Narrow Microchannels Research Articles

Effect of elastic deformation on squeezing film lubrication properties of soft tribocontacts with microstructured surface

PurposeInspired by the high-friction performance of the soft toe pads of tree frogs, this study aims to investigate the effect of elastic deformation on the lubrication properties of squeezing films inside soft tribocontacts with microstructured surface under wet conditions.Design/methodology/approachA one-dimensional hydrodynamic extrusion model was used to study the film lubrication characteristics of conformal contact. The lubrication characteristics of the extruded film, including load-carrying capacity, liquid flow and surface elastic deformation, were obtained through the simultaneously iterative solution of the fluid-governing and deformation equations.FindingsThe results show that the hydrodynamic pressure is approximating parabolically and symmetrically distributed in the contact area, and the peak value appears in the center of the extrusion surface. Elastic deformation increases the thickness of the liquid film, weakens the bearing capacity and homogenizes the liquid flow rate of inside soft friction contact. The magnitude of this effect greatly increases as the initial liquid film thickness decreases. Moreover, the elastic deformation directly affects the average film thickness of the extrusion contact. Narrow and shallow microchannels are found to result in a more prominent elastic deformation on the microstructured soft surface.Originality/valueThese results present a design for soft tribocontacts suitable for submerged or wet environments involving high friction, such as wiper blades, in situ flexible electrons and underwater robots.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-02-2024-0049/

Physical confinement and cell proximity increase cell migration rates and invasiveness: A mathematical model of cancer cell invasion through flexible channels

Cancer cell migration between different body parts is the driving force behind cancer metastasis, which is the main cause of mortality of patients. Migration of cancer cells often proceeds by penetration through narrow cavities in locally stiff, yet flexible tissues. In our previous work, we developed a model for cell geometry evolution during invasion, which we extend here to investigate whether leader and follower (cancer) cells that only interact mechanically can benefit from sequential transmigration through narrow micro-channels and cavities. We consider two cases of cells sequentially migrating through a flexible channel: leader and follower cells being closely adjacent or distant. Using Wilcoxon’s signed-rank test on the data collected from Monte Carlo simulations, we conclude that the modelled transmigration speed for the follower cell is significantly larger than for the leader cell when cells are distant, i.e. follower cells transmigrate after the leader has completed the crossing. Furthermore, it appears that there exists an optimum with respect to the width of the channel such that cell moves fastest. On the other hand, in the case of closely adjacent cells, effectively performing collective migration, the leader cell moves 12% faster since the follower cell pushes it. This work shows that mechanical interactions between cells can increase the net transmigration speed of cancer cells, resulting in increased invasiveness. In other words, interaction between cancer cells can accelerate metastatic invasion.

Floating-Electrode-Enabled Impedance Cytometry for Single-Cell 3D Localization

As a label-free, low-cost, and noninvasive tool, impedance measurement has been widely used in single-cell characterization analysis. However, due to the tiny volume of cells, the uncertainty of the spatial position in the microchannel will bring measurement errors in single-cell electrical parameters. To overcome the issue, we designed a novel microdevice configured with a coplanar differential electrode structure to accurately resolve the spatial position of single cells without constraining techniques such as additional sheath fluids or narrow microchannels. The device precisely localizes single cells by measuring the induced current generated by the combined action of the floating electrode and the differential electrodes when single cells flow through the electrode-sensing area. The device was experimentally validated by measuring 6 μm yeast cells and 10 μm particles, achieving spatial localization with a resolution down to 2.1 μm (about 5.3% of the channel width) in lateral direction and 1.2 μm (about 5.9% of the channel height) in the vertical direction at a flow rate of 1.2 μL/min. In addition, by comparing measurement of yeast cells and particles, it was demonstrated that the device not only localizes the single cells or particles but also simultaneously characterizes their status properties such as velocity and size. The device offers a competitive electrode configuration in impedance cytometry with the advantages of simple structure, low cost, and high throughput, promising cell localization and thus electrical characterization.

Epithelial monolayers develop density and effective temperature differentials to migrate across confined matrices

Collective migration of epithelial cells across heterogeneous environments regulates vital physiological and pathological processes, such as embryogenesis, wound healing, tissue regeneration, and cancer metastasis. In response to matrix stiffness and topography, epithelial monolayers undergo jamming-unjamming transitions. Although previous studies have shown different migration phenotypes separately in confined and unconfined environment, physical transitions required for collective cell populations to negotiate confined regions remain unknown. To address this question, we fabricated PDMS substrates with contiguous wide (200 μm) and narrow (50 μm) microchannels, and studied collective migration of MCF10A (WT) epithelial cells. We found that cells develop a density differential in wide regions, relative to narrow channels, which could generate enough pressure for their eventual migration into confinement. To understand whether cell-cell cohesivity alters this requirement of density differential, we used cells with constitutively-active RhoA (+RhoA) for higher cohesivity and overexpressed ErbB2 (+ErbB2) for cancer-like reduced cohesion. We found that more cohesive cells (+RhoA) developed higher density differential relative to less cohesive cells for migration across confinement. While confined channels allowed for faster migration in WT cells, this confinement sensitivity was diminished for both alternations in cell cohesivity. With migration into confinement, cells deformed their shapes and increased their effective temperature, calculated by treating cells as a granular thermodynamic system. Further, more cohesive cells (+RhoA) and those in confined regions were effectively warmer. Overall, our results show that collective cell populations build a pressure differential between narrow and wide matrices to enter confined environments and these requirements vary for cell types of varying cohesivity. These findings show that collective cell migration across confinements can be interpreted in terms of basic physical principle of pressure and temperature of granular matter, which could inform our understanding of more complex living systems.

Pore-scale study on flow characteristics and enhanced oil recovery mechanisms of polymer microspheres in microchannels

Polymer microsphere flooding is a promising technology for enhancing oil recovery in high water cut reservoirs. Nevertheless, the microscopic flow characteristics and oil recovery mechanisms remain unclear. In this paper, the retention and flow diversion characteristics were investigated by single-microchannel and dual-microchannel chips, respectively. On this basis, the microscopic conformance control and oil recovery performance were studied by pore network model experiments. The influence of concentration and injection rate was discussed. The findings indicate that the polymer microspheres present the discontinuous phase flow characteristics of “temporary plugging-pressurizing-deformation-remigration” in microchannels. They realize flow diversion by alternately plugging wide and narrow microchannels. In porous media, they can inhibit fluid fingering and improve sweep efficiency. As the concentration increases, the sweep efficiency and the oil recovery improve, while the increase in injection rate leads to the opposite result. This work provides microscopic insights into the flow and oil recovery mechanisms of polymer microspheres.

Compartmentalized organ-on-a-chip structure for spatiotemporal control of oxygen microenvironments

Hypoxia is a condition where tissue oxygen levels fall below normal levels. In locally induced hypoxia due to blood vessel blockage, oxygen delivery becomes compromised. The site where blood flow is diminished the most forms a zero-oxygen core, and different oxygenation zones form around this core with varying oxygen concentrations. Naturally, these differing oxygen microenvironments drive cells to respond according to their oxygenation status. To study these cellular processes in laboratory settings, the cellular gas microenvironments should be controlled rapidly and precisely. In this study, we propose an organ-on-a-chip device that provides control over the oxygen environments in three separate compartments as well as the possibility of rapidly changing the corresponding oxygen concentrations. The proposed device includes a microfluidic channel structure with three separate arrays of narrow microchannels that guide gas mixtures with desired oxygen concentrations to diffuse through a thin gas-permeable membrane into cell culture areas. The proposed microfluidic channel structure is characterized using a 2D ratiometric oxygen imaging system, and the measurements confirm that the oxygen concentrations at the cell culture surface can be modulated in a few minutes. The structure is capable of creating hypoxic oxygen tension, and distinct oxygen environments can be generated simultaneously in the three compartments. By combining the microfluidic channel structure with an open-well coculture device, multicellular cultures can be established together with compartmentalized oxygen environment modulation. We demonstrate that the proposed compartmentalized organ-on-a-chip structure is suitable for cell culture.

Dense nanoarrays LDHs film evenly dividing the Cl− diffusion path into longitudinal microchannels in favor of quasi-uniform corrosion of biomedical magnesium alloys

The correlation of orientation-controlled LDHs-sheet on pitting susceptibility and stability was investigated. The LDHs-array film (ab-plane perpendicular to substrate) was constructed by pre-nucleation coupled complex-assisted hydrothermal growth, where LDHs microcrystals uniformly grown face-to-face into nano-array film in a dot-line-flake growth manner. The results demonstrate that ab-plane parallel LDHs film tends to exhibit filiform rather than pitting corrosion; ab-plane vertical LDHs film, Cl− selectively penetrating/attacking extreme intersect-gaps, emerges pitting corrosion in advance. Interestingly, the LDHs-array film appears dense but tiny corrosion pits (quasi-uniform corrosion), attributing to uniformly dividing the Cl− diffusion path into numerous narrow longitudinal microchannels by its flakes.

Sensing of Fluidic Features Using Colloidal Microswarms.

Sensing of key parameters in fluidic environments has attracted extensive attention because the physical features of body fluids could be used for point-of-care disease diagnosis. Although various sensing methods have been investigated, effective sensing strategies of microenvironments remains a major challenge. In this paper, we propose an approach that can realize sensing of fluidic viscosity and ionic strength using microswarms formed by simple colloidal nanoparticles. The influences of fluidic ionic strength and viscosity on two swarm behaviors are analyzed (i.e., the spreading of circular vortex-like swarms and the elongation of elliptical swarms). The data models for quantifying the fluidic viscosity and ionic strength are obtained from experiments, and the fluidic features can be sensed successfully using the swarm behaviors. Furthermore, we demonstrate that the microswarms have the capability of passing through tortuous and narrow microchannels for sensing. Continuous sensing of different fluidic environments using swarms is also realized. Finally, the sensing of viscosity and ionic strength of porcine whole blood is presented, which also validates the feasibility of the sensing strategy.

Aligned Magnetic Nanocomposites for Modularized and Recyclable Soft Microrobots.

Creating reconfigurable and recyclable soft microrobots that can execute multimodal locomotion has been a challenge due to the difficulties in material processing and structure engineering at a small scale. Here, we propose a facile technique to manufacture diverse soft microrobots (∼100 μm in all dimensions) by mechanically assembling modular magnetic microactuators into different three-dimensional (3D) configurations. The module is composed of a cubic micropillar supported on a square substrate, both made of elastomer matrix embedded with prealigned magnetic nanoparticle chains. By directionally bonding the sides or backs of identical modules together, we demonstrate that assemblies from only two and four modules can execute a wide range of locomotion, including gripping microscale objects, crawling and crossing solid obstacles, swimming within narrow and tortuous microchannels, and rolling along flat and inclined surfaces, upon applying proper magnetic fields. The assembled microrobots can additionally perform pick-transfer-place and cargo-release tasks at the microscale. More importantly, like the game of block-building, the microrobots can be disassembled back to separate modules and then reassembled to other configurations as demanded. The present study not only provides a versatile and economic manufacturing technique for reconfigurable and recyclable soft microrobots, enabling unlimited design space for diverse robotic locomotion from limited materials and module structures, but also extends the functionality and dexterity of existing soft robots to microscale that should facilitate practical applications at such small scale.

Acoustic streaming in Maxwell fluids generated by standing waves in two-dimensional microchannels

In this work, a semianalytic solution for the acoustic streaming phenomenon, generated by standing waves in Maxwell fluids through a two-dimensional microchannel (resonator), is derived. The mathematical model is non-dimensionalized and several dimensionless parameters that characterize the phenomenon arise: the ratio between the oscillation amplitude of the resonator and the half-wavelength ( $\eta =2A/\lambda _{a}$ ); the product of the fluid relaxation time times the angular frequency known as the Deborah number ( $De=\lambda _{1}\omega$ ); the aspect ratio between the microchannel height and the wavelength ( $\epsilon =2 H_{0}/\lambda _{a}$ ); and the ratio between half the height of the microchannel and the thickness of the viscous boundary layer ( $\alpha =H_{0}/\delta _{\nu }$ ). In the limit when $\eta \ll 1$ , we obtain the hydrodynamic behaviour of the system using a regular perturbation method. In the present work, we show that the acoustic streaming speed is proportional to $\alpha ^{2.65}De^{1.9}$ , and the acoustic pressure varies as $\alpha ^{6/5}De^{1/2}$ . Also, we have found that the growth of inner vortex is due to convective terms in the Maxwell rheological equation. Furthermore, the velocity antinodes show a high dependency on the Deborah number, highlighting the fluid's viscoelastic properties and the appearance of resonance points. Due to the limitations of perturbation methods, we will only analyse narrow microchannels.

Pool boiling heat transfer and bubble dynamics over V-shaped microchannels and micropyramids: Does high aspect ratio always benefit boiling?

Metallic microchannels have been widely applied in two-phase heat transfer devices to satisfy the high heat flux cooling requirements of compact devices, but the pool boiling heat transfer and bubble dynamics over metallic microchannels have been rarely reported. Herein, we investigate the pool boiling heat transfer and bubble dynamics over V-shaped open microchannels prepared on copper by ultrafast laser micromachining. The open microchannels are of various channel widths, 100 μm and 50 μm, and of depth-to-width ratios varying from 0.5 to 2. In addition to the conventional methods used for studying boiling, we utilized the captive bubble method and the capillary tube method to analyze the bubble dynamics and the surface wickability, respectively. We find that for the microchannels with a channel width of 100 μm, the critical heat flux (CHF) and the heat transfer coefficient (HTC) increase with the increase of depth-to-width ratio; but the opposite trend is observed for the microchannels with a channel width of 50 μm. We propose that the surface wickability affects the boiling heat transfer more significantly with the increase of heat flux. For narrow and deep V-shaped microchannels, there is a mismatch between the bubble removal rate and the liquid supplying rate and thus leading to the deteriorated boiling heat transfer. We also demonstrate that the wickability of pyramidal microstructures is much higher than V-shaped microchannels. The CHF and HTC of pyramidal microstructures increase by 5–40% and 35–157%, respectively, compared to the V-shaped microchannels with identical structural spacing and depth.

Fluid compressional properties sensing at microscale using a longitudinal bulk acoustic wave transducer operated in a pulse-echo scheme

Acoustic devices have been widely used as smart chemical and biochemical sensors since they are sensitive to mechanical, chemical, optical or electrical perturbations on their surfaces; making them a reliable option for noninvasive detection of changes in physical properties of liquid samples for real-time applications. Here we present a longitudinal acoustic wave device for study of compressional properties of liquids in microfluidic systems, with the particularity of pulse-echo mode of operation. We have studied at a microscale the interaction between longitudinal acoustic waves and the compressional properties of liquid samples, interrogating the fluids with short pulses of ultrasound at GHz, finding a direct relationship between the magnitude of the bulk modulus or the specific acoustic impedance of liquids and the amplitude of the output voltage produced by acoustic echoes received by the aluminum nitride transducer. Analytical expressions and FEM simulations support the detection mechanism, while applications such as classification of liquids and detection of concentration change in solutions experimentally demonstrate the method. This contribution overcomes current restrictions of film acoustic resonators such as fragility of operation in liquid environments, high manufacturing cost or limitations regarding narrow microchannels; offering an alternative to applications that demand ultra-low consumption, miniaturization, versatility (it offers multi-frequency operation in 1 – 10 GHz range) and ease of readout (peak voltage).

Numerical investigation of compressibility effects on friction factor in rectangular microchannels

This study numerically investigated the effect of gas compressibility on the friction factor in rectangular microchannels. The numerical model adopted in the study was validated against experimental data obtained by testing rectangular microchannels with a hydraulic diameter of 295 μm. The numerical model was used to evaluate the Reynolds number at which the compressibility effects on the friction factor became significant by analyzing the role of both hydraulic diameter and aspect ratio of the microchannel. To this end, three values of the hydraulic diameter (100, 295, and 500 μm) and five different aspect ratios (from 0.25 to 1) were investigated. The results showed that compressibility effects became increasingly stronger by reducing the hydraulic diameter and that they led to an increase in the average friction factor. For smaller microchannels, the Reynolds numbers at which the compressibility effects became significant tended to reduce and were in the laminar regime. The gas compressibility could not be ignored when friction factors needed to be accurately determined. Moreover, for narrow microchannels (low aspect ratio), compressibility effects became important for higher values of the Reynolds number than those observed for nearly squared microchannels.

Magnetically propelled soft microrobot navigating through constricted microchannels

Recent strides in microfabrication technologies offer important possibilities for developing microscale robotic systems with enhanced power, functionality and versatility. Previous microrobots fabricated by lithographic techniques usually lack the ability to adaptively deform in confined and constricted spaces and navigate through, therefore hindering their applications in complex biological environments. Here, a microfluidic strategy is combined with a dip-coating process for continuous fabrication of soft helical structures with controllable mechanical property as magnetically propelled microrobots, capable of actively propelling through narrow and sinuous microchannels. Because of their self-adaptive deformation capability, the magnetically propelled soft microrobots can actively navigate through a narrow opening, 2.21 times smaller than the sectional area of the microrobot, and a U-shape-bent capillary, directed by a programmed magnetic field. Additionally, the soft microrobot demonstrates increased swimming speed in a fluid of high viscosity, because of the adaptive tightening deformation of the helix when swimming. This new magnetically propelled soft microrobot and its attractive performance will open up new possibilities for biomedical operation at the micro and nanoscale.

Critical heat flux characteristics of R1234yf, R1234ze(E) and R134a during saturated flow boiling in narrow high aspect ratio microchannels

This experimental study investigated narrow and high aspect ratio multi-microchannels that were micro-machined in copper with thin separating walls during saturated flow boiling of refrigerants. The hypothesis was that these channels could increase the footprint critical heat flux and support the future development of thermal management systems for power electronics and other electronic packages. The measured footprint critical heat flux was as high as 678.5 W/cm2, which is twice as high as other investigations in the literature concerning saturated flow boiling of refrigerants. Two test samples were fabricated with a footprint area of (10 × 10) mm. The first had 25 rectangular channels (198 μm wide, 1167 μm high). The second had 17 rectangular channels (293 μm wide, 1176 μm high). The experimental investigation covered low-GWP replacement refrigerants (R1234yf, R1234ze(E)) as well as the well-examined R134a serving as a benchmark. A large data bank was obtained with 432 data points covering a wide range of typical inlet subcooling (1.3–14.7) K and mass fluxes (333–1260) kg/m2s as well as two nominal saturation temperatures (30 and 40) °C.The effect of inlet subcooling was found to be consistently significant at the higher mass fluxes. This was contradictory to other investigations that found moderate or insignificant effects. The result is suggested to be attributed to the two-phase stability induced by the upstream throttle valve, inlet orifices, isolated refrigerant in the inlet and outlet plenums as well as the short heated length causing higher importance of orifice to channel pressure drop importance. Finally, a new modified Katto and Ohno correlation including the effect of subcooling was proposed, achieving a 4.0% mean average error and predicting 93.3% of the data points within 10% error.

Robust and easy-to-use microchip electrophoresis within sub-millimeter channels for fast and highly efficient separation

Microchip capillary electrophoresis (MCE) is a powerful technique for rapid separation; however, its acceptance in routine laboratories is still limited. Compromises caused by the efforts for solving different problems, such as reducing its cost of fabrication and ensuring high separation efficiency, undermine the competitiveness of this technology compared to other separation techniques. Contrary to the conventional pursuit of narrow microchannels, this study investigated the suitability of microchips with channels at the sub-millimeter level, targeting the simplification of the overall operation, cost reduction, and robustness improvement. To this effect, we considered the influence of pressurized flow and Joule heating on the separation. The suppression of pressurized flow with viscous solutions was confirmed through a combination of simulations and experimental results, indicating that the buffer viscosity was enough for successful separation. We fabricated channels of 200 μm × 230 μm using computer numerical controlled (CNC) machining and obtained theoretical plate numbers of 4.8 × 105 m−1 and 5.3 × 105 m−1 for fluorescein isothiocyanate (FITC) labeled small molecules and DNA fragments, respectively, with a buffer viscosity of 168 mPa s (0.5 % hydroxypropyl methylcellulose, HPMC). These values are comparable with that of narrow-bore microchips. Furthermore, we did not observe any deleterious effects with low-conductivity buffers. We investigated the rapid and highly sensitive detection of mycoplasma contamination and the real samples of circulating cell-free DNA (cfDNA), which gave a limit of detection (LOD) as low as 2.3 ng mL−1. Owing to the significant reduction in cost, ease of operation, and fast separation capabilities demonstrated in this work, MCE can be a viable alternative to the usual slab gel electrophoresis running in most biological laboratories.

Application of a Simple Microfluidic Chip Analysis Technology to Evaluate the Inhibitory Role of Protocatechuic Acid on Shear-Induced Platelet Aggregation.

This study aimed to develop a simple microfluidic chip analysis technology to study the inhibitory effect of protocatechuic acid on shear-induced platelet aggregation. The microfluidic chip designed in this study simulates 80% fixed narrow microchannels. This microchannel narrow model uses the finite element analysis module of the three-dimensional modeling software solidwork to analyze fluid dynamic behavior. Blood treated with protocatechuic acid at 1, 2, 4, 8, or 16 µg/mL was passed through the microchannel stenosis model at a shear rate of 10,000 s−1. The platelet adhesion and aggregation behaviors were then measured using fluorescence microscopy and observed in real time. Simultaneously, the antiplatelet aggregation effect of protocatechuic acid was analyzed using thromboelastography and photoelectric turbidimetry. The designed stenosis model of the microfluidic chip can produce a gradient of fluid shear rate, and the gradient of fluid shear rate can induce platelet aggregation. Under this model, the degree of platelet adhesion and aggregation increased as the shear rate increased. In the experimental concentration range of 0–8 µmol/mL, protocatechuic acid exerted a concentration-dependent inhibition of platelet aggregation. In contrast, thromboelastography and photoelectric turbidimetry failed to demonstrate an inhibitory effect. The microfluidic chip analysis technology developed in this study can be used to study the effect of protocatechin in inhibiting platelet aggregation induced by shear rate in vitro. This technology is simple to operate and can be used as a new type of antiplatelet aggregation analysis technology for screening studies of novel potential antiplatelet aggregation drugs.

Soft strain gauge tactile sensor array using liquid metal

In recent years, soft tactile sensors comprising liquid metal and silicone rubber have been studied for tactile sensing of soft robots. In order to obtain detailed information about objects, the sensor array was configured by utilizing crossing of orthogonal microchannels filled with liquid metal as sensing elements in previous research. In this study, we propose the soft tactile sensor array with independent sensing elements. This sensor embeds narrow microchannels standing vertically in silicone rubber. Microchannels are filled with liquid metal and used as sensing elements. This structure realizes miniaturization and high-density placement of sensing elements while maintaining the independence of the sensing elements. We fabricated the sensor with 16 sensor elements of 0.4mm width embedded at 2.0mm intervals.

Formation and Deformation of Liquid Drops in Microchannels

The laws of drop formation are experimentally studied in narrow horizontal microchannels with rectangular section with heights from 50 to 150 μm. It is shown that there exists a new flow regime when the drops in the form of vertical liquid bridges move along the microchannel. Three mechanisms of formation of such drops are distinguished: formation directly near the liquid nozzle, departure of drops from liquid moving at the lateral sides of channel, and drop formation due to destruction of severely deformed drops and horizontal liquid bridges. It is established that the deformation of drops increases with increasing Weber number. It is demonstrated that the drops begin to deform when the first critical value of the Weber number is achieved and begin to be destroyed when the second one is achieved.

Underwater Bubble and Oil Repellency of Biomimetic Pincushion and Plastron-Like Honeycomb Films.

Underwater highly bubble and oil-repellent surfaces were prepared based on honeycomb- and pincushion-structured films prepared by breath figure technique and post modifications including UV-ozone treatment and peeling the top layer. Furthermore, bubble generation from the plastron-like honeycomb gas chamber by attaching oil droplet onto the surface of honeycomb films was first observed. Both controlling gas bubbles and oil droplets underwater are important issues in the field of microfluidics since they are useful and may solve the maleficence to liquid transportation in narrow microchannels.