Channel Surface Properties Research Articles

Determination Of Two-Phase Flow Regimes Of A Condensing Water-Air Mixture In An Array Of Micro Channels

Proton exchange membrane (PEM) full cells (FCs) are a promising alternative to combustion engines for mobile applications. Their signature low operating temperature leads to the possible existence of product water in its liquid form. Liquid water formation and transport are important phenomena to understand and model in order to facilitate the development of new, more efficient PEMFCs. This contribution investigates the two-phase flow phenomena separately by introducing a gaseous air-vapor mixture into an optically accessible fuel cell flow plate with 14 parallel micro channels (confinement number Co = 7.3) and a carbon paper inlay representing the FC's gas diffusion layer and bringing it to the point of onset of condensation. Special care is taken to provide a controlled and reproducible air-vapor mixture with low pressure pulsation. One key feature of the experiment is that the flow plate used is designed to be as similar to a regular research fuel cell flow plate as possible, retaining its channel surface properties and electrical parameters. It can also be heated to a given temperature to mimic the fuel cell operating conditions. Therefore, the same plate can also be used in in-situ experiments in an operated fuel cell. An air mass flow rate of 500 - 1000 mLn / min with relative humidities of 75% - 95% at 65°C and 75°C was investigated. The emerging flow patterns were captured using a high-speed camera at 250 Hz and categorized using canonical two-phase flow regime nomenclature. It is shown that the present setup can produce the relevant two-phase flow patterns that can be found in operated fuel cells as reported by previous studies and is furthermore equipped to investigate flow states not easily reached with an operated fuel cell (e.g. flooded channels).

Bubble generation and growth mechanism in PMMA microfluidic chip

Bubbles often impede microfluidic chip functionality, especially in areas where heating is required. In this article, the mechanism of bubble generation and growth in polymethyl methacrylate microchannels was analyzed. The sources of bubbles were taken into consideration: (i) dissolved gas in the liquid, (ii) insoluble gas in cavities of the channel, and (iii) the gas produced by the phase change of the liquid. The factors that affect bubbles in microchannels were analyzed, including the liquid flow rate, surface modification of channels, and pressure on the fluid in the microchannel. Three sets of experiments were designed and carried out, and the results demonstrated that the bubbles in the microchannel can be shrunk and even eliminated by improving the flow rate, modifying channel surface properties, and increasing the liquid pressure.

In situ monitoring of protein transfer into nanoscale channels

Protein transfer into nanoscale compartments is critical for many cellular/life processes, yet there are few reports on how compartment properties impact the protein orientation during a transfer. Such a knowledge gap limits a deeper understanding of the protein transfer mechanism, which could be bridged using nanoporous materials. Here, we use a mesoporous silica, a covalent organic framework, and a metal-organic framework with charged, hydrophobic, and neutral surfaces, respectively, to elucidate the impact of channel properties on the transfer of a model protein, lysozyme. Using site-directed spin labeling and time-resolved electron paramagnetic resonance spectroscopy, we reveal that the transfer can be a multi-step process depending on channel properties and depict the relative orientation changes of lysozyme upon transfer into each channel. To the best of our knowledge, this is the first structural insight into protein orientation upon transfer into different compartments, meaningful for the rational design of synthetic materials to host enzymes or mimic the cellular compartments. Protein transfer into nanoscale channels differing in hydrophobicity is demonstrated Multi-step protein transfer is observed for charged and hydrophobic channels Protein changes its orientation depending on the channel surface properties Time-resolved EPR spectroscopy can reveal residue-level details of protein transfer Pan et al. report the orientation changes of a model protein upon transfer into three nanoscale channels that differ in surface hydrophobicity (but a similar diameter). The findings may be useful for guiding the rational design of synthetic materials to host enzymes and/or mimic the cellular compartments.

Proton Activity in Nanochannels Revealed by Electron Paramagnetic Resonance of Ionizable Nitroxides: A Test of the Poisson–Boltzmann Double Layer Theory

Chemical and physical processes occurring within the nanochannels of mesoporous materials are known to be determined by both the chemical nature of the solution inside the pores/channels as well as the channel surface properties, including surface electrostatic potential. Such properties are important for numerous practical applications such as heterogeneous catalysis and chemical adsorption including chromatography. However, for solute molecules diffusing inside the pores, the surface potential is expected to be effectively screened by counter ions for the distances exceeding the Debye length. Here, we employed electron paramagnetic resonance spectroscopy of ionizable nitroxide spin probes to experimentally examine the conditions for the efficient electrostatic surface potential screening inside the nanochannels of chemically similar silica-based mesoporous molecular sieves (MMS) filled with water at ambient conditions and a moderate ionic strength of 0.1 M. Three silica MMS having average channel diamet...

Determination of dynamic contact angles within microfluidic devices

Here, we report a single-point detection method for the determination of dynamic surface conditions inside microfluidic channels. The proposed method is based on monitoring fluorescence amplitude as a function of the convolution of a laser beam with segmented flow consisting of two immiscible liquids, one containing fluorescent dye. The fluorescence amplitude is determined by the flow rate and the droplet shape, which is affected by the channel surface properties. We modeled the interaction of a droplet and a laser beam via computer-aided design software, using the laser beam location in relation to the droplet shape as a parameter. The method was applied to fused silica capillaries with both unmodified and modified surfaces, with segmented flow exhibiting water contact angles of ≈ 30° and ≈ 100°, respectively. The method allows discrimination between hydrophillic and hydrophobic surfaces, as well as the quality of the treatment. The results were verified using fluorescence imaging of the droplets via a stroboscopic technique. We also applied this method to the analysis of microfabricated channels with non-circular cross sections. We demonstrated that the technique enables the determination of the hydrophobicity of channel surfaces, a crucial property required for the generation of segmented flow or emulsions for applications such as digital PCR.

Modular microfluidics for double emulsion formation.

For many engineering applications such as manipulating two phase flows, generating single and double emulsions, and passively propelling liquids through channels, control over the surface energy of microfluidic channels is essential. In particular, double emulsion formation, which benefits from alternating hydrophobic and hydrophilic sections of channel, represents a challenge in fabricating controlled microfluidic channel surface properties. As double emulsions find further applications in single-cell handling and analysis, straightforward methods for generating them increase in value. Here, we present a method for generating double emulsions in microfluidic channels fabricated from modular fluidic blocks. By using a vapor-phase polymer coating technology-initiated chemical vapor deposition-we are able to fabricate blocks with varying surface properties. Assembling these blocks together then creates step-like changes in surface energy within a microchannel.

A numerical investigation on the effects of water inlet location and channel surface properties on water transport in PEMFC cathode channels

Water motions in simplified, straight cathode channels are investigated numerically to study the dynamics of water in a proton exchange membrane (PEM) fuel cell. The computational domain consists of a channel with four walls, with the bottom wall representing the surface of gas diffusion layer. The impact of channel orientation (a horizontal channel with water inlet pore on the bottom wall, side wall and top wall respectively, and a vertical channel with water inlet pore on the bottom wall) on gas transport and water removal is discussed, taking into account the wettability of the bottom wall and air and water inlet velocity. Our numerical results show that water drains out more quickly for a channel with a hydrophobic bottom wall than for a channel with a hydrophilic bottom wall. The former case yields a greater pressure loss mainly because water appears in the form of droplets, versus as film in the latter case. The cases with the inlet pore located on the top wall and the vertical channel have the least water coverage on the bottom wall and relatively little volumetric fraction of water inside the channel. However, the pressure drop is slightly higher for the cases with the inlet pore located on the top wall and the vertical channel than that in other cases. Gravity has some effect on water motions in the channels, while with the increase of air and water inlet velocity, gravity plays a slight part in water motions. Our observations are in good agreement with other experimental and computational works in the literature.

The physical origins of transit time measurements for rapid, single cell mechanotyping.

The mechanical phenotype or 'mechanotype' of cells is emerging as a potential biomarker for cell types ranging from pluripotent stem cells to cancer cells. Using a microfluidic device, cell mechanotype can be rapidly analyzed by measuring the time required for cells to deform as they flow through constricted channels. While cells typically exhibit deformation timescales, or transit times, on the order of milliseconds to tens of seconds, transit times can span several orders of magnitude and vary from day to day within a population of single cells; this makes it challenging to characterize different cell samples based on transit time data. Here we investigate how variability in transit time measurements depends on both experimental factors and heterogeneity in physical properties across a population of single cells. We find that simultaneous transit events that occur across neighboring constrictions can alter transit time, but only significantly when more than 65% of channels in the parallel array are occluded. Variability in transit time measurements is also affected by the age of the device following plasma treatment, which could be attributed to changes in channel surface properties. We additionally investigate the role of variability in cell physical properties. Transit time depends on cell size; by binning transit time data for cells of similar diameters, we reduce measurement variability by 20%. To gain further insight into the effects of cell-to-cell differences in physical properties, we fabricate a panel of gel particles and oil droplets with tunable mechanical properties. We demonstrate that particles with homogeneous composition exhibit a marked reduction in transit time variability, suggesting that the width of transit time distributions reflects the degree of heterogeneity in subcellular structure and mechanical properties within a cell population. Our results also provide fundamental insight into the physical underpinnings of transit measurements: transit time depends strongly on particle elastic modulus, and weakly on viscosity and surface tension. Based on our findings, we present a comprehensive methodology for designing, analyzing, and reducing variability in transit time measurements; this should facilitate broader implementation of transit experiments for rapid mechanical phenotyping in basic research and clinical settings.

Two-phase flow dynamics in a micro channel with heterogeneous surfaces

In this paper, two-phase flows in a micro channel with heterogeneous surfaces are investigated both experimentally and numerically. The heterogeneity in a micro rectangular channel is characterized by three hydrophilic channel walls plus one hydrophobic one; and the hydrophobic surface is either smooth or rough (roughness: 226–550nmrms for the smooth surface and 21.5–28.9μmrms for the rough). Results on an entirely hydrophilic smooth channel (roughness ∼1.3nmrms) are also presented for comparison. Two-phase flows are visualized through two different angles: (1) a top view; and (2) a cross-sectional view. It is observed that the surface wetting property and roughness affect the presence site of liquid water flow: liquid water is preferentially present in the two hydrophilic corners when one channel wall is hydrophobic. As a result, two-phase pressure, patterns, and stability differ from the purely hydrophilic channel. Rough surface is found to affect two-phase flow. Model predictions, obtained from both empirical formula based on the Lockhart–Martinelli parameter and two-fluid approach, along with model parameters optimization using experimental data. Real-time pressures are presented to show the effects of channel surface properties on pressure drop.

Hall-Effect Thruster Channel Surface Properties Investigation

Surface properties of Hall-effect thruster channel walls play an important role in the performance and lifetime of the device. Physical models of near-wall effects are beginning to be incorporated into thruster simulations, and these models must account for evolution of channel surface properties due to thruster operation. Results from this study show differences in boron nitride channel surface properties from beginning-of-life and after 100’s of hours of operation. Two worn thruster channels of different boron nitride grades are compared with their corresponding pristine and shadow-shielded samples. Pristine HP grade boron nitride surface roughness is 9000±700 A, while the worn sample is 110,900±8900 A at the exit plane. Pristine M26 grade boron nitride surface roughness is 18400±1400 A, while the worn sample is 52300±4200 A at the exit plane. Comparison of pristine and worn channel surfaces also show surface properties are dependent on axial position within the channel. For example, surface roughness increases by as much as a factor of 5.4 and surface atom fraction of carbon and metallic atoms decreases by a factor of 2.9 from anode to exit plane. Macroscopic striations at the exit plane angled 10o to 30 o from axial are found to be related to the electron gyroradius and give rise to anisotropic surface roughness. Smoothing of ceramic grains at the microscopic level is also evident.

A Unified Approach for Flow Analysis of Magnetorheological Fluids

This study presents a new approach for flow analysis of magnetorheological (MR) fluids through channels with various surface topologies. Based on an experimental study an analytical method is developed to predict the pressure loss of a MR fluid as a function of the applied magnetic field strength, volumetric flow rate, and surface topology, without utilizing the concept of shear yield stress. A channel flow rheometer with interchangeable channel walls is built to demonstrate that the pressure loss across the MR fluid flow channel is significantly affected by the channel surface properties. Based on the experimental study it is concluded that a unique shear yield stress cannot be defined for a given MR fluid, since its pressure drop depends on the surface topology of the device. Therefore, a relation for nondimensional friction factor associated with MR fluid channel flow is developed in terms of a modified Mason number and dimensionless surface topology parameters. Using the nondimensional model, the pressure loss for various magnetic fields and volumetric flow rates can be represented by a single master curve for a given channel surface topology without the assumption of a constitutive model for MR fluids.

Graft linker immobilization for spatial control of protein immobilization inside fused microchips

Fused silica glass microchips have several attractive features for lab-on-a-chip applications; they can be machined with excellent precision down to nanospace; are stable; transparent and can be modified with a range of silanization agents to change channel surface properties. For immobilization, however, ligands must be added after bonding, since the harsh bonding conditions using heat or hydrofluoric acid would remove all prior immobilized ligands. For spatial control over immobilization, UV-mediated immobilization offers several advantages; spots can be created in parallel, the feature size can be made small, and spatial control over patterns and positions is excellent. However, UV sensitive groups are often based on hydrophobic chemical moieties, which unfortunately result in greater non-specific binding of biomolecules, especially proteins. Here, we present techniques in which any -CH(x) (x=1,2,3) containing surface coating can be used as foundation for grafting a hydrophilic linker with a chemical anchor, a carboxyl group, to which proteins and amine containing molecules can be covalently coupled. Hence, the attractive features of many well-known protein and biomolecule repelling polymer coatings can be utilized while achieving site-specific immobilization only to pre-determined areas within the bonded microchips.

Applications of Microfluidic Devices in Food Engineering

The design of novel food micro-structures aimed at the quality, health and pleasure markets will probably require unit operations where the scale of the forming device is closer to the size of the structural elements (i.e., 1–100 μm). One emerging possibility is microfluidics or devices that employ small amounts of fluids (10−6 to 10−9 l) flowing in channels where at least one dimension is less than 1 mm. However, under these conditions, the predominant effects are not necessarily those present in conventional macroscopic unit operations. Dominant physical effects at the microfluidic scale are introduced through the use of dimensionless numbers. Different types of geometries to generate multi-phase flows in micro-channels, techniques and materials to construct the micro-devices, principally soft lithography and laser ablation, as well as methods used to modify surface properties of channels, are reviewed. The operation of micro-devices, the role of flow regimes, rheological behaviour of fluids in micro-channels and of transient time is discussed. Finally, systems developed to generate emulsions and foams, fluid mixing and dispersion, and future applications of these devices in food processing and food analysis are presented.

Fabrication of microchip electrophoresis devices and effects of channel surface properties on separation efficiency

This paper describes the influence of chip material on the separation efficiency in microchip electrophoresis using quartz, glass, polydimethylsiloxane (PDMS), polymethylmethacrylate (PMMA), and glass hybrid as the microchip material. The fabrication processes for each microchip are described in detail. We introduce a new fabrication method for a glass chip using amorphous silicon on the glass surface as an etch mask and a bonding interface for anodic bonding. The microchannel structures in each microchip were compared using a scanning electron microscope or profilometer. We investigated the mobilities and bandwidths of electroosmotic flow (EOF) and analytes in microchip electrophoresis. Significant band broadening was observed in a microchip composed of a hybrid material (glass and silicon dioxide membrane) compared with a microchip of single material due to the nonuniformity of the surface charge density.

Biocompatible channels for field-flow fractionation of biological samples: correlation between surface composition and operating performance

Biocompatible methods capable of rapid purification and fractionation of analytes from complex natural matrices are increasingly in demand, particularly at the forefront of biotechnological applications. Field-flow fractionation is a separation technique suitable for nano-sized and micro-sized analytes among which bioanalytes are an important family. The objective of this preliminary study is to start a more general approach to field-flow fractionation for bio-samples by investigation of the correlation between channel surface composition and biosample adhesion. For the first time we report on the use of X-ray photoelectron spectroscopy (XPS) to study the surface properties of channels of known performance. By XPS, a polar hydrophobic environment was found on PVC material commonly used as accumulation wall in gravitational field-flow fractionation (GrFFF), which explains the low recovery obtained when GrFFF was used to fractionate a biological sample such as Staphylococcus aureus. An increase in separation performance was obtained first by conditioning the accumulation wall with bovine serum albumin and then by using the ion-beam sputtering technique to cover the GrFFF channel surface with a controlled inert film. XPS analysis was also employed to determine the composition of membranes used in hollow-fiber flow field-flow fractionation (HF FlFFF). The results obtained revealed homogeneous composition along the HF FlFFF channel both before and after its use for fractionation of an intact protein such as ferritin.