Microscale Flow Research Articles

Simulation study on gas flow in curved capillary used in laser wakefield acceleration

<sec>Based on the standard <i>k</i>-<i>ε</i> model, a gas flow calculation model in a curved capillary is established, and the flow process of helium working medium in a curved capillary with gradually changing curvature is numerically simulated. Compared with other methods of studying micro-scale gas flow, this simulation obtains the gas density distribution in the curved capillary more conveniently, and has the same variation trend as the experimental measurement of the plasma electron density distribution, and can predict the gas flow distribution in the tube more accurately. The situation provides a theoretical basis for designing the discharge capillary experiment. Based on this model, the gas flow process in the capillary of the one-sided direct flushing, double-sided hedging and “straight + curved” cascade acceleration structures are numerically simulated. The results and conclusions are summarized as follows.</sec><sec>1) Comparing with the single-sided straight-bent capillary structure, the gas density fluctuation between the left gas inlet and the right gas inlet of the double-sided hedging-bend capillary is smaller, the gas flow is more stable, and a relatively stable plasma density channel can be generated.</sec><sec>2) In the double-sided hedged curved capillary, a relatively uniform gas density distribution is formed between the two inlets of the capillary under the same inflation back pressure; further research results show that a more uniform plasma density distribution with different lengths can be obtained by controlling the position of the gas inlet.</sec><sec>3) In the “traight + curved” cascaded accelerating capillary structure, the diameter of the electron injection channel will affect the gas density distribution in the bend. When the diameter of the electron injection channel is small, the absolute pressure in the capillary is low. The larger pressure difference between them will lead to a higher gas flow rate in the elbow, which will increase the fluctuation of the gas density in the elbow; the final research shows that the diameters of the electron injection channel, 100 μm and 150 μm are more suitable for the application in the “direct + bend” cascade acceleration capillary structure design.</sec><sec>In summary, the calculation model of gas flow in the curved capillary constructed in this paper can accurately predict the gas flow distribution in the tube. The double-sided hedged curved capillary can generate a relatively stable plasma density channel, and the electron injection channel diameters, 100 μm and 150 μm, are more suitable for application in the “straight + curved” cascade accelerating capillary structure design. The research results obtained are expected to provide theoretical guidance and technical support for the laser wake cascade acceleration experiment based on the curved capillary with gradually changing curvature.</sec>

Simulation of the Production Performances of Horizontal Wells with a Fractured Shale Gas Reservoir

The production performances of a well with a shale gas reservoir displaying a complex fracture network are simulated. In particular, a micro-seismic cloud diagram is used to describe the fracture network, and accordingly, a production model is introduced based on a multi-scale flow mechanism. A finite volume method is then exploited for the integration of the model equations. The effects of apparent permeability, conductivity, Langmuir volume, and bottom hole pressure on gas well production are studied accordingly. The simulation results show that ignoring the micro-scale flow mechanism of the shale gas leads to underestimating the well gas production. It is shown that after ten years of production, the cumulative gas production difference between the two scenarios with and without considering the micro-scale flow mechanisms is 19.5%. The greater the fracture conductivity, the higher the initial gas production of the gas well and the cumulative gas production. The larger the Langmuir volume, the higher the gas production rate and the cumulative gas production. With the reduction of the bottom hole pressure, the cumulative gas production increases, but the growth rate gradually decreases.

Innovations in exploiting photo-controlled Marangoni flows for soft matter actuations.

The generation and control of microscale flows are crucial for fundamental as well as applied aspects of microfluidics. Commonly employed strategies for creating microflows are based on electric field, magnetic field, surface tension, temperature, pressure, light, etc. Among them, light as an external stimulus is gaining increased attention as it offers non-contact actuation, high spatial and temporal resolution, tunable wavelength and intensity, ease of miniaturization, and fast response. Optically tuning the surface tension is promising because a surface tension gradient of a few mN m-1 along the liquid surface is sufficient to create a strong Marangoni flow. This can be done by either optically heating (thermocapillary) or modulating the chemical composition (soluto-capillary) at the liquid surface, which could be exploited to realize the transport/assembly of colloidal particles, droplets, bubbles and liquid marbles. In this perspective article, we focus on the innovative approaches in modulating surface tension using light, in the context of soft matter and microfluidic applications. First, we discuss the photo-controlled Marangoni flow-driven patterning and assembly of colloidal particles in different microfluidic systems, such as liquid droplets and liquid films. Next, we review the various methods in which the photo-controlled Marangoni flows are being exploited for the actuation of liquid droplets and liquid marbles over solid as well as soft substrates. Finally, we highlight the recent trends in optically controlled flow-based reconfigurable optics and energy harvesting systems.

Molecular Simulation of Multiphase Interface Characteristics and Microscale Flow Mechanisms of Oil and Brine in Carbonate Reservoir

Molecular Simulation of Multiphase Interface Characteristics and Microscale Flow Mechanisms of Oil and Brine in Carbonate Reservoir

Design and characterization of a flow reaction calorimeter based on FlowPlate® Lab and Peltier elements

Microscale flow reaction calorimeter based on commercially available hastelloy C-22 microreactor for isoperibolic and isothermal operation mode.

Gradient-based optimization of micro-scale pressurized volumetric receiver geometry and flow rate

This study focuses on the design optimization of a micro-scale pressurized volumetric receiver by changing geometry and flow rate constrained by the volume, outlet air temperature, and outer surface temperature. The pressurized volumetric receiver model is replicated from an existing model, which assumes constant air pressure and neglects the convection loss from the cavity. The existing model is revised from a solver to a design optimizer.The replicated model is restructured using OpenMDAO (Open-source MultiDisciplinary Analysis and Optimization) framework, and analytical derivatives are implemented for efficient derivative calculation to increase optimization performance. The replicated model is verified, and the maximum outlet air temperature difference is less than 0.05%. Optimization performance, selection of optimizers, the effect of the domain size, and radiative methods are discussed. The combined impact of the design variables is observed by selecting SLSQP (Sequential Least SQuares Programming) and trust-region optimizers. Optimization performance is tested in different domain sizes and compared with a design of experiments analysis. For testing the impact of radiative heat transfer methods to design optimization, the Rosseland approximation, and P1 method are selected. Depending on the design domain, a solution methodology is suggested for future receiver design optimizations applicable for macro-scale pressurized volumetric receivers.

Building predictive disease models using extracellular vesicle microscale flow cytometry and machine learning.

Extracellular vesicles (EVs) are highly abundant in human biofluids, containing a repertoire of macromolecules and biomarkers representative of the tissue of origin. EVs released by tumours can communicate key signals both locally and to distant sites to promote growth and survival or impact invasive and metastatic progression. Microscale flow cytometry of circulating EVs is an emerging technology that is a promising alternative to biopsy for disease diagnosis. However, biofluid-derived EVs are highly heterogeneous in size and composition, making their analysis complex. To address this, we developed a machine learning approach combined with EV microscale cytometry using tissue- and disease-specific biomarkers to generate predictive models. We demonstrate the utility of this novel extracellular vesicle machine learning analysis platform (EVMAP) to predict disease from patient samples by developing a blood test to identify high-grade prostate cancer and validate its performance in a prospective 215 patient cohort. Models generated using the EVMAP approach significantly improved the prediction of high-risk prostate cancer, highlighting the clinical utility of this diagnostic platform for improved cancer prediction from a blood test.

Reactor performance estimation in microscale flow calorimeter for rapid characterization of exothermic reactions

Continuous flow calorimeters are a promising tool in process development and safety engineering, especially for flow chemistry applications to characterize the heat release and kinetic parameters of rapid chemical reactions. In this study, the digital accompaniment of an isoperibolic flow calorimeter for characterization of exothermic reactions is presented. To support experimental planning and evaluation, computational fluid dynamic simulations are carried out for single-phase flow in the microreactor. The residence time distribution is obtained and used for estimation of conversion and temperature profiles along the microreactor channel. This leads to an integration of CFD simulations into the calorimeter’s software-guided workflow reducing the experimental effort regarding the determination of thermokinetic data. The approach is tested for a highly exothermic test reaction, which provides further hints for future investigations.Article highlights• Estimation of conversion and temperature profiles within a microscale calorimeter• Combination of CFD simulations and reactor performance estimation• Approach was tested for highly oxidation of sodium thiosulfate• Estimated conversion and temperature profiles are in good agreement with experimental dataGraphical abstractComputational Fluid Dynamics simulations are combined with reactor performance estimation to support experimental design for the determination of thermokinetic data. Insights of the dynamic behavior are gained to assist the operator in the investigation of exothermic reactions.

Mesoscopic simulation of liquid bridge spreading under squeezing of parallel plates

The spreading behavior of a droplet under squeezing between parallel plates is seen in the adhesion of microelectronic components and the lubrication of human joints, which is a process involving complex micro-scale flow behaviors, such as three-phase contact line movement. In this study, a many-body dissipative particle dynamics method is employed to account for this process. The method has been first validated by comparing with Cox's theory of contact lines. Two stages have been identified during the process of squeezing: a contact line retraction state and a symmetrical spreading state, which can also be reflected by the change of the system's surface energy. The combined effects of the squeezing velocity and plate's wettability on the appearance of the first stage have been investigated, showing that a large enough squeezing velocity and a hydrophobic enough substrate will lead to no contraction of the contact line. This study provides a valuable tool to explore the possibility of controlling the droplet squeezing behavior and, thus, is helpful for optimizing the adhesion and lubrication process.

Using Machine Learning to Predict Multiphase Flow through Complex Fractures

Multiphase flow properties of fractures are important in engineering applications such as hydraulic fracturing, evaluating the sealing capacity of caprocks, and the productivity of hydrocarbon-bearing tight rocks. Due to the computational requirements of high fidelity simulations, investigations of flow and transport through fractures typically rely on simplified assumptions applied to large fracture networks. These simplifications ignore the effect of pore-scale capillary phenomena and 3D realistic fracture morphology (for instance, tortuosity, contact points, and crevasses) that lead to macro-scale effective transport properties. The effect of these properties can be studied through lattice Boltzmann simulations, but they require high performance computing clusters and are generally limited in their domain size. In this work, we develop a technique to represent 3D fracture geometries and fluid distributions in 2D without losing any information. Using this innovative approach, we present a specialized machine learning model which only requires a few simulations for training but still accurately predicts fluid flow through 3D fractures. We demonstrate our technique using simulations of a water filled fracture being displaced by supercritical CO2. By generating highly efficient simulations of micro-scale multiphase flow in fractures, we hope to investigate a wide range of fracture types and generalize our method to be incorporated into larger discrete fracture network simulations.

Leveraging spreadsheet analysis tool for electrically actuated start-up flow of non-Newtonian fluid in small-scale systems

In this article, we demonstrate the solution methodology of start-up electrokinetic flow of non-Newtonian fluids in a microfluidic channel having square cross-section using Spreadsheet analysis tool. In order to incorporate the rheology of the non-Newtonian fluids, we take into consideration the Ostwald-de Waele power law model. By making a comprehensive discussion on the implementation details of the discretized form of the transport equations in Spreadsheet analysis tool, and establishing the analytical solution for a special case of the start-up flow, we compare the results both during initial transience as well as in case of steady-state scenario. Also, to substantiate the efficacy of the proposed spreadsheet analysis in addressing the detailed flow physics of rheological fluids, we verify the results for several cases with the corresponding numerical results. It is found that the solution obtained from the Spreadsheet analysis is in good agreement with the numerical results—a finding supporting spreadsheet analysis's suitability for capturing the fine details of microscale flows. We strongly believe that our analysis study will open up a new research scope in simulating microscale transport process of non-Newtonian fluids in the framework of cost-effective and non-time consuming manner.

A Novel Equivalent Numerical Simulation Method for Non-Darcy Seepage Flow in Low-Permeability Reservoirs

The low permeability and submicron throats in most shale or tight sandstone reservoirs have a significant impact on microscale flow. The flow characteristics can be described with difficultly by the conventional Darcy flow in low-permeability reservoirs. In particular, the thickness of the boundary layer is an important factor affecting the formation permeability, and the relative permeability curve obtained under conventional conditions cannot accurately express the seepage characteristics of porous media. In this work, the apparent permeability and relative permeability were calculated by using non-Darcy-flow mathematical modeling. The results revealed that the newly calculated oil–water relative permeability was slightly higher than that calculated by the Darcy seepage model. The results of the non-Darcy flow based on the conceptual model showed that the area swept by water in non-Darcy was smaller than that in Darcy seepage. The fingering phenomenon and the high bottom hole pressure in the non-Darcy seepage model resulted from the larger amount of injected water. There was a large pressure difference between the injection and production wells where the permeability changed greatly. A small pressure difference between wells resulted in lower variation of permeability. Consequently, the non-Darcy simulation results were consistent with actual production data.

Microfluid Switching-Induced Transient Refractive Interface.

The laminar flow interface (LFI) developed at low Reynolds numbers is one of the most prominent features of microscale flows and has been employed in a diverse range of optofluidic applications. The formation of LFIs usually requires the manipulation of multiple streams within a microchannel using a complex hydrodynamic pumping system. Herein, we present a new type of LFI that is generated by fluid switching within a three-dimensional (3D) microlens-incorporating microfluidic chip (3D-MIMC). Since Poiseuille flows exhibit a parabolic velocity profile, the LFI is cone-like in shape and acts as a transient refractive interface (TRI), which is sensitive to the refractive index (RI) and the Péclet number (Pe) of the switching fluids. In response to the TRI, the intensity of the transmitted light can be intensified or attenuated depending on the sequence of fluid switching operations. By incorporating three-dimensional (3D) microlenses and increasing the Pe values, the profile and amplitude of the intensity peak are both significantly improved. The limit of detection (LoD) for a sodium chloride (NaCl) solution at Pe = 1363 is as low as 0.001% (w/w), representing an improvement of 1-2 orders of magnitude when compared to existing optofluidic concentration sensors based on intensity modulation. Fluid switching of a variety of inorganic and organic sample fluids confirms that the specific optical response (Kor) correlates positively with both Pe and the specific RI (Knc), obeying a linear relationship. This model is further verified through cross-validations and used to estimate the molecular diffusion coefficient (D) of a range of species. Furthermore, by virtue of the TRI, we achieve a sensitive measurement of optical-equivalent total dissolved solids (OE-TDS) for environmental samples.

Characteristics of boundary layer under microscale flow and regulation mechanism of nanomaterials on boundary layer

As a new functional material that can effectively reduce displacement pressure and increase injection, nanomaterials have attracted much attention in the development and research of tight oil and gas reservoirs with a low permeability. At present, the research on the technology of decreasing displacement pressure and increasing injection by nanomaterials, mostly focuses on improving the interface properties of nanomaterials such as wall wettability and roughness, and evaluates the performance from the macro-level to clarify the influence and application prospect of nanomaterials. However, there are relatively few studies on the micro influence of nanomaterials on fluid flow. At the same time, owing to the existence of a boundary layer in the pores of a tight reservoir with a low permeability, the fluid flow characteristics are also substantially different from those under the condition of a conventional-size flow channel. In this study, a porous medium was constructed with glass beads to simplify the physical model, and the variation trend of the boundary layer thickness under different treatment conditions and flow channel sizes was clarified through centrifugal experiments, and a formula that can effectively describe the variation in boundary layer thickness with displacement pressure was established, consistent with the functional relationship δ = δ0 + A × exp [− (Grad ΔP − B) / C]. Using the relationship, the static boundary layer thickness δ0 can be obtained. In addition, the regulation of nanomaterials on the thickness of the boundary layer was clarified. The results show that nanomaterials can effectively reduce the thickness of the boundary layer of the fluid in the flow channel, and their performance for this thickness reduction increases with the increase in the wall hydrophobicity after nanomaterials are adsorbed. The smaller the scale of the flow channel, the more clear the regulation effect. In this study, the influence of nanomaterials on the regulation of boundary layer during the fluid flow under a microscale flow channel was clarified from the micro-level. The mechanism for displacement pressure decrease and injection increase of nanomaterials was further elucidated, which is important for the application of nanomaterials in tight reservoirs with a low permeability.

Numerical Modelling of a Valveless Impedance Pump with Various Pinch Locations

Microfluidics devices offers reliable transport mechanism to drive fluid within a microscale flow. This research work aims to identify the characteristic of flow in valveless impedance pump which uses acoustic impedance mismatch to drive flow using numerical simulations. The focus of the studies is designing a well performed pump where the required displacement of elastic wall can still be achieved even when the force applied in different locations and also a mechanism that requires low input current. Shear rate resulting from the pumping mechanism were measured at different input boundary conditions. Important parameters such as pinch location can affect the direction of the velocity magnitude and shear rate. It is found that the maximum shear rate occurred at 2 mm pinch location at 400 s-1. Hence, the pump design from this study can be used as the best application for microfluidic systems where a high pumping amount is desired.

Hysteresis in permeability evolution simulated for a sandstone by mineral precipitation and dissolution

Abstract. Mineral dissolution and precipitation can substantially affect rock permeability, which is a critical parameter for a broad range of geological subsurface applications. Virtual experiments on digital pore-scale samples represent a powerful and flexible approach to understand the impact of microstructural alterations on evolving hydraulic rock behaviour and quantify trends in permeability. In the present study, porosity-permeability relations are simulated for a precipitation-dissolution cycle within a typical reservoir sandstone. A hysteresis in permeability is observed depending on the geochemical process and dominating reaction regime, whereby permeability of the six investigated reaction paths varies by more than two orders of magnitude at a porosity of 17 %. Controlling parameters for this hysteresis phenomenon are the closure and re-opening of micro-scale flow channels, derived from changes in pore throat diameter and connectivity of the pore network. In general, a transport-limited regime exhibits a stronger impact on permeability than a reaction-limited regime, which uniformly alters the pore space. In case of mineral precipitation, higher permeability reduction results from successive clogging of pore throats, whereas in case of dissolution, permeability significantly increases due to a widening of existing flow paths. Both, the geochemical process and dominating reaction regime govern characteristic microstructural alterations, which cannot be simply reversed by the inversion of the geochemical processes itself. Hence, permeability evolution clearly depends on the hydrogeochemical history of the sample.

Flows of healthy and hardened RBC suspensions through a micropillar array.

Red blood cell (RBC) deformability is an important haemorheological factor; it is impaired in many pathologies leading to microvascular complications. Several microfluidic platforms have been utilized to examine the role of deformability in RBC flows but their geometries tend to be simplified. In the present study, we extend our previous work on healthy RBC flows in micropillar arrays [1] to probe the effect of impaired RBC deformability on the velocity and haematocrit distributions in microscale RBC flows. Healthy and artificially hardened RBC suspensions at 25% haematocrit were perfused through the micropillar array at various flow rates and imaged. RBC velocities were determined by Particle Image Velocimetry (PIV) and haematocrit distributions were inferred from the image intensity distributions. The pillars divide the flow into two distinct RBC streams separated by a cell-depleted region along the centreline and in the rear/front stagnation points. RBC deformability was not found to significantly affect the velocity distributions; the shape of the velocity profiles in the interstitial space remained the same for healthy and hardened RBCs. Time-averaged and spatiotemporal intensity distributions, however, reveal differences in the dynamics and local distributions of healthy and hardened cells; hardened cells appear to enter the cell-depleted regions more frequently and their interstitial distributions are more uniform. The study highlights the importance of local RBC distributions and the impact of RBC deformability on cell transport in complex microscale flows.

Experimental study of loss coefficients for laminar oil-water two-phase flow through micro-scale flow restrictions

Micro-scale flow restrictions have significant effect on the flow performances in microfluidic devices, porous media and biological processes. Two pairs of micro channels were fabricated by microfluidic method to experimentally investigate the irreversible pressure losses of laminar oil–water two-phase flow through two micro-scale flow restrictions. The hydraulic diameters of these two micro-scale flow restrictions are 72 μm and 33 μm, respectively. The oil contents of oil–water mixture vary from 0 % to 100 % and all the flow patterns are dispersed. By comparing the pressure drops between a micro channel with a built-in micro-scale flow restriction and the one without micro-scale flow restriction for a pair of micro channels, the irreversible pressure losses through a micro-scale flow restriction were determined. The results indicate that the local irreversible pressure loss is principally generated from the frictional dissipation effect instead of the acceleration and capillary effects for dispersed oil–water two-phase flow through the micro-scale restrictions with identical contraction and expansion geometries. As a dimensionless expression of irreversible pressure loss, the dependencies of local loss coefficient on the dimensionless parameters such as capillary number, Knudsen number, Reynolds number and restriction area ratio are concerned and discussed for the laminar oil–water two-phase flow through micro-scale flow restrictions. A new expression of local loss coefficient was proposed for laminar oil–water flow through a whole micro-scale flow restriction, by which the prediction results show good consistency with the experimental data.

Droplet microfluidics for studying surfactant-rich interfaces found in aerosols, emulsions and foams

<h2>Abstract</h2> The presence of droplets suspended in a liquid or gas is known to play an essential role in fields ranging from materials to atmospheric science. Often, the dispersed phase is stabilized by surface active compounds and surfactants, resulting in complex chemical composition and material properties at the fluid-fluid interface. In this talk, recent advancements in use of microscale flow fields will be highlighted for measuring properties of aqueous multiphase, interface-rich systems. Systems studied will include atmospheric aerosol droplets and liquid-liquid emulsions. Microfluidic contractions, traps, and wells are used to measure surface-bulk partitioning and temperature-dependent liquid-liquid phase separation of aqueous droplets, towards better understanding of suspensions of aerosol droplets in our atmosphere. Advanced measurements of droplet shape deformations and coalescence will also be presented for both water-in-fuel and oil-in-water emulsions, for improved treatment and separation of emulsions. Dynamic interfacial tensions measurements were performed using a microfluidic tensiometer, demonstrating a dependence on if the surfactant approaches the interface from inside (dispersed) versus outside (continuous), implying phase dependent surfactant transport to curved interfaces at the microscale. Droplet coalescence and film drainage experiments are also performed in a microfluidic Stokes trap across a range of viscosity ratios and surfactant concentrations. Results are used to explore the influence of interfacial mobility and Marangoni stresses on film stability with soluble surfactants.

Permeable Biofilms Can Support Persistent Hyporheic Anoxic Microzones

AbstractExperimental observations reveal the existence of anoxic microzones within oxygen‐rich aquatic sediments. The microzones are hotspots of enhanced removal of nitrate and production of greenhouse gas nitrous oxide. Here we established a new microscale flow, reactive transport and biofilm growth model and explore how anoxic microzones form and evolve within hyporheic zones under dynamic interactions among pore‐scale flow, reactive transport and biofilm growth. The microzones can form and evolve by two patterns as streambeds are progressively clogged due to biofilm growth: (a) physical and biogeochemical conditions‐dependent perennial microzones for permeable biofilms, and (b) physical and biogeochemical conditions‐independent ephemeral microzones for low permeability biofilms. During biofilm growth, persistent transport‐dominated systems sustain for the former, whereas the transformation of transport‐dominated to reaction‐dominated systems occurs for the latter. These findings are significantly different from previous single pattern of ephemeral microzones.