Microscale Devices Research Articles

A Primer on Stochastic Partial Differential Equations with Spatially Correlated Noise

With the growing number of microscale devices from computer memory to microelectromechanical systems, such as lab-on-a-chip biosensors and the increased ability to experimentally measure at the micro- and nanoscale, modeling systems with stochastic processes is a growing need across science. In particular, stochastic partial differential equations (SPDEs) naturally arise from continuum models—for example, a pillar magnet's magnetization or an elastic membrane's mechanical deflection. In this review, I seek to acquaint the reader with SPDEs from the point of view of numerically simulating their finite-difference approximations, without the rigorous mathematical details of assigning probability measures to the random field solutions. I will stress that these simulations with spatially uncorrelated noise may not converge as the grid size goes to zero in the way that one expects from deterministic convergence of numerical schemes in two or more spatial dimensions. I then present some models with spatially correlated noise that maintain sampling of the physically relevant equilibrium distribution. Numerical simulations are presented to demonstrate the dynamics; the code is publicly available on GitHub.

Quantized Seebeck coefficient of quasi-ballistic gold nanowires

The behavior of the Seebeck coefficient in the intermediate regime between atomic scale ballistic conduction and bulk-like diffusive conduction remains unclear. To address this, we have developed a microscale device capable of simultaneously measuring the Seebeck coefficient and electrical conductance of gold nanowires in an adiabatic environment. The nanowires were made in situ by electromigration from lithographically prepared bow-tie electrodes, yielding a wide range of wire thicknesses down to a few hundred atoms. We observed quantization of the Seebeck coefficient, a phenomenon previously observed only at the Ångstrom scale, in relatively thick wires with a thickness of several tens of nanometers. The quantized Seebeck coefficient was proportional to the reciprocal of the electrical conductance with a slope of −47.8 μV/K, indicating that electrons are spatially confined due to the electronic shell structure of the nanowire, similar to the quantization of electrical conductance.

A New Two-Phase Design Process for a Compliant Mechanism Gripper

The structural design of the components with high accuracy and controllable motion is the focus of precision industries. As a result, researchers found that components created via compliant mechanisms were much preferable. Monolithic structures known as compliant mechanisms allow motion to be achieved without the need of traditional joints. The compliant mechanisms make it easier to build microscale devices because they do not have hard junctions. In this study, a novel two-phase design methodology for compliant mechanism forceps is proposed. In order to eliminate high-stress areas, forceps are often constructed as a distributed compliant mechanism. To offer a distributed planar design, topological optimization is introduced with new approach. Design domain introduced with pattern of holes restricts the single point contact formation and large area formation which yield distributed compliant mechanism. The parametrization technique is implemented to convert the conceptual design to working design. The design of compliant forceps is assessed using finite element analysis (FEA) based on structural considerations. Finally, a handle-equipped microgripper prototype has been developed. Experimental verification demonstrates the gripper's performance and variation is less than 3% with numerical results. Integerated force sensor measure the gripping force and compared the reaction force estimated through FEA.

Effects of wall material, working fluid, and barriers on performance of a nano flat-plate heat pipe: Molecular dynamics simulation

The use of microscale heat generating or heat transfer equipment with higher capacity and smaller dimensions requires more accurate management and better disposal of their produced heat. This makes the necessity of designing and manufacturing superconductors completely clear. In the meantime, the nano-grooved flat plate heat pipes (FPHP) have gained the industry's attention. Due to the provision of the conditions for manufacturing devices on a micro scale, it is possible to integrate this type of heat pipe with other microscale devices. In the meantime, understanding their behavior on small scales requires more studies. In this paper, the effect of using barriers to improve the thermal performance of a nano FPHP (1050 × 220 × 95 Å) is evaluated. Simulations are performed on a molecular scale through molecular dynamics (MD) simulations using LAMMPS® software. Platinum (Pt), copper (Cu), and aluminum (Al) are used for HP's body. In addition, argon (Ar), water (H2O), and ethanol (EtOH) are used as working fluids. The results show that increasing the number of barriers leads to improved thermal performance. Different cases include different teeth called barriers augmented inside HP are simulated. The combination of Cu-EtOH showed the best thermal performance (about 18 % better than other cases). Cubical barriers have more heat flux improvement than conical ones (from 3.5 % up to about 10.7 %). Among the three fluids used, EtOH leads to a better heat flux (about 6 % for Pt and up to 15 % for Cu). Using 24 barriers, a very favorable result of 1992 W/cm2 heat flux is achieved. In this case, the minimum heat flux is obtained by using Pt and Ar and is 1609 W/cm2. Using Ar and the cub shape barriers, the highest mass transfer rates for Pt, Cu, and Al are about 35.2 %, 38.9 %, and 38 %, respectively.

A graphite thermal Tesla valve driven by hydrodynamic phonon transport.

The Tesla valve benefits the rectification of fluid flow in microfluidic systems1-6 and inspires researchers to design modern solid-state electronic and thermal rectifiers referring to fluid-rectification mechanisms in a liquid-state context. In contrast to the rectification of fluids in microfluidic channels, the rectification of thermal phonons in micro-solid channels presents increased complexity owing to the lack of momentum-conserving collisions between phonons and the infrequent occurrence of liquid-like phonon flows. Recently, investigations and revelations of phonon hydrodynamics in graphitic materials7-10 have opened up new avenues for achieving thermal rectification. Here we demonstrate a phonon hydrodynamics approach to realize the rectification of heat conduction in isotopically enriched graphite crystals. We design a micrometre-scale Tesla valve within 90-nm-thick graphite and experimentally observe a discernible 15.2% difference in thermal conductivity between opposite directions at 45 K. This work marks an important step towards using collective phonon behaviour for thermal management in microscale and nanoscale electronic devices, paving the way for thermal rectification in solids.

Non-Volatile Resistive Switching in Nanoscaled Elemental Tellurium by Vapor Transport Deposition on Gold.

Two-dimensional (2D) materials are promising for resistive switching in neuromorphic and in-memory computing, as their atomic thickness substantially improve the energetic budget of the device and circuits. However, many 2D resistive switching materials struggle with complex growth methods or limited scalability. 2D tellurium exhibits striking characteristics such as simplicity in chemistry, structure, and synthesis making it suitable for various applications. This study reports the first memristor design based on nanoscaled tellurium synthesized by vapor transport deposition (VTD) at a temperature as low as 100°C fully compatible with back-end-of-line processing. The resistive switching behavior of tellurium nanosheets is studied by conductive atomic force microscopy, providing valuable insights into its memristive functionality, supported by microscale device measurements. Selecting gold as the substrate material enhances the memristive behavior of nanoscaled telluriumin terms of reduced values of set voltage and energy consumption. In addition, formation of conductive paths leading to resistive switching behavior on the gold substrate is driven by gold-tellurium interface reconfiguration during the VTD process as revealed by energy electron loss spectroscopy analysis. These findings reveal the potential of nanoscaled tellurium as a versatile and scalable material for neuromorphic computing and underscore the influential role of gold electrodes in enhancing its memristive performance.

Towards Design Automation of Microfluidic Mixers: Leveraging Reinforcement Learning and Artificial Neural Networks.

Microfluidic mixers, a pivotal application of microfluidic technology, are primarily utilized for the rapid amalgamation of diverse samples within microscale devices. Given the intricacy of their design processes and the substantial expertise required from designers, the intelligent automation of microfluidic mixer design has garnered significant attention. This paper discusses an approach that integrates artificial neural networks (ANNs) with reinforcement learning techniques to automate the dimensional parameter design of microfluidic mixers. In this study, we selected two typical microfluidic mixer structures for testing and trained two neural network models, both highly precise and cost-efficient, as alternatives to traditional, time-consuming finite-element simulations using up to 10,000 sets of COMSOL simulation data. By defining effective state evaluation functions for the reinforcement learning agents, we utilized the trained agents to successfully validate the automated design of dimensional parameters for these mixer structures. The tests demonstrated that the first mixer model could be automatically optimized in just 0.129 s, and the second in 0.169 s, significantly reducing the time compared to manual design. The simulation results validated the potential of reinforcement learning techniques in the automated design of microfluidic mixers, offering a new solution in this field.

A System Built for Both Deterministic Transfer Processes and Contact Photolithography

A home‐built compact system that functions as both a transfer stage for deterministic transfer processes and a mask aligner for contact photolithography, is constructed. The precision translation sample stage and optical microscope are shared between the two modes. In the transfer mode, assisted by either an adhesive or a heating element, the setup has been used to deterministically transfer freestanding semiconductors, 2D materials, and van der Waals electrodes. When configured in photolithography mode, the proposed instrument has been employed to fabricate various microscale patterns and devices, with minimum feature sizes of 1–2 μm achieved. The prototype instrument provides a feasible solution for performing high‐quality deterministic transfer and photolithography processes on one single tool in‐house.

Advances in Microfluidic Systems and Numerical Modeling in Biomedical Applications: A Review.

The evolution in the biomedical engineering field boosts innovative technologies, with microfluidic systems standing out as transformative tools in disease diagnosis, treatment, and monitoring. Numerical simulation has emerged as a tool of increasing importance for better understanding and predicting fluid-flow behavior in microscale devices. This review explores fabrication techniques and common materials of microfluidic devices, focusing on soft lithography and additive manufacturing. Microfluidic systems applications, including nucleic acid amplification and protein synthesis, as well as point-of-care diagnostics, DNA analysis, cell cultures, and organ-on-a-chip models (e.g., lung-, brain-, liver-, and tumor-on-a-chip), are discussed. Recent studies have applied computational tools such as ANSYS Fluent 2024 software to numerically simulate the flow behavior. Outside of the study cases, this work reports fundamental aspects of microfluidic simulations, including fluid flow, mass transport, mixing, and diffusion, and highlights the emergent field of organ-on-a-chip simulations. Additionally, it takes into account the application of geometries to improve the mixing of samples, as well as surface wettability modification. In conclusion, the present review summarizes the most relevant contributions of microfluidic systems and their numerical modeling to biomedical engineering.

Transition to oscillatory instability of lean methane–air flames in microchannels

Micro-flow reactors and porous media burners are prone to instability in the form of flame with repetitive extinction and ignition (FREI) near the region of a temperature gradient. Although significant progress has been made in this field, there is still a lack of understanding regarding the nature of such stable-to-unstable transition. Some studies reported a smooth and continuous instability transition in microchannels, interpreting this phenomenon as a supercritical bifurcation. Meanwhile, others have observed that the transition is subcritical, with rapid, stepwise evolution of the stable flame to FREI. This study aims to rigorously determine the transition character theoretically and numerically using a two-dimensional model with a precise resolution of the bifurcation parameter (flow velocity) near the critical point and gradual wall temperature ramp. The results indicate that the transition is a subcritical bifurcation with strong hysteresis. However, the amplitude of the limit cycle born in the bifurcation point could be small compared to FREI-like oscillations. Moreover, further development of the instability depends significantly on the channel width. In relatively wide channels, the stable regime transforms immediately into the extinction/ignition one with rapid growth in amplitude. In moderate channels, complex transitional dynamics were observed with an evident pulsating regime, which evolved into FREI through mixed-mode oscillations. In narrow channels, the amplitude of transitional pulsating flame becomes comparable to the fully-developed extinction/ignition cycles, and the bifurcation diagram has a continuous character with no significant jumps or discontinuities. Such qualitative variation of transition character provides a possible explanation for contradictory interpretations presented in the literature.Novelty and Significance StatementCurrently, there is no consensus about the character of transition between the stable and extinction/ignition regimes in microchannels. Some studies have reported a continuous instability transition with an evident pulsating regime, interpreting it as a supercritical bifurcation, while others have observed subcritical bifurcation with a rapid transition. The novelty of this research lies in the rigorous determination of the bifurcation type and further instability development in microchannels of different widths based on an accurate simulation of flame dynamics near the critical point with very small steps of the flow velocity variation, along with the bifurcation theory. The study contributes to understanding the instability transition nature and may explain the different interpretations of the transitional phenomenon found in the literature. The results offer valuable insights for designing micro-scale and porous media combustion devices.

The significance of quadratic thermal radiative scrutinization of a nanofluid flow across a microchannel with thermophoretic particle deposition effects

Abstract The investigation of thermal radiation and thermophoretic impacts on nano-based liquid circulation in a microchannel has a significant impact on the cooling of microscale equipment, microliquid devices, and many more. These miniature systems can benefit from the improved heat transfer efficiency made possible by the use of nanofluids, which are designed to consist of colloidal dispersion of nanoparticles in a carrier liquid. Understanding and precisely modeling the thermophoretic deposition (TPD) of nanoparticles on the channel surfaces is of utmost importance since it can greatly affect the heat transmission properties. This work examines the complex interaction between quadratic thermal radiation, magnetohydrodynamics, and TPD in a permeable microchannel. It aims to solve a significant knowledge gap in microfluidics and thermal and mass transport. The governing equations are simplified by applying suitable similarity restrictions, and computing solutions to the resulting equations is done using the Runge‒Kutta Fehlberg fourth‒fifth-order scheme. The results are shown using graphs, and significant engineering metrics are analyzed. The outcomes show that increased Eckert number, magnetic, and porous factors will improve the thermal distribution. Quadratic thermal radiation shows the greater thermal distribution in the presence of these parameters, while Linear thermal radiation shows the least thermal distribution. The rate of thermal distribution is higher in the linear thermal distribution case and least in the nonlinear thermal radiation case in the presence of radiation and solid fraction factors. The outcomes of the present research are helpful in improving the thermal performance in microscale devices, electronic devices cooling, health care equipment, and other microfluidic applications.

Microscale Chiral Rectennas for Energy Harvesting.

Wireless radiofrequency rectifiers have the potential to power the billions of "Internet of Things" (IoT) devices currently in use by effectively harnessing ambient electromagnetic radiation. However, the current technology relies on the implementation of rectifiers based on Schottky diodes, which exhibit limited capabilities for high-frequency and low-power applications. Consequently, they require an antenna to capture the incoming signal and amplify the input power, thereby limiting the possibility of miniaturizing devices to the millimeter scale. Here, the authors report wireless rectification at the GHz range in a microscale device built on single chiral tellurium with extremely low input powers. By studying the crystal symmetry and the temperature dependence of the rectification, the authors demonstrate that its origin is the intrinsic nonlinear conductivity of the material. Additionally, the unprecedented ability to modulate the rectification output by an electrostatic gate is shown. These results open the path to developing tuneable microscale wireless rectifiers with a single material.

Impinging Flow Reactor as a New Approach to Scaling-Up Microreactors for Enhanced Extraction and Separation of Zinc from Cadmium and Manganese

ABSTRACT Despite significant progress in microreactor research, limited throughput hinders their industrial application. The impinging flow method offers a novel approach to scaling up microreactors. In this study, a T-shaped impinging flow reactor was used to investigate the improvement of the throughput of micro-scale devices for the enhanced extraction and separation of zinc from cadmium and manganese using the impinging flow method. Computational fluid dynamics (CFD) simulations were performed to analyze the velocity fields and concentration distributions and gain a better understanding of the mixing and mass transfer phenomena. A comparison of experimental and simulation results under the same conditions was then carried out. Results revealed that decreasing mass transfer efficiency with increased inner diameter, though higher inlet flow rates partially offset this, however beyond a certain threshold, larger channel sizes significantly decrease efficiency. The study also demonstrated that impinging flow reactors exhibit superior volumetric mass transfer coefficients compared to segmented flow microreactors. By increasing the volumetric flow rate and channel inner diameter simultaneously, the millimeter impinging flow reactor achieved the same extraction efficiency as the micrometer segmented flow reactor with a scale-up factor of 1312.5, which indicates a significant increase in device capacity. Overall, this study demonstrates the potential for significant scale-up in device capacity using impinging flow reactors.

A Photolithographable Electrolyte for Deeply Rechargeable Zn Microbatteries in On-Chip Devices.

Zn batteries show promise for microscale applications due to their compatibility with air fabrication but face challenges like dendrite growth and chemical corrosion, especially at the microscale. Despite previous attempts in electrolyte engineering, achieving successful patterning of electrolyte microscale devices has remained challenging. Here, successful patterning using photolithography is enabled by incorporating caffeine into a UV-crosslinked polyacrylamide hydrogel electrolyte. Caffeine passivates the Zn anode, preventing chemical corrosion, while its coordination with Zn2+ ions forms a Zn2+-conducting complex that transforms into ZnCO3 and 2ZnCO3·3Zn(OH)2 over cycling. The resulting Zn-rich interphase product significantly enhances Zn reversibility. In on-chip microbatteries, the resulting solid-electrolyte interphase allows the Zn||MnO2 full cell to cycle for over 700 cycles with an 80% depth of discharge. Integrating the photolithographable electrolyte into multilayer microfabrication creates a microbattery with a 3D Swiss-roll structure that occupies a footprint of 0.136 mm2. This tiny microbattery retains 75% of its capacity (350 µAh cm-2) for 200 cycles at a remarkable 90% depth of discharge. The findings offer a promising solution for enhancing the performance of Zn microbatteries, particularly for on-chip microscale devices, and have significant implications for the advancement of autonomous microscale devices.

Ultrathin MXene film interaction with electromagnetic radiation in the microwave range

The quick progress in communication technologies demands superior electromagnetic interference (EMI) shielding materials. However, achieving a high shielding effectiveness (SE) with thin films, which is needed for microscale, flexible, and wearable devices, through absorption of EM radiation remains a challenge. 2D titanium carbide MXene, Ti3C2Tx, has been shown to efficiently reflect electromagnetic waves. In this paper, we investigated the electromagnetic shielding of ultrathin printed Ti3C2Tx films and recorded absorption up to 50% for 4 nm-thick films. This behavior is explained by impedance matching. Analysis of the sheet impedance in the X-band frequency range allows us to correlate the EMI shielding mechanism with the electrical conductivity measured within the same range. The average bulk in-plane conductivity for 4 to 40 nm-thick films reaches 106 S/m, while the average relaxation time is estimated at around 2.3 ps. Our figures of merit are similar to those reported for ultrathin metal films, such as gold, showing that an abundant MXene material can replace noble metals. We demonstrate that the MXene conductivity mechanism does not change from direct current to THz. The conventional method of reporting EMI SE is correlated with absolute values of transmitted, reflected, and absorbed power, which allows us to interpret previous results on MXene EMI shielding. Considering the easy deposition of thin MXenes films from solution onto a variety of surfaces, our findings offer an attractive alternative for shielding microscale devices and personal electronics.

Direct laser synthesis and patterning of high entropy oxides from liquid precursors

High entropy oxides are a class of materials distinguished by the use of configurational entropy to drive material synthesis. These materials are being examined for their exciting physiochemical properties and hold promise in numerous fields, such as chemical sensing, electronics, and catalysis. Patterning and integration of high entropy materials into devices and platforms can be difficult due to their thermal sensitivity and incompatibility with many conventional thermally-based processing techniques. In this work, we present a laser-based technique, laser-induced thermal voxels, that combines the synthesis and patterning of high entropy oxides into a single process step, thereby allowing patterning of high entropy materials directly onto substrates. As a proof-of-concept, we target the synthesis and patterning of a well-characterized rock salt-phase high entropy oxide, (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O, as well as a spinel-phase high entropy oxide, (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)Cr2O4. We show through electron microscopy and x-ray analysis that the materials created are atomically homogenous and are primarily of the rock salt or spinel phase. These findings show the efficacy of laser induced thermal voxel processing for the synthesis and patterning of high entropy materials and enable new routes for integration of high entropy materials within microscale platform and devices.

Sound wave propagation in rarefied molecular gases

Sound wave propagation in rarefied flows of molecular gases confined in micro-channels is investigated numerically. We first validate the employed kinetic model against the experimental results and then systematically study the gas damping and surface force on the transducer as well as the resonance/anti-resonance in confined space. To quantify the impact of the finite relaxation rates of the translational and internal energies on wave propagation, we examine the roles of bulk viscosity and thermal conductivity in depth over a wide range of rarefactions and oscillation frequencies. It is found that the bulk viscosity only exerts influence on the pressure amplitude and its resonance frequency in the slip regime in high oscillations. In addition, the internal degree of freedom is frozen when the bulk viscosity of a molecular gas is large, resulting in the pressure amplitude of sound waves in the molecular gas being the same as in a monatomic gas. Meanwhile, the thermal conductivity has a limited influence on the pressure amplitude in all the simulated flows. In the case of the thermoacoustic wave, we prove that the Onsager–Casimir reciprocal relation also holds for molecular gases, i.e. the pressure deviation induced by the temperature variation is equal to the heat flux induced by the plate oscillation. Our findings enable an enhanced understanding of sound wave propagation in molecular gases, which may facilitate the design of nano-/micro-scale devices.

Significant catalyst savings and reaction kinetics enhancement for catalytic micro-combustors via non-uniform catalyst segmentation

A recent prioritization of energy-efficient and sustainable technologies has fueled an ongoing demand for improving the heat sources necessary to drive thermally dependent processes in portable and small-scale devices. Previous efforts have explored hydrocarbon-based catalytic combustion (and its kinetics and fuel usage efficiency) for reliable heating in microscale devices (TPVs, TEGs, etc.). However, for mesoscale systems that require a larger length scale combustor, fuel management and the ability to maintain consistent heat flux across the combustor become challenging due to kinetics and mass diffusion limitations, which are extrinsic to microscale combustors. In this work, we present the design and experimental evaluation of a compact parallel plate catalytic micro-combustor with non-uniform, segmented catalytic zones that allow for direct control of heating, spatially. Specifically, we performed a comparative FEM study of various catalyst segmentation patterns to further optimize catalyst bed spacing for decreased fuel consumption and improved combustion reaction kinetics. Due to these enhancements, the proposed patterning scheme can achieve similar temperature profiles to those of a fully-coated surface while using 77.8% less catalyst and reducing extreme thermal gradients during the early transient combustion phase. Finally, we demonstrated the capabilities of the proposed catalytic combustor by testing the combustor in a controlled environment.

Experimental investigation of inflow-outflow asymmetry in induced-charge electro-osmosis

Induced-charge electro-osmosis (ICEO) is a research hotspot in bioengineering and analytical chemistry. Inflow-outflow asymmetry of ICEO was reported in the existing literatures, but systematic study on this phenomenon is insufficient. In this experimental study, we found that in strong electric fields, not only the velocity magnitude but also the vortex positions of ICEO are asymmetrical along the inflow and outflow directions because of the pronounced non-uniform surface electrokinetic transport. On the inflow and outflow directions, the amplitudes of velocities are unequal, ICEO maximum velocity positions change depending on the electric field intensity and sodium chloride (NaCl) concentration. Additionally, the distances between vortex centers are different. At NaCl solution concentration of 0.001 mol·L–1, the outflow velocity almost vanishes. The asymmetry rises with the increasing electric field intensity. The new discoveries can direct the application of microscale devices.

Enhanced mechanical property through high-yield fabrication process with double laser scanning method in two-photon lithography

Two-photon lithography (TPL) is a representative fabrication process for micro 3D structures with a sub-micron fabrication resolution. As the TPL process is developed and commercialized, the TPL process requires higher process yield to be a considered reliable manufacturing technique for mass production. However, delamination is one of the main factors that reduce the yield of the TPL process. In this study, a double-step laser scanning method is proposed as a reliable high-yield TPS process to reduce the delamination effect. The method entails a simple process that polymerizes the initial interfacial layer between the structures and substrate twice. The additional laser scanning step induces a strong crosslinking in the polymer, and thereby preventing swelling and delamination. Furthermore, the effectiveness of the method is evaluated with large-scale microlens arrays. Although the process time is slightly increased, the process yield is significantly improved through the use of the double-step laser scanning method. Hence, the double-step laser scanning process can be employed as a reliable additive manufacturing process for various 3D microscale devices, and more importantly, it can be used to increase the manufacturing yield through the reduction of defects caused by surface delamination.