Micromechanical Applications Research Articles

Shape-Morphing in Oxide Ceramic Kirigami Nanomembranes.

Interfacial strain engineering in ferroic nanomembranes can broaden the scope of ferroic nanomembrane assembly as well as facilitate the engineering of multiferroic-based devices with enhanced functionalities. Geometrical engineering in these material systems enables the realization of 3-D architectures with unconventional physical properties. Here, 3-D multiferroic architectures are introduced by incorporating barium titanate (BaTiO3, BTO) and cobalt ferrite (CoFe2O4, CFO) bilayer nanomembranes. Using photolithography and substrate etching techniques, complex 3-D microarchitectures including helices, arcs, and kirigami-inspired frames are developed. These 3-D architectures exhibit remarkable mechanical deformation capabilities, which can be attributed to the superelastic behavior of the membranes and geometric configurations. It is also demonstrated that dynamic shape reconfiguration of these nanomembrane architectures under electron beam exposure showcases their potential as electrically actuated microgrippers and for other micromechanical applications. This research highlights the versatility and promise of multi-dimensional ferroic nanomembrane architectures in the fields of micro actuation, soft robotics, and adaptive structures, paving the way for incorporating these architectures into stimulus-responsive materials and devices.

Droplet-based mechanical transducers modulated by the symmetry of wettability patterns

Asymmetric mechanical transducers have important applications in energy harvesting, signal transmission, and micro-mechanics. To achieve asymmetric transformation of mechanical motion or energy, active robotic metamaterials, as well as materials with asymmetric microstructures or internal orientation, are usually employed. However, these strategies usually require continuous energy supplement and laborious fabrication, and limited transformation modes are achieved. Herein, utilizing wettability patterned surfaces for precise control of the droplet contact line and inner flow, we demonstrate a droplet-based mechanical transducer system, and achieve multimodal responses to specific vibrations. By virtue of the synergistic effect of surface tension and solid-liquid adhesion on the liquid dynamics, the droplet on the patterned substrate can exhibit symmetric/asymmetric vibration transformation when the substrate vibrates horizontally. Based on this, we construct arrayed patterns with distinct arrangements on the substrate, and employ the swarm effect of the arrayed droplets to achieve three-dimensional and multimodal actuation of the target plate under a fixed input vibration. Further, we demonstrate the utilization of the mechanical transducers for vibration management, object transport, and laser modulation. These findings provide a simple yet efficient strategy to realize a multimodal mechanical transducer, which shows significant potential for aseismic design, optical molding, as well as micro-electromechanical systems (MEMS).

Organic-inorganic piezoelectric single-crystal TMCM-2CdCl3 with high piezoelectric properties

Organic-inorganic piezoelectric material is a kind of composite material with piezoelectricity, flexibility and plasticity. The crystals grown at room temperature can be shaped arbitrarily without polarization, and have unique advantages in shaped devices and wearable systems. In this paper, organic–inorganic piezoelectric single crystals TMCM-2CdCl3 with a perovskite molecular structure have been prepared by solution evaporation growth using trimethylammonium halide as the organic part and CdCl3 as the inorganic part. It has a monoclinic structure, and the space group is P21/n. The piezoelectric coefficient of unpolarized piezoelectric single crystals can reach 213 pC/N, making this material a competitive candidate in medical, micromechanical, and biomechanical applications.

Averaging material tensors of any rank in textured polycrystalline materials: Extending the scope beyond crystallographic proper point groups

In many modern micromechanical applications, there is usually a need to perform the orientational averaging of certain material tensors weighted by an orientation distribution function (ODF). The computation of these averages is seen to be very simplified by means of the so-called generalized spherical harmonic method (GSHM), which is based on the classical assumption that the ODFs are defined in the rotation group SO(3). A priori, this makes the averaging strictly applicable only to polycrystals with crystallite symmetry defined by one of the 11 proper point groups. Despite the interest in the study of materials belonging to any of the other 21 point groups, few studies have properly considered such cases. This is crucial for physical properties represented by odd-order tensors such as the third-order linear piezoelectric tensor. The goal of this work is to extend the applicability of the GSHM to crystallites with symmetry that belongs to the orthogonal group O(3). Thus, a simple formula is provided for the averaging of material tensors of any rank in textured polycrystals containing crystallites with symmetry defined by any of the 32 crystallographic point groups. Our work presents a simple yet rigorous method for indirect averaging on the orthogonal group, using averaging on SO(3). We demonstrate the full equivalence between our proposed method and averaging properly on O(3) through a detailed proof provided in this paper. This proof represents a significant contribution to the field, providing a practical and reliable approach for researchers working with crystalline materials. The results reported here confirm the validity of closed-form expressions previously derived by the authors for piezoelectric materials.

Towards Two-Photon Polymerization-Compatible Diffractive Optics for Micro-Mechanical Applications.

Diffractive optics are structured optical surfaces that manipulate light based on the principles of interference and diffraction. By carefully designing the diffractive optical elements, the amplitude, phase, direction, and polarization of the transmitted and reflected light can be controlled. It is well-known that the propagation of light through diffractive optics is sensitive to changes in their structural parameters. In this study, a numerical analysis is conducted to evaluate the capabilities of slanted-wire diffraction gratings to function opto-mechanically in the infrared spectral range. The slanted wire array is designed such that it is compatible with fabrication by two-photon polymerization, a direct laser-writing approach. The modeled optical and mechanical capabilities of the diffraction grating are presented. The numerical results demonstrate a high sensitivity of the diffracted light to changes in the slant angle of the wires. The compressive force by which desired slant angles may be achieved as a function of the number of wires in the grating is investigated. The ability to fabricate the presented design using two-photon polymerization is supported by the development of a prototype. The results of this study suggest that slanted-wire gratings fabricated using two-photon polymerization may be effective in applications such as tunable beam splitting and micro-mechanical sensing.

Theoretical justification of Tsai's modulus based on micromechanical analysis

This paper deals with the application of micromechanics for the determination of lamina effective properties. Micromechanical models can be considered as the first step of the composite material mechanical design and the traditional estimative of the effective elastic properties presents the drawback of uncertainties related to the fibers' properties and matrix curing effect. On this basis, Tsai's modulus was proposed based on average normalized properties for carbon fiber reinforced polymers (CFRP). On one hand, this approach has the advantage to be a simple procedure with good results; on the other hand, its generality needs to be confirmed because it is based on empirical approach. The present work develops a theoretical justification of the Tsai's modulus by comparing it with two micromechanical models: asymptotic homogenization, a rigorous analytical model; and VSPKc, a model based on mechanical principles of the traditional rule of mixture. Ranges of average properties for polymeric matrices and carbon fibers are selected, generating 65,536 data with the micromechanical models to be employed for each one of the five independent lamina's elastic properties. Results show that the difference between Tsai's modulus and the average normalized properties estimated by micromechanical models is smaller than 15% for all the properties. In this regard, the investigation provides a theoretical justification of Tsai's modulus approach.

Strong dependency of the tribological behavior of CuZr-based bulk metallic glasses on relative humidity in ambient air

Thanks to their outstanding mechanical properties, Bulk Metallic Glasses (BMGs) are new alternatives to traditional crystalline metals for mechanical and micromechanical applications including power transmission. However, the tribological properties of BMGs are still poorly understood, mostly because their amorphous nature induces counter intuitive responses to friction and wear. In the present study, four different BMGs (Cu47Zr46Al7, Zr46Cu45Al7Nb2, Zr60Cu28Al12, and Zr61Cu25Al12Ti2) underwent ball-on-disc friction tests against 100Cr6 steel balls (American Iron and Steel Institute (AISI) 52100) at different relative humidities (RHs) ranging from 20% to 80%. Controlling humidity enabled to observe a high repeatability of the friction and wear responses of the BMG. Interestingly, the friction coefficient decreased by a factor of 2 when the humidity was increased, and the wear rate of BMGs was particularly low thanks to a 3rd-body tribolayer that forms on the BMG surface, composed of oxidized wear particles originating from the ball. The morphology of this tribolayer is highly correlated to humidity. The study also identifies how the tribolayer is built up from the initial contact until the steady state is achieved.

Thin-film dynamics unveils interplay between light momentum and fluid mechanics.

We quantitatively measure the nanomechanical dynamics of a water surface excited by the radiation pressure of a Gaussian/annular laser beam of incidence near total internal reflection (TIR). Notably, the radiation pressure near TIR allowed us to induce a pushing force (Abraham's momentum of light) for a wide annular Gaussian beam excitation of the thin-film regime of water, which, to the best of our knowledge, has never been observed with nanometric precision previously. Our finding suggests that the observation of either/both Abraham's and Minkowski's theories can be witnessed by the interplay between optics and fluid mechanics. Furthermore, we demonstrate the first, to the best of our knowledge, simultaneous measurement of Abraham's and Minkowski's momenta emerging in a single setup with a single laser shot. Our experimental results are strongly backed by numerical simulations performed with realistic experimental parameters and offer a broad range of light applications in optofluidics and light-actuated micromechanics.

Morphological Effects of Strain Localization in the Elastic Region of Magnetorheological Elastomers.

Strain localization is a significant issue that poses interesting research challenges in viscoelastic materials because it is difficult to accurately predict the damage evolution behavior. Over time, the damage mechanism in the amorphous structure of viscoelastic materials leads to subsequent localization into a shear band, gradually jeopardizing the materials' elastic sustainability. The primary goal of this study is to further understand the morphological effects and the role of shear bands in viscoelastic materials precipitated by strain localization. The current study aims to consolidate the various failure mechanisms of a sample and its geometry (surface-to-volume ratio) used in torsional testing, as well as to understand their effects on stress relaxation durability performance. A torsional shear load stress relaxation durability test was performed within the elastic region on an isotropic viscoelastic sample made of silicon rubber and a 70% weight fraction of micron-sized carbonyl iron particles. Degradation was caused by a shear band of localized plasticity that developed microscopically due to stress relaxation durability. The failure pattern deteriorated as the surface-to-volume ratio decreased. A field-emission scanning electron microscope (FESEM) and a tapping-mode atomic force microscope (AFM) were used for further observation and investigation of the sample. After at least 7500 cycles of continuous shearing, the elastic sustainability of the viscoelastic materials microstructurally degraded, as indicated by a decline in stress performance over time. Factors influencing the formation of shear bands were observed in postmortem, which was affected by simple micromanipulation of the sample geometry, making it applicable for practical implementation to accommodate any desired performance and micromechanical design applications.

Non-Hookean Droplet Spring for Enhancing Hydropower Harvest.

Nonlinear elastic materials are significant for engineering and micromechanics. Droplets with the merits of easy-accessibility, diversity, and energy-absorption capability exhibit a variety of non-Hookean elastic behaviors. Herein, benefiting from the confinement of heterogeneous-wettable parallel plates, the non-Hookean mechanics of the droplet-based spring are systematically investigated. Experimental results and theoretical analysis reveal that the force generated by the spring varies nonlinearly with its deformation, and a force model is accordingly built to depict the mechanics of springs with different sized/numbered droplets and confined by different wettability patterns. Importantly, for the droplet-based spring, the droplet-plate contact area expands nonlinearly with the pressing force, which is employed to optimize the output performance of the droplet-based triboelectric nanogenerator to 226% compared with the control test. This finding deepens the understanding of the non-Hookean behavior of droplet-based springs, and sheds light on applications in energy harvesting, micromechanics, and miniature optic/electric devices.

A viscoelastic constitutive model for shape memory polymer composites: Micromechanical modeling, numerical implementation and application in 4D printing

In this work, a novel micromechanics-based thermo-viscoelastic constitutive model for shape memory polymer composites (SMPCs) is proposed and applied to four-dimensional (4D) printed SMPCs. The multi-branch constitutive model is used to simulate the time- and temperature-dependent mechanical behavior of the shape memory polymer matrix. According to the elastic–viscoelastic correspondence principle, the equivalent viscoelastic stiffness tensor of the composite is obtained in the micromechanics framework of energy-based effective strain theory and the Mori-Tanaka homogenization scheme, in which a two-parameter interfacial damage model is adopted to consider the displacement discontinuity and traction continuity conditions at the interface between the inclusion and the matrix. The numerical integration scheme for the three-dimensional (3D) viscoelastic constitutive model of SMPCs is presented, and the finite element application is implemented by the user material subroutine UMAT of ABAQUS. After identifying the model parameters with experimental data, the tensile stress-strain curves and stress relaxation phenomena of 4D printed unidirectional SMPCs are successfully described by theoretical simulations. Besides, theoretical simulations also adequately predict the shape memory behavior of 4D printed composites and complex members.

P34. CONTINUOUS NEGATIVE PRESSURE WOUND THERAPY REGULATES HEALING AND FIBROSIS IN MURINE EXCISIONAL WOUNDS

PURPOSE: Hypertrophic scarring is a fibroproliferative disorder that can occur post-operatively resulting in significant functional and esthetic morbidity. Mechanical forces are known to modify the wound microenvironment and influence cellular behavior and organization. The potential effects of micromechanical force application through Negative Pressure Wound Therapy (NPWT) on skin scarring have not yet been explored in vivo. Here we investigate the application of continuous NPWT in a murine excisional wound model. METHODS: 20 obese and diabetic (db/db) mice received a 1×1 cm2 full thickness excisional dorsal skin wound and were separated into two groups, treated either with occlusive covering (Control, n=10), or NPWT (continuous, -125 mmHg, 7 days; NPWT, n=10). The wounds were photographed on days 0, 7, and 10, and the animals sacrificed and tissue harvested on day 10 for histological and molecular analysis. RESULTS: Although use of NPWT was shown to significantly increase Yes-associated protein (YAP), Engrailed-1 (En1) was significantly decreased. Use of NPWT significantly increased fibronectin deposition and improved collagen deposition and organization. Western blotting showed significantly increased RhoA and YAP and decreased Caspase-3 in the NPWT group. CONCLUSION: Taken together, our findings indicate that the mechanotransduction forces exerted by NPWT uncouple YAP, a mechanical signaling protein, from En1. Fibroblasts of an En1 lineage are known to mediate fibrotic processes that lead to scarring, while increased expression of caspase-3 has been linked to keloid formation. Therefore, through its actions on fibroblast proliferation and apoptosis, NPWT may play a significant role in final scar appearance.

Propagation characteristics of elliptic vortex beams in nonlocal nonlinear media

Optical patterns with the orbital angular momentum (OAM) can serve as “optical spanners”, and have attracted much attention for their applications in biophysics, micromechanics or microfluidics. In this work, we theoretically demonstrate the propagation characteristics of elliptic vortex beams carrying the OAM in nonlocal nonlinear media. The elliptic vortex beams cannot evolve as solitons due to their non-circular-symmetry, but can evolve as breathers in the nonlinear media without anisotropy. During breathing evolutions, the rotating phenomenon is observed. The rotations take place in both the vortex cores and the beam envelopes, which exhibit opposite rotating directions. The breathing and rotating evolutions both have close relations with the input power and the beam size. The analytical results are well confirmed by the numerical simulations. The results obtained in the paper might find potential applications in beam shaping, and in optical manipulations of rod-shaped micro-particles.

Mechanical Properties of Organic Electronic Polymers on the Nanoscale

AbstractOrganic semiconducting polymers have attractive electronic, optical, and mechanical properties that make them materials of choice for large area flexible electronic devices. In these devices, the electronically active polymer components are micrometers in size, and sport negligible performance degradation upon bending the centimeter‐scale flexible substrate onto which they are integrated. A closer look at the mechanical properties of the polymers, on the grain‐scale and smaller, is not necessary in large area electronic applications. In emerging micromechanical and electromechanical applications where the organic polymer elements are flexed on length scales spanning their own micron‐sized active areas, it becomes important to characterize the uniformity of their mechanical properties on the nanoscale. In this work, the authors use two precision nanomechanical characterization techniques, namely, atomic force microscope based PeakForce quantitative nanomechanical mapping (PF‐QNM) and nanoindentation‐based dynamical mechanical analysis (nano‐DMA), to compare the modulus and the viscoelastic properties of organic polymers used routinely in organic electronics. They quantitatively demonstrate that the semiconducting near‐amorphous organic polymer indacenodithiophene‐co‐benzothiadiazole (C16‐IDTBT) has a higher carrier mobility, lower modulus, and greater nanoscale modulus areal uniformity compared to the semiconducting semicrystalline organic polymer poly[2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene] (C14‐PBTTT). Modulus homogeneity appears intrinsic to C16‐IDTBT but can be improved in C14‐PBTTT upon chemical doping.

Nonlinear feedback self-excitation of modal oscillations in a class of under-actuated two degrees-of-freedom mechanical systems

Recently, scientists have utilized self-excited oscillations in many mechanical and micromechanical applications and accordingly, several experimental and theoretical research activities have been undertaken with the objective of artificially inducing self-excited oscillation in mechanical systems. This paper considers some theoretical aspects of generation of self-excited oscillation in a two degrees-of-freedom under-actuated spring–mass–damper system by centralized nonlinear feedback. The control force depends on the response of both masses. The proposed control law is slightly general in nature as it entails both collocated and non-collocated control as special cases. The response of the nonlinear feedback control is studied with the help of the method of averaging. The present study shows that by appropriately selecting the control gains it is possible to excite the desired mode(s) of oscillation irrespective of initial conditions. Even it is possible to excite dual mode oscillation having the characteristics of quasiperiodic oscillations. The theoretical analysis of the dynamics of the system with the control is validated by simulation results, performed in MATLAB Simulink.

PROGRESS AND APPLICATION PROSPECT OF FUNDAMENTAL RESEARCH ON CONCRETE MICROMECHANICS

Concrete is a fundamental and key structural material for modern civil engineering construction. The development of modern engineering construction poses new challenges to the performance of concrete. Calcium-silicate-hydrate (C-S-H) is the most important product of cement hydration, which occupies about 70% of the hydration products. It is also the most important cementitious material in concrete. C-S-H plays a key role in cementing sands and coarse aggregates, and exerting the strength. At the microscale, C-S-H is a porous material with a complex microstructure. Therefore, the analysis and design of C-S-H micromechanical properties is essential for understanding and improving the macroscale mechanical properties of cement-based materials. It is also a fundamental scientific problem in the field of concrete research. This paper aims to introduce the micromechanical characterization method of concrete, the micromechanical theory of concrete, and the new findings obtained from the applications of micromechanics into concrete, and to forecast the application of concrete micromechanics in engineering construction.

Microstructural evolution and its outcome on the photo induced micro actuation effect and mechanical properties of copper doped Co-Ni-Al FSMA

In this work, we studied the effect of doping with copper to seek improvement in mechanical and photo induced micro actuation (PIMA) properties of a Co-Ni-Al ferromagnetic shape memory alloy (FSMA). Unique and unexplored PIMA property of FSMA system has opened up the scope of various laser controlled micro-mechanical engineering applications in miniature scale. Transmission Electron Microscopy and X-ray diffraction technique were used to characterize and correlate the evolved microstructure with the corresponding changes in the PIMA and mechanical properties. Magnetic properties of these samples were studied by performing hysteresis experiments at room temperature. It was found that partial substitution of cobalt with copper resulted in samples with lower magnetic and PIMA properties but better mechanical property, however, in case of substituting aluminum, it resulted in improvement of the ferromagnetic property but destroyed the PIMA effect altogether. In order to understand these properties changes, the electron density of each of the samples was calculated and compared. These studies should help in deciding on use of new alloys with optimization of PIMA and mechanical property for desired and customized engineering applications.

Interferometric measurements of graphene-based membranes for micromechanical applications

Recent studies on piezoelectricity in 2D materials, such as graphene, have revealed their potential in multiple applications including microelectromechanical systems. In this work, we reveal piezoelectricity in few-layer graphene sheets deposited on amorphous Si3N4 membranes. The response in the center of square membrane of 6.7 × 6.7 mm2 size reaches about 14 nm at resonance and could be further enhanced by adjusting the composite structure. Resonance piezoelectric phenomena are fully consistent with the earlier piezoresponse force microscopy observations on free-standing piezoelectric graphene and open up a possibility of using 2D materials in micromechanical applications.

Observation of Vortex Domains in a Two-Dimensional Lead Iodide Perovskite Ferroelectric.

Topological defects, such as vortices and skyrmions, provide a wealth of splendid possibilities for new nanoscale devices because of their marvelous electronic, magnetic, and mechanical behaviors. Recently, great advances have been made in the study of the ferroelectric vortex in conventional perovskite oxides, such as BaTiO3 and BiFeO3. Despite extensive interest, however, no intriguing ferroelectric vortex structures have yet been found in organic-inorganic hybrid perovskites (OIHPs), which are desirable for their mechanical flexibility, ease of fabrication, and low acoustical impedance. We observed the robust vortex-antivortex topological configurations in a two-dimensional (2D) layered OIHP ferroelectric (4,4-DFPD)2PbI4 (4,4-DFPD is 4,4-difluoropiperidinium). This provides future directions for the study of perovskites and makes it a promising alternative for nanoscale ferroelectric devices in medical, micromechanical, and biomechanical applications.

Studies on the Effect of Temperature on the Photo-Induced Microactuation Effect of a Co-based FSMA System

In this work, we have studied the effect of open air annealing on photo-induced microactuation (PIMA) property of a Co–Ni–Al melt-spun FSMA ribbon sample. This is one of the alloy systems that demonstrate the special type of PIMA effect while interacting with laser. This property has opened up the scope of various laser-controlled micromechanical engineering applications. Due to this importance, a detailed study has been made to explore the applicability of this alloy in different environmental conditions, where temperature plays an important role. The correlation between microstructure and the photo-induced microactuation (PIMA) properties has also been addressed here. The variation in actuation of the alloy with different heat-treated conditions has been studied in details. Transmission electron microscopy (TEM) was used to characterize the evolved microstructure for both the as-spun and the heat-treated alloy samples. The correlation between the evolution of the structural effects and resulting degradation of the PIMA property changes was also followed systematically to understand the correlation.