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Bistable composite structures have a wide application prospect in the design of deformable structures. In this study, the snap behaviors of unsymmetric cross-ply bistable composites with different ...

Self-assembled films (SAFs) have been proposed to be a promising candidate for molecularly thin lubricants. However, the frictional performance of SAFs is sensitively dependent on their molecular structures that are susceptible to external environments. Taking erucamide, a fatty amide widely used as a macroscale slip additive, as an example, we demonstrate that SAFs can be readily formed on various substrates, including silicon oxide, sapphire, copper, and graphite. Through high-resolution topography and friction measurements, two types of erucamide SAFs are identified. The first type is atomically flat and exhibits clear atomic stick–slip friction behavior and ultra-low frictional dissipation; while the second type has a stripe-like nanoscale pattern and shows much higher (8 times higher) frictional dissipation. The sharp contrast between these two types of SAFs is attributed to their distinct molecular structures originating from different interactions between erucamide molecules and substrates. We further demonstrate that, by proper surface functionalization, we can actively and reliably control the molecular structures of SAFs through guided self-assembling, achieving rational friction tuning with patterning capability.

Collective cell migration is extensively observed in embryo development and cancer invasion. During these processes, the interactions between cells with distinct identities and fates are of importance for boundary formation and host defense against cancer. In this paper, we explore the collective dynamics of a two-dimensional cell mixing monolayer consisting of non-tumorigenic mammary epithelial cells and breast cancer cells. We show that the epithelial–cancerous cell mixing system displays unique sorting behaviors. The epithelial cells aggregate to form scattered clusters, which perform random motion with simultaneous translation and rotation, strikingly distinct from the classical persistent random walk of individual migratory cells. The motility of cancer cells is markedly promoted by the epithelial clusters, exhibiting remarkable contact enhancement of locomotion. A discrete model based on the Johnson–Kendall–Roberts contact mechanics is proposed to identify the influence of intercellular interactions, active migration forces and cell–substrate friction forces on the collective cell dynamics. These findings could advance our understanding of many biological processes, such as cancer metastasis and tissue morphogenesis. The epithelial–cancerous cell mixing system displays unique collective dynamics. The epithelial cells aggregate to form scattered clusters, which perform random motion with simultaneous translation and rotation, strikingly distinct from the classical persistent random walk of individual migratory cells. The interactions between the two cell types drive the evolution of collective migration dynamics and give rise to a behavior akin to the contact enhancement of locomotion. This study revealed the distinctive dynamic features and the underlying regulatory mechanisms arising from epithelial–cancerous interactions.

When two bodies are in contact, various surface/subsurface damage problems may arise, which significantly shorten the service lives of functional components. These damage problems may further interact with one another and be influenced by such factors as friction and lubrication. Contact mechanics plays an important role in understanding the complex phenomena and analyzing the damage evolution. This paper aims to provide a comprehensive review of the recent progress in contact mechanics for tribological and contact damage–related problems. After a brief introduction of the contact mechanics, the applications of contact mechanics in various problems are systematically elaborated. Finally, the review concludes with future perspectives and major challenges to be addressed. Nanyang Technological University National Research Foundation (NRF) The authors acknowledge the financial support from the National Natural Science Foundation of China (12072272 and 11921002) and the SMRT-NTU Smart Urban Rail Corporate Laboratory with funding support from the National Research Foundation, Singapore, SMRT, Singapore, and Nanyang Technological University, Singapore.

Biological materials have evolved various degrees of robustness against microscopic defects and structural randomness. Of particular interest here is whether and how nacre's brick–mortar structure suppresses the adverse effect of microstructural randomness. To this end, a tension–shear–chain (TSC) network model, combined with the virtual internal bond concept, is adopted to investigate the effects of microstructural randomness of nacre, where we show that the ensemble strength and failure behaviors of a larger TSC model exhibit substantially lower randomness. Our results indicate that the staggered brick–mortar structure renders nacre insensitive to microstructural randomness, resulting in enhanced resistance to strain localization and crack initiation at weaker interfaces. The influence of microstructural randomness on the size effect of the ensemble mechanical properties of nacre is also revealed. This study provides further insights and guidelines for designing strong and robust nacre-mimic composites. This work is sponsored by the National Natural Science Foundation of China (Grant Nos. 12002184, 12032104, and 11921002) and the fellowship of China Postdoctoral Science Foundation (Grant Nos. 2020TQ0173 and 2021M691797).

Evaporation of virus-loaded droplets and liquid nanofilms plays a significant role in the pandemic of COVID-19. The evaporation mechanism of liquid nanofilms has attracted much attention in recent decades. In this minireview, we first introduce the relationship between the evaporation process of liquid nanofilms and the pandemic of COVID-19. Then, we briefly provide the frontiers of liquid droplet/nanofilm evaporation on solid surfaces. In addition, we discuss the potential application of machine learning in liquid nanofilm evaporation studies, which is expected to be helpful to build up a more accurate molecular model and to investigate the evaporation mechanism of liquid nanofilms on solid surfaces.

This paper reports (1) an application of the modified hyperbolic sine law to P92 for the minimum creep strain rate over a wider range of stress level, and 2) the calibration of the creep ca...

Dorsal closure is an essential developmental process of Drosophila embryogenesis, during which the ectoderm fuses the two sides of a gap into a complete ectodermal epithelium. A defective closure may cause scar formation or even embryonic lethality. In this paper, a multiscale fracture mechanics model is established by treating the dorsal closure as a crack healing process. We investigate how the F-actin dynamics at the subcellular level and the cell-pair fusion at the cellular level are orchestrated to accomplish the tissue-level closure. A spatiotemporal cohesive law is proposed to characterize the active contractions of filopodial and lamellipodial protrusions, which involve F-actin retrogradation. The contribution of active forces to dorsal closure is evaluated in terms of the stress intensity factors at the canthi of the gap. The proposed model can well predict both the evolutionary zipping zone shape and the sealing speed during dorsal closure, and the theoretical results are in consistency with relevant experiments. This work can not only elucidate the multiscale mechanisms underlying dorsal closure, but also provide a new perspective of facture mechanics to understand some physiological and pathological processes during the development of tissues and organs.

Functional fluorescent silkworm silk holds promise for many important applications in biomedical engineering, optics, and photonics. However, it remains a challenge to obtain fluorescent silk in scale-up with both good mechanical properties and highly stable fluorescence simultaneously. In this work, we report a highly efficient strategy to produce fluorescent silk through directly feeding silkworm larvae with graphene quantum dots or CdSe/ZnS core–shell quantum dots. The obtained quantum dots-reinforced luminescent silkworm silk has superior mechanical strength and toughness, stable fluorescence, and good biocompatibility in comparison with the normal or fluorescent dye-colored silk. The strategy proposed in this work is environmental and economical and, also importantly, can generate superior luminescent silks in large scale. This study also provides possible cues for fabricating durable, fluorescent microdevices, and fabrics.

Growth shapes soft tissues not only through mass addition or volume expansion but also through deformation instabilities and consequent morphological evolution. In this paper, we probe the torsion instability of an anisotropically growing tube with fiber reinforcement, which mimics many tubular organs in animals or plants. We derive the Stroh formulation for the incremental boundary value problem and numerically solve it using the surface impedance method. A linear stability analysis is conducted to investigate the critical condition for the onset of wrinkling. The thresholds of helical wrinkling are calculated in terms of growth ratio and external load. The effect of fibers on the critical state under axial stretching is examined. It is found that the tangential growth tends to enhance the critical torsion angle but has a weak influence on the critical longitudinal mode of wrinkling, which, however, can be remarkably affected by the axial growth. Our study can help understand the formation of helical morphologies in biological materials and provide cues for engineering desired structures or devices.

The environmental adaption, growth, motion, and other dynamic behaviors of bacteria are closely associated with the coupled mechanical–electrochemical properties of their subcellular structures, but the underlying regulatory mechanisms remain unclear. The mechanical responses of bacteria are difficult to elucidate by traditional models without considering the mechanical, chemical, and electric coupling effects. In this paper, a mechanical–electrochemical theory is constructed to investigate the deformation behavior of bacterial cells. A bacterium is treated as a bilayer structure consisting of a negatively charged polysaccharide capsule and an elastic envelope subjected to turgor pressure. This model is used to reveal the regulating roles of the electrostatic double-layer force and osmosis under different electrolyte conditions. Good agreement is found between the theoretical predictions and the experimentally observed three-stage nanoindentation responses of Klebsiella pneumoniae (K. pneumoniae) bacterial cells. Furthermore, we investigate the mechanical–electrochemical coupling mechanisms in the compression resistance of the bacterial capsule. The results reveal that the osmosis induced by ionic imbalance and polysaccharide–solvent aggregates plays a significant role in the compression resistance of the capsule. The present model not only deepens our understanding of the mechanical–electrochemical coupling mechanisms of bacterial cells at the subcellular scale, but also holds promise for applications in characterizing their mechanical properties.

Deciphering the bio–chemo–mechanical mechanisms in tissue growth and deformation helps understand the morphogenesis of organs and organisms under physiological and pathological conditions. In this paper, we present a finite element method that can account for the interplay of volumetric growth, chemical transport, and mechanical deformation in soft tissues, such as tumors. The poroelastic theory and the volumetric growth model are combined to capture the essential growth and deformation traits of biological tissues. This method can not only simulate the bio–chemo–mechanical coupling processes in growing tissues, but also track their morphological instabilities and evolutions. The deformation and instability of interacting blood vessels in a growing tumor are considered as an example. The mechanisms underpinning the collapse of blood vessels observed in vascular solid tumors are revealed. This work holds promise for applications in the diagnosis and therapy of diseases such as cancer.

The thermocapillary motion of a drop on a solid substrate is a common phenomenon in daily life and many industrial fields. The motion can be significantly affected by the temperature gradient of the substrate and the properties of the liquid, such as surface tension, viscosity, thermal coefficient, density, and diffusivity. In this study, a numerical model based on modified many-body dissipative particle dynamics was developed to capture correctly the temperature dependence of a fluid. The momentum, thermal diffusivity, viscosity, and surface tension of liquid water at various temperatures ranging from 273 to 373 K were used as examples to verify the proposed model. The results calculated with this model for heat conduction in a liquid–solid system are in good agreement with those calculated with Fourier's law. The approach successfully modeled the thermocapillary motion of a liquid water droplet on a hydrophobic substrate with a temperature gradient. The migration of the droplet on a flat solid substrate was induced by the difference in surface tension due to the temperature gradient. The migration velocity increased with the temperature difference, which is in agreement with the present theoretical analysis and dynamic van der Waals theory. The modified numerical model proposed in this work could be used to study heat and mass transfer across a free interface, such as Marangoni convection in multiphase fluid flow.

The sine-Wiener noise, as one new type of bounded noise and a natural tool to model fluctuations in dynamical systems, has been applied to problems in a variety of areas, especially in biomolecular networks and neural models. In this paper, by virtue of the Novikov theorem, Fox’s approach, and the ansatz of Hanggi, an approximate Fokker–Planck equation is derived for an one-dimensional Langevin-type equation with cross-correlated sine-Wiener noise. Meanwhile, the dynamical characters of a bistable system driven by cross-correlated sine-Wiener noise are investigated by applying the approximate theoretical method. For the bistable system, the cross-correlation intensity $$\lambda $$ can induce the reentrance-like phase transition, while the other noise intensities and the self-correlation time, except for the self-correlation time of additive bounded noise, can induce the first-order-like phase transition. The transition from the stable state to another one can be accelerated by $$\alpha $$ (additive bounded noise intensity), $$\tau _1$$ (the self-correlation time of the multiplicative bounded noise), and $$\tau _2$$ (the self-correlation time of the additive bounded noise) and can be restrained with $$\lambda $$ and $$\tau _3$$ (self-correlation time of the cross-correlation bounded noise). It is interesting that the noise-enhanced stability phenomenon is observed with D (multiplicative bounded noise intensity) varying for the positive correlation ($$\lambda >0$$) and is enhanced as $$\lambda $$ increases. The numerical results are in basic agreement with the theoretical predictions.

It remains a puzzling issue why and how the organs in plants living in the same natural environment evolve into a wide variety of geometric architecture. In this work, we explore, through a combination of experimental and numerical methods, the biomechanical morphogenesis of the leaves and stalks of representative emergent plants, which can stand upright and survive in harsh water environments. An interdisciplinary topology optimization method is developed here by integrating both mechanical performance and biological constraint into the bi-directional evolutionary structural optimization technique. The experimental and numerical results reveal that, through natural selection over many million years, these leaves and stalks have been optimized into distinctly different cross-sectional shapes and aerenchyma tissues with intriguing anatomic patterns and improved load-bearing performance. The internal aerenchyma is an optimal compromise between the mechanical performance and functional demands such as air exchange and nutrient transmission. We find that the optimal distribution of the internal material depends on multiple biomechanical factors such as the cross-sectional geometry, hierarchical structures, boundary condition, biological constraint, and material property. This work provides an in-depth understanding of the property–structure–performance–function interrelations of biological materials. The proposed topology optimization method and the presented biophysical insights hold promise for designing highly efficient and advanced structures (e.g., airplane wings and turbine blades) and analyzing other biological materials (e.g., bones, horns, and beaks).

Material anisotropy regulates the instabilities of film–substrate systems at different length scale. In this paper, we investigate the surface wrinkling and morphological evolution of an orthotropic thin film resting on a compliant substrate. Under different loading conditions, the system may buckle into various surface patterns, e.g., stripe, checkerboard, and herringbone, which are analyzed by using the Foppl–von Karman plate theory. The Fourier spectral method is employed to simulate the morphological evolutions of surface patterns under different loading biaxialities. We find that both loading biaxiality and material anisotropy affect the characteristics of surface wrinkling patterns and their evolutions. Stripe and checkerboard modes often emerge at the critical buckling and they tend to transform into herringbone and labyrinth patterns during postbuckling. Phase diagrams are established to reveal the dependence of surface patterns on material anisotropy, Poisson's effect, and loading biaxiality. This study may help design diverse functional surfaces and deepen our understanding of the morphogenesis of some soft biological tissues and organs.

Understanding the buckling behaviors of rods confined in a finite space is of paramount importance in a diversity of engineering fields. In this paper, theoretical analyses, numerical simulations and experimental measurements are combined to investigate the buckling and postbuckling of a long rod confined in a circular tube. Under uniaxial compression, the initially straight rod first buckles into a sinusoidal shape, followed by the occurrence of snap-through instability or helical buckling, which leads to a complicated, three-dimensional configuration consisting of serially connected sinusoids and helices. A new theoretical model is presented to analyze the sinusoidal and helical buckling processes of the confined rod. The complete load–displacement curve of the buckled system during loading and unloading can be well predicted by the present theory. The critical conditions are obtained for the occurrence of the sinusoidal buckling and the sinusoid–helix transitions of buckling modes. It is found, both theoretically and experimentally, that the morphological evolution during unloading exhibits distinctly different features from that during loading due to the peculiar energetic features of snap-through instability. A flexible multibody dynamics method on the basis of the geometrically exact beam theory is employed to numerically explore the buckling behavior of slender rods and to reveal the underlying energetic mechanisms. The theoretical, numerical, and experimental results agree with each other very well. The theoretical model and the numerical method presented in this work are expected to analyze some other problems of confined rods, beams, and their combined systems.

Cellulose nanopaper exhibits superior mechanical properties with both high strength and toughness, and the crack bridging mechanism of nanofibers makes the most significant contribution to its fracture toughness. In this paper, we investigate the fracture toughness of a mode-I crack in cellulose nanopaper by using a modified crack-bridging model. Different from previous crack-bridging models, we account for the effect of nanofibers inclined to the crack surfaces. Particular attention is given to the dependence of fracture toughness on the orientation distribution of nanofibers in the cellulose nanopaper. We use a cohesive law to account for the interfacial shear stress of nanofibers, which involve self-healing of hydrogen bonds at their interfaces. Two representative orientation distributions are considered, in which nanofibers are aligned or randomly oriented, respectively. The theoretical results agree well with relevant experiments. This work helps understand the structure–property relationship of cellulose nanopaper and design other fiber-reinforced nanocomposites.

Periodic oscillations of collective cells occur in the morphogenesis and organogenesis of various tissues and organs. In this paper, an oscillating cytodynamic model is presented by integrating the chemomechanical interplay between the RhoA effector signaling pathway and cell deformation. We show that both an isolated cell and a cell aggregate can undergo spontaneous oscillations as a result of Hopf bifurcation, upon which the system evolves into a limit cycle of chemomechanical oscillations. The dynamic characteristics are tailored by the mechanical properties of cells (e.g., elasticity, contractility, and intercellular tension) and the chemical reactions involved in the RhoA effector signaling pathway. External forces are found to modulate the oscillation intensity of collective cells in the monolayer and to polarize their oscillations along the direction of external tension. The proposed cytodynamic model can recapitulate the prominent features of cell oscillations observed in a variety of experiments, including both isolated cells (e.g., spreading mouse embryonic fibroblasts, migrating amoeboid cells, and suspending 3T3 fibroblasts) and multicellular systems (e.g., Drosophila embryogenesis and oogenesis).

Twisted fiber-reinforced structures that resemble plywood, also called Bouligand structures, are widely observed biological materials in organisms such as lobsters, crabs, mantis shrimp and scorpions, where they exhibit outstanding fracture toughness and damage resistance. In this paper, we develop a three-dimensional crack twisted-bridging model to correlate fracture toughness with Bouligand microstructures and reveal the underlying toughening mechanisms. Depending on their orientation, some nanofibers bridge the crack surfaces in a twisting arrangement within the fracture process zone at the crack tip. The crack resistance of the structures assembled by soft biopolymers and physical cross-links is parameterized in terms of the nanofiber pitch angle and interfacial properties. Bouligand structures exhibit high and direction-independent fracture toughness in the nanofiber rotating plane, a notable advantage that endows material with superior load-bearing capacity in all directions. The crack resistance of Bouligand structures increases with the pitch angle of nanofibers, and the highest value is achieved at 90 degrees. Within a reasonable structural motif, an increase in the nanofiber length or interfacial strength results in an enhancement of the fracture toughness. Compared with the finite deformation of biopolymers, the facile formation and reformation mechanism of physical cross-links at the interfaces is a more efficient toughening strategy for materials with network-like structures of nanofibers. Our theoretical predictions agree well with relevant experimental measurements. This work can aid in designing biomimetic structural materials with high performance.

Contact stiffness is a fundamental mechanical index of solid surfaces and relevant to a wide range of applications. Although the correlation between contact stiffness, contact size and load has long been explored for single-asperity contacts, our understanding of the contact stiffness of rough interfaces is less clear. In this work, the contact stiffness of hexagonally patterned multi-asperity interfaces is studied using a discrete asperity model. We confirm that the elastic interaction among asperities is critical in determining the mechanical behavior of rough contact interfaces. More importantly, in contrast to the common wisdom that the interplay of asperities is solely dictated by the inter-asperity spacing, we show that the number of asperities in contact (or equivalently, the apparent size of contact) also plays an indispensable role. Based on the theoretical analysis, we propose a new parameter for gauging the closeness of asperities. Our theoretical model is validated by a set of experiments. To facilitate the application of the discrete asperity model, we present a general equation for contact stiffness estimation of regularly rough interfaces, which is further proved to be applicable for interfaces with single-scale random roughness.

The ability to control adhesion with a high tuning ratio and robust performance in a reversible manner is highly desirable for many engineering applications. Here, by embedding low-melting-point alloy (LMPA) blocks in a polymer matrix, we design a composite structure, whose surface adhesion can be actively switched between high and low when the LMPA is cooled to solid state or heated to liquid state. The proposed strategy does not need to modify the surface/interface directly yet can easily achieve an adhesion switching ratio about 100. As revealed by a contact mechanics model and finite element simulation, the adhesion tuning capability is enabled by rational regulation of the interface decohesion mode. More specifically, the interface will separate uniformly and exhibit high adhesion when the LMPA is solid; however, the interface will detach through crack-like propagation and exhibit much weaker adhesion when the LMPA is liquid. Based on this essential mechanism, we proceed to design a composite array structure, demonstrating that programmable inhomogeneous adhesion can be potentially achieved with pixel-by-pixel control capability.

Mechanical interactions between cells and extracellular matrix are known to regulate cellular processes ranging from cell signaling, spreading, migration, tissue morphogenesis, to cell differentiation, which may even alter cell phenotype and change physical properties of cells. Moreover, understanding cell contact, adhesion, and cellular mechanotransduction has great significance to cell cultures, muscle growth, and wound healing, and some related diseases such as cancer and fibrosis. For these reasons, cell mechanobiology research has become a focal point in the field of molecular and cell biology research receiving much attention from both biologists and biophysicists in recent years. In fact, cellular mechanobiology is an emerging multidisciplinary field that encompasses molecular cell biology, cell developmental biology, bioengineering and biophysics, and soft matter physics and mechanics. In this document, we would like to present an overview on the recent research developments on mechanics of cells and cellular mechanotransduction through the viewpoint of soft matter physics and biophysics, particularly from the perspective of mechanics of soft materials. Specifically, we review the recent research activities in mechanics of soft matter contact and cell behaviors involving experimental observations, mathematical modeling, and computational methods. Finally, the paper provides author’s perspectives on future issues and challenges on modeling and computational aspects.

Pump drill is an easily constructed ancient device that has been used for centuries to start fires and bore holes. It can effectively transfer rhythmic translational motions into vibratory, bi-directional rotary insertions. Here we explore, both experimentally and theoretically, the kinematics, dynamics, and potential applications of pump drills. The theoretical model, validated by experimental measurements, enables us to obtain the optimal structural geometries (e.g., the thread length and the crossbar span) of pump drills that maximize the mechanical responses such as the winding angle of the threads. Furthermore, the dependence of its rotational speed and piercing force on the loading conditions is investigated. Finally, manually powered devices, including an electric generator and a centrifugal separator, are developed based on the pump drill. This study paves a way towards promising applications of the pump drill in, for instance, energy harvesting and centrifugation.

A common spinning toy, called “buzzer”, consists of a perforated disk and flexible threads. Despite of its simple construction, a buzzer can effectively transfer translational motions into high-speed rotations. In the present work, we find that the disk can be spun by hand at an extremely high rotational speed, e.g., 200,000 rpm, which is much faster than the previously reported speed of any manually operated device. We explore, both experimentally and theoretically, the detailed mechanics and potential applications of such a thread–disk system. The theoretical prediction, validated by experimental measurements, can help design and optimize the system for, e.g., easier operation and faster rotation. Furthermore, we investigate the synchronized motion of multiple disks spinning on a string. Distinctly different twist waves can be realized by the multi-disk system, which could be exploited in the control of mechanical waves. Finally, we develop two types of manually-powered electric generators based on the thread–disk system. The high-speed rotation of the rotors enables a pulsed high current, which holds great promise for potential applications in, for instance, generating electricity and harvesting energy from ocean waves and other rhythmic translational motions.

Destruction of microstructural stability caused by topologically close-packed (TCP) phase precipitation restricted the industrial application of advanced single crystal superalloys. However, systematic investigations on microstructural nature and evolution of Co-rich TCP phases in Ru-containing single crystal superalloys with high level of Co addition have been rarely reported. In this study, experimental results indicated that the main TCP phase was μ phase after thermal exposure at 950 °C for 1000 h, while R phase was the main phase after thermal exposure at 1100 °C for 1000 h in the alloys containing high level of Co and different levels of Cr and Mo. The experimental and thermodynamic calculation results showed that R phase was more stable at 1100 °C than that of μ phase in the experimental alloys. Meanwhile, μ phase started to transform to R phase at 1100 °C for 50 h, and the stress accelerated the transformation of μ phase and R phase during creep test at 1100 °C/140 MPa in an alloy containing high levels of Co, Cr and Mo additions. Both R phase and μ phase were enriched in Re, W, Mo, Cr and Co, while R phase contained more Cr and μ phase was more enriched in Mo. This study contributes to better understand the microstructural evolution of TCP phases at different temperatures and to optimize the alloy design and thermodynamic database of Ru-containing single crystal superalloys with high level of Co addition.

We examine the contribution of crack bridging and surface elasticity to the elastic interaction between a finite crack and an edge dislocation. The surface effect on the crack faces is incorporated by using the continuum-based surface/interface model of Gurtin and Murdoch. The crack faces are subjected to both normal and shear bridging forces, and the bridging stiffnesses are allowed to vary arbitrarily along the crack. The residual surface tension is taken to be zero in our discussion. The Green’s function method is utilized to reduce the boundary value problem to three first-order Cauchy singular integro-differential equations, which are solved numerically by combining the Chebyshev polynomials and the collocation method. A general formula is derived for calculating the image force acting on the edge dislocation. Our analysis indicates that the stresses exhibit both the weak logarithmic and the strong square root singularities at the crack tips. We note that both crack bridging and surface elasticity influence the magnitude and direction of the image force acting on the edge dislocation. Particularly, the results show that the dislocation may have four stable and two unstable equilibrium positions due to the presence of surface elasticity. In addition, the number and location of the equilibrium positions depend on both surface elasticity and crack bridging.

Significantly enhanced electric field in plasmonic hot spots can dramatically increase the linear and nonlinear absorption of light, leading to a high-temperature electron gas which radiates, through mainly intraband transition, a broadband luminescence quite similar to blackbody radiation. Here, we demonstrate that such hot-electron intraband luminescence (HEIL) can also be achieved by exploiting the significantly enhanced electric field at the magnetic dipole resonances of gallium arsenide (GaAs) nanospheres (NSs). We show that monocrystalline GaAs NSs with distinct electric and magnetic dipole (ED and MD) resonances can be obtained by using femtosecond laser ablation and annealing. Significantly enhanced second harmonic generation and broadband HEIL are observed when the MD resonances of such GaAs NSs are resonantly excited. The lifetime of the HEIL is found to be as short as ∼82 ps, indicating a significant enhancement in radiative intraband transition rate. We reveal that the slope extracted from the dependence of the HEIL intensity on the irradiance is linearly proportional to the energy of the emitted photon. The existence of distinct ED and MD resonances in combination with a direct bandgap makes GaAs NSs an attractive candidate for constructing novel all-dielectric metamaterials and active photonic devices.

Surface wrinkling of thin films on substrates offers an effective strategy to create controllable surface patterns of wide application. In this article, both theoretical analysis and numerical simulations are performed to study the surface wrinkling of thin films bonded on a microstructured soft substrate under compression. We consider two typical kinds of substrates that have different mechanical properties. One possesses a periodic array of micropillars, and the other has a period of alternating unbonded—bonded regions at the film—substrate interface. The characteristics of surface wrinkling and postbuckling evolution of films are revealed. It is found that the interfacial microstructures modulate the normal mechanical properties of the substrate and, thus, tailor the buckling behavior of the thin film atop it. Under lateral compression, the film on the substrate shows periodic arrays of micropillars exhibiting sinusoidal wrinkling first, and then with further compression, the wrinkles give way ...

The extracellular matrix (ECM) of a solid tumor not only affords scaffolding to support tumor architecture and integrity but also plays an essential role in tumor growth, invasion, metastasis, and therapeutics. In this paper, a non-equilibrium thermodynamic theory is established to study the chemo-mechanical behaviors of tumor ECM, which is modeled as a poroelastic polyelectrolyte consisting of a collagen network and proteoglycans. By using the principle of maximum energy dissipation rate, we deduce a set of governing equations for drug transport and mechanosensitive enzymatic degradation in ECM. The results reveal that osmosis is primarily responsible for the compression resistance of ECM. It is suggested that a well-designed ECM degradation can effectively modify the tumor microenvironment for improved efficiency of cancer therapy. The theoretical predictions show a good agreement with relevant experimental observations. This study aimed to deepen our understanding of tumor ECM may be conducive to novel anticancer strategies.

The conflict between strength and toughness is a long-standing challenge in advanced materials design. Recently, a fundamental bottom-up material design strategy has been demonstrated using cellulose nanopaper to achieve significant simultaneous increase in both strength and toughness. Fertile opportunities of such a design strategy aside, mechanistic understanding is much needed to thoroughly explore its full potential. To this end, here we establish a multiscale crack-bridging model to reveal the toughening mechanisms in cellulose nanopaper. A cohesive law is developed to characterize the interfacial properties between cellulose nanofibrils by considering their hydrogen bonding nature. In the crack-bridging zone, the hydrogen bonds between neighboring cellulose nanofibrils may break and reform at the molecular scale, rendering a superior toughness at the macroscopic scale. It is found that cellulose nanofibrils exhibit a distinct size-dependence in enhancing the fracture toughness of cellulose nanopaper. An optimal range of the length-to-radius ratio of nanofibrils is required to achieve higher fracture toughness of cellulose nanopaper. A unified law is proposed to correlate the fracture toughness of cellulose nanopaper with its microstructure and material parameters. The results obtained from this model agree well with relevant experiments. This work not only helps decipher the fundamental mechanisms underlying the remarkable mechanical properties of cellulose nanopaper but also provides a guide to design a wide range of advanced functional materials.

The dynamic response of a rectangular nanoplate subjected to the low velocity impact by a nanoparticle is theoretically investigated. The van der Waals interaction between the particle and the plate is taken into account. Using the theory of surface elasticity and Hamilton's principle, the governing equations of the nanoplate are derived. Both the effects of surface elasticity and residual surface stress of the nanoplate are incorporated. Numerical examples are given for a silicon nanoplate impinged by a carbon nanoparticle. The physical mechanism of the impact behavior at the nanoscale is explained, and it is found that the interaction between the nanoparticle and the nanoplate exhibits different characteristics in comparison with the classical macroscopic impact. The surface effect of the nanoplate, the impact velocity and mass of the nanoparticle play a significant role in the impact force and dynamic response of the nanoplate.

Intercellular interactions play a significant role in a wide range of biological functions and processes at both the cellular and tissue scales, for example, embryogenesis, organogenesis, and cancer invasion. In this paper, a dynamic cellular vertex model is presented to study the morphomechanics of a growing epithelial monolayer. The regulating role of stresses in soft tissue growth is revealed. It is found that the cells originating from the same parent cell in the monolayer can orchestrate into clustering patterns as the tissue grows. Collective cell migration exhibits a feature of spatial correlation across multiple cells. Dynamic intercellular interactions can engender a variety of distinct tissue behaviors in a social context. Uniform cell proliferation may render high and heterogeneous residual compressive stresses, while stress-regulated proliferation can effectively release the stresses, reducing the stress heterogeneity in the tissue. The results highlight the critical role of mechanical factors in the growth and morphogenesis of epithelial tissues and help understand the development and invasion of epithelial tumors.

Living thin structures such as cell cortex layers and multicellular sheets often exhibit intricate morphologies and dynamic behaviors, including Turing’s pattern, periodic oscillation, and wave propagation. In this paper, we present a bio-chemo-mechanical theoretical framework to model these morphogenetic processes and to unveil the underlying mechanisms. On the basis of the nonlinear non-Euclidean shell model, we formulate an active solid shell theory which couples the excitation and transmission of biochemical signals with mechanical forces. Through linear stability analysis, it is found that Hopf and Pitchfork bifurcations induced by chemomechanical feedback are two primary mechanisms that may trigger pattern formation. A numerical scheme is developed to solve the coordinated mechanical and biochemical fields in the active shell system and thus, to track the dynamic pattern evolution beyond the stationary state. For illustration, the proposed theory is applied to starfish oocytes and to decipher the synchronous protein dynamics and surface contraction pattern emerging in the anaphase of meiosis, which involves both global oscillation and traveling waves. This study underscores the crucial role of bio-chemo-mechanical feedback in driving morphogenetic pattern evolution.

Surface microstructures greatly affect many physical and mechanical properties of solid materials. Surface constitutive relations are often required to analyze the mechanical responses of materials and structures at the micro scales, but it remains a challenge to determine the surface constitutive parameters of microstructured materials. In this paper, a machine learning-based approach is proposed to predict the surface elastic properties of materials with complex microstructures on their surfaces. Using a data set generated from the finite element method, we demonstrate that the trained deep neural network can efficiently provide an accurate mapping between the equivalent surface elastic properties (e.g., elastic moduli and Poisson’s ratios) and the mechanical and geometric features of surface microstructures. This study provides an efficient way to evaluate the surface properties of materials and promises for wide applications in the analysis of micro-/nanostructured materials and devices.

Cell dynamics is of crucial significance for the morphogenesis, self-repair, and other physiological and pathological processes of tissues. Collective cells exhibit greatly different dynamic behaviors from isolated cells. In this lecture, some recent advances in experimental and theoretical researches on collective cell dynamics will be presented, with particular attention paid to the biomechanical mechanisms underlying the morphomechanics of developing embryos and tumors. First, a cell division model is established for the division of interconnecting cells in a biological tissue. Coupled mechanical–chemical mechanisms involved in the multi-phase cell division are taken into account. Second, we explain why spontaneous oscillation of collective cells may occur in such biological tissues as Drosophila amnioserosa during development. It is revealed that the collective cell oscillation in an epithelium-like monolayer results from the dynamic bifurcation induced by negative feedback between mechanical strains and chemical cues. Further, we combine experimental measurements, theoretical analysis, and numerical simulations to investigate the migration modes and the underlying biomechanical mechanisms of collective cells. Universal statistical laws are derived for such dynamic parameters as energies and velocities of collective cells. EML Webinar speakers and videos are updated at https://imechanica.org/node/24098

The interplay between interfacial shear stress and adhesion has been an active but controversial subject of adhesive contact mechanics, which is currently plagued by diverse, sometimes contradicting, predictions. Recently, McMeeking et al. showed that a reversible interface slip parameter plays an essential role in determining how interfacial shear stress affects adhesion for a Johnson-Kendall-Roberts (JKR) contact interface. In this paper, adhesive contact between a rigid spherical indenter and an elastic half-space is studied with a generalized Maugis-Dugdale (M-D) model, where a constant frictional shear stress presents in the intimate contact area while a constant adhesive stress exists in a cohesive zone near the contact edge. The model solution predicts that the contact behavior is governed by a non-dimensional reversible shear index α τ ¯ 2 as well as the Maugis parameter λ . More specifically, it is found that the impact of interfacial shear stress on adhesion is most significant when the model approaches the JKR limit, and it gets less pronounced in the transitional regime and eventually becomes negligible in the Derjaguin-Mulller-Toporov (DMT) limit. Such behavior is in distinct contrast to Johnson's phenomenological solution. Finally, the proposed model is experimentally validated by adhesion tests on contact interfaces with varying Maugis parameters, where the reversible slip factor is experimentally extracted for the first time.

The elastic wave band structure in a phononic crystal (PC) is usually affected by the deformations in its soft constituent phase. In this work, hyperelastic transformation materials are proposed in the design of PCs in order to achieve stable elastic band-gaps that do not vary with deformation. It is demonstrated that one-dimensional PCs with a semi-linear soft phase can keep all elastic wave modes unchanged with respect to external deformations. However, only S-wave modes can be precisely retained in the PCs made of a neo-Hookean soft material. The theoretical results and the robustness of the proposed PCs are validated by numerical simulations.

Mechanical self-assembly of ordered patterns via spontaneous buckling of thin-film/soft-substrate systems has received considerable interests in recent years. Here we study the wrinkling of a stiff film resting on a fiber-filled soft substrate. In particular, the effects of the cross-section dimension, spacing, and positions of fibers on the wrinkling patterns in the film/substrate bilayer system are investigated. We show that diverse wrinkling patterns, including sinusoidal wrinkling, period-doubling, period-tripling and mountain ridge modes, may occur at a small or moderate overall compression strain due to the inhomogeneous deformation in the substrate and they can be well controlled by tuning geometrical and physical parameters of the system. To illustrate the potential use of the wrinkling patterns revealed in this study, we investigate the elastic wave propagation in such a wrinkled bilayer using the Bloch wave theory. Our computational results show that diverse stress patterns generated in the soft composites give rise to a rich variety of band structures. Desired bandgaps of elastic waves can be achieved and tuned by simply designing the geometric parameters and controlling the external stimuli imposed on the soft metamaterials.

Overheating exposures of turbine blades during service increased the risk of service safety in aircraft engines. Nevertheless, limited investigations about microstructural degradation induced by the overheating temperature and the effects on mechanical property of wrought superalloys were reported. In this paper, wrought superalloy GH4037 sectioned from an un-used 1st stage turbine blade of an aircraft engine was adopted to investigate the microstructural degradation caused by short-time thermal exposure at very high temperature (higher than normal service temperature) and the microstructural evolution during the mechanical tests after the short-time thermal exposures. The effects of microstructural degradation on mechanical properties including hardness, high temperature tensile properties and creep properties were also analyzed. The results indicate that gradual dissolutions of γ′ phase and grain boundary (GB) carbides were the typical manifestation of microstructural degradation in GH4037 alloy during the short-time thermal exposures at 1000–1140 °C for 3 min and 5 min. The dissolved γ′ phase and GB carbides re-precipitated in a very short time (less than 12 min) as ultra-fine particles and continuous/cellular GB carbides during 850 °C tensile tests and creep tests at 850 °C/196 MPa. Due to the re-precipitated ultra-fine γ′ phase, the tensile strength at 850 °C after exposing to 1140 °C for 3 min was analogous to those of specimens without the thermal exposure. But the re-precipitated continuous/cellular GB carbides would cause the significant decrease of tensile ductility at 850 °C and the creep elongation at 850 °C/196 MPa. On the other hand, the dissolution of γ′ phase and GB carbides during the short-time thermal exposures reduced the creep properties at 700°C/471 MPa significantly, as it took much longer time (more than 18.1 h) for the re-precipitation of γ′ phase when the overheated alloys were exposed at 700°C/471 MPa. The work provides the guidance for overheating-inspection and diagnosis of service safety for blades made of wrought superalloys.