Microscale Effects Research Articles

Micromixing performance of a miniaturized annular rotating flow mixer (MARFM)

Miniaturization of mixers is becoming an increasingly significant topic in process intensification. In this work, we develop a novel miniaturized annular rotating flow mixer (MARFM) by integrating the rotating flow field and micro-scale effect. The micromixing performance of MARFM is evaluated by the Villermaux/Dushman reaction system. Effects of various structural parameters on the micromixing performance are investigated. Two flow regions named stratification region and entrainment region have been firstly distinguished. Increasing rotational speed can remarkably improve micromixing performance under the stratification region rather than the entrainment region. In addition, the mixing performance at extreme flow conditions including high fluid viscosity and high flow rate ratio can be substantially improved in the MARFM. Segregation index is at least an order of magnitude smaller than that without the rotating flow field. Moreover, the micromixing time could range from 10−5 to 10−3 s in the MARFM, which is 1-3 orders of magnitude smaller than that of conventional mixers. Relationship between micromixing time and mean energy dissipation rate is established, which shows that higher energy utilization efficiency has been reached.

Modified couple stress theory for wave propagation in viscoelastic sandwich microplates with FG-GPLRC core and piezoelectric face sheets as sensor and actuator

In the present investigation, wave propagation of the viscoelastic sandwich microplates with functionally graded graphene nanoplatelets’ reinforced composite (FG-GPLRC) core and piezoelectric face sheets as sensor and actuator has been analyzed by the refined zigzag theory (RZT). Electromagnetic field is exposed to core and piezoelectric layers. Young's modulus, mass density, and Poisson's ratio are calculated based on the modified Halpin–Tsai model and rule of mixtures because FG-GPLs are arranged as uniform, parabolic, and linear patterns along the thickness of core. In addition, viscoelastic properties of the structure are simulated by the Kelvin–Voigt model. The system is located on an orthotropic visco Pasternak medium. The modified couple stress theory (MCST) is applied to simulate the microscale effects of the structure. Hamilton's principle is utilized for deriving the motion equations. Phase velocity, cut-off, and escape frequencies are computed with an analytical solution. The influence of valuable parameters, such as geometrical dimensions of the system, magnetic field, external voltage, small scale parameter, viscoelastic structural damping coefficient, various foundation types, weight fraction, and different distributions of GPLs on phase velocity, will be studied. The results of this study have a significant effect on the design and fabrication of sandwich micro-structures used in various industrial equipment.

Investigating the micro-scale thermal effects of natural underlying surfaces on adjacent spaces in a subtropical zone with an optimized method

Micro-scale thermal interactions between underlying surfaces and their surrounding air in urban spaces critically influence urban residents' thermal comfort. On the micro-scale, heat can induce substantial changes in the air temperature (Ta) at the pedestrian height. To characterize the spatiotemporal distribution of the Ta for spaces adjacent to the underlying surfaces, an accurate measurement method is essential. This study explored the proper spatial interpolation methods (SIMs) through strategically-arranged measurement points and then assessed the spatiotemporal distribution of Ta in a tiny experimental area. Results showed that the selection of SIMs and interpolation points significantly affected the results for the micro-scale spatial interpolation. The Ta might fluctuate precipitously within extremely short distances between adjacent underlying surfaces, depending on their thermal properties or on the locations of shadows. At the pedestrian height, the Ta readings registered a maximum difference of 1.31 °C for the same areas with and without arbor shading. However, the arbors' cooling extent and location depended on their shading's position, which constantly changed during the day. The shrubs and the lawn under direct sunlight hardly regulated the microclimate, even in the periphery. Additionally, solar radiation and wind speeds influenced the Ta spatial distribution on the micro-scale. Therefore, future micro-scale urban designs should take into consideration the dynamic positioning of arbor shading, given its significant cooling effect.

Quantitative Characterization of Micro-Scale Pore-Throat Heterogeneity in Tight Sandstone Reservoir

Nanoscale pore-throat systems are widely developed in the pore-throat of tight reservoirs. The pore-throat structures of different microscales are complex and diverse with obvious microscale effects. Taking the Chang 63 tight sandstone reservoir of the Huaqing area in Ordos basin as an example, under the guidance of information entropy theory, the quantitative characterization model of pore-throat micro-scale heterogeneity in a tight oil reservoir is established based on casting thin sections, physical properties analysis, constant velocity mercury injection, and NMR technology. Moreover, the correlation between pore-throat heterogeneity and porosity, permeability and movable fluid saturation is analyzed. The results show that there are obvious differences in pore-throat heterogeneity between different reservoirs, and the throat uniformity of macro pore-fine-throat reservoir, macro pore–micro throat reservoir, and macro pore–micro throat reservoir becomes worse, successively. There is a negative correlation between porosity uniformity and porosity, permeability and movable fluid saturation. However, there is a positive correlation between throat uniformity and combined pore throat uniformity and porosity, permeability and movable fluid saturation. Therefore, the uniformity of the throat controls the seepage capacity and fluid mobility in the pore system of the Chang 63 tight sandstone reservoir in the study area. This has important theoretical and practical significance to enhance oil recovery and promote the efficient development of a tight oil and gas reservoir.

Damage Evolution Simulations via a Coupled Crystal Plasticity and Cohesive Zone Model for Additively Manufactured Austenitic SS 316L DED Components

This study presents a microstructural model applicable to additively manufactured (AM) austenitic SS 316L components fabricated via a direct energy deposition (DED) process. The model is primarily intended to give an understanding of the effect of microscale and mesoscale features, such as grains and melt pool sizes, on the mechanical properties of manufactured components. Based on experimental observations, initial assumptions for the numerical model regarding grain size and melt pool dimensions were considered. Experimental observations based on miniature-sized 316L stainless steel DED-fabricated samples were carried out to shed light on the deformation mechanism of FCC materials at the grain scale. Furthermore, the dependency of latent strain hardening parameters based on the Bassani–Wu hardening model for a single crystal scale is investigated, where the Voronoi tessellation method and probability theory are utilized for the definition of the grain distribution. A hierarchical polycrystalline modeling methodology based on a representative volume element (RVE) with the realistic impact of grain boundaries was adopted for fracture assessment of the AM parts. To qualify the validity of process–structure–property relationships, cohesive zone damage surfaces were used between melt pool boundaries as the predefined initial cracks and the performance of the model is validated based on the experimental observations.

Fast deoxygenation in a miniaturized annular centrifugal device

Corrosion of steel devices and pipelines by dissolved oxygen (DO) is of major concern in many industries. Miniaturization of devices is becoming increasingly important significant for process intensification and distributed production. In this context, we attempted to realize efficient deoxygenation of water using a miniaturized annular centrifugal device (MACD) based on the centrifugal force and microscale effect. The influence of different operational variables on the deoxygenation performance was studied. It was found that the optimal volume mass transfer coefficient and deoxygenation efficiency are 1.01 s−1 and 97.8 %, respectively. The gas-liquid flow patterns in the mixing cavity, liquid film thickness distribution and residence time of water in the separation cavity of the MACD were investigated. The mechanism of fast deoxygenation and phase separation in MACD is explained. Additionally, a dimensionless correlation was obtained to calculate the volume mass transfer coefficient, and the predicted values were in good agreement with the experimental data. The deoxygenation performance of the MACD was also compared to those of other similar rotating deoxygenation devices, proving that MACD has the advantages of efficient deoxygenation performance, fast phase separation, and low energy consumption.

Study on the Stability of Functionally Graded Simply Supported Fluid-Conveying Microtube under Multi-Physical Fields.

The stability of functionally graded simply supported fluid-conveying microtubes under multiple physical fields was studied in this article. The strain energy of the fluid-conveying microtubes was determined based on strain gradient theory, and the governing equation of the functionally graded, simply supported, fluid-conveying microtube was established using Hamilton’s principle. The Galerkin method was used to solve the governing equation, and the effects of the dimensionless microscale parameters, temperature difference, and magnetic field intensity on the stability of the microtube were investigated. The results showed that the dimensionless microscale parameters have a significant impact on the stability of the microtube. The smaller the dimensionless microscale parameters were, the stronger the microscale effect of the material and the better the microtube stability became. The increase in the temperature difference decreased the eigenfrequency and critical velocity of the microtube and reduced the microtube stability. However, the magnetic field had the opposite effect. The greater the magnetic field intensity was, the greater the eigenfrequency and critical velocity were, and the more stable the microtube became.

Rapid demulsification and phase separation in a miniaturized centrifugal demulsification device

To achieve effective demulsification of stable emulsions, we proposed a miniaturized centrifugal demulsification device (MCDD) based on the centrifugal force and microscale effect. Various parameters were studied to determine the demulsification efficiency, phase separation time, and mean energy dissipation rate per unit volume of the reactor. Detailed investigations of the controlled mechanism of demulsification efficiency and phase separation time are discussed. A demulsification efficiency of up to 99.9% was achieved. The relationship between phase separation time and demulsification efficiency was obtained. In addition, the mean energy dissipation rate per unit volume of the reactor primarily ranged from 0.32 × 104 to 4.76 × 104 W/m3, which is one order of magnitude lower than that of other active demulsification equipment. Furthermore, competitive demulsification for both water-in-oil and oil-in-water emulsions can be achieved with the MCDD. This study is the first to propose an MCDD with high demulsification efficiency, fast separation time, and low energy consumption.

Holistic determination of physical fracture toughness values and numerical parameters for delamination analysis considering multidirectional-interfaces

Delaminations are a concern for the structural integrity of fibre composite structures. The decisive material parameter for delaminations is fracture toughness. Structures usually have a multidirectional ply stacking. However, interface orientation dependant fracture toughness values and R-curve effects are neglected in numerical delamination analysis. This work investigates interface orientation specific fracture toughness values and considers R-curve effects in mode I to improve simulation accuracy. Therefore, mode I, mode II and mixed mode fracture toughness values of different interface ply orientations are determined experimentally and verified using numerical analysis of the characterisation specimens with cohesive zone modelling. In this way, an engineering methodology is provided for the experimental characterisation and comprehensive numerical modelling of delaminations in mesoscale progressive damage analysis of multidirectional composite structures. In addition, a full parameter set for the simulation of four different interfaces of M21-T700GC prepreg material is given. It can be shown that the use of standard 0°//0°-values leads to very conservative results. The use of interface specific values increases the accuracy. Ultrasonic scans of the DCB specimens are used to compare the crack front shapes for validation. Not only the load displacement curves of the characterisation specimens are well captured, but also the crack front shapes. This demonstrates that by smearing the microscale effects, the material behaviour can be captured phenomenologically correct by mesoscale modelling suitable for industrial use.

Length effect on the plastic deformation of SiO2 microcantilevers

SiO2 films are of much interest for the realization of suspended micromechanical structures such as microcantilevers and micro-bridges owing to their superior characteristics when compared to conventional c-Si. However, presence of residual stress induced bending and structural plastic deformation in these devices especially at elevated temperatures hinder their potential for real-time field applications. Moreover, the impact of micro-scale dimensional effects on the magnitude of deformation in these devices is not well understood. In this work, we have investigated the effect of post-release high temperature annealing on the bending and resonance frequency characteristics of SiO2 microcantilevers (MCs) with a specific focus on the length effect. For this purpose, SiO2 MCs of various lengths (Length: 50–190 μm, Width: 37 μm and thickness: 0.98 μm) were fabricated using direct laser writer and wet chemical etching methods and were subjected to high temperature annealing under N2 and O2 environments. We demonstrate through our experiments that the curvature of SiO2 MCs increases with both annealing temperature and length square of MCs. Our experiments revealed that, apart from onset of plastic deformation, stress accumulation at the fixed end of SiO2 MCs is also responsible for the observed curvature increase. From the measured curvature, mean (σ0) and gradient (σ1) residual stress values were estimated and found to increase with annealing temperature. Interestingly, there exists a critical length of MCs (90 μm) below which both σ0 and σ1 were found to increase further with reducing MC length and is shown to be due to the existence of large transverse curvature at these dimensions. High temperature annealing was also found to increase the resonance frequency of MCs and the increase was maximum ∼ 8% for 130 μm long MC. Various possible reasons for the frequency shift is discussed in detail.

Nonlinear random vibrations of micro-beams with fractional viscoelastic core

This work investigates the nonlinear vibration of a fractional viscoelastic micro-beam exposed to random excitation. For the micro-scale effect, the micro-beam model is established based on the Modified Couple Stress Theory (MCST). The fractional nonlinear partial differential equation is obtained using Von-Karman’s nonlinear strains and the Kelvin–Voigt fractional viscoelastic model. The micro-beam is exposed to a random force which its amplitude is considered as the Gaussian white noise. The Finite Difference Method (FDM) and the Galerkin method are applied to discretize the stochastic governing equation of motion. To solve the stochastic partial differential equation, a numerical discretization for the white noise excitation is applied. Moreover, the Statistical Linearization (SL) method and the Spectral Representation (SR) method are also employed and a comparison is done between these methods. The simulations show good agreement between the numerical method and other methods. In addition, it is shown that the proposed method can be used for different boundary conditions of the micro-beam. The impacts of the fractional derivative order, white noise intensity, viscoelastic model, and length-scale parameter on the statistical properties of the micro-beam are shown. The numerical simulations indicate that the high values of fractional-order can significantly affect the variance and the Root Mean Square (RMS) of the transverse amplitude, However, this influence for small values of the fractional-order is negligible; the impact of the fractional-order derivative on the statistical characteristics of the micro-beam is very different from 0<α<0.6, and 0.6<α<1.

Laser induced 3D porous graphene dots: Bottom-up growth mechanism, multi-physics coupling effect and surface wettability

One-step conversion of three-dimensional (3D) porous laser-induced graphene (LIG) from carbon precursors opens a facile way to prepare graphene-related materials. Herein, a series of dots is fabricated by CO2 laser irradiation of polyimide (PI, Kapton) films to investigate the in-situ growth mechanism of LIG. Complex phase transitions combined with various physical fields lead to morphologic changes from mastoid to volcanic structure. The cross-section profile varying with laser parameters are fitted to measure the physical characteristics (e.g., top/bottom width, depth, and tilt angle) under Gauss energy distribution. With considering the highly localized heating effect and non-uniform thermal stress, a coupled physical field is established to theoretically analyze the temperature, thermal stress and deformation distribution. The micro-scale spatial effects such as nucleation, growth, escape and explosion of bubbles in the air flow field are responsible for the 3D porous structure of LIG. Inspired by the “lotus effect”, a superhydrophobic dot matrix is manufactured with static contact angle of 151.5 ± 1.3° and roll-off angle of 15.7 ± 1.2°, laying a foundation for large-scale production of LIG.

Effects of cell permeability on distribution and penetration of drug into biological tissues: A multiscale approach

This article presents a finite volume heterogeneous multiscale method (FVHMM-p) to study drug delivery into biological tissues. The FVHMM-p accounts for the passive diffusion across the cell membrane, and the algorithm involves models at two scales: macroscale (tissue scale) and microscale (cell scale). The passive diffusion across membranes is treated in the microscale model which is apparently a three-compartment model; the first compartment involves solute diffusion in extracellular space, the second one includes diffusivity across the cell membrane, and the third one is meant for diffusion in the intracellular space. For the microscale model simulation, a permeable interface method based on central-type finite difference discretization is developed. This model is coupled with the tissue scale model to enforce the cellular (microscale) effects. Furthermore, the effects of cell membrane permeability and drug particle size on distribution and penetration of drug nanoparticles in tissues are investigated by conducting numerical experiments. The cell is considered to be elliptical, and the simulations with different configurations lead to the pronounced effects. The proposed model will be able to identify the optimal strategies in drug delivery.

On the use of ultra-high resolution PIC methods to unveil microscale effects of plasma kinetic instabilities: electron trapping and release by electrostatic tidal effect

Abstract Ultra-high resolution particle-in-cell coupled to Monte-Carlo collisions modelling unveils microscale instabilities in non-equilibrium plasmas fulfilling Penrose’s instability criterion. The spontaneous development of ion turbulence in the phase-space generated by charge exchange collisions leads to finite amplitude modulations of the local electric field. The latter are responsible for the trapping of low energy electrons and their transport from the plasma volume to the sheath vicinity. Electrostatic tidal effect occurring near the sheath is responsible for the release of the trapped electrons as a monochromatic bunch, accelerated back towards the source. This instability provides an additional theoretical ground for the anomalous enrichment of low-energy electrons observed by Langmuir probes in similar conditions. The present results demonstrate that marginally fulfilling PIC criteria is insufficient to study the microscale instabilities effects on the electrons dynamics in non-equilibrium low temperature plasmas.

Relative permeability of gas and water flow in hydrate-bearing porous media: A micro-scale study by lattice Boltzmann simulation

The water-gas relative permeability is an important parameter to characterize multiphase flow in sediments. To study the water-gas relative permeability of hydrate-bearing porous media, multiphase flow simulations were carried out at the pore scale using the lattice Boltzmann method. The effects of hydrate saturation and hydrate-growth habits on the water-gas relative permeability, which is scaled by the relative permeability considering the hydrate only, were evaluated in a two-dimensional porous medium. Results show that the increase of hydrate saturation causes the decrease of water-gas effective permeability as expected. However, the effect of hydrate saturation on the water-gas relative permeability is different from that of hydrate saturation on the water-gas effective permeability. The water-gas relative permeability increases with the increase of hydrate saturation in the pore-filling case. The water-gas relative permeability decreases with the increase of hydrate saturation in the grain-coating case. The wettability of solid phase has a different effect on the relative permeability of wetting phase and nonwetting phase. The Jamin effect (phase blocking) was observed and may exist in the production of gas from natural gas hydrate reservoirs. This seriously affects the multiphase flow characteristics. The changes of microscale fluid distribution effect the changes of water-gas relative permeability. The relationship between the water-gas relative permeability and the characterization parameters of microscale fluid distribution was analyzed.

Liquid-liquid flow and mass transfer characteristics in a miniaturized annular centrifugal device

• A MACD with efficient mass transfer, fast phase separation and low energy consumption is proposed. • The dimensionless correlation of maximum throughput is obtained. • Operation region and controlled mechanism of liquid–liquid phase separation are obtained. • the maximum η and k L a are up to 99.8 % and 0.31 s −1 , respectively. • The mean energy dissipation rate is lower than that of similar rotating equipment. Miniaturization of chemical equipment is becoming an important topic in process intensification and microscale flow chemistry. In this context, we developed a miniaturized annular centrifugal device (MACD) based on the microscale effect and centrifugal force. Various parameters were investigated to determine the maximum throughput of the single-phase, operating region of liquid–liquid phase separation, mass transfer performance, and mean energy dissipation rate. Detailed investigations of the controlled mechanism of the liquid–liquid flow process showed that a very fast phase separation can be achieved. The extraction efficiency and volume mass transfer coefficient were up to 99.8 % and 0.31 s −1 , respectively. The mean energy dissipation rate ranged from 0.8 × 10 4 to 3.9 × 10 4 W/m 3 , which is lower than that of the similar rotating equipment. In addition, dimensionless correlations were obtained to predict the maximum throughput of the single-phase, the operating region of liquid–liquid phase separation, and the volume mass transfer coefficient. This study is the first time to propose a MACD with fast phase separation, efficient mass transfer, and low energy consumption, and provides a highly promising liquid–liquid process intensification tool.

Coupling mesoscale transport to catalytic surface reactions in a hybrid model.

In heterogeneous catalysis, reactivity and selectivity are not only influenced by chemical processes occurring on catalytic surfaces but also by physical transport phenomena in the bulk fluid and fluid near the reactive surfaces. Because these processes take place at a large range of time and length scales, it is a challenge to model catalytic reactors, especially when dealing with complex surface reactions that cannot be reduced to simple mean-field boundary conditions. As a particle-based mesoscale method, Stochastic Rotation Dynamics (SRD) is well suited for studying problems that include both microscale effects on surfaces and transport phenomena in fluids. In this work, we demonstrate how to simulate heterogeneous catalytic reactors by coupling an SRD fluid with a catalytic surface on which complex surface reactions are explicitly modeled. We provide a theoretical background for modeling different stages of heterogeneous surface reactions. After validating the simulation method for surface reactions with mean-field assumptions, we apply the method to non-mean-field reactions in which surface species interact with each other through a Monte Carlo scheme, leading to island formation on the catalytic surface. We show the potential of the method by simulating a more complex three-step reaction mechanism with reactant dissociation.

Electrostatic-Fluid-Structure 3D Numerical Simulation of a MEMS Electrostatic Comb Resonator.

The reliability and stability of MEMS electrostatic comb resonators have become bottlenecks in practical applications. However, there are few studies that comprehensively consider the nonlinear dynamic behavior characteristics of MEMS systems and devices in a coupled field so that the related simulation accuracy is low and cannot meet the needs of design applications. In this paper, to avoid the computational complexity and the uncertainty of the results of three-field direct coupling and take into the damping nonlinearity caused by coupled fields, a novel electrostatic-fluid-structure three-field indirect coupling method is proposed. Taking an actual microcomb resonant electric field sensor as an example, an electrostatic-fluid-structure multiphysics coupling 3D finite element simulation model is established. After considering the influence of nonlinear damping concerning the large displacement of the structure and the microscale effect, multifield coupling dynamics research is carried out using COMSOL software. The multiorder eigenmodes, resonant frequency, vibration amplitude, and the distribution of fluid load of the microresonator are calculated and analyzed. The simulated data of resonance frequency and displacement amplitude are compared with the measured data. The results show that the fluid load distribution of the microelectrostatic comb resonator along the thickness direction is high in the middle and low on both sides. The viscous damping of the sensor under atmospheric pressure is mainly composed of the incompressible flow damping of the comb teeth, which is an order of magnitude larger than those of other parts. Compared with the measured data, it can be concluded that the amplitude and resonance frequency of the microresonator considering the nonlinear damping force and residual thermal stress are close to the experimental values (amplitude error: 15.47%, resonance frequency error: 12.48%). This article provides a reference for studies on the dynamic characteristics of electrostatically driven MEMS devices.

Characterization of wax valving and μPIV analysis of microscale flow in paper-fluidic devices for improved modeling and design.

Paper-fluidic devices are a popular platform for point-of-care diagnostics due to their low cost, ease of use, and equipment-free detection of target molecules. They are limited, however, by their lack of sensitivity and inability to incorporate more complex processes, such as nucleic acid amplification or enzymatic signal enhancement. To address these limitations, various valves have previously been implemented in paper-fluidic devices to control fluid obstruction and release. However, incorporation of valves into new devices is a highly iterative, time-intensive process due to limited experimental data describing the microscale flow that drives the biophysical reactions in the assay. In this paper, we tested and modeled different geometries of thermally actuated valves to investigate how they can be more easily implemented in an LFIA with precise control of actuation time, flow rate, and flow pattern. We demonstrate that bulk flow measurements alone cannot estimate the highly variable microscale properties and effects on LFIA signal development. To further quantify the microfluidic properties of paper-fluidic devices, micro-particle image velocimetry was used to quantify fluorescent nanoparticle flow through the membranes and demonstrated divergent properties from bulk flow that may explain additional variability in LFIA signal generation. Altogether, we demonstrate that a more robust characterization of paper-fluidic devices can permit fine-tuning of parameters for precise automation of multi-step assays and inform analytical models for more efficient design.

Antarctic Hairgrass Rhizosphere Microbiomes: Microscale Effects Shape Diversity, Structure, and Function.

The rhizosphere microbiome of the native Antarctic hairgrass Deschampsia antarctica from the central maritime Antarctic was investigated using 16S RNA metagenomics and compared to those of the second native Antarctic plant Colobanthus quitensis and closely related temperate D. cespitosa. The rhizosphere microbial communities of D. antarctica and D. cespitosa had high taxon richness, while that of C. quitensis had markedly lower diversity. The majority of bacteria in the rhizosphere communities of the hairgrass were affiliated to Proteobacteria, Bacteroidetes, and Actinobacteria. The rhizosphere of C. quitensis was dominated by Actinobacteria. All microbial communities included high proportions of unique amplicon sequence variants (ASVs) and there was high heterogeneity between samples at the ASV level. The soil parameters examined did not explain this heterogeneity. Bacteria belonging to Actinobacteria, Bacteroidetes, and Proteobacteria were sensitive to fluctuations in the soil surface temperature. The values of the United Soil Surface Temperature Influence Index (UTII, Iti) showed that variations in most microbial communities from Galindez Island were associated with microscale variations in temperature. Metabolic predictions in silico using PICRUSt 2.0, based on the taxonomically affiliated part of the microbiomes, showed similarities with the rhizosphere community of D. antarctica in terms of the predicted functional repertoire. The results obtained indicate that these communities are involved in the primary processes of soil development (particularly the degradation of lignin and lignin-derived compounds) in the central maritime Antarctic and may be beneficial for the growth of Antarctic vascular plants. However, due to the limitations associated with interpreting PICRUSt 2.0 outputs, these predictions need to be verified experimentally.