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Active topological phase transitions widely occur in active matters and biological systems, such as developing embryos. Since the discovery of the intriguing bulk-boundary effects of topological insulators in Hermitian and non-Hermitian systems, various electric, optical, acoustic, and mechanical topological metamaterials with efficient energy transmission and robust defect-immunization have been designed. To date, however, it remains a challenge to precisely and fast manipulate the topological phase transitions in elastic topological insulators. In this paper, on the basis of theoretical analysis and numerical simulations, we propose an active strategy to achieve this aim through a combination of pneumatic actuation and liquid metals. The proposed method can precisely tune the connecting stiffness and vertex mass in the tight Su–Schrieffer–Heeger model. Thus, we realize the effective and fast control of topological phase transitions and elastic wave bandgap switching. We also uncover the active spinning bulk-boundary effects and higher-order topological states in the elastic topological insulators, demonstrating the high effectiveness and practicability of the proposed method. In addition, the differences between the 1D edge and 0D corner higher-order states are specified by information entropy theory. This work not only gains insights into the active manipulation of topological phase transitions but also inspires novel strategies to design active topological materials through untethered methods, e.g., magnetism or biological cells.

Graphynes are the allotrope of graphene. In this work, extensive molecular dynamics simulations are performed on four different graphynes ( α-, β-, γ-, and 6,6,12-graphynes) to explore their mechanical properties (shear modulus, shear strength, and bending rigidity) under shearing and bending. While the shearing properties are anisotropic, the bending rigidity is almost independent of the chirality of graphynes. We also find that the shear modulus and shear fracture strength of graphynes decrease with increasing temperature. The effect of the percentage of the acetylenic linkages on the shear mechanical properties and bending rigidity is investigated. It is shown that the fracture shear strengths and bending rigidities of the four types of graphynes decrease, while the fracture shear strain increases, with increasing percentages of the acetylenic linkages. Significant wrinkling is observed in graphyne under shear strain. The influence of the temperatures and percentages of the acetylenic linkages on the r...

Almost all solid surfaces have roughness on different length scales, from macro, micro to nano. In the conventional fractal contact model, the macroscopic Hertzian contact theory is employed to predict the contact load-area relation for all sizes of contact spots. However, when the contact radius of an asperity shrinks to nanometers, surface tension may greatly alter the contact behavior. In the present paper, we address surface effects on the contact between a rigid sphere and an elastic half space, and we demonstrate that the contact load-area relation is size-dependent, especially for nanosized asperities. Then, the refined contact relation is incorporated into the Majumdar-Bhushan fractal contact model. It is found that the presence of surface tension requires higher load than the conventional fractal contact model to generate the same real contact area.

Owing to their unique phase change property, GeSbTe alloys hold promise for applications as a candidate material for nonvolatile electronic data storage. In this paper, we theoretically investigate the dislocation mechanisms underlying the phase change phenomenon of GeSbTe alloys under electric pulses. On the basis of the recent experiments by Nam et al. (Science 336, 1561–1566 (2012)), a theoretical model is presented to rationalize the dislocation-templated amorphization process under the action of electric pulses. The physical mechanisms of the nucleation, movement, and multiplication of dislocations in the electric field are analyzed. Using the model, the evolutions of temperature and dislocation density in a Ge2Sb2Te5 nanowire under electric pulses are computed and the critical voltage of amorphization is predicted.

The mechanisms of force generation and transference via microfilament networks are crucial to the understandings of mechanobiology of cellular processes in living cells. However, there exists an enormous challenge for all-atom physics simulation of real size microfilament networks due to scale limitation of molecular simulation techniques. Following biophysical investigations of constitutive relations between adjacent globular actin monomers on filamentous actin, a hierarchical multiscale model was developed to investigate the biomechanical properties of microfilament networks. This model was validated by previous experimental studies of axial tension and transverse vibration of single F-actin. The biomechanics of microfilament networks can be investigated at the scale of real eukaryotic cell size (10 μm). This multiscale approach provides a powerful modeling tool which can contribute to the understandings of actin-related cellular processes in living cells.

Nanoporous metals are a class of novel nanomaterials with potential applications in many fields such as sensing, catalysis, and fuel cells. The present paper is aimed to investigate atomic mechanisms associated with the uniaxial tensile deformation behavior of nanoporous gold. A phase field method is adopted to generate the bicontinuous open-cell porous microstructure of the material. Molecular dynamics simulations then reveal that the uniaxial tensile deformation in such porous materials is accompanied by an accumulation of stacking faults in ligaments along the loading direction and their junctions with neighboring ligaments, as well as the formation of Lomer–Cottrell locks at such junctions. The tensile strain leads to progressive necking and rupture of some ligaments, ultimately resulting in failure of the material. The simulation results also suggest scaling laws for the effective Young's modulus, yield stress, and ultimate strength as functions of the relative mass density and average ligament size in the material.

We investigated numerically and experimentally the transmission of terahertz (THz) waves through single and multiple metallic defects created in a one-dimensional (1D) photonic crystal (PC) by inserting single metallic wires or arrays of parallel metallic wires into the air-gap defect of the 1D PC. The transmission properties of the metallic defect modes generated in the photonic bandgap (PBG) were characterized by using THz time-domain spectroscopy. For single metallic defects, it was found that the appearance the defect mode depends not only on the diameter of the metallic wires but also on the polarization of the THz wave. For transverse magnetic (TM) polarized waves whose electric fields are parallel to the metallic wires, the incident THz wave is generally split into two identical parts. In sharp contrast, the excitation of surface plasmon polaritons (SPPs) with enhanced field intensity is observed for transverse electric (TE) polarized waves whose electric fields are perpendicular to the metallic wi...

An approximate continuum theory is developed to effectively handle the problem of interaction between dislocations and inhomogeneity of any shape and properties. The inhomogeneity is, based on the Eshelby equivalent inclusion theory, equivalent to a homogenous one with a transformation strain. The interaction force between dislocation and the inhomogeneity can then be evaluated from the work done by the dislocation stress field during the transformation. The proposed continuum theory is applicable to a variety of inhomogeneities, such as pore, gas bubble, shear band and plastically deformed zone. It can be reduced to the classical continuum theory for some special cases.

The wetting angle of a liquid droplet on a dielectric substrate can be tuned by an applied electric field. Recent experiments revealed an intriguing phenomenon where the contact line between a droplet and a substrate may lose stability under a relatively large field, leading to the ejection of small droplets from the edge of the mother droplet. While this behavior may pose a limit on the achievable wetting angle, it also provides an interesting approach to produce patterns of tiny droplets. We propose a phase field model to explain the mechanism, which combines thermodynamics and convective viscous flow. Our study suggests that instability is preceded by a contact angle reduction and extrusion of a thin layer from the edge of the droplet. While instability appears when the electric field is above a critical value, it can be suppressed with increased surface energy of the droplet.

The modification of the defect mode in a one-dimensional (1D) photonic crystal (PC) in terahertz (THz) spectral region was systematically investigated by using THz time-domain spectroscopy (THz-TDS). The 1D PC was constructed by periodically arranging glass slides into an ordered structure. A defect was created by intentionally increasing the separation (i.e., the air gap) between two neighboring glass slides located in the middle of the PC. The tuning of the defect mode in the band gap was demonstrated by changing the thickness of the air gap. Modification of the defect mode was realized by inserting a silicon wafer into the defect. It was found that the appearance of the defect mode in the band gap depends not only on the relative position of the inserted silicon wafer with respect to the beam center but also on the resistivity of the silicon wafer. For the silicon wafer with a high resistivity, a transition of the defect mode from one frequency to the other was observed. When the front end of the silic...

The optical trapping and manipulation of magnetic holes (MHs) dispersed in a magnetic fluid is systematically investigated. It is found that the gradient force, which tends to attract MHs to the beam center, can be completely counteracted by the repulsive force between MHs induced by a magnetic field. As a result, a depletion region is created at the laser beam spot for a sufficiently strong magnetic field. This phenomenon can be easily observed for large MHs with a diameter of 11 μm. However, it does not appear for MHs with a smaller diameter of 4.3 μm. It is revealed that the enhancement in the concentration of magnetic nanoparticles in the laser spot region as well as the clustering of these nanoparticles leads to a much stronger interaction between MHs when a magnetic field is applied. Consequently, the magnetic field strength necessary to create the depletion region is significantly reduced. We also find that the trapping behavior of MHs depends strongly on the thickness of the sample cells. For thin...

The dependence of the transmission behavior of magnetic fluids on the incident power density of a laser beam is investigated and the conventional Z-scan technique is employed to continuously vary the optical forces induced by the laser beam. We calculate the optical forces exerted on magnetic nanoparticles and compare them with those for gold and silica nanoparticles. It is found that the optical forces for magnetic nanoparticles are comparable to those for gold nanoparticles. In addition, the calculation results show that the absorption force is dominant at low incident power densities while the gradient and scattering forces become significant at high incident power densities when the clustering of magnetic nanoparticles occurs. In Z-scan experiments, it is observed that the evolution of the Z-scan trace of a magnetic fluid with increasing incident power density cannot be explained only by the nonlinear absorption of the magnetic fluid induced by the thermal diffusion of magnetic nanoparticles. Instead,...

We investigate the dependence of the differential reflection on the structure parameters of quantum dot (QD) heterostructures in pump-probe reflection measurements by both numerical simulations based on the finite-difference time-domain technique and theoretical calculations based on the theory of dielectric films. It is revealed that the value and sign of the differential reflection strongly depend on the thickness of the cap layer and the QD layer. In addition, a comparison between the carrier dynamics in undoped and p-doped InAs∕GaAs QDs is carried out by pump-probe reflection measurements. The carrier capture time from the GaAs barrier into the InAs wetting layer and that from the InAs wetting layer into the InAs QDs are extracted by appropriately fitting differential reflection spectra. Moreover, the dependence of the carrier dynamics on the injected carrier density is identified. A detailed analysis of the carrier dynamics in the undoped and p-doped QDs based on the differential reflection spectra i...

For soft films with a thickness on the order of microns or nanometers, the long-range surface∕interface interaction can be sufficiently strong to induce their surface instability or even rupture. By using the bifurcation theory of elasticity, we here present a three-dimensional theoretical model to study the spontaneous surface instability of a soft elastic thin film supported by a rigid substrate. By accounting for the competition of van der Waals interaction energy with elastic strain energy and surface energy, we obtain the analytical solutions for the critical conditions of three-dimensional surface morphology instability. The effects of surface energy, thickness, and elastic properties of the film on the characteristic wavelength of surface wrinkling are examined. It is found that the characteristic wavelength of the deformation bifurcation mode depends on the film thickness via an exponential relation, with the power index in the range of 0.75–1.0, which mainly depends on the ratio between the surfa...

Effects of surfaces/interfaces become prominent in micro- and nanosized materials and devices. In the present paper, the diffraction of plane harmonic compressional waves (P wave) by a spherical nanoinclusion is studied theoretically using the surface/interface elasticity theory. The results demonstrated that when the inclusion size shrinks to nanometers, surface/interface elasticity plays a significant role in the diffraction of elastic waves. For incident waves of different frequencies, the interface effects on the dynamic stress concentration around the spherical inclusion are examined in detail.

Based on the surface elasticity theory, we examined the effects of surface stresses on nanosized contact problems. The Fourier integral transform method is adopted to derive the general solution for the contact problem under pressure. As two examples, the deformations induced, respectively, by a uniform distributed pressure and a concentrated force are analyzed in detail. The results indicate some interesting characteristics in contact mechanics, which are distinctly different from those in classical elasticity theory. Both the contact normal stress and the deformation gradient on the deformed surface vary smoothly across the loading boundary as a result of surface stress. In addition, the indent depth and the maximum normal contact stress depend strongly on the surface stress for nanoindentation.