Microscale Flow Research Articles

An immersed boundary-lattice Boltzmann method for gaseous slip flow

An immersed boundary (IB)-lattice Boltzmann (LB) method is proposed for microchannel slip flow encountered in microfluidics applications such as microelectromechanical Systems, filtration applications with nanofibers, polymer processing, and unconventional shale gas and coal seam gas applications. The LB method is theoretically analyzed to have an intrinsic ability to model velocity discontinuities at finite Knudsen numbers (Kn) when a sufficiently fine grid spacing and an external continuous perturbation, e.g., the body force of an IB method, are applied. Based on this analysis, an IB method coupled with a LB framework without ghost grids in nonfluid areas is proposed to study gaseous slip flow at finite Kn. In addition, an improved treatment to the suspending grids in nonfluid areas is proposed to assist the IB-LB method. In the simulations of two-dimensional Poiseuille and Couette flows for 0.01 ≤ Kn ≤ 1, the slip flow predicted by the proposed nonghost-grid IB-LB method achieves good agreement with that predicted by the linearized Boltzmann and/or Direct Simulation Monte Carlo methods up to Kn = 0.2. Since the proposed IB-LB method is free of adjustable parameters and slip velocity models, it provides a simple and promising pathway for modeling microscale flow applications for the validated regime, i.e., Kn < = 0.2.

Characterization of reaction enthalpy and kinetics in a microscale flow platform

We report an isothermal flow calorimeter for characterization of reaction enthalpy and kinetics.

Microscale Flow Field Analysis and Flow Prediction Model Exploration of Dry Gas Seal

The characteristics of high speed, high pressure, microscale, and rotating flow fields result in very complicated flow between the dynamic and static rings of a dry gas seal. In particular, the existence of a groove, dam, and weir further contribute to the uncertainty of a microscale flow field. Classically, the flow regime of pipe flow is determined based on the critical Reynolds number (Pipe model) while the rotating flow field is sometimes determined by the flow factor (Oval model). In this study, a new method was developed that can be used for prediction of flow regime of a dry gas seal rotating flow field based on a three-dimensional velocity component (Ellipsoid model). The aim was to make the given model reflect the flow regime more realistically based on the consideration of the axial velocity component. When modeling dry gas seal rotating flow fields, we showed that compared to the one-dimensional Pipe model and the two-dimensional Oval model, the three-dimensional Ellipsoid model proposed in this paper can produce more accurate results.

Numerical study on micro-scale extensional viscoelastic flows

The capillary thinning dynamics can be considered one of the fingerprints of extensionally-dominated viscoelastic flows. Notably, the rheological behavior of complex fluids in extensional flow has been investigated in different rheometric devices, as for instance in the Capillary Breakup Extensional Rheometer (CaBER), the Dripping-onto-Substrate (DoS) rheometry, or the Rayleigh Ohnesorge Jetting Extensional Rheometer (ROJER). In recent years, the Computational Rheology community has made a considerable effort to better understand the interplay of viscoelasticity and capillarity effects in such transient rheometric experiments. In this work, we present a numerical study on the dynamics of extensional flows of dilute polymeric solutions at small scales. Our numerical investigation focus primarily on the potential of using small scale two-phase extensional viscoelastic flows as suitable platforms for performing rheometry of weakly viscoelastic polymer solutions. In particular, we have adopted the setup used in the experiments of Sousa et al. [Rheol. Acta 56 (2017) 11–20]. In such set up, the filament stretching is conducted using oil as an outer phase, avoiding sample evaporation, or allowing visualization of the filament interior. In order to handle with the moving interface problem, we have employed a two-phase viscoelastic fluid flow solver, based on a finite differences scheme. In this methodology, the interface between the fluids is approximated by the volume-of-fluid interface reconstruction algorithm, and a second-order operator-split method is used to solve the advection equation. We observed a negligible influence of the use of different types of low viscosity oils as outer fluid on the measurement of the sample relaxation time. We also found that the use of an external low viscosity immiscible oil did not prevent the formation of beads-on-a-string structures.

Inner Surface Design of Functional Microchannels for Microscale Flow Control.

Fluidic flow behaviors in microfluidics are dominated by the interfaces created between the fluids and the inner surface walls of microchannels. Microchannel inner surface designs, including the surface chemical modification, and the construction of micro-/nanostructures, are good examples of manipulating those interfaces between liquids and surfaces through tuning the chemical and physical properties of the inner walls of the microchannel. Therefore, the microchannel inner surface design plays critical roles in regulating microflows to enhance the capabilities of microfluidic systems for various applications. Most recently, the rapid progresses in micro-/nanofabrication technologies and fundamental materials have also made it possible to integrate increasingly complex chemical and physical surface modification strategies with the preparation of microchannels in microfluidics. Besides, a wave of researches focusing on the ideas of using liquids as dynamic surface materials is identified, and the unique characteristics endowed with liquid-liquid interfaces have revealed that the interesting phenomena can extend the scope of interfacial interactions determining microflow behaviors. This review extensively discusses the microchannel inner surface designs for microflow control, especially evaluates them from the perspectives of the interfaces resulting from the inner surface designs. In addition, prospective opportunities for the development of surface designs of microchannels, and their applications are provided with the potential to attract scientific interest in areas related to the rapid development and applications of various microchannel systems.

Flow Field Image Based Attitude Control for Small Unmanned Aerial Vehicles

In recent years, microscale flow sensors have been extensively studied and developed, which can measure local flow information such as pressure or wall-shear stress over aerial vehicle surfaces in real-time. It is expected that with those sensors onboard, SUAVs can potentially mimic birds or bats in achieving more stable and agile flights than purely relying on rigid body sensors. However, it is challenging to utilize such a rich amount of surface airflow information to enable agile SUAV flights, which could be in the form of 2-D or 3-D images. In this paper, a flow field image based approach is developed for SUAV attitude control. The proposed robust controllers work directly on flow field images through defined image operators. The asymptotically stability of controllers are proven for the closed-loop systems under bounded uncertainties. The effectiveness of the controllers are demonstrated in simulations for both pitching motion and three-axis attitude motion even under gust wind conditions.

Assessing On-Road Emission Flow Pattern under Car-Following Induced Turbulence Using Computational Fluid Dynamics (CFD) Numerical Simulation

Research assessing on-road emission flow patterns from motor vehicles is essential in monitoring urban air quality, since it helps to mitigate atmospheric pollution levels. To reveal the influence of vehicle induced turbulence (VIT) caused by both front- and rear-vehicles on traffic exhaust and verify the applicability of the simplified line source emission model, a Computational Fluid Dynamics (CFD) numerical simulation was used to investigate the micro-scale vehicle pollutant flow patterns. The simulation results were examined through sensitivity analysis and compared with the field measured carbon monoxide (CO) concentration. Conclusions indicate that the vehicle induced turbulence caused by the airflow blocking effect of both front- and rear-vehicles impedes the diffusion of front-vehicle traffic exhaust, compared with that of the rear vehicle. The front-vehicle isosurface with the CO mass fraction of 0.0012 extended to 6.0 m behind the vehicle, while that of the rear-vehicle extends as far as 12.7 m. But for the entire motorcade, VIT is beneficial to the diffusion of pollutants in car-following situations. Meanwhile, within the range of 9 m behind the rear of the lagging vehicle lies a vehicle induced turbulence zone. Furthermore, the influence of vehicle induced turbulence on traffic exhaust flow pattern is obvious within a range of 1 m on both sides of the vehicle body, where the concentration gradient of on-road emission is larger and contains severe mechanical turbulence. As a result, in the large concentration gradient area of the pollutant flow field, which accounts for 99.85% of the total concentration gradient, using the line source models to represent the on-road emission might introduce considerable errors due to neglecting the influence of vehicle induced turbulence. Findings of this study may shed lights on predicting emission concentrations in multiple locations by selecting appropriate on-road emission source models.

Experimental Study on Liquid Flow and Heat Transfer in Rough Microchannels

Although roughness is negligible for laminar flow through tubes in classic fluid mechanics, the surface roughness may play an important role in microscale fluid flow due to the large ratio of surface area to volume. To further verify the influence of rough surfaces on microscale liquid flow and heat transfer, a performance test system of heat transfer and liquid flow was designed and built, and a series of experimental examinations are conducted, in which the microchannel material is stainless steel and the working medium is methanol. The results indicate that the surface roughness plays a significant role in the process of laminar flow and heat transfer in microchannels. In microchannels with roughness characteristics, the Poiseuille number of liquid laminar flow relies not only on the cross section shape of the rough microchannels but also on the Reynolds number of liquid flow. The Poiseuille number of liquid laminar flow in rough microchannels increases with increasing Reynolds number. In addition, the Nusselt number of liquid laminar heat transfer is related not only to the cross section shape of a rough microchannel but also to the Reynolds number of liquid flow, and the Nusselt number increases with increasing Reynolds number.

Lattice Boltzmann Simulation of Micro-scale Couette Flow of Two-component Gas Mixtures in the Slip Regime

Lattice Boltzmann method (LBM) is considered as a promising numerical method for simulating the micro-scale flows. In this work, the LBM is employed to simulate the micro-scale Couette flow of two-component gas mixtures in the slip regime. The bounce-back and full diffusive boundary condition is proposed to deal with the gas-wall interaction. Based on the simulations, linear velocity profiles and obvious slip at the wall are observed, which are affected by the molar fraction, gas components, and Knudsen number. In comparison with the molar fraction and gas components, the Knudsen number has a more significant effect on the velocity profiles. The LBM provides an efficient way to capture the flow behavior of the micro-scale Couette flow of two-component gas mixtures. Furthermore, the proposed LBM can also be used to simulate other complex micro-scale flows of two-component gas mixtures.

A Microparticle Image Velocimetry Based on Light Field Imaging

Accurate 3D flow characterization in a microchannel is becoming increasingly important for the design and development of microfluidic chips. In recent years, a light held camera that can simultaneously record the direction and position information of rays in a single photographic exposure has been developed and employed in the held of computer graphics. In this paper, a microparticle image velocimetry based on light held imaging (light held μPIV) is proposed to reconstruct the 3D velocity held of a microscale flow. Both simulations and experiments are performed to verify the proposed method. The light held image of tracer particles and the point spread function (PSF) of a light held microscopic imaging system are numerically calculated based on the Abbe imaging principle. The 3D positions of the tracer particles in a flow held are then reconstructed by the Lucy-Richardson 3D deconvolution algorithm. Furthermore, a light held μPIV system based on an assembled cage light held camera with a microscope is developed, and calibrations are performed to obtain the geometric parameters of the μPIV system accurately. The simulation and experimental results demonstrate the feasibility of the proposed light held μPIV. Compared with the synthetic refocusing reconstruction method, the Lucy-Richardson 3D deconvolution algorithm greatly improves the lateral and the axial resolutions of the flow held.

Braess’s paradox and programmable behaviour in microfluidic networks

Microfluidic systems are now being designed with precision as miniaturized fluid manipulation devices that can execute increasingly complex tasks. However, their operation often requires numerous external control devices owing to the typically linear nature of microscale flows, which has hampered the development of integrated control mechanisms. Here we address this difficulty by designing microfluidic networks that exhibit a nonlinear relation between the applied pressure and the flow rate, which can be harnessed to switch the direction of internal flows solely by manipulating the input and/or output pressures. We show that these networks-implemented using rigid polymer channels carrying water-exhibit an experimentally supported fluid analogue of Braess's paradox, in which closing an intermediate channel results in a higher, rather than lower, total flow rate. The harnessed behaviour is scalable and can be used to implement flow routing with multiple switches. These findings have the potential to advance the development of built-in control mechanisms in microfluidic networks, thereby facilitating the creation of portable systems and enabling novel applicationsin areas ranging from wearable healthcare technologies to deployable space systems.

MRT-LBM modelling of the oscillatory gas slide film damping in the transition regime

In this paper, the slide film damping (SFD) of micro-scale oscillatory Couette flow has been researched by an effective multiple relaxation time lattice Boltzmann method (MRT-LBM). The validity and effectiveness of MRT-LBM for solving SFD have been verified through contrasting the velocity distribution between the upper plate and the substrate of MRT-LBM and the direct simulation Monte Carlo (DSMC) model. The impacts of the vibration frequency and gap of plates on the damping are discussed, and the result shows that the nonlinear character of velocity profiles is manifest and SFD increases substantially for a larger vibration frequency in the oscillatory gas flow; SFD reduces obviously with the gap increases. Consequently, the results further confirm the implementation of LBM in analysis of the non-equilibrium micro-scale gas flow.

Optical Feedback Interferometry Based Microfluidic Sensing: Impact of Multi-Parameters on Doppler Spectral Properties

As a compact and simple sensing technique, optical feedback interferometry (OFI) can be a promising flowmetry method in various microfluidic applications. In this paper, OFI-based flowmetry sensor performance in a microscale flow scheme is studied theoretically and experimentally. An innovating model and different numerical methods are investigated, where the scattering light angle distribution is involved to predict the Doppler frequency distribution. For the first time, our model describes the influences of multiple OFI sensor system characteristics, such as flowing particle size, concentration, channel interface reflectivity and channel dimension, on the OFI signal spectral performances. In particular, a significant OFI signal level enhancement was achieved by deposing a high reflectivity gold layer on the rear channel interface due to the increased forward scattered light reflection. The consistent experimental validation associated with the simulations verifies this numerical simulation method’s reliability. The numerical methods presented here provide a new tool to design novel microfluidic reactors and sensors.

Spatially Resolved Genetic Analysis of Tissue Sections Enabled by Microscale Flow Confinement Retrieval and Isotachophoretic Purification.

We have developed a method for spatially resolved genetic analysis of formalin-fixed paraffin-embedded (FFPE) cell block and tissue sections. This method involves local sampling using hydrodynamic flow confinement of a lysis buffer, followed by electrokinetic purification of nucleic acids from the sampled lysate. We characterized the method by locally sampling an array of points with a circa 200 μm diameter footprint, enabling the detection of single KRAS and BRAF point mutations in small populations of RKO and MCF-7 FFPE cell blocks. To illustrate the utility of this approach for genetic analysis, we demonstrate spatially resolved genotyping of FFPE sections of human breast invasive ductal carcinoma.

Spatially Resolved Genetic Analysis of Tissue Sections Enabled by Microscale Flow Confinement Retrieval and Isotachophoretic Purification

AbstractWe have developed a method for spatially resolved genetic analysis of formalin‐fixed paraffin‐embedded (FFPE) cell block and tissue sections. This method involves local sampling using hydrodynamic flow confinement of a lysis buffer, followed by electrokinetic purification of nucleic acids from the sampled lysate. We characterized the method by locally sampling an array of points with a circa 200 μm diameter footprint, enabling the detection of single KRAS and BRAF point mutations in small populations of RKO and MCF‐7 FFPE cell blocks. To illustrate the utility of this approach for genetic analysis, we demonstrate spatially resolved genotyping of FFPE sections of human breast invasive ductal carcinoma.

The effect of deformability on the microscale flow behavior of red blood cell suspensions

Red blood cell (RBC) deformability is important for tissue perfusion and a key determinant of blood rheology. Diseases such as diabetes, sickle cell anemia, and malaria, as well as prolonged storage, may affect the mechanical properties of RBCs altering their hemodynamic behavior and leading to microvascular complications. However, the exact role of RBC deformability on microscale blood flow is not fully understood. In the present study, we extend our previous work on healthy RBC flows in bifurcating microchannels [Sherwood et al., “Viscosity and velocity distributions of aggregating and non-aggregating blood in a bifurcating microchannel,” Biomech. Model. Mechanobiol. 13, 259–273 (2014); Sherwood et al., “Spatial distributions of red blood cells significantly alter local hemodynamics,” PLoS One 9, e100473 (2014); and Kaliviotis et al., “Local viscosity distribution in bifurcating microfluidic blood flows,” Phys. Fluids 30, 030706 (2018)] to quantify the effects of impaired RBC deformability on the velocity and hematocrit distributions in microscale blood flows. Suspensions of healthy and glutaraldehyde hardened RBCs perfused through straight microchannels at various hematocrits and flow rates were imaged, and velocity and hematocrit distributions were determined simultaneously using micro-Particle Image Velocimetry and light transmission methods, respectively. At low feed hematocrits, hardened RBCs were more dispersed compared to healthy ones, consistent with decreased migration of stiffer cells. At high hematocrit, the loss of deformability was found to decrease the bluntness of velocity profiles, implying a reduction in shear thinning behavior. The hematocrit bluntness also decreased with hardening of the cells, implying an inversion of the correlation between velocity and hematocrit bluntness with loss of deformability. The study illustrates the complex interplay of various mechanisms affecting confined RBC suspension flows and the impact of both deformability and feed hematocrit on the resulting microstructure.

Directionally controlled open channel microfluidics

Free-surface microscale flows have been attracting increasing attention from the research community in recent times, as attributable to their diverse fields of applications ranging from fluid mixing and particle manipulation to biochemical processing on a chip. Traditionally, electrically driven processes governing free surface microfluidics are mostly effective in manipulating fluids having characteristically low values of the electrical conductivity (lower than 0.085 S/m). Biological and biochemical processes, on the other hand, typically aim to manipulate fluids having higher electrical conductivities (>0.1 S/m). To circumvent the inherent limitation of traditional electrokinetic processes in manipulating highly conductive fluids in free surface flows, here we experimentally develop a novel on-chip methodology for the same by exploiting the interaction between an alternating electric current and an induced thermal field. We show that the consequent local gradients in physical properties as well as interfacial tension can be tuned to direct the flow toward a specific location on the interface. The present experimental design opens up a new realm of on-chip process control without necessitating the creation of a geometric confinement. We envisage that this will also open up research avenues on open-channel microfluidics, an area that has vastly remained unexplored.

A detailed method to estimate inter-regional capital flows using inter-firm transaction and person flow big data

This study develops a method to estimate inter-regional capital flows in the real economy across Japan, which is important for understating the regional economy with as much detail as possible, and that existing censuses do not monitor adequately. The method can monitor spatial distributions of three kinds of capital flow: inter-firm transactions (F2F), firm-to-consumer flows as salaries (F2C), and consumer-to-firm flows as consumption behavior (C2F) using inter-firm transaction big data (IF data) and person flow big data (PF data). First, we estimated F2F using IF data that reports all capital flows between a company’s headquarters. Second, we estimate the home, work place, and consumption locations of all consumers using PF data collected by the auto-GPS function on mobile phones. Third, we estimate the F2C of each consumer by estimating their salaries based on the locations of firms in the IF data. Finally, we estimate the C2F of each consumption area by estimating the consumption of F2C in each consumption location. We develop a dataset that can monitor micro-scale capital flows across Japan. This study is one of the first attempts in any country to develop a dataset that can monitor the spatial distribution and time-series changes of nationwide capital flows at a very high-resolution scale. We expected that our data will contribute to policy-making to address the socioeconomic problems in the private sector and among local governments in Japan.

Identification and illustration of relationships between produced gas and water in marcellus under different spatial and temporal domains through data-driven analytics - Nonparametric model

Understanding the relationship between produced gas and flowback/produced water is important for both shale gas well operation optimization and well performance enhancement; however, it is not fully understood due to complex flow mechanisms and interactions/feedback among various geoscience and engineering controls. Data-driven analysis, as an alternative way, could inversely provide valuable insights on the relationship between produced gas and water, to adjust future development plans for well/regional optimal economic production.An auto-updated nonlinear data analytics method was applied to evaluate the relationship between water and gas in different spatial and temporal domains, to inversely correlate the micro-scale flow mechanisms from macro-scale data. In this study, the cases were covered with both wet and dry shale gas regions, as well as short and long shale gas production periods. Fracture-fluid flowback is defined as water production within one month, following a fracture treatment (exclusive of well shut-in time), and the produced water is 1–3 years cumulative water production. 114 wells from the Marcellus Formation, in wet and dry gas regions of northwestern West Virginia, were selected to investigate the relationship between fracture-fluid flowback and corresponding one-month gas production. 67 Marcellus wells in Pennsylvania were selected to study the relationship between produced water and corresponding gas production across different time periods ranging from one to three years. The results indicate that the relationship between gas and fracture-fluid flowback in the wet gas region is positive while negative in the dry gas region. The discussion of water flow format was done by comparing the water gas ratio at reservoir conditions and at the surface condition. WGR (water gas ratio) is high (>9 bbl/mmcf) during the 1st-year and then leveled off at 3 bbl/mmcf after the 1st-year, indicating water initially be produced through displacement, and then be produced through evaporation.

Heat transfer and flow characteristics of forced convection in PDMS microchannel heat sink

Recently, microfluidic devices are being widely used in various heat transfer applications and, thermal management in microchannels is a challenging issue. As a solution, different types of microchannel heat sinks have been employed to enhance the heat transfer. The flow characteristics in the microchannel heat sink have influence on the heat transfer rate and thermal management performance. Therefore, the simultaneous measurement of temperature and velocity fields is highly required to understand the single-phase heat transfer phenomena in the PDMS microchannel heat sink. In this study, μPIV-μLIF technique which can provide the velocity and temperature field information of micro-scale thermofluid flow was newly developed to analyze the heat transfer of forced convection in PDMS microchannel heat sink. Especially, flow characteristics in the microchannel heat sink were systematically investigated. At a high Re flow (Re = 10) with heating, a high-speed crossflow occurs in the microchannel heat sink. Spanwise vorticity and Reynolds shear stress, which contribute to heat transfer in the microchannel, exhibit higher values at Re = 10 with heating compared to the non-heating case. Variations in velocity and local Nu number were also compared to explain heat transfer enhancement in the microchannel heat sink. Temporal variations in the temperature field were evaluated to demonstrate the effect of flow on heat transfer. The results presented herein would be helpful for understanding heat transfer in microscale thermofluid flows and for designing microscale heat transfer devices.