Flow In Microfluidic Channels Research Articles

Modified trident-shaped electrode design for particle lateral position detection in microfluidic impedance flow cytometry

Modified trident-shaped planar electrodes were developed for the detection of particles and their lateral positions inside a microfluidic flow channel. The electrodes generated time-of-flight measurements of individual moving particles in the form of a double peak, where each peak varies in amplitude and duration depending on particle trajectories. The amplitude ratio of the two peaks indicates the lateral positions of particles when crossing the electrode pairs. In particular, the ratio is 1 for particles near the channel centerline, >1 for particles displaced laterally to one-half of the channel, and < 1 for particles to the opposite half. This was confirmed by numerical simulations and detection of microspheres (6 – 10 µm ø) using proposed electrodes embedded in a polydimethyl siloxane (PDMS) microfluidic device. The device could count individual microspheres at a rate of at least 1151 per minute while identifying microspheres of the same material but differing in size based on peak amplitudes.

Polarization-insensitive multifunctional reconfigurable band-notched absorber with wideband switchable absorption properties utilizing Galinstan.

A dual-polarized multifunctional reconfigurable band-notched absorber (MRBNA) based on Galinstan is presented in this paper. The proposed MRBNA comprises a liquid metal transmission/reflection switchable layer (LM-T/RSL) and a wideband band-notched absorber (BNA). The MRBNA represents a paradigm shift in adaptive electromagnetic (EM) solutions, offering unprecedented wideband switching capabilities between superior band-notched absorption and full-band reflection states. By harnessing the potential of LM-T/RSL and innovative polarization selection mechanisms, this design sets a new benchmark for advanced stealth systems. The impact of LM-T/RSL on transmission and reflection characteristics is investigated, followed by experimental investigation of fluid flow in microfluidic channels. Then, the structural conception of the MRBNA is examined, employing the equivalent circuit method (ECM) and surface current distributions for a better understanding of its operating mechanism. Finally, a prototype is fabricated and empirical validation is conducted to demonstrate the simulated results. Positioning it at the forefront, the seamless adaptability of the proposed MRBNA to changing EM conditions promises to revolutionize modern stealth operations.

Influence of surface roughness on the fluid flow in microchannel

Surface roughness is a crucial factor of fluid flow in microfluidic channels. Most of the existing studies focus on the effect of regular microstructured surfaces on fluid flow, while researchs about the impact of random rough surfaces on fluid flow remains limited. Therefore, in this study, a random rough surface model and two microstructured surface models were established, and the effects surface roughness on the fluid flow at Wenzel state and Cassie-Baxter state were studied using COMSOL multi-physics simulation software. Our findings reveal that both the flow velocity and the fluid flow rate at the microchannel outlet decreases with the surface roughness increases. Notably, the flow rate and velocity of the fluid flow at Cassie-Baxter state is higher than that of fluid flow at Wenzel state.

Observing root growth and signalling responses to stress gradients and pathogens using the bi-directional dual-flow RootChip.

Plants respond to environmental stressors with adaptive changes in growth and development. Central to these responses is the role of calcium (Ca2+) as a key secondary messenger. Here, the bi-directional dual-flow RootChip (bi-dfRC) microfluidic platform was used to study defence signalling and root growth. By introducing salinity as sodium chloride (NaCl) treatment via a multiplexed media delivery system (MMDS), dynamic gradients were created, mimicking natural environmental fluctuations. Signal analysis in Arabidopsis thaliana plants showed that the Ca2+ burst indicated by the G-CaMP3 was concentration dependent. A Ca2+ burst initiated in response to salinity increase, specifically within the stele tissue, for 30 seconds. The signal then intensified in epidermal cells directly in contact with the stressor, spreading directionally towards the root tip, over 5 minutes. Inhibition of propidium iodide (PI) stain transport through the xylem was observed following salinity increase, contrasting with flow observed under control conditions. The interaction of Phytophthora capsici zoospores with A. thaliana roots was also studied. An immediate directional Ca2+ signal was observed during early pathogen recognition, while a gradual, non-directional increase was observed in Orp1_roGFP fluorescent H2O2 levels, over 30 min. By adjusting the dimensions of the bi-dfRC, plants with varying root architectures were subjected to growth analysis. Growth reduction was observed in A. thaliana and Nicotiana benthamiana roots when exposed to salinity induced by 100 mM NaCl, while Solanum lycopersicum exhibited growth increase over 90 minutes at the same NaCl concentration. Furthermore, novel insights into force sensing in roots were gained through the engineering of displaceable pillars into the bi-dfRC channel. These findings highlight the vital role of controlling fluid flow in microfluidic channels in advancing our understanding of root physiology under stress conditions.

Asymmetric bistability of chiral particle orientation in viscous shear flows.

The migration of helical particles in viscous shear flows plays a crucial role in chiral particle sorting. Attaching a nonchiral head to a helical particle leads to a rheotactic torque inducing particle reorientation. This phenomenon is responsible for bacterial rheotaxis observed for flagellated bacteria as Escherichia coli in shear flows. Here, we use a high-resolution microprinting technique to fabricate microparticles with controlled and tunable chiral shape consisting of a spherical head and helical tails of various pitch and handedness. By observing the fully time-resolved dynamics of these microparticles in microfluidic channel flow, we gain valuable insights into chirality-induced orientation dynamics. Our experimental model system allows us to examine the effects of particle elongation, chirality, and head heaviness for different flow rates on the orientation dynamics, while minimizing the influence of Brownian noise. Through our model experiments, we demonstrate the existence of asymmetric bistability of the particle orientation perpendicular to the flow direction. We quantitatively explain the particle equilibrium orientations as a function of particle properties, initial conditions and flow rates, as well as the time-dependence of the reorientation dynamics through a theoretical model. The model parameters are determined using boundary element simulations, and excellent agreement with experiments is obtained without any adjustable parameters. Our findings lead to a better understanding of chiral particle transport and bacterial rheotaxis and might allow the development of targeted delivery applications.

Three-dimensional porous Cu-Zn/NF catalysts for efficient photocatalytic reduction of CO2 in alkaline environment for methanol preparation in a monolithic microreactor

The sluggish energy of photocatalytic CO2 reduction and the low methanol yields got are straightforwardly connected with the applied photocatalyst and reactor design, restricting the broad utilization of this technology. In light of this, the aim of this work is to innovate a new application of this Cu-Zn/Ni foam (Cu-Zn/NF) with a rod-like structure, which combines a intact foam-based photocatalyst with a microfluidic flow channel. This design ensures a larger specific surface area, besides a uniform phase distribution and efficient CO2 reduction. A novel foam-based monolithic photomicroreactor was evaluated by measuring methanol concentration and yield. Under the conditions of 25 mL·min−1, 0.5 M NaOH, 5 mm thickness of Ni foam, and 350 W xenon lamp irradiation conditions were applied, the methanol yield could reach 15.88 μmol·g−1·h−1. This outcome is superior to the announced values in most CO2 photoreactors. This work guides the optimization of photomicroreactors, besides provides new ideas for the use of monolithic microreactors for CO2 reduction.

Modelling and Experimental Validation of the Flow and Mixing Processes in a Microfluidic Membraneless X-Cell with Flowing Vanadium Electrolytes

Microfluidic devices are characterized by submillimeter channel cross-sections and tiny flow rates, typically below 100 µL/min. This results in rather small Reynolds numbers that maintain the flow laminar and steady, preventing chaotic mixing from turbulence [1-4]. This orderly flow can be used to remove the ionomeric membrane in redox flow batteries, generating membraneless microfluidic cells with low internal electric resistance. These devices could be manufactured at a lower cost than their macro counterparts. However, without a membrane, the mixing region where the electrolytes come into contact must be accurately controlled to avoid unwanted interface deflections leading to increased cross-contamination of liquid and species.In this work, we compare experimental and CFD realizations of the flow in a membraneless microfluidic flow channel [5], including in the simulations the effect of viscosity variations across the positive and negative vanadium electrolytes. The apparatus includes a microfluidic control system that uses piezoelectric pumps and valves for flow regulation, an X-cell as flow channel, and small tanks with discharged positive (VIV) and negative (VIII) electrolytes. We use a UV/Visible spectrometer to measure the amount of mixing at the outlet of the cell [6]. This allows us to validate the CFD model against the experimentally measured cross-contamination and liquid transfer. To our knowledge, these results are unique in using spectrometry data to quantify the mixing ratio of VIII/VIV, and provide valuable data to validate CFD models that could later be used to engineer the flow in membraneless microfluidic vanadium redox flow batteries. Acknowledgments This work has been partially funded by FEDER/Ministerio de Ciencia, Innovación y Universidades – Agencia Estatal de Investigación Projects PID2019-106740RB-I00 and RTC-2017-5955-3/AEI/10.13039/501100011033, and by Grant IND2019/AMB-17273 of the Comunidad de Madrid.

The role of surface adhesion on the macroscopic wrinkling of biofilms.

Biofilms, bacterial communities of cells encased by a self-produced matrix, exhibit a variety of three-dimensional structures. Specifically, channel networks formed within the bulk of the biofilm have been identified to play an important role in the colonies' viability by promoting the transport of nutrients and chemicals. Here, we study channel formation and focus on the role of the adhesion of the biofilm matrix to the substrate in Pseudomonas aeruginosa biofilms grown under constant flow in microfluidic channels. We perform phase contrast and confocal laser scanning microscopy to examine the development of the biofilm structure as a function of the substrates' surface energy. The formation of the wrinkles and folds is triggered by a mechanical buckling instability, controlled by biofilm growth rate and the film's adhesion to the substrate. The three-dimensional folding gives rise to hollow channels that rapidly increase the effective volume occupied by the biofilm and facilitate bacterial movement inside them. The experiments and analysis on mechanical instabilities for the relevant case of a bacterial biofilm grown during flow enable us to predict and control the biofilm morphology.

Pressure Drop Measurements in Microfluidic Devices: A Review on the Accurate Quantification of Interfacial Slip (Adv. Mater. Interfaces 5/2022)

Pressure Drop Measurements in Microfluidic Devices Huge hydrodynamic resistance slows down fluid flow in microfluidic channels. This resistance can be reduced through the design of nanostructured surfaces that induce interfacial slip. Quantifying the conditions for flow in microfluidic devices enables the design of new functional surfaces that will reduce hydrodynamic drag and increase mass transport, as reviewed in article number 2101641 by Christopher Vega-Sánchez and Chiara Neto. Cover image by Vega-Sánchez and Neto.

Optically accessible microfluidic flow channels for non-invasive high-resolution biofilm imaging using lattice light sheet microscopy

An imaging system that can achieve long-term, high-resolution imaging of biofilms is essential for studying cellular level dynamics within bacterial biofilms. Combining high spatial-temporal resolution and low phototoxicity, lattice light sheet microscopy (LLSM) has proven powerful for non-invasive 3D imaging of living cells and tissues. However, its application in biofilm research is limited because the open-on-top imaging geometry using water-immersion objective lenses is not compatible with living bacterial specimens; bacterial growth in the media basin and on the microscope's objective lenses makes long-term time-lapse imaging impossible.

Optically Accessible Microfluidic Flow Channels for Noninvasive High-Resolution Biofilm Imaging Using Lattice Light Sheet Microscopy.

Imaging platforms that enable long-term, high-resolution imaging of biofilms are required to study cellular level dynamics within bacterial biofilms. By combining high spatial and temporal resolution and low phototoxicity, lattice light sheet microscopy (LLSM) has made critical contributions to the study of cellular dynamics. However, the power of LLSM has not yet been leveraged for biofilm research because the open-on-top imaging geometry using water-immersion objective lenses is not compatible with living bacterial specimens; bacterial growth on the microscope’s objective lenses makes long-term time-lapse imaging impossible and raises considerable safety concerns for microscope users. To make LLSM compatible with pathogenic bacterial specimens, we developed hermetically sealed, but optically accessible, microfluidic flow channels that can sustain bacterial biofilm growth for multiple days under precisely controllable physical and chemical conditions. To generate a liquid- and gas-tight seal, we glued a thin polymer film across a 3D-printed channel, where the top wall had been omitted. We achieved negligible optical aberrations by using polymer films that precisely match the refractive index of water. Bacteria do not adhere to the polymer film itself, so that the polymer window provides unobstructed optical access to the channel interior. Inside the flow channels, biofilms can be grown on arbitrary, even nontransparent, surfaces. By integrating this flow channel with LLSM, we were able to record the growth of S. oneidensis MR-1 biofilms over several days at cellular resolution without any observable phototoxicity or photodamage.

Characterization of Mixing Performance Induced by Double Curved Passive Mixing Structures in Microfluidic Channels

Functionalized sensor surfaces combined with microfluidic channels are becoming increasingly important in realizing efficient biosensing devices applicable to small sample volumes. Relaxing the limitations imposed by laminar flow of the microfluidic channels by passive mixing structures to enhance analyte mass transfer to the sensing area will further improve the performance of these devices. In this paper, we characterize the flow performance in a group of microfluidic flow channels with novel double curved passive mixing structures (DCMS) fabricated in the ceiling. The experimental strategy includes confocal imaging to monitor the stationary flow patterns downstream from the inlet where a fluorophore is included in one of the inlets in a Y-channel microfluidic device. Analyses of the fluorescence pattern projected both along the channel and transverse to the flow direction monitored details in the developing homogenization. The mixing index (MI) as a function of the channel length was found to be well accounted for by a double-exponential equilibration process, where the different parameters of the DCMS were found to affect the extent and length of the initial mixing component. The range of MI for a 1 cm channel length for the DCMS was 0.75–0.98, which is a range of MI comparable to micromixers with herringbone structures. Overall, this indicates that the DCMS is a high performing passive micromixer, but the sensitivity to geometric parameter values calls for the selection of certain values for the most efficient mixing.

PyOIF: Computational tool for modelling of multi-cell flows in complex geometries

A user ready, well documented software package PyOIF contains an implementation of a robust validated computational model for cell flow modelling. The software is capable of simulating processes involving biological cells immersed in a fluid. The examples of such processes are flows in microfluidic channels with numerous applications such as cell sorting, rare cell isolation or flow fractionation. Besides the typical usage of such computational model in the design process of microfluidic devices, PyOIF has been used in the computer-aided discovery involving mechanical properties of cell membranes. With this software, single cell, many cell, as well as dense cell suspensions can be simulated. Many cell simulations include cell-cell interactions and analyse their effect on the cells. PyOIF can be used to test the influence of mechanical properties of the membrane in flows and in membrane-membrane interactions. Dense suspensions may be used to study the effect of cell volume fraction on macroscopic phenomena such as cell-free layer, apparent suspension viscosity or cell degradation. The PyOIF module is based on the official ESPResSo distribution with few modifications and is available under the terms of the GNU General Public Licence. PyOIF is based on Python objects representing the cells and on the C++ computational core for fluid and interaction dynamics. The source code is freely available at GitHub repository, runs natively under Linux and MacOS and can be used in Windows Subsystem for Linux. The communication among PyOIF users and developers is maintained using active mailing lists. This work provides a basic background to the underlying computational models and to the implementation of interactions within this framework. We provide the prospective PyOIF users with a practical example of simulation script with reference to our publicly available User Guide.

Targeting shear gradient activated von Willebrand factor by the novel single-chain antibody A1 reduces occlusive thrombus formation in vitro.

Intraluminal thrombus formation precipitates conditions such as acute myocardial infarction and disturbs local blood flow resulting in areas of rapidly changing blood flow velocities and steep gradients of blood shear rate. Shear rate gradients are known to be pro-thrombotic with an important role for the shear-sensitive plasma protein von Willebrand factor (VWF). Here, we developed a single-chain antibody (scFv) that targets a shear gradient specific conformation of VWF to specifically inhibit platelet adhesion at sites of shear rate gradients (SRG) but not in areas of constant shear. Microfluidic flow channels with stenotic segments were used to create SRG during blood perfusion. VWF-GPIbα interactions were increased at sites of SRG compared to constant shear rate of matched magnitude. The scFv-A1 specifically reduced VWF-GPIbα binding and thrombus formation at sites of SRG but did not block platelet deposition and aggregation under constant shear rate in upstream sections of the channels. Significantly, the scFv A1 attenuated platelet aggregation only in the later stages of thrombus formation. In the absence of shear, direct binding of scFv-A1 to VWF could not be detected and scFVA1 did not inhibit ristocetin induced platelet agglutination. We have exploited the pro-aggregatory effects of SRG on VWF dependent platelet aggregation and developed the shear gradient-sensitive scFv-A1 antibody that inhibits platelet aggregation exclusively at sites of SRG. The lack of VWF inhibition in non-stenosed vessel segments places scFV-A1 in an entirely new class of anti-platelet therapy for selective blockade of pathological thrombus formation while maintaining normal hemostasis.

Graphdiyne tubular micromotors: Electrosynthesis, characterization and self-propelled capabilities

Graphdiyne (GDY) micromotors have been synthesized by direct electrochemical deposition, avoiding harsh conditions or sophisticated equipment. GDY is directly electrodeposited in a membrane template by cyclic voltammetry, followed by the deposition of diverse inner catalytic layers (Pt/Ni, MnO2, or Pd/Cu) conferring the micromotors with structural stability to avoid nanoscale deformation. The best performance in terms of catalytic activity is observed for Pt-based micromotors, with up to 4-fold speed increase on average. In addition, compared with similarly prepared graphene oxide (GO) micromotors, a 2-fold speed enhancement is observed both in water and complex samples (saliva, blood serum, milk, and wine). The combination of the unique GDY properties with the moving capabilities of micromotors is also illustrated in the micromotor navigation against fluid flows in microfluidic channels. Practical applications are illustrated in fluorescent assay experiments for bacteria toxins detection and for the capture of a fluorescent probe, also used as model environmental pollutant. The template electrodeposition route holds great potential for the preparation of a myriad of GDY micromotors for diverse applications.

Longitudinal position dependence of the second-harmonic generation of optically trapped silica microspheres.

We report the design of a setup combining the simultaneous and independent optical trapping and second-harmonic generation (SHG) of 1 µm diameter silica microspheres with two independent laser beams. Optical trapping is achieved with a tightly focused continuous wave infrared laser beam whereas the SHG intensity from the trapped microparticles is obtained with a 810 nm femtosecond pulsed laser. The silica microparticles are dispersed in an aqueous solution, and a microfluidic channel flow is used to remove untrapped microparticles. We show by the perpendicular displacement of the optical trap from the microfluidic channel wall that it is possible to control the contribution of the channel wall/solution interface to the overall SHG intensity. Stable trapping and SHG detection of two microparticles is also demonstrated. Combining the independent trapping of centrosymmetrical silica microparticles with SHG offers new avenues for analytical studies like surface sensing or all-optical devices where the SHG intensity is controlled by the trapping beam.

In Vitro Microfluidic Disease Model to Study Whole Blood-Endothelial Interactions and Blood Clot Dynamics in Real-Time.

The formation of blood clots involves complex interactions between endothelial cells, their underlying matrix, various blood cells, and proteins. The endothelium is the primary source of many of the major hemostatic molecules that control platelet aggregation, coagulation, and fibrinolysis. Although the mechanism of thrombosis has been investigated for decades, in vitro studies mainly focus on situations of vascular damage where the subendothelial matrix gets exposed, or on interactions between cells with single blood components. Our method allows studying interactions between whole blood and an intact, confluent vascular cell network. By utilizing primary human endothelial cells, this protocol provides the unique opportunity to study the influence of endothelial cells on thrombus dynamics and givesvaluable insights into the pathophysiology of thrombotic disease. The use of custom-made microfluidic flow channels allows application of disease-specific vascular geometries and model specific morphological vascular changes. The development of a thrombus is recorded in real-time and quantitatively characterized by platelet adhesion and fibrin deposition. The effect of endothelial function in altered thrombus dynamics is determined by postanalysis through immunofluorescence staining of specific molecules. The representative results describe the experimental setup, data collection, and data analysis. Depending on the research question, parameters for every section can be adjusted including cell type, shear rates, channel geometry, drug therapy, and postanalysis procedures. The protocol is validated by quantifying thrombus formation on the pulmonary artery endothelium of patients with chronic thromboembolic disease.

Parallel High Throughput Single Molecule Kinetic Assay for Site-Specific DNA Cleavage.

Site-specific DNA cleavage (SSDC) is a key step in many cellular processes, and it is crucial to gene editing. This work describes a kinetic assay capable of measuring SSDC in many single DNA molecules simultaneously. Bead-tethered substrate DNAs, each containing a single copy of the target sequence, are prepared in a microfluidic flow channel. An external magnet applies a weak force to the paramagnetic beads. The integrity of up to 1,000 individual DNAs can be monitored by visualizing the microbeads under darkfield imaging using a wide-field, low magnification objective. Injecting of a restriction endonuclease, NdeI, initiates the cleavage reaction. Video microscopy is used to record the exact moment of each DNA cleavage by observing the frame in which the associated bead moves up and out of the focal plane of the objective. Frame-by-frame bead counting quantifies the reaction, and an exponential fit determines the reaction rate. This method allows collection of quantitative and statistically significant data on single molecule SSDC reactions in a single experiment.

Experimental Investigation of Air Compliance Effect on Measurement of Mechanical Properties of Blood Sample Flowing in Microfluidic Channels.

Air compliance has been used effectively to stabilize fluidic instability resulting from a syringe pump. It has also been employed to measure blood viscosity under constant shearing flows. However, due to a longer time delay, it is difficult to quantify the aggregation of red blood cells (RBCs) or blood viscoelasticity. To quantify the mechanical properties of blood samples (blood viscosity, RBC aggregation, and viscoelasticity) effectively, it is necessary to quantify contributions of air compliance to dynamic blood flows in microfluidic channels. In this study, the effect of air compliance on measurement of blood mechanical properties was experimentally quantified with respect to the air cavity in two driving syringes. Under periodic on–off blood flows, three mechanical properties of blood samples were sequentially obtained by quantifying microscopic image intensity (<I>) and interface (α) in a co-flowing channel. Based on a differential equation derived with a fluid circuit model, the time constant was obtained by analyzing the temporal variations of β = 1/(1–α). According to experimental results, the time constant significantly decreased by securing the air cavity in a reference fluid syringe (~0.1 mL). However, the time constant increased substantially by securing the air cavity in a blood sample syringe (~0.1 mL). Given that the air cavity in the blood sample syringe significantly contributed to delaying transient behaviors of blood flows, it hindered the quantification of RBC aggregation and blood viscoelasticity. In addition, it was impossible to obtain the viscosity and time constant when the blood flow rate was not available. Thus, to measure the three aforementioned mechanical properties of blood samples effectively, the air cavity in the blood sample syringe must be minimized (Vair, R = 0). Concerning the air cavity in the reference fluid syringe, it must be sufficiently secured about Vair, R = 0.1 mL for regulating fluidic instability because it does not affect dynamic blood flows.

A Counter Propagating Lens-Mirror System for Ultrahigh Throughput Single Droplet Detection.

Fluorescence-based detection schemes provide for multiparameter analysis in a broad range of applications in the chemical and biological sciences. Toward the realization of fully portable analysis systems, microfluidic devices integrating diverse functional components have been implemented in a range of out-of-lab environments. That said, there still exits an unmet and recognized need for miniaturized, low-cost, and sensitive optical detection systems, which provide not only for efficient molecular excitation, but also enhanced photon collection capabilities. To this end, an optofluidic platform that is adept at enhancing fluorescence light collection from microfluidic channels is presented. The central component of the detection module is a monolithic parabolic mirror located directly above the microfluidic channel, which acts to enhance the number of emitted photons reflected toward the detector. In addition, two-photon polymerization is used to print a microscale-lens below the microfluidic flow channel and directly opposite the mirror, to enhance the delivery of excitation radiation into the channel. Using such an approach, it is demonstrated that fluorescence signals can be enhanced by over two orders of magnitude, with component parallelization enabling the detection of pL-volume droplets at rates up to 40 000 droplets per second.