Size-scale Effects Research Articles

Cortical synapses of the world's smallest mammal: An FIB/SEM study in the Etruscan shrew.

The main aim of the present study was to determine if synapses from the exceptionally small brain of the Etruscan shrew show any peculiarities compared to the much larger human brain. We analyzed the cortical synaptic density and a variety of structural characteristics of 7,239 3D reconstructed synapses, using using Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM). We found that someof the general synaptic characteristics are remarkably similar to those found in the human cerebral cortex. However, the cortical volume of the human brain is about 50,000 times larger than the cortical volume of the Etruscan shrew, while the total number of cortical synapses in human is only 20,000 times the number of synapses in the shrew, and synaptic junctions are 35% smaller in the Etruscan shrew. Thus, the differences in the number and size of synapses cannot be attributed to a brain size scaling effect but rather to adaptations of synaptic circuits to particular functions.

Cosserat continuum modelling of chiral scale-size effects and their influence on effective constitutive laws

We derive effective models from heterogeneous Cosserat media using asymptotic homogenization and taking into account size effects observed experimentally. The classification of the effective equations depends on the hierarchy of four characteristic lengths: the size of the heterogeneities ℓ, the Cosserat intrinsic lengths of the constituents ℓcoss and ℓchiral, and the overall characteristic length of the domain L. Depending on the different scale interactions between ℓcoss, ℓchiral, ℓ, and L we obtain either an effective Cauchy continuum or an effective Cosserat continuum with or without centrosymmetry. Herewith, we extend the work of Forest, Pradel, and Sab (Int. J. Solids Structures 38 (26–27): 4585–4608 ’01) by accounting for chiral effects.

(Invited) Pressure-Induced New Carbon Materials with New Properties

Design and synthesis of new carbon materials have been attracting intensive attention due to their application in various fields. The study of carbon nanostructures under high pressure provides us a very effective method to create new carbon material which is hardly discovered at ambient conditions because of the appearance of nanometer scale size effect and the novel high pressure behavior. In this presentation, several examples on high pressure induced novel structures in recently studied fullerene based molecular cocrystals will be outlined. Particularly, anomalous negative volume compressibility observed in the eutectic Sc3N@C80 -cubane crystal under high pressure will be introduced. The charge transfer from fullerene cage to cubane promotes the configuration change of cubane, which transforms into a “low density” configuration, resulting in an abnormal lattice expansion of the crystal. This is the first experimentally discovered crystal which has negative volume compressibility. We will also present the synthesis of nearly all sp3 amorphous carbon bulk material from high temperature and high pressure (HPHT)-treated C60 for the first time. This new sp3 amorphous carbon material shows an optical band gap of ~2.7eV and extremely high Vickers hardness of 102 GPa under 9.8N load, close to that of diamond, which is a multifunctional superhard amorphous carbon material with the highest thermal conductivity and hardness known in the amorphous materials up to now.

Mechanically durable anti-bacteria non-fluorinated superhydrophobic sponge for highly efficient and fast microplastic and oil removal

Microplastics (MPs) pollution evolves into a global environmental problem to be solved urgently. Although many studies are exploring ways to remove MPs from water environment, most of them are lack of selectivity and low efficiency. Herein, considering the fascinating absorption selectivity of superwetting materials, a robust magnetic-responsive superhydrophobic and superoleophilic sponge was firstly used to quickly eliminate MPs from water with very high efficiency. The functional sponge was fabricated by a non-fluorinated coating technique that consisted of polydimethylsiloxane (PDMS) grafted Fe3O4 particle, PDMS grafted halloysite nanotubes, and PDMS binder. The coated sponge achieved excellent mechanically durable and chemically stable superhydrophobicity that resisted a series of severe treatments. It was unquestionable to show very fast oil absorption. What's more, it especially showed very high adsorption capacity (24.3–48.2 mg/g) and could quickly adsorb almost 100% MPs (polypropylene, polyvinyl chloride, and polyethylene) from aqueous suspensions. Moreover, the removal rates remained almost 100% for these MPs after 50 cycles. Besides, the coated sponge had excellent salt tolerance and antibacterial activity to Escherichia coli (E. coli) (99.91%) and Staphylococcus aureus (S. aureus) (90.46%). The adsorption mechanism of the coating was discussed from the perspectives of molecular structure, electronic effect, steric hindrance, and size-scale effect. The absorption driving force mainly derived from the intra-particle diffusion under capillary attraction, whilst slight electrostatic interaction, hydrogen bond interaction, and σ-p (or p-p) conjugation between PDMS and MPs. This functional sponge was destined to be a new strategy in the removal of MPs and other solid pollutants, especially in the high-salinity and rich-microorganism water environment.

Scale effects in prestressed concrete structures: Maximum reinforcement percentage to avoid brittle crushing

The structural behaviour of prestressed concrete beams is considerably affected by different nonlinear phenomena occurring in post-cracking and crushing regimes, such as snap-back or snap-through instabilities. Design procedures included in current technical Standards are not able to take into account the actual crushing regime, since the adopted constitutive laws overlook the strain-localization and softening behaviour of the concrete matrix. Moreover, design provisions are usually based on Plasticity Theory, leading to completely disregard size-scale effects and ductile-to-brittle transitions as functions of the beam depth.In the present paper, the above-mentioned shortcomings are overtaken by means of a Fracture Mechanics approach. The Cohesive/Overlapping Crack Model is able to simulate the strain-localization and softening regime of concrete both in tension and compression, predicting the nonlinear crushing behaviour of prestressed concrete beams. Then, a scale-dependent maximum reinforcement percentage formulation is provided in order to avoid concrete crushing failure. In this way, the field in which prestressed concrete structures can develop a safe ductile behavior can be defined, formulating new standard requirements for an effective structural design.

Bandstructure and Size-Scaling Effects in the Performance of Monolayer Black Phosphorus Nanodevices.

Nanodevices based on monolayer black phosphorus or phosphorene are promising for future electron devices in high density integrated circuits. We investigate bandstructure and size-scaling effects in the electronic and transport properties of phosphorene nanoribbons (PNRs) and the performance of ultra-scaled PNR field-effect transistors (FETs) using advanced theoretical and computational approaches. Material and device properties are obtained by non-equilibrium Green’s function (NEGF) formalism combined with a novel tight-binding (TB) model fitted on ab initio density-functional theory (DFT) calculations. We report significant changes in the dispersion, number, and configuration of electronic subbands, density of states, and transmission of PNRs with nanoribbon width (W) downscaling. In addition, the performance of PNR FETs with 15 nm-long channels are self-consistently assessed by exploring the behavior of charge density, quantum capacitance, and average charge velocity in the channel. The dominant consequence of W downscaling is the decrease of charge velocity, which in turn deteriorates the ON-state current in PNR FETs with narrower nanoribbon channels. Nevertheless, we find optimum nanodevices with W > 1.4 nm that meet the requirements set by the semiconductor industry for the “3 nm” technology generation, which illustrates the importance of properly accounting bandstructure effects that occur in sub-5 nm-wide PNRs.

Evaluation of heater size and external enhancement techniques in pool boiling heat transfer with dielectric fluids

With the advent of digital and information-based economies and increased social interaction on digital media, electronic components have become ever-pervasive, smaller and more powerful, resulting in increased thermal loads. The cooling systems need to become more efficient in extracting the heat and allowing the devices to sustain peak performance for a longer duration. Available pool boiling data is mainly available on heater sizes of 10 mm square, but larger chip sizes of 34.5 × 32 mm are generally employed in data server applications. The present work focuses on pool boiling performance on plain and microchannel chips with dielectric fluids and also brings out the effect of heater size scaling. The pool boiling experiments were conducted in a closed system at atmospheric pressure using PP1, PP1C and FC-87 as the working fluids. A flow pattern with a large plume of bubbles was found to be responsible for generating macroconvection currents and improve the performance of the larger-sized heater, although additional study is recommended to see the transition point and its relation to the bubble flow patterns. Additionally, pool boiling enhancement using a patented hollow volcano-shaped conical enhancement structure was investigated. The conical structure placed above the heater surface similarly generated macroconvection currents that were responsible for boiling enhancement. However, the hollow conical structure did not always improve the performance since the liquid supply mechanism is already present in the microchannel flow passages.

Static and dynamic modeling of functionally graded Euler–Bernoulli microbeams based on reformulated strain gradient elasticity theory using isogeometric analysis

Strain Gradient Theories (SGTs) are often considered to capture the intrinsic microstructural behaviors of the microbeams. In the present work, the recent Reformulated Strain Gradient Theory (RSGT), which accounts the effect of the strain gradient, couple stress (rotation gradients) and velocity gradients collectively, is extended for the static and dynamic modeling of Functionally Graded (FG) Euler–Bernoulli microbeams to account for the effect of the strain gradient, couple stress (rotation gradients) and velocity gradients collectively. The present formulation also considers the shift of neutral axis due to material inhomogeneity in the FG microbeam. The problems of static deflection and free vibrations with different boundary conditions are addressed. Numerical solutions are obtained using isogeometric analysis. Analytical solutions of the present formulation for simply supported FG microbeam for static deflection and free vibration problems are also obtained and employed to validate the numerical results. The influence of the material index, different length scale parameters on the static deflection and free vibration responses of the FG microbeams are discussed. The present FG RSGT microbeam shows the prominent size scale effect in thin beams and predicts stiffer response than the FG microbeam based on modified couple stress theory.

Size-dependent vibration of single-crystalline rectangular nanoplates with cubic anisotropy considering surface stress and nonlocal elasticity effects

The presented paper investigates size-dependent vibration of single-crystalline rectangular nanoplates with cubic anisotropy based on combined surface stress and nonlocal elasticity effects. For this, the fundamental equations are derived with a discussion about the anisotropy formulations. The generalized differential quadrature method (GDQM) is used to solve the differential equation for applicable boundary conditions. The convergence and stability of the results are verified to obtain a suitable number of grid points. At first, the effective Young’s modulus versus the thickness is verified by the existing experiments. Then, the problem is simulated for a complete set of single crystal orientation and a wide range of size scale parameters and material types. The results indicate that size scale parameters have different vibration effects at different material orientations and boundary conditions. The size-scale effects decrease the natural frequency of CCFF (two edges clamped) and CCCC (fully clamped) nanoplates rotates from [100] to [110] axis for FCC (Face Centered Cubic ) materials in contrast with cantilevered nanoplates (CFFF).

Dynamics of perforated higher order nanobeams subject to moving load using the nonlocal strain gradient theory

The goal of this manuscript is to develop a nonclassical size dependent model to study and analyze the dynamic behaviour of the perforated Reddy nanobeam under moving load including the length scale and microstructure effects, that not considered before. The kinematic assumption of the third order shear deformation beam theory in conjunction with modified continuum constitutive equation of nonlocal strain gradient (NLSG) elasticity are proposed to derive the equation of motion of nanobeam included size scale (nonlocal) and microstructure (strain gradient) effects. Mathematical expressions for the equivalent geometrical parameters due to the perforation process of regular squared pattern are developed. Based on the virtual work principle, the governing equations of motion of perforated Reddy nanobeams are derived. Based on Navier's approach, an analytical solution procedure is developed to obtain free and forced vibration response under moving load. The developed methodology is verified and checked with previous works. Impacts of perforation, moving load velocity, microstructure parameter and nonlocal size scale effects on the dynamic and vibration responses of perforated Reddy nanobeam structures have been investigated in a wide context. The obtained results are supportive for the design of MEMS/NEMS structures such as frequency filters, resonators, relay switches, accelerometers, and mass flow sensors, with perforation.

Decoupling the effects of material thickness and size scale on the transverse free vibration of BNNTs based on beam models

Here we establish a new model to decouple the material thickness and elastic property of single-walled boron nitride nanotubes, and thus we can evaluate natural frequencies of transverse free vibration by directly using the Timoshenko beam and Euler-Bernoulli beam models. Full atomic simulations regarding on the vibration behaviors are also presented for comparison. The determination procedure of thickness and the physical meaning of scale parameter for one-atomic thick material are clearly defined. In order to comprehensively unfold the influence of the selection of tubular thickness on natural frequencies, a series of assumed tubular thickness is also adopted to analyze the sources of error and its trend, and finally confirms the accuracy of our proposed result. The accuracy of the beam models is also found to be dependent on the type of boundary constraints. The precision of the beam theory improves for weaker boundary constraints due to the release of shear stresses. The paper heals the rift between atomic simulations and continuum theories.

DPM model segregation validation and scaling effect in a rotary drum

Discrete phase method (DPM) model was used to analyse rotary drum systems for segregation behavior. DPM simulations were performed for comparison with a dynamic segregation experimental measurement from the literature. This included dynamic segregation and time-averaged particle velocity field, which were validated with experimental data. In addition, a direct DPM and parcel scaled DPM simulation study was performed to analyse the effect of drum and particle parcel size scaling. The segregation dynamics was evaluated using the Lacey mixing index. This work shows segregation dynamics decreases with increasing drum size while keeping the same particle size. It further shows that for a given drum size the segregation dynamics deviate after a certain particle parcel scaling limit. The parcel scaling limit also increases with increasing drum size.

Effect of vessel size scale on the aerosol spray scavenging efficiency with water mist

Research interest in eliminating airborne aerosol particles has been growing for decades. Aerosol particles with aerodynamic diameters between 0.2 and 1 μm are difficult to remove by traditional spray systems. To improve the aerosol scavenging efficiency, a new method is proposed that employs the addition of water mist. Water mist is supposed to aggregate with aerosol particles to form large-sized coagulated aerosol-mist particle clusters, which can subsequently be removed by spray droplets with a higher efficiency. To investigate the effect of the vessel's size scale on the aerosol collection efficiency, aerosol spray scavenging experiments without and with water mist were conducted in both the UTARTS facility and TOSQAN facility. The cylindrical vessels of the two facilities have the same internal diameter, but the vessel of the TOSQAN facility has a larger height. Experimental results show that water mist has the potential to improve aerosol scavenging efficiency in both facilities but to a higher degree inside the TOSQAN facility. Numerical simulations of aerosol removal by spray droplets without mist were conducted, and different scaling criteria for the geometry of the vessel and gaseous entrainment rate were discussed to explain the experimental results. With water mist, the coagulated aerosol-mist particles have a stronger inertia and a larger Stokes number; these particles are easier to capture by using spray droplets and a water film. The contribution of the spray injection height, spray spread volume, and water film to aerosol removal in cases with mist increased to a larger degree than that of the gaseous entrainment rate, finally resulting in a higher improvement ratio of aerosol removal efficiency in the TOSQAN facility.

A nanoscale study of size scale, strain rate, temperature, and stress state effects on damage and fracture of polyethylene

The mechanical properties and damage behavior of amorphous polyethylene are investigated using Molecular Dynamics (MD) simulations. The influence of time, temperature, size scale, and stress state are extensively studied to gather insights into the complex nature of glassy polymers and the related damage process of void formation during dynamic loading conditions. The Modified Embedded Atom Method (MEAM) is employed to model interatomic interactions, balancing computational efficiency with a superior description of free surfaces and void formation to provide a detailed analysis of the progression of damage in the form of nucleation, growth, and coalescence of voids. The MD simulations capture the evolution of the number and size of voids and are shown to correlate to specific regimes of the stress-strain response. Particular attention is given to the quantification of void coalescence and the influence coalescence has on the growth and nucleation rates of voids. A nanoscale parameter, the critical Inter-void Ligament Distance (ILD), is defined for the initiation of void coalescence, and is found to be about 0.75 void diameters resulting in the enhancement of void growth and the restriction of void nucleation once the critical ILD is achieved. This study provides important insight into the damage progression of glassy polymers that cannot be defined through current experimental techniques.

Size effect on void coalescence under intense shear

Void interaction, leading to coalescence, is the mechanism leading to ductile failure under intense shearing. Published unit cell model studies have demonstrated that micron-size voids collapse to form micro-cracks while continuous elongation and rotation of the voids thin the intervoid ligaments. At a final stage, the deformation leads to plastic flow localization in the ligament, and the material loses the load-carrying capacity. The micro-mechanism of void collapse, elongation, and rotation has been studied using 2D and 3D unit cell simulations but only within a conventional strain hardening material and, thereby, not accounting for micron scale size effects. However, the severe plastic deformation near the voids implies the development of significant plastic strain gradients, which must be accommodated by geometrically necessary dislocations (GNDs) that strengthens the matrix locally and elevates the stress level. The present research accounts for such gradient strengthening within the matrix in order to investigate the material size effect in ductile shear failure. The work presented leans on the unit cell model approach by Tvergaard (2009), but enables a constitutive length parameter to enter the analysis by substituting the matrix with a Fleck–Willis gradient enhanced material. The results reflect the combined effect of applied load, strain hardening, initial void volume fraction, and microstructure size. The general conclusion is that matrix strengthening, governed by size, delays the loss of load-carrying capacity and leads to less concentrated localization around small void. The results also show that the void collapse, elongation, and rotation mechanism is more sensitive to changes to the applied load, hardening, and initial void volume fraction at small scales.

Size dependent nano-spherical pressure vessels based on strain gradient theory

This study investigates the effect of size scale material parameters on stress distribution and radial displacement of nanosphere based on strain gradient theory. This model is more capable of studying mechanical behavior than classical elasticity theory as the size scale effect of the nanosphere is also considered. Minimum total potential energy is used to derive governing differential equation of nanosphere under internal hydrostatic pressure. Using the efficient numerical generalized differential quadrature (GDQ) method, the governing equation and corresponding boundary conditions are solved. The classical elasticity equation is obtained by setting the value of size scale material parameters to zero. With the comparison of these theories, the importance of the size scale material parameters is achieved. It is found that the radial displacement of nanosphere predicted by strain gradient theory is less than those predicted by classical elasticity theory but comparing the distribution of stress components along radius is more complex. The effect of the size of the nanosphere on the radial stress components is also studied. With an increasing outer radius of the nanosphere, the mechanical behavior predicted by strain gradient theory tends toward those in classical elasticity theory.

Room temperature magnetization dynamics of Y3Fe5O12 films capped with a Cr2O3 layer

Spin-transport across antiferromagnetic insulators (AFMIs) is a novel way to reduce power consumption in pure spin current based devices. In this paper, room temperature magnetization dynamics of Yttrium-Iron-Garnet (YIG) interfaced with an AFMI Cr2O3 has been investigated in order to study the spin transport across the AFMI. After capping with 20 nm thick Cr2O3, the Gilbert damping parameter of YIG layer is not changed enormously as compared to the non-capped sample. Cr2O3 of 20 nm thickness is in a spin disordered state at room temperature due to finite size scaling effect. So, it was expected that the spin current from YIG layer can be transmitted through the Cr2O3 layer. However, due to large spin diffusion length and large domain size of Cr2O3 (>20 nm), spin current cannot lose its angular momentum during propagation in Cr2O3 as a result of which, no change in Gilbert damping parameter is noticed. This observation will pave a way for further investigation of spin current transport across AFMIs.

Dynamics of perforated nanobeams subject to moving mass using the nonlocal strain gradient theory

In the present manuscript, based on the nonlocal strain gradient theory, a nonclassical dynamic finite element model is developed to study and analyze the dynamic behavior of perforated nanobeam structures under moving mass/load. In the context of nonclassical continuum mechanics and Timoshenko beam theory, dynamic equations of motion of perforated nanobeams are derived including both size scale (nonlocal) and microstructure (strain gradient) effects. The modification of the geometrical parameters due to the perforation process is included in the equations of motion for squared holes arranged in the arrayed form. The effect of the moving mass (the inertia, Coriolis and centripetal forces, and the gravity force) or moving load are included in the proposed model. To remove shear locking problem in slender nanobeams, finite element model on nonclassical shape function basis is developed. Elements stiffness and mass matrices and force vector including the nonlocal and strain gradient effects are derived. The proposed model is verified and checked with previous works. Impacts of perforation, mass/load velocities, inertia of mass, microstructure parameter and nonlocal size scale effects on the dynamic and vibration responses of perforated nanobeam structures have been investigated in a wide context. The following model is beneficial for the design of MEMS/NEMS structures such as frequency filters, resonators, relay switches, accelerometers, and mass flow sensors, with perforation.

Size effects in plain-weave Astroquartz® deployable thin shells

The general scaling trend for brittle materials, in which the strength increases when the sample size decreases, is reversed in plain-weave laminates of Astroquartz® and cyanate ester resin. Specifically, both the shear stiffness and the compressive strength decrease for test samples with widths smaller than 15 times the wavelength of the fabric, and observations at the microscale explain this behavior. The derived scaling is applied to the analysis of a deployable thin shell forming a 90∘ corner hinge with five cutouts on each side. The cutouts leave narrow strips of material with width as small as one fabric wavelength, forming structural ligaments whose strength and stiffness are subject to strong size-scaling effects. A numerical simulation of the folding process followed by a failure analysis is presented, using two alternative material models and failure criteria. The size independent model predicts that the structure will remain damage-free after it is folded and deployed, whereas the size-scaled model predicts that failure will occur. The correctness of the size-scaled model prediction is verified by measuring localized damage in a physical prototype, using x-ray CT scans.

Free vibration of a cracked FG microbeam embedded in an elastic matrix and exposed to magnetic field in a thermal environment

A mathematical model is developed, though this article, to investigate a vibrational behaviour of functionally graded (FG) cracked microbeam rested on elastic foundation and exposed to thermal and magnetic fields. The model includes a size scale effect and temperature dependent material properties, for the first time. The crack is modelled as a rotating spring, that is connecting the two parts of the microbeam at the crack’s position. The equation of motion of the FG microbeam is obtained by using the Euler-Bernoulli beam theory for kinematic assumption and nonlocal elasticity theory for size-dependency effects. The transverse Lorentz force induced from the magnetic field is derived using Maxwell's equations. By adding the effects of thermal loading and foundation parameters on the cracked micro beam, the motion equation of the entire system is obtained using the Hamilton’s principle and then solved with a Navier type solution method. Eight constraints equations are used to derived the frequency equation, which are boundary conditions at the end points and the displacement, slope, bending moment and transverse force continuity in the section where the crack is located. The resulting system of equations is solved sequentially, and natural frequencies and vibration modes of the cracked microbeam are obtained. The model is verified with previous published work. Numerical results are presented to illustrate influences of temperature, material composition, foundation parameters and magnetic field on the dynamics of the cracked FG microbeam.