Microscale Systems Research Articles

Research on the performance characteristics of an oil-free scroll expander that is applied to a micro-scale compressed air energy storage system

The oil-free scroll expander, which is the power component of the micro-scale compressed air energy storage (CAES) system, exhibits a satisfactory application prospect. The expander's performance directly influences the efficiency of the expansion power generation system. To enhance the work efficiency of the power generation system, we built a thermodynamic model that facilitates the oil-free scroll expansion mechanism by considering heat transfer and leakage; furthermore, for the micro-scale CAES, we constructed an experiment bench that utilizes air as the working fluid. Based on computational fluid dynamics (CFD), we conducted a three-dimensional unsteady numerical simulation of the fluid flow that occurs in the working chamber of the scroll expander. We observed that the suction pressure loss that occurs during the suction process, which is initiated by the scroll expander, leads to inlet pressure drop, and we noted that the suction pressure directly influences the work efficiency of the power generation system. When the setup is subjected to rotational speeds of 1200–3200 r/min, the difference between the theoretical and measured values of volumetric flow decreases from 0.0226 to 0.000527 m3/min. The pressure, velocity, and temperature distribution of the fluid that exists in the working chamber of the expander are not uniform, and the mass exchange that occurs between adjacent working chambers considerably impacts the distribution of velocity and temperature. As gas flows from the back-pressure chamber to the outlet pipe, most of it flows through the non-deforming domain before flowing into the outlet pipe. The fluid flow in the deforming domain is highly complicated, and even secondary flow can occur.

Unsteady and three-dimensional computational fluid dynamics modelling of scroll expander for low-grade waste heat recovery transcritical carbon dioxide micro-scale power system

Recent developments in the field of renewable energy have led to a renewed interest in low-grade heat (< 500 K). The low-grade heat is widely wasted by the lack of efficient heat recovery technologies. It is also limited by the system size, which defines as the micro to small-scale (< 50 kW). Although ORC based unit has been implemented in this field, the CO2 based waste heat recovery units can be more capable in the size construction. The performance of the expander plays a vital role in the system’s efficiency. Thus, the current paper provides thermodynamic and CFD analysis of a scroll expander regarding a micro-scale T-CO2 recovery system (< 10 kW) with a 400 K low-grade heat source. In the current CFD model, all the fluid domains were constructed by structural mesh. It also successfully integrated with the thermodynamic table to simulate two-phase T-CO2. This model can be the first scroll expander model for T-CO2 power system and gap the bridge of utilising the scroll machinery in this field. The CFD methodology was successfully validated by the new-built testing platform and previous data. The energy performance of T-CO2 and ORC (R123) based scroll expanders are compared by isentropic and exergy efficiency. The results showed that isentropic and exergy efficiencies of T-CO2 were 7% and 14% higher than the R123. It also identified higher irreversibilities of T-CO2 by the exergy of the working fluids. The pressure and temperature distributions identified the over-expansion and reversed flow characteristics, and the pressure imbalance of the initial expansion chambers denoted the reversed flow.

The dynamics of partially encapsulated microbubbles subjected to ultrasound

Acoustically excited microbubbles generate various nonlinear forces that can be leveraged in microscale systems for actuation and manipulation. To obtain optimal performances, bubbles should be characterized; however, so far, they were studied indirectly by measuring downstream phenomena. Here, we present a novel scheme to measure the vibrations of a bubble at the water-air interface using a laser vibrometer and the impinging pressure using a hydrophone. A custom-built optical setup couples the vibrometer to an inverted microscope. The microstructures encapsulating the bubbles are 3D nanoprinted on glass slides, which allows the realization of complex configurations with various polymers. The overall platform enables us to study the dynamics of bubbles with single or multiple interfaces, and their interactions. The measurements are also used to refine the analytical model of the multi-physics system and find optimal operating conditions. The conditions depend on the bubble geometry, boundaries, and excitation source. The measurements reveal that it is easier to excite certain vibration modes when the acoustic wavelength is much larger than the bubble. We demonstrate the controllable motion of 3D nanoprinted flexible structures by harnessing nonlinear forces that are produced by acoustically excited microbubbles. These structures will serve as building blocks for all-mechanical soft microrobots.

Diverse behaviors in non-uniform chiral and non-chiral swarmalators

We study the emergent behaviors of a population of swarming coupled oscillators, dubbed swarmalators. Previous work considered the simplest, idealized case: identical swarmalators with global coupling. Here we expand this work by adding more realistic features: local coupling, non-identical natural frequencies, and chirality. This more realistic model generates a variety of new behaviors including lattices of vortices, beating clusters, and interacting phase waves. Similar behaviors are found across natural and artificial micro-scale collective systems, including social slime mold, spermatozoa vortex arrays, and Quincke rollers. Our results indicate a wide range of future use cases, both to aid characterization and understanding of natural swarms, and to design complex interactions in collective systems from soft and active matter to micro-robotics.

Analysis of Tangential Leakage Flow Characteristics of Oil-Free Scroll Expander for a Micro-Scale Compressed Air Energy Storage System.

Tangential leakage loss is the primary factor that significantly affects the output performance of oil-free scroll expanders. A scroll expander can function under different operating conditions, and the flow of tangential leakage and generation mechanism is different. This study employed computational fluid dynamics to investigate the unsteady flow characteristics of the tangential leakage flow of a scroll expander with air as the working fluid. Consequently, the effects of different radial gap sizes, rotational speeds, inlet pressures, and temperatures on the tangential leakage were discussed. The tangential leakage decreased with increases in the scroll expander rotational speed, inlet pressure, and temperature, and decreased with decrease in radial clearance. With an equal-proportional increase in radial clearance, the flow form of the gas in the first expansion and back-pressure chambers became more complicated; when the radial clearance increased from 0.2 to 0.5 mm, the volumetric efficiency of the scroll expander decreased by approximately 5.0521%. Moreover, because of the large radial clearance, the tangential leakage flow maintained a subsonic flow. Further, the tangential leakage decreased with increase in rotational speed, and when the rotational speed increased from 2000 to 5000 r/min, the volumetric efficiency increased by approximately 8.7565%.

Design and performance analysis of a novel displacement-based temperature sensor

Abstract In this paper, we present a proof-of-concept for a novel temperature sensing approach that combines the thermal expansion and a compliant mechanism. The objective is first to demonstrate its feasibility at the macroscale, develop and validate an FEM model at the macroscale and then scale down the FEM model to verify the possible implementation of the mechanism at the microscale. The sensing approach relies on a mechanical compliant mechanism that amplifies the thermal expansion of a structure. A testing platform equipped with an IR thermometer, thermocouple, a power supply, and laser distance sensors, is implemented to demonstrate the operability of the proposed sensing mechanism. A numerical model of the sensor is developed using the FE software Ansys. The numerical results show a good agreement with their experimental counterparts at the macro scale. The model is then used to numerically investigate several configurations, namely single, double, triple and quadruple compliant mechanisms. The amplification factor is found the highest when using the double compliant mechanism. A temperature sensitivity of 28.5 μm/°C is achieved for this compliant mechanism. The numerical analysis also demonstrated that the performance obtained at the macro scale, can be conserved for microscale devices. However, buckling of some elements is observed for the microscale system which degrades the performance of the sensor when subjected to relatively large displacements. The microscale FEM model shows the possible prevention of buckling issues by slightly modifying the geometry of the compliant mechanisms. The present study is expected to provide baseline and guidance for the implementation of the sensing approach for MEMS devices.

Anisotropic property of turbulent flow control through multiple stenosed microtubes

Rapid development in micro-scale systems and devices for drug delivery and biotechnical analysis has been scurrying. Microtube or microchannel roughness is another challenging factor in microfabrication technology. Moreover, the irregular roughness occurs due to the multiple depositions of cholesterol and other substances in the endothelium of the arterial wall. To investigate those issues, the effect of the irregular roughness of the wall on the anisotropy of the turbulent blood flow through an atherosclerotic artery has been analyzed. The flow through a stenosed microtube has been numerically simulated to understand the blood flow phenomenon through arterial stenosis and different turbulent states. The Reynolds stress components are estimated and plotted an invariant map and Eigen map to examine the turbulent states. The comparison results have been studied to show the nature of turbulent flow in low Reynolds numbers due to irregular roughness. The turbulent kinetic energy and turbulent viscous dissipation rate are also examined. Due to wall shear stress, the flow deserves the turbulence flow characteristic downstream of the abnormally narrowed regions of the microtube. The nature of the wall shear rate and the high viscosity of blood impact the disturbance in the flow velocity in microtubes.

Combined electromagnetohydrodynamic flow in microchannels with consideration of the surface charge-dependent slip

In microscale systems, hydrodynamic slip is considered to significantly influence the fluid flow field. Existing theories of electromagnetohydrodynamic flow in hydrophobic microchannels have postulated a constant slip length and ignored the effect of the surface charge on slip. In this study, we extended prior models by considering a combined pressure-driven and electromagnetohydrodynamic flow in microchannels with consideration of surface charge-dependent slip. An analytical solution for this simple model was derived. After a detailed discussion of the obtained results, we demonstrate that the more realistic surface-charge-dependent case has smaller velocities and flow rates than the surface-charge-independent slip case. Considering the effect of the surface charge on slip, the flow rate can be reduced by up to 7% in the currently selected parameter range. Our results are useful for optimizing electromagnetohydrodynamic flow models in microchannels.

Evolution of the electrical double layer with electrolyte concentration probed by second harmonic scattering.

Investigating the electrical double layer (EDL) structure has been a long-standing challenge and has seen the emergence of several sophisticated techniques able to probe selectively the few molecular layers of a solid/water interface. While a qualitative estimation of the thickness of the EDL can be obtained using simple theoretical models, following experimentally its evolution is not straightforward and can be even more complicated in nano- or microscale systems, particularly when changing the ionic concentration by several orders of magnitude. Here, we bring insight into the structure of the EDL of SiO2 nanoparticle suspensions and its evolution with increasing ionic concentration using angle-resolved second harmonic scattering (AR-SHS). Below millimolar salt concentrations, we can successively characterize inner-sphere adsorption, diffuse layer formation, and outer-sphere adsorption. Moreover, we show for the first time that, by appropriately selecting the nanoparticle size, it is possible to retrieve information also in the millimolar range. There, we observe a decrease in the magnitude of the surface potential corresponding to a compression in the EDL thickness, which agrees with the results of several other electroanalytical and optical techniques. Molecular dynamics simulations suggest that the EDL compression mainly results from the diffuse layer compression rather than outer-sphere ions (Stern plane) moving closer to the surface.

A microscale system for in situ investigation of immobilized microalgal cell resistance against liquid flow in the early inoculation stage.

In attached microalgae cultivation systems, cell detachment due to fluid hydrodynamic flow is not a subject matter that is commonly looked into. However, this phenomenon is of great relevance to optimizing the operating parameters of algae cultivation and feasible reactor design. Hence, this current work miniaturizes traditional benchtop assays into a microfluidic platform to study the cell detachment of green microalgae, Chlorella vulgaris, from porous substrates during its early cultivation stage under precisely controlled conditions. As revealed by time lapse microscopy, an increase in bulk flow velocity facilitated nutrient transport but also triggered cell detachment events. At a flow rate of 1000 μL min-1 of growth medium for 120 min, the algal cell coverage was up to 5% lower than those at 5 μL min-1 and 50 μL min-1. In static seeding, the evolution of attached cell resistance toward liquid flows was dependent on hydrodynamic zones. The center zone of the microchannel was shown to be a "comfortable zone" of the attached cells to sequester nutrients effectively at lower medium flow rates but there was a profile transition where outlet zones favored cell attachment the most at higher flow rates (1.13 times higher than the center zone for 1000 μL min-1). Besides, computational fluid dynamics (CFD) simulations illustrated that the focusing band varied between cross-sections and depths, while the streamline was the least concentrated along the side walls and bottom plane of the microfluidic devices. It was intriguing to learn that cell detachment was not primarily happening along the symmetry streamline. Insight gained from this study could be further applied in the optimization of operating conditions of attached cultivation systems whilst preserving laminar flow conditions.

Acoustofluidic large-scale mixing for enhanced microfluidic immunostaining for tissue diagnostics.

The usage of microfluidics for automated and fast immunoassays has gained a lot of interest in the last decades. This integration comes with certain challenges, like the reconciliation of laminar flow patterns of micro-scale systems with diffusion-limited mass transport. Several methods have been investigated to enhance microfluidic mixing in microsystems, including acoustic-based fluidic streaming. Here, we report both by numerical simulation and experiments on the beneficiary effect of acoustic agitation on the uniformity of immunostaining in large-size and thin microfluidic chambers. Moreover, we investigate by numerical simulation the impact of reducing the incubation times and the concentrations of the biochemical detection reagents on the obtained immunoassay signal. Finally, acoustofluidic mixing was successfully used to reduce by 80% the incubation time of the Her2 (human epidermal growth factor receptor 2) and CK (cytokeratins) biomarkers for the spatial immunostaining of breast cancer cell pellets, or reducing their concentration by 66% and achieving a higher signal-to-background ratio than comparable spatially resolved immunostaining with static incubation.

A Hierarchy of Network Models Giving Bistability Under Triadic Closure

Triadic closure describes the tendency for new friendships to form between individuals who already have friends in common. It has been argued heuristically that the triadic closure effect can lead to bistability in the formation of large-scale social interaction networks. Here, depending on the initial state and the transient dynamics, the system may evolve towards either of two long-time states. In this work, we propose and study a hierarchy of network evolution models that incorporate triadic closure, building on the work of Grindrod, Higham, and Parsons [Internet Mathematics, 8, 2012, 402--423]. We use a chemical kinetics framework, paying careful attention to the reaction rate scaling with respect to the system size. In a macroscale regime, we show rigorously that a bimodal steady-state distribution is admitted. This behavior corresponds to the existence of two distinct stable fixed points in a deterministic mean-field ODE. The macroscale model is also seen to capture an apparent metastability property of the microscale system. Computational simulations are used to support the analysis.

Evolving and generalising morphologies for locomoting micro-scale robotic agents

Designing locomotive mechanisms for micro-scale robotic systems could enable new approaches to tackling problems such as transporting cargos, or self-assembling into pre-programmed architectures. Morphological factors often play a crucial role in determining the behaviour of micro-systems, yet understanding how to design these aspects optimally is a challenge. This paper explores how the morphology of a multi-cellular micro-robotic agent can be optimised for reliable locomotion using artificial evolution in a stochastic environment. We begin by establishing the theoretical mechanisms that would allow for collective locomotion to emerge from contractile actuations in multiple connected cells. These principles are used to develop a Cellular Potts model, in order to explore the locomotive performance of morphologies in simulation. Evolved morphologies yield significantly better performance in terms of the reliability of the travel direction and the distance covered, compared to random morphologies. Finally, we demonstrate that patterns in evolved morphologies are robust to small imperfections and generalise well to larger morphologies.

Power spectral density analysis of a scaled model simulation of an optical tweezer

The majority of microscale and nanoscale mechanical systems encountered in the world are stiff. One example of such a system is a microbead trapped in an optical tweezer. For this example, the ratio of a body’s mass to the viscous damping coefficient is negligible. Apart from being stiff, this system is also stochastic meaning that the differential equations representing the system include a random variable. This type of model can be solved with existing stochastic differential equation (SDE) solvers. Addressing the stochastic nature of the model usually requires averaging the results of multiple simulations. The computational time required to run the hundreds of simulations that may be necessary to obtain a useful average can be prohibitive. This paper presents a scaling approach that significantly reduces the computational time required to obtain these averages. However, this scaling changes the model and therefore the power spectral density (PSD) of the predicted motion. This work provides a general analysis of a mass-spring-damper model, such as an optical tweezer, that clearly shows the effect of scaling on the frequency content of the stochastic system. This analysis shows that the scaling technique can be used effectively without much loss of information. An experimental dataset of a 2 μm diameter microbead falling into an optical trap is compared with the simulation for validating the scaling method.

Recent Advances in Solution‐processed Inorganic Thermoelectric Thin Films

AbstractRecent advancements in microscale electronics and wearable devices have led to the concept of energy autonomy, which requires self‐powered energy sources that can be integrated into systems. Thermoelectric generators have received significant attention as direct energy conversion systems between heat and electrical energy. However, typical bulk‐scale inorganic thermoelectric materials have limited utility in these systems, and the traditional fabrication methods of generators pose difficulty in their miniaturization. Thin‐film thermoelectric materials can address these issues owing to their easy integration into microscale systems. Moreover, solution processing can provide a cost‐effective and scalable production approach for inorganic thin films. In this paper, we review the recent progress in solution‐processed inorganic thermoelectric thin films, including the synthesis of ink solutions, fabrication of thin films, and thermoelectric performance of materials and devices.

Cryogel-Templated Fabrication of n-Al/PVDF Superhydrophobic Energetic Films with Exceptional Underwater Ignition Performance.

The rapid heat loss and corrosion of nano-aluminum limits the energy performance of metastable intermolecular composites (MICs) in aquatic conditions. In this work, superhydrophobic n-Al/PVDF films were fabricated by the cryogel-templated method. The underwater ignition performance of the energetic films was investigated. The preparation process of energetic materials is relatively simple, and avoids excessively high temperatures, ensuring the safety of the entire experimental process. The surface of the n-Al/PVDF energetic film exhibits super-hydrophobicity. Because the aluminum nanoparticles are uniformly encased in the hydrophobic energetic binder, the film is more waterproof and anti-aging. Laser-induced underwater ignition experiments show that the superhydrophobic modification can effectively induce the ignition of energetic films underwater. The results suggest that the cryogel-templated method provides a feasible route for underwater applications of energetic materials, especially nanoenergetics-on-a-chip in underwater micro-scale energy-demanding systems.

Numerical and experimental assessment of a novel SOFC-based system for micro-power generation

In this work, a novel micro-scale system based on solid oxide fuel cells (SOFCs) is studied for operation at low temperature, assessing the expected performance numerically and the actual cell operation experimentally. The potential application is powering gas-connected but off-electric-grid remote electronic devices with nominal capacity of tens of watts, such as those necessary to monitor and operate the gas grid, using methane as feed. Since water is generally not available in such distributed uses, feed pre-treating is critical and poses technical challenges for the high and stable performance of the SOFC. Several promising design configurations for the system are defined. The use of commercial YSZ-based SOFCs is envisioned, and recirculation is needed to avoid carbon deposition, which is predicted by the calculation of carbon activity and confirmed by the experimental observations. A higher anodic recirculation is required at low (≤50%) fuel utilization in order to decrease the estimated carbon activity, due to a lower content of oxygen atoms in the anode outlet stream. The anode inlet composition was numerically estimated assuming a fuel utilization factor of 70% with 60% anodic recirculation, then was experimentally tested, achieving a stable power generation without carbon deposition.

Bonding Strategies for Thermoplastics Applicable for Bioanalysis and Diagnostics.

Microfluidics is a multidisciplinary science that includes physics, chemistry, engineering, and biotechnology. Such microscale systems are receiving growing interest in applications such as analysis, diagnostics, and biomedical research. Thermoplastic polymers have emerged as one of the most attractive materials for microfluidic device fabrication owing to advantages such as being optically transparent, biocompatible, cost-effective, and mass producible. However, thermoplastic bonding is a key challenge for sealing microfluidic devices. Given the wide range of bonding methods, the appropriate bonding approach should be carefully selected depending on the thermoplastic material and functional requirements. In this review, we aim to provide a comprehensive overview of thermoplastic fabricating and bonding approaches, presenting their advantages and disadvantages, to assist in finding suitable microfluidic device bonding methods. In addition, we highlight current applications of thermoplastic microfluidics to analyses and diagnostics and introduce future perspectives on thermoplastic bonding strategies.

Investigating the hybridisation of micro-scale concentrated solar trigeneration systems and wind turbines for residential applications using a dynamic model

Solar and wind energy sources are envisioned to play a key role in the transition towards fully renewable energy systems but both suffer from their intermittent nature, which can be relieved by hybridisation. Hence, the benefits of the complementarity of these sources are investigated hereunder by considering a small-scale hybrid trigenerative system consisting of a Linear Fresnel Reflector solar field with a 440 m2 collector area, a 20 kWe/100 kWt organic Rankine cycle unit combined with a latent heat thermal energy storage system, an absorption chiller, and a 20 kWe wind turbine for the provision of electricity, heating and cooling to 10 residential apartments. To this end, a dynamic model and suitable control logic are developed to assess and compare the performances of the proposed hybrid system concerning the solar plant and wind turbine separately.The results show that hybridisation can extend the annual thermal coverage by renewables by 3.7–6.1% for the investigated locations by increasing the operation of the organic Rankine cycle unit, especially in winter. In particular, the surplus electrical energy from the wind turbine is used to satisfy 6% and 4% more than the separate configuration of the annual thermal and electrical demands for Ancona. As a result of hybridisation, the annual thermal energy consumption of the boiler reduces by 8.3%, 9.9%, and 37.6% for Perugia, Ancona, and Messina respectively, which shows the impact of the profile of the energy sources. At the same time, hybridisation improves grid stability by reducing the excess electricity production 18.2%, 19.2%, and 26.3% for Perugia, Ancona, and Messina respectively.A sensitivity analysis for Ancona has revealed that the size of the water thermal energy storage tank does not have a significant impact on the electrical and thermal coverage of the hybrid system, while electrical and thermal coverages improve 11.1% and 6.7% respectively by doubling the wind turbine capacity along with a 14.8% drop in the thermal energy consumption by the boiler. In brief, the analysis has shown the complementarity of these energy sources and the benefits of hybridisation for trigenerative systems for residential buildings.

Confinement-induced accumulation and de-mixing of microscopic active-passive mixtures

Understanding the out-of-equilibrium properties of noisy microscale systems and the extent to which they can be modulated externally, is a crucial scientific and technological challenge. It holds the promise to unlock disruptive new technologies ranging from targeted delivery of chemicals within the body to directed assembly of new materials. Here we focus on how active matter can be harnessed to transport passive microscopic systems in a statistically predictable way. Using a minimal active-passive system of weakly Brownian particles and swimming microalgae, we show that spatial confinement leads to a complex non-monotonic steady-state distribution of colloids, with a pronounced peak at the boundary. The particles’ emergent active dynamics is well captured by a space-dependent Poisson process resulting from the space-dependent motion of the algae. Based on our findings, we then realise experimentally the de-mixing of the active-passive suspension, opening the way for manipulating colloidal objects via controlled activity fields.