamic response and deformation laws of CSPC arches, reinforced concrete (RC) arches and plain concrete (PC) arches under different blast loads were comprehensively compared and analyzed. The test results show that under this test condition, the plane air shock wave overpressure peak with the increase in charge is basically a linear growth, but the shock wave pressure duration of action and impulse growth rate gradually decreased, the test device simulated by the explosion shock wave pressure duration of the maximum action time of more than 340 ms. Comparing the differences in the dynamic response of the three structures, it was found that when the peak long-duration shock wave overpressure acting on the structure reached 0.09 MPa, the PC model had plastic deformation, and the combined reinforced concrete and CSPC structure exhibited fully elastic deformation. When the peak shock wave overpressure reaches 0.238 MPa, the RC model begins to show plastic deformation. When the peak shock wave overpressure increased to 0.315 MPa, all models produced significant plastic deformation. The deformation resilience of the CSPC composite structure under the above-mentioned blast shock wave load is greater than that of RC and PC structures, which have good deformation recovery ability and can effectively reduce the damage to the structure by blast shock. On the contrary, the peak structural vibration acceleration caused by the blast shock wave is greater in the CSPC combined structure than in the RC and PC structure, and the deformation rebound and oscillation of the corrugated steel plate is the main way of impact energy dissipation. The results of the study provide an experimental basis for the simulation of long-duration plane blast waves and the optimal design of CSPC blast-resistant structures.

Study of Typical Ruptured and Unruptured Intracranial Aneurysms Based on Fluid–Structure Interaction

Most intracranial aneurysms (IAs) will be abnormal bulges on the walls of intracranial arteries that result from the dynamic interaction of geometric morphology, hemodynamics, and pathophysiology. Hemodynamics plays a key role in the origin, development, and rupture of IAs. In the past, hemodynamic studies of IAs were mostly based on the rigid wall hypothesis of computational fluid dynamics, and the influence of arterial wall deformation was ignored. We used fluid-structure interaction (FSI) to study the features of ruptured aneurysms, because it can solve this problem very well and the simulation will be more realistic. A total of 12 IAs, 8 ruptured and 4 unruptured, at the middle cerebral artery bifurcation were studied using FSI to better identify the characteristics of ruptured IAs. We studied the differences in the hemodynamic parameters, including the flow pattern, wall shear stress (WSS), oscillatory shear index (OSI), and displacement and deformation of the arterial wall. Ruptured IAs had a larger low WSS area and more complex, concentrated, and unstable flow. Also, the OSI was higher. In addition, the displacement deformation area at the ruptured IA was more concentrated and larger. A large aspect ratio; a large height/width ratio; complex, unstable, and concentrated flow patterns with small impact areas; a large low WSS region; large WSS fluctuation, high OSI; and large displacement of the aneurysm dome could be risk factors associated with aneurysm rupture. If similar cases are encountered when simulation is used in the clinic, priority should be given to diagnosis and treatment.

Gaborheometry: Applications of the discrete Gabor transform for time resolved oscillatory rheometry

Oscillatory rheometric techniques such as small amplitude oscillatory shear (SAOS) and, more recently, medium amplitude oscillatory shear and large amplitude oscillatory shear (LAOS) are widely used for rheological characterization of the viscoelastic properties of complex fluids. However, in a time-evolving or mutating material, the build-up or breakdown of microstructure is commonly both time- and shear-rate (or shear-stress) dependent, and thixotropic phenomena are observed in many complex fluids including drilling fluids, biopolymer gels, and many food products. Conventional applications of Fourier transforms for analyzing oscillatory data assume the signals are time-translation invariant, which constrains the mutation number of the material to be extremely small. This constraint makes it difficult to accurately study shear-induced microstructural changes in thixotropic and gelling materials, and it is becoming increasingly important to develop more advanced signal processing techniques capable of robustly extracting time-resolved frequency information from oscillatory data. In this work, we explore applications of the Gabor transform (a short-time Fourier transform combined with a Gaussian window), for providing optimal joint time-frequency resolution of a mutating material’s viscoelastic properties. First, we show using simple analytic models and measurements on a bentonite clay that the Gabor transform enables us to accurately measure rapid changes in both the storage and/or loss modulus with time as well as extract a characteristic thixotropic/aging time scale for the material. Second, using the Gabor transform we demonstrate the extraction of useful viscoelastic data from the initial transient response following the inception of oscillatory flow. Finally, we consider extension of the Gabor transform to nonlinear oscillatory deformations using an amplitude-modulated input strain signal, in order to track the evolution of the Fourier–Tschebyshev coefficients of thixotropic fluids at a specified deformation frequency. We refer to the resulting test protocol as Gaborheometry (Gabor-transformed oscillatory shear rheometry). This unconventional, but easily implemented, rheometric approach facilitates both SAOS and LAOS studies of time-evolving materials, reducing the number of required experiments and the data postprocessing time significantly.

MT-InSAR Unveils Dynamic Permafrost Disturbances in Hoh Xil (Kekexili) on the Tibetan Plateau Hinterland

Hoh Xil is an uninhabited extremity secluded on the Tibetan Plateau hinterland. A complete mapping of ground motion variation in Hoh Xil is essential for in-depth understanding of terrain’s responses to climate change on the Tibetan Plateau. However, the inaccessibility and extremely harsh environment impeded extensive field investigations on landform alteration and its formative process. Such difficulty can be resolved by Interferometric Synthetic Aperture Radar (InSAR), which enables a broad detection of subtle permafrost motions at millimeter precision. This study, for the first time, accomplished a Multi-temporal InSAR (MT-InSAR) deformation mapping from 2015 to 2020 in Hoh Xil, with a wide coverage of about 200,000 km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . 1,592 Sentinel-1 images were processed based on the small baseline subset (SBAS) technique. The results show that Hoh Xil was experiencing dynamic permafrost disturbances. Thawing permafrost with both the linear subsidence rate higher than 2 mm/yr and the periodic amplitude over 2 mm was primarily detected in areas of flat or gentle slopes. The InSAR cumulative deformation is highly correlated with permafrost thawing depth. Significant lag times were identified between seasonal oscillation of InSAR deformation and land surface temperature (LST). Thermokarst landforms of retrogressive thaw slumps and thermokarst lakes broadly formed and dynamically evolved as a consequence of permafrost degradation. Particularly, widespread thawing permafrost characterized by the spatial clustering of thermokarst lakes appeared to occur in areas adjacent to large lakes. The discovered dynamic permafrost disturbances in Hoh Xil manifested even the secluded Tibetan Plateau hinterland was facing the threat of climate change.

Impact of Particle Sedimentation in Pendant Drop Tensiometry.

Understanding the interface-stabilizing properties of surface-active components is key in designing stable macroscopic multiphase systems, such as emulsions and foams. When poorly soluble materials are used as an interface stabilizer, the insoluble material may sediment and interfere with the analysis of interfacial properties in pendant (or hanging) drop tensiometry. Here, the impact of sedimentation of particles on the interfacial properties determined by pendant drop tensiometry was evaluated using a model system of whey protein isolate and (non surface-active) glass beads (2.2–34.7 μm). Although the glass beads did not adsorb to the air–water interface, a 1% (w/w) glass bead solution appeared to decrease the surface tension by nearly 12 mN/m after 3 h. A similar effect was shown for a mixture of whey proteins and glass beads: the addition of 1% (w/w) of glass beads led to an apparent surface tension decrease of 31 mN/m rather than the 20 mN/m observed for pure whey proteins. These effects are attributed to the sedimentation of particles near the apex of the droplet, leading to droplet shape changes, which are interpreted as a decrease in surface tension using tensiometer software. The droplet density at the apex increases due to sedimentation, and this density increase is not accounted for when fitting the droplet shape with the Young–Laplace equation. The result is the observed apparent decrease in surface tension. In contrast to the significant impact of sedimenting material on the surface tension measurements, the impact on the results of oscillatory deformations was limited. These findings show that the impact of sedimentation should be considered when studying the interface-stabilizing properties of materials with reduced solubility, such as certain plant protein extracts. The presence of such particles should be carefully considered when conducting pendant drop tensiometry.

Constraining the Ellipticity of the Newborn Magnetar with the Observational Data of Long Gamma-Ray Bursts

The X-ray plateau emission observed in many long gamma-ray bursts (LGRBs) has been usually interpreted as the spin-down luminosity of a rapidly spinning, highly magnetized neutron star (millisecond magnetar). If this is true, then the magnetar may emit extended gravitational wave (GW) emission associated with the X-ray plateau due to nonaxisymmetric deformation or various stellar oscillations. The advanced LIGO and Virgo detectors have searched for long-duration GW transients for several years; no evidence of GWs from any magnetar has been found until now. In this work, we attempt to search for signatures of GW radiation in the electromagnetic observation of 30 LGRBs under the assumption of the magnetar model. We utilize the observations of the LGRB plateau to constrain the properties of the newborn magnetar, including the initial spin period P 0, dipole magnetic field strength B p , and the ellipticity ϵ. We find that there are some tight relations between magnetar parameters, e.g., and . In addition, we derive the GW strain for the magnetar sample via their spin-down processes, and find that the GWs from these objects may not be detectable by the aLIGO and Einstein Telescope (ET) detectors. For a rapidly spinning magnetar (P ∼ 1 ms, B ∼ 1015 G), the detection horizon for the advanced LIGO O5 detector is ∼180 Mpc. The detection of such a GW signal associated with the X-ray plateau would be a smoking gun that the central engine of a GRB is a magnetar.

A spatio-temporal in-situ investigation of the Payne effect in silica-filled rubbers in Large Amplitude Oscillatory Extension

The present work deals with the structural and dynamic characteristics of a silica filler network within an industrial elastomeric composite with time-resolved Ultra-Small-Angle-X-ray Scattering (USAXS), collected for the first time in-situ under periodic uniaxial deformation. The in-situ configuration allows a unique correlation between the x-ray patterns highlighting the structure of the filler and the dynamic-mechanical stress-strain response that characterizes the full composite. A scattering model is applied to quantitatively identify the filler network evolution as a function of dynamic strain. To address the Payne effect and the underlying structural modifications correlated to intra- and inter-filler-aggregate effects, all rubbers were pre-conditioned to suppress stress-softening related to the Mullins effect. Sector-averaged scattering intensities along the parallel strain direction reveal a jamming/de-jamming transition between clusters. The dynamic stress response of the full composite shows onsets of non-linear behavior with contributions assigned to both filler and rubber phase separately. • Scattering analysis of silica-filled SBR elastomers. • Study of the Payne effect through in-situ combination of USAXS and DMA methods. • Relationship between structural and dynamic-mechanical properties. • Identification of jamming/de-jamming transition upon oscillatory deformation.

Interaction of trans-tetrachlorodi-m-carboxylates of dirhenium(III) with dipeptides of the glycyl series

The paper reports the methods for the synthesis of new cluster compounds of trans-tetrachlorodi--carboxylates of dirhenium(III) with dipeptides of the glycyl series. The compositions and the structures were determined by electron absorption and IR spectroscopies as well as by elemental analysis for the following newly synthesized compounds: trans-[Re2(H3N+–CH2–СO–NH–CH(COO)–CH2–CH(CH3)–CH3)2Cl42СН3CN]Cl2 (І) and trans-[Re2(H3N+–CH2–СO–NH–CH(COO)–CH2–C6H5)2Cl42СН3CN]Cl2 (ІІ). Analysis of the electronic absorption spectra of the solutions of the synthesized substances showed the presence of a doublet pattern (12500 and 16129 сm–1), which corresponds to the *-electronic transition of the Re–Re quadruple bond characteristic of solutions of trans-tetrachlorodi--carboxylates of dirhenium(III). The appearance of oscillations at 1485 сm–1 for І and 1486 сm–1 for ІІ, characteristic of the s(CO) of the coordinated carboxylate group, was detected on the IR spectra, which indicates the bridging coordination of this group to the Re26+ binuclear fragment. The protonation of the amino group is indicated by the appearance of a wide band of valence oscillations (NH3+) in the range of 3400–3350 сm–1 and deformation oscillations (NH3+) at 1557 сm–1 and 1614 сm–1 for I and II, respectively. The stability of the obtained complex compounds in aqueous solutions was determined. It was shown that the hydrolysis of the synthesized substances takes place in 4–5 days, which is accompanied by a decrease in the pH of the reaction solution.

Robust adaptive direct speed control of PMSG-based airborne wind energy system using FCS-MPC method

Electricity generation using airborne wind energy system (AWES) tends to be more efficient than that of conventional wind turbines due to their ability at extracting higher speeds from wind at higher altitudes by the tethered kite generator system (TKGS). The speed regulation of a permanent synchronous machine (PMSM) attached to a kite to maximize harvested power from wind is the topic of the present research. To track the optimal speed produced by TKGS, a finite-control-set model predictive control (FCS-MPC) with a new cost function is directly employed to generate appropriate gate signals for a 3-level neutral point clamped (3-L NPC) inverter that drives a PMSM. A novel adaptive algorithm is also provided for adjusting the weight coefficients of the cost function to improve the system’s performance across a wider range of change in the operating conditions. Moreover, a terminal sliding mode disturbance observer (TSMDO) is designed to estimate the load torque. A terminal sliding mode speed controller (TSMSC) is also used to produce the current reference for the optimizing algorithm in FCS-MPC. The proposed control scheme eliminates the need for a cascade structure while significantly reduces current deformation and torque oscillations. Additionally, the controller operates as fast as a traditional direct speed controller without the use of a cascade structure and is more robust against parameter uncertainties. To illustrate the success of the suggested controller, the results of simulations are presented in MATLAB/SIMULINK® software. The performance of the proposed technique is evaluated in three situations which simulates a grid-connected PMSM fed by a back to back 3-L NPC. In the first scenario, it is demonstrated that the suggested technique performs well when the system’s operational conditions vary. The performance of the adaptive tuning mechanism of the cost function weight factors is investigated in the second scenario, which takes into account model parameter uncertainty. The impacts of the disturbance observer (DO) on the system performance are explored in the final scenario, and it is discovered that the disturbance observer may increase the speed response and DC link voltage tracking performance. All of the simulation results show that the new technique outperformed the traditional FSC-MPC.

Theory of defect-mediated morphogenesis.

Growing experimental evidence indicates that topological defects could serve as organizing centers in the morphogenesis of tissues. Here, we provide a quantitative explanation for this phenomenon, rooted in the buckling theory of deformable active polar liquid crystals. Using a combination of linear stability analysis and computational fluid dynamics, we demonstrate that active layers, such as confined cell monolayers, are unstable to the formation of protrusions in the presence of disclinations. The instability originates from an interplay between the focusing of the elastic forces, mediated by defects, and the renormalization of the system’s surface tension by the active flow. The posttransitional regime is also characterized by several complex morphodynamical processes, such as oscillatory deformations, droplet nucleation, and active turbulence. Our findings offer an explanation of recent observations on tissue morphogenesis and shed light on the dynamics of active surfaces in general.

Numerical simulation on free oscillation interfacial dynamics of single-core compound droplet driven by shell deformation

Compound droplets are often used as microcontainers in biomedical, material encapsulation, and so on, and a good understanding of their free oscillatory behavior is essential for controllable product preparation. Herein, the free oscillatory deformation behavior of single-core compound droplet induced by shell deformation is studied computationally using the volume-of-fluid (VOF) method. The effects of the inner droplet diameter, the outer droplet initial deformation and physical properties are considered. The results show that outer droplet oscillation has obvious periodicity and damping attenuation mechanism, and the inner droplet has two oscillation forms: large-deformation forced oscillation and small-deformation free oscillation. The oscillation period is positively correlated with the inertial force, while the damped attenuation mechanism is dominated by the viscous force, and the presence of the inner droplet and the increase of its diameter will enhance the inertial and viscous forces of the outer droplet. The deformation degree of the inner and outer droplet is related to the energy transfer between the two and the initial interfacial energy, which is generally greater for the outer droplet than for the inner droplet, but the larger diameter ratio makes the opposite situation, and the critical value lies between the diameter ratio of 0.625–0.75. The obtained results will provide the possibility to control the morphological structure of the compound droplets after preparation.

Study on the interfacial dynamics of free oscillatory deformation and breakup of single-core compound droplet

Compound droplets are usually taken as microcontainers for biomedical and material encapsulation applications in which a good understanding of the free oscillatory deformation and breakup behavior is essential. In this work, the dynamics of free oscillatory deformation and breakup of a single-core compound droplet with an initial ellipsoidal shell was investigated numerically using the volume-of-fluid method. The effects of droplet diameter and the outer droplet initial deformation parameter are considered. Four outcomes are identified: oscillatory deformation, separation, separation breakup, and breakup. The evolution of the kinetic energy and pressure field of the compound droplet for the four typical outcomes is also analyzed in detail. A clear boundary exists between the first and the latter three outcomes (initial deformation parameters of 0.600–0.773), while the critical factor for the latter three outcomes is the inner and outer droplet diameter ratio. The oscillatory deformation is characterized by the inner and outer droplet undergoing a finite deformation and subsequent oscillatory behavior, with the maximum deformation of the inner and outer droplets being related to the energy transfer between the two, and the outer droplet being a periodic decaying oscillation, while the inner droplet is a large deformation oscillation interspersed with a small deformation oscillation. Separation, separation breakup, and breakup are characterized by breakup at the inner or outer interface during deformation; separation and breakup times are largely dependent on droplet diameter and the initial deformation parameter of the outer droplet; and the neck width at separation is also analyzed in detail.

High-Efficiency Directional Ejection of Coalesced Drops on a Circular Groove.

Coalescence-induced drop jumping has received significant attention in the past decade. However, its application remains challenging as a result of the low energy conversion efficiency and uncontrollable drop jumping direction. In this work, we report the high-efficiency coalescence-induced drop jumping with tunable jumping direction via rationally designed millimeter-sized circular grooves. By increasing the surface-droplet impact site area and restricting the oscillatory deformation, the energy conversion efficiency of the jumping droplet reaches 43.5%, 600% as high as the conventional superhydrophobic surfaces. The droplet jumping direction can be tuned from 90° to 60° by varying the principal curvature of the circular groove, while the energy conversion efficiency remains unchanged. We show through theoretical analysis and numerical simulations that the directional jumping mainly originates from reallocation of droplet momentum enabled by the asymmetric liquid bridge impact. Our study demonstrates a simple yet effective method for fast, efficient, and directional droplet removal, which warrants promising applications in jumping droplet condensation, water harvesting, anti-icing, and self-cleaning.

Covalently cross‐linked hydrogels: Mechanisms of nonlinear viscoelasticity

AbstractGelatin‐based hydrogels have been widely used in tissue engineering, three‐dimensional cell culture, drug delivery, and cell therapy. The mechanical behaviour of hydrogels combined with their chemical properties determines their functionality and efficacy. With respect to the mechanical behaviour of hydrogels, the vast majority of publications have reported their linear viscoelastic response. However, for practical conditions in the body, these materials experience large deformations beyond the linear viscoelastic limit. Herein, to mimic practical conditions and to evaluate the mechanical response of the hydrogels subjected to large deformations, we report inter‐ and intra‐cycle nonlinear viscoelastic behaviour of a gelatin methacryloyl (GelMA) hydrogel with different concentrations of the hydrogel precursor (10%–20% [w/v]) under large amplitude oscillatory shear deformation. To achieve this, we used a novel technique by chemically bonding the hydrogels to treated glass slides, which were attached to the oscillating metal plates using a double‐sided tape to alleviate any error arising from wall slip during rheological measurements. The results show that the elasticity of the covalently cross‐linked hydrogels at large deformations obeys a nonlinear force‐extension law and that the viscous intra‐cycle nonlinearity at moderate deformations stems from the dual cross‐linked (DC; i.e., physical and chemical) nature of the GelMA hydrogel. It was also shown that viscoelastic parameters can be tuned by the concentration of the hydrogel precursor, that is, yield stress increased from 2.6–7.1 kPa, critical strain amplitude decreased from γ0 = 100%–70%, and the onset of inter‐cycle nonlinearity shifted from γ0 = 50%–20% upon increasing the concentration of the hydrogel precursor. These insights have important implications for the rational development of hydrogel‐based biomaterials to design biocompatible scaffolds in tissue engineering applications.

The fate of shear-oscillated amorphous solids.

The behavior of shear-oscillated amorphous materials is studied using a coarse-grained model. Samples are prepared at different degrees of annealing and then subjected to athermal and quasi-static oscillatory deformations at various fixed amplitudes. The steady-state reached after several oscillations is fully determined by the initial preparation and the oscillation amplitude, as seen from stroboscopic stress and energy measurements. Under small oscillations, poorly annealed materials display shear-annealing, while ultra-stabilized materials are insensitive to them. Yet, beyond a critical oscillation amplitude, both kinds of materials display a discontinuous transition to the same mixed state composed of a fluid shear-band embedded in a marginal solid. Quantitative relations between uniform shear and the steady-state reached with this protocol are established. The transient regime characterizing the growth and the motion of the shear band is also studied.

Adaptive Robust Control for a Spatial Flexible Timoshenko Manipulator Subject to Input Dead-Zone

This article investigates the adaptive robust spatial vibration control for a flexible Timoshenko manipulator subject to input dead-zone nonlinearity characteristic. The “disturbance-like” terms and dead-zone nonlinearity are first incorporated into the context of control design, and the new boundary robust adaptive control laws are constructed to reduce the shear deformation and elastic oscillation, ensure the expected angle orientation, handle the input dead-zone, and estimate the upper bound of compound disturbances. The convergence of states and the stability of the system are analyzed and proven without simplifying the infinite dimensional dynamics. In the end, the effectiveness of the presented scheme is demonstrated by the result of simulation research.

OrthoChirp: A fast spectro-mechanical probe for monitoring transient microstructural evolution of complex fluids during shear

Shear-induced microstructural changes are observed ubiquitously in materials as diverse as hydrogels, or drilling fluids, as well as many consumer products. Superposition rheometry, consisting of superimposing Small Amplitude Oscillatory Shear (SAOS) on top of a constant rate unidirectional shear has often been used to gain insight into these shear-induced changes. Orthogonal superposition (OSP), in which the two modes of deformation are perpendicular, has been preferred over parallel superposition to avoid non-linear cross-coupling of the steady shear and small amplitude oscillatory deformation fields. This cross-coupling can lead to unphysical sign changes in the measured material properties, and makes it difficult to unambiguously interpret the flow-induced mechanical properties. Recently, orthogonal superposition has been used to investigate the shear-induced anisotropy produced in colloidal gels by comparing the transient evolution of the orthogonal moduli with the parallel moduli immediately after cessation of shear. However, probing transient evolution using the OSP technique can be challenging for rapidly mutating complex materials which evolve on time scales comparable to the time scale of the experiment. Using a weakly associated alginate gel, we demonstrate the potential to substantially reduce the measurement time by superimposing fast Optimally Windowed Chirp (OWCh) deformations orthogonally to the steady shearing deformation. We evaluate the changes in the rate-dependent relaxation spectrum in the direction of the applied unidirectional shear rate, as well as in the orthogonal direction, by measuring the damping function and shear-induced changes in the orthogonal moduli respectively. We observe systematic differences between the two sets of spectra that we measure in the two orthogonal directions, enabling us to quantify the flow-induced generation of microstructural anisotropy in the alginate gel. • Reduction of measurement time for orthogonal superposition (OSP) rheometry. • Possible to superpose optimally windowed chirps (OWCh) orthogonally to steady shear. • Shear flow induces microstructural anisotropy in weakly associating alginate gels. • Observation of transient evolutions in rapidly mutating complex materials • Different rate-dependent relaxation spectra in orthogonal directions.

Pulsation, translation and P1 deformation of two aspherical bubbles in liquid.

In this work, the interactions between the axial translational motions and aspherical oscillations of two gas bubbles in an incompressible liquid are considered. Representing the surface function by the Legendre polynomial of first order, we derive a dynamic model to describe the motions of two aspherical bubbles in Lagrangian mechanics. An apple-shaped bubble from simulations based on the model can be well consistent with known experimental observation. The bubble appears as the shape of a sphere at maximum expansion. The maximum asymmetry of the bubbles occurs during collapse. The surface tension is a key factor to stable oscillatory deformation. It is also found that the aspherical amplitudes of two bubbles decrease with increasing distance or decreasing driving pressure.

Buckling versus Crystal Expulsion Controlled by Deformation Rate of Particle-Coated Air Bubbles in Oil.

Oil foams stabilized by crystallizing agents exhibit outstanding stability and show promise for applications in consumer products. The stability and mechanics imparted by the interfacial layer of crystals underpin product shelf life, as well as optimal processing conditions and performance in applications. Shelf life is affected by the stability against bubble dissolution over a long time scale, which leads to slow compression of the interfacial layer. In processing flow conditions, the imposed deformation is characterized by much shorter time scales. In practical situations, the crystal layer is therefore subjected to deformation on extremely different time scales. Despite its importance, our understanding of the behavior of such interfacial layers at different time scales remains limited. To address this gap, here we investigate the dynamics of single, crystal-coated bubbles isolated from an oleofoam, at two extreme time scales: the diffusion-limited time scale characteristic of bubble dissolution, ∼104 s, and a fast time scale characteristic of processing flow conditions, ∼10–3 s. In our experiments, slow deformation is obtained by bubble dissolution, and fast deformation in controlled conditions with real-time imaging is obtained using ultrasound-induced bubble oscillations. The experiments reveal that the fate of the interfacial layer is dramatically affected by the dynamics of deformation: after complete bubble dissolution, a continuous solid layer remains; after fast, oscillatory deformation of the layer, small crystals are expelled from the layer. This observation shows promise toward developing stimuli-responsive systems, with sensitivity to deformation rate, in addition to the already known thermoresponsiveness and photoresponsiveness of oleofoams.