Microchannel Surface Research Articles

Separation of particles using the focused acoustic sorting chip based on the wettability treatment

Combining the focusing energy characteristics of the focused interdigital transducer with the self-cleaning function of the wettability surface, a focused acoustic sorting chip based on the surface wettability treatment is proposed in this work. In the laboratory, two kinds of focused acoustic sorting chips based on the hydrophilic and hydrophobic wall characteristics were designed and fabricated separately. The corresponding separation experiments were carried out using polystyrene microparticles with diameters of 1 and 10 µm. Moreover, the particle adhesion characteristics based on different surface wettability and the effects of average velocity (Va) and input power (P) on particle deflection were investigated. The relevant optimum separation conditions were confirmed, namely under the hydrophilic treatment of the micro-channel surface, when the work frequency (f) was 131.83 MHz, Va = 4 mm/s, and P = 320 mW. Under these conditions, the optimal separation efficiency can reach 99.17%. The proposed chip has the advantages of simple structure, high separation accuracy, self-cleaning, and focusing energy.

Functionalized microchannels as xylem-mimicking environment: Quantifying X. fastidiosa cell adhesion

Microchannels can be used to simulate xylem vessels and investigate phytopathogen colonization under controlled conditions. In this work, we explore surface functionalization strategies for polydimethylsiloxane and glass microchannels to study microenvironment colonization by Xylella fastidiosa subsp. pauca cells. We closely monitored cell initial adhesion, growth, and motility inside microfluidic channels as a function of chemical environments that mimic those found in xylem vessels. Carboxymethylcellulose (CMC), a synthetic cellulose, and an adhesin that is overexpressed during early stages of X. fastidiosa biofilm formation, XadA1 protein, were immobilized on the device’s internal surfaces. This latter protocol increased bacterial density as compared with CMC. We quantitatively evaluated the different X. fastidiosa attachment affinities to each type of microchannel surface using a mathematical model and experimental observations acquired under constant flow of culture medium. We thus estimate that bacterial cells present ∼4 and 82% better adhesion rates in CMC- and XadA1-functionalized channels, respectively. Furthermore, variable flow experiments show that bacterial adhesion forces against shear stresses approximately doubled in value for the XadA1-functionalized microchannel as compared with the polydimethylsiloxane and glass pristine channels. These results show the viability of functionalized microchannels to mimic xylem vessels and corroborate the important role of chemical environments, and particularly XadA1 adhesin, for early stages of X. fastidiosa biofilm formation, as well as adhesivity modulation along the pathogen life cycle.

Application of the Trefftz Method for Pool Boiling Heat Transfer on Open Microchannel Surfaces

In the paper, the results of tests on pool boiling heat transfer on open microchannel surfaces were discussed. Nucleate pool boiling from copper microchannel surfaces was examined using ethanol as a working fluid. Parallel microchannels fabricated by machining were about 0.2 to 0.4 mm wide and 0.2 to 0.5 mm deep. In the mathematical model the heat transfer process was in steady state. The heat transfer in copper block was assumed axisymmetric while the specimen temperature distribution was not. The temperature of the copper block and the specimen were assumed to satisfy Laplace’s equations with adequate boundary conditions. The problem was solved by the Trefftz method with two sets of Trefftz functions. Graphs were used to represent the copper block and the specimen temperature distributions, local values of heat transfer coefficients as a function of the heat flux and fin temperature distribution. Uncertainty analysis was presented.

Effect of outlet plenum design on flow boiling heat transfer in microchannel heat sinks

Instabilities during flow boiling in microchannel lead to fluctuations in temperature, pressure, mass flux. It is well known that the instabilities can be suppressed by effective extraction of the accumulated vapor away from the outlet plenum. Here we investigate the effect of outlet plenum volume on instabilities during flow boiling in a heat sink comprising of fourteen parallel microchannels within a footprint area of 1 × 1 cm2. Plain and nanostructured microchannel surfaces are investigated with two outlet plenum designs including the one with same volume (baseline case) as the inlet plenum and another one with eight times the volume of the inlet plenum. An eight-fold increase in the outlet plenum volume facilitates a significant reduction in amplitude of fluctuations of surface temperature from 6.6 to 2.8 °C at heat flux of 137 W/cm2 and mass flux of 500 kg/m2s. Efficient vapor removal due to the increase in the manifold volume also lowers the pressure drop, thereby allowing maximum reduction in surface temperature of 4.8 °C and 3 °C for mass flux of 250 and 500 kg/m2s, respectively, compared to that of the baseline case. The enhancement in heat transfer by implementing nanostructured surface with larger outlet plenum volume is marginal with a temperature reduction of about 0.5 °C in comparison to that with plain microchannel with similar outlet plenum volume. Overall, it is shown that the performance improvement obtained with simple outlet plenum design modifications can be comparable or better in comparison to the enhancement due to complex surface modification strategies.

Hydrogel-Based Iontronics on a Polydimethylsiloxane Microchip.

In response to the extensive utilization of ionic circuits, including in iontronics and wearable devices, a new method for fabricating a hydrogel-based ionic circuit on a polydimethylsiloxane (PDMS) microchip is reported. Prolonged UV/ozone oxidation combined with proper surface functionalizations and a novel microchip bonding method using thiol-epoxy click reaction enable the robust attachment of the photopolymerized hydrogel to the microchannel surface for eventual operation in electrolytes as an ionic circuit. The stretchable ionic diode constructed on the PDMS microchip shows a superior rectification ratio even under tensile stress and long-term storage stability. Furthermore, the combination of the ionic circuit and unique material properties of PDMS allows us to maximize the versatility and diversify the functionalities of the iontronic device, as demonstrated in a pressure-driven ionic switch and chip-integrated ionic regulator. Its iontronic signal transmission mimicking the excitatory and inhibitory synapses also evinces the potential of the hydrogel-based iontronics on the PDMS microchip as developed toward an aqueous neuromimetic information processor while opening up new opportunities for various bioinspired applications.

Multiplex MicroRNA Detection on a Surface-Functionalized Power-Free Microfluidic Chip.

Circulating microRNAs (miRNAs) have emerged as promising cancer biomarkers because their concentration profiles in body fluids are associated with the type and clinical stage of cancer. For multiplex miRNA detection, a novel surface-functionalized power-free microfluidic chip (SF-PF microchip) has been developed. The inner surface of the SF-PF microchip microchannels was functionalized via electron beam-induced graft polymerization and immobilization of capture probe DNAs. Simultaneous and specific duplex miRNA detection was achieved on the line-type SF-PF microchip with detection limits of 19.1 and 47.6 nmol L-1 for hsa-miR-16 and hsa-miR-500a-3p, respectively. Moreover, simultaneous and specific triplex miRNA detection was achieved on the stripe-type SF-PF microchip. The sample volume required for this microchip was 0.5 μL, and the time required for detection was 17 min. These results indicate that up to six types of miRNAs could be detected without compromising the advantages of the previous SF-PF microchips for cancer point-of-care testing.

Formation of bionic surface textures composed by micro-channels using nanosecond laser on Si3N4-based ceramics

Bionic surface textures exhibit excellent properties in improving the friction and wear resistance. To obtain the micro-channel bionic textures with high-quality on Si3N4/TiC ceramic surface, the effect of laser process parameters by using a nanosecond laser with 1064 nm wavelength on micro-channel surface quality and geometry is studied. The surface morphology, depth and width of the micro-channels are examined by scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM). The surface compositions of heat-affected are detected by energy dispersive spectrometer (EDS). Results show that the surface morphology and geometry of the micro-channels are seriously influenced by the laser process parameters. Higher laser power, lower scanning speed, higher frequency and overscan are prone to produce larger channel depth and width due to more accumulated energies. The optimal laser process parameters of the micro-channels are obtained, correspondingly the micro-channel depth and width are about 40–45 μm and 55–60 μm, respectively; meanwhile, it is found that the oxidation occurs in the heat-affected zone. Finally, five kinds of bionic surface textures orderly composed by micro-channels (square, circle, diamond, hexagon and sector) are fabricated on Si3N4/TiC ceramic surface based on the optimal laser process parameters.

An ultrafast enzyme-free acoustic technique for detaching adhered cells in microchannels.

Adherent cultured cells are widely used biological tools for a variety of biochemical and biotechnology applications, including drug screening and gene expression analysis. One critical step in culturing adherent cells is the dissociation of cell monolayers into single-cell suspensions. Different enzymatic and non-enzymatic methods have been proposed for this purpose. Trypsinization, the most common enzymatic method for dislodging adhered cells, can be detrimental to cells, as it can damage cell membranes and ultimately cause cell death. Additionally, all available techniques require a prolonged treatment duration, typically on the order of minutes (5–10 min). Dissociation of cells becomes even more challenging in microfluidic devices, where, due to the nature of low Reynolds number flow and reduced mixing efficiency, multiple washing steps and prolonged trypsinization may be necessary to treat all cells. Here, we report a novel acoustofluidic method for the detachment of cells adhered onto a microchannel surface without exposing the cells to any enzymatic or non-enzymatic chemicals. This method enables a rapid (i.e., on the order of seconds), cost-effective, and easy-to-operate cell detachment strategy, yielding a detachment efficiency of ∼99% and cellular viability similar to that of the conventional trypsinization method. Also, as opposed to biochemical-based techniques (e.g., enzymatic), in our approach, cells are exposed to the dissociating agent (i.e., substrate-mediated acoustic excitation and microstreaming flow) only for as long as they remain attached to the substrate. After dissociation, the effect of acoustic excitation is reduced to microstreaming flow, therefore, minimizing unwanted effects of the dissociating agent on the cell phenotype. Additionally, our results suggest that cell excitation at acoustic powers lower than that required for complete cell detachment can potentially be employed for probing the adhesion strength of cell–substrate attachment. This novel approach can, therefore, be used for a wide range of lab-on-a-chip applications.

Numerical Study of Nanoparticle Deposition in a Gaseous Microchannel under the Influence of Various Forces

Nanoparticle deposition in microchannel devices inducing contaminant clogging is a serious barrier to the application of micro-electro-mechanical systems (MEMS). For micro-scale gas flow fields with a high Knudsen number (Kn) in the microchannel, gas rarefaction and velocity slip cannot be ignored. Furthermore, the mechanism of nanoparticle transport and deposition in the microchannel is extremely complex. In this study, the compressible gas model and a second-order slip boundary condition have been applied to the Burnett equations to solve the flow field issue in a microchannel. Drag, Brownian, and thermophoretic forces are concerned in the motion equations of particles. A series of numerical simulations for various particle sizes, flow rates, and temperature gradients have been performed. Some important features such as reasons, efficiencies, and locations of particle deposition have been explored. The results indicate that the particle deposition efficiency varies more or less under the actions of forces such as Brownian force, thermophoretic force, and drag force. Nevertheless, different forces lead to different particle motions and deposition processes. Brownian or thermophoretic force causes particles to move closer to the wall or further away from it. The drag force influence of slip boundary conditions and gas rarefaction changes the particles’ residential time in the channel. In order to find a way to decrease particle deposition on the microchannel surface, the deposition locations of different sizes of particles have been analyzed in detail under the action of thermophoretic force.

Effect of a protective coating on the surface integrity of a microchannel produced by microultrasonic machining

In micro-ultrasonic machining (μUSM), slight lateral vibration at the end of the tool inevitably occurs due to tool manufacturing installation errors and mechanical system vibrations. These vibrations cause overcutting and edge breakage, creating undesirable effects on heat transfer and flow deflection. Hence, this paper presented a new fabrication method that ensures high surface integrity by applying a protective coating on the substrate. To study the material removal modes during fabrication and the causes of overcutting and edge damage, a mathematical model of the microchannel surface integrity was established. The experimental parameters—feed rate, ultrasonic power, slurry viscosity, and protective coating—had a large impact on the surface integrity. The optimal parameters were a PE-Wax coating of 400 μm thickness, feed rate of 4 μm/s, 60 % ultrasonic power and 25 % slurry concentration. Under these conditions, the amount of overcutting decreased from 74.3 μm to 28 μm (62.3 % improvement) and the weighted edge damage decreased from 52.3 μm to 16 μm (69.4 % improvement), which led to a 65.85 % improvement in the surface integrity index. The experimental investigation revealed that the surface integrity of silicon microchannels and the quality of μUSM can be significantly improved by applying the protective coating.

Laser Surface Texturing on Nickel-Aluminium-Bronze Alloy for Improving the Hydrophobicity

The laser-assisted surface engineering approach to produce hydrophobic surfaces on the metals is exponentially increasing over the last decade. In this study, nanosecond pulsed Nd3+:YAG laser was used to form a linear and non-linear micro-channel textured surface on the surface of Nickel-Aluminium-Bronze metal which is extensively used for marine applications. Contact angle measurement was performed on both non-textured and textured surfaces using a contact angle meter and its hydrophobic behavior was studied. A linear and non-linear textured surface was produced. Initially, the effect of linear micro-channel spacing and re-deposition layer on hydrophobicity was analyzed. Then the inside surface of the linear micro-channel was produced as a periodic surface structure and its influence on hydrophobicity was studied. A non-linear micro-channel containing protruding type structure was also laser textured and its hydrophobic nature was studied. The plain non-textured surface showed a contact angle of 92o. A maximum contact angle of 166o was observed on a non-linear micro-channel containing protruding type pattern.

Endoscopic Visualization of Contact Line Dynamics during Pool Boiling on Capillary‐Activated Copper Microchannels

AbstractMicrochannel surfaces are common to microfluidics, biofluidics, thermal management, and energy applications. Due to processing limitations for the majority of metallic materials, the majority of hyperfine microchannels used in microfluidics and thermo‐fluids are fabricated on non‐metallic substrates, for example, silicon and polydimethylsiloxane. Here, a technique to fabricate ultrasmall microchannels on arbitrary metallic materials is developed using photolithography in combination with electrochemical deposition. The technique is used to prepare copper microchannels and to investigate the pool boiling heat transfer performance with a focus on the three‐phase contact line dynamics. The hydrodynamics of nucleating bubbles during boiling are observed in situ using in‐liquid endoscopy. The results show that the variation of critical heat flux enhancement has a linear relationship with the contact line increase ratio. The scalable microchannel surfaces exhibit superior heat transfer performance with a maximum heat transfer coefficient) enhancement of 930% with ultra‐low wall superheat of 5 °C. This work not only develops a scalable manufacturing method to develop ultra‐small microchannels on metallic materials, it outlines design guidelines for structure optimization of pool boiling heat transfer for temperature sensitive applications, such as electronics thermal management.

Polymer Microchannel and Micromold Surface Polishing for Rapid, Low-Quantity Polydimethylsiloxane and Thermoplastic Microfluidic Device Fabrication.

Polymer-based micromolding has been proposed as an alternative to SU-8 micromolding for microfluidic chip fabrication. However, surface defects such as milling marks may result in rough microchannels and micromolds, limiting microfluidic device performance. Therefore, we use chemical and mechanical methods for polishing polymer microchannels and micromolds. In addition, we evaluated their performance in terms of removing the machining (milling) marks on polymer microchannel and micromold surfaces. For chemical polishing, we use solvent evaporation to polish the sample surfaces. For mechanical polishing, wool felt polishing bits with an abrasive agent were employed to polish the sample surfaces. Chemical polishing reduced surface roughness from 0.38 μm (0 min, after milling) to 0.13 μm after 6 min of evaporation time. Mechanical polishing reduced surface roughness from 0.38 to 0.165 μm (optimal pressing length: 0.3 mm). As polishing causes abrasion, we evaluated sample geometry loss after polishing. Mechanically and chemically polished micromolds had optimal micromold distortion percentages of 1.01% ± 0.76% and 1.10% ± 0.80%, respectively. Compared to chemical polishing, mechanical polishing could better maintain the geometric integrity since it is locally polished by computer numerical control (CNC) miller. Using these surface polishing methods with optimized parameters, polymer micromolds and microchannels can be rapidly produced for polydimethylsiloxane (PDMS) casting and thermoplastic hot embossing. In addition, low-quantity (15 times) polymer microchannel replication is demonstrated in this paper.

Experimental Investigation of Microchannel Copper Plate Surfaces on Manufacturing and Wettability

In this study, static contact angle measurement experiments were conducted on smooth copper surface and microchannel structure surfaces under different droplet sizes. The results showed that shallow processing will more easily lead to the deformation of the processed interface. And the wettability of the microchannel surfaces with the same width and different depth were obviously different. Moreover, the contact angles on three microchannel surfaces were almost the same when the size of droplet was 12μl duo to action of gravity. The optimal droplet size is 9μl to measure contact angle in this study.

THERMOCAPILLARY MONITORING USING PARTICLE IMAGE VELOCIMETRY ASSOCIATED WITH DYNAMIC LASER SPECKLE

Thermocapillarity is a physical phenomenon used in many industrial processes, mainly in the field of miniaturization. Thermocapillary forces are the base of thermocapillary pumping (TCP), in which a drop of liquid moves through a microchannel or flat surface after temperature gradient occurs. The objective of this work was to study the thermocapillary convection during pumping without inserting external particles, monitored by the Particle Image Velocimetry (PIV) technique. The experiment consisted of a Pasteur tube containing a yellow fluorescein solution (0.02, 0.04, 0.06, 0.08, and 0.1 Molar), illuminated by a laser beam of 545 nm, 40 mW. After 2 minutes of illumination, the thermocapillary movement occurred at a distance of 0.05 mm below the meniscus when using a laser beam of 545 nm, 3 mW. The images were captured by a charge-coupled device (CCD) camera and processed using the PIV technique. The results showed an internal conversion capacity between the intersystem crossing, vibrational, and relaxation phenomena, also demonstrating the potential for applying the proposed approach. The images presented velocity distribution caused by thermocapillarity. The PIV was a useful tool for convective flow analysis if connected to appropriate image processing and enhancement techniques. In conclusion, the research showed the images with velocity distribution caused by thermocapillarity.

Boiling of Argon flow in a microchannel by considering the spherical geometry for roughness barriers using molecular dynamics simulation

In this research, the method of molecular dynamic (MD) is used to consider the influences of the spherical roughness barrier on the boiling flow regime of Argon particles, flowing into microchannel with a square cross-section. To prepare boiling conditions, different constant temperatures from 84 K to 133 K are applied on the walls of microchannels. Results show that roughness elements help boiling thermal forces to the distribution of fluid atoms in central layers of a microchannel, especially at high time steps between 750,000 and 1,000,000. This phenomenon is noticeable under applied high wall temperatures of 114 K and 133 K. Also, a summation of velocity values indicated that increasing boundary wall temperatures of rough microchannels results in a small reduction of the velocity of fluid flow, whereas; it is almost unchanged in the smooth microchannel. The statistical approach shows that the presence of spherical roughness does not have a destructive influence on the boiling flow properties of Argon into the microchannel. Also, they are useful to enhance effective surfaces of heat transfer, which empower the boiling process. Therefore, investing in polishing microchannel surfaces and removing the spherical shape of roughness components can even be a serious mistake for some of the practical applications.

A Microfluidic Platform Equipped With Magnetic Nano Films for Organizing Bio-Particle Arrays and Long-Term Studies

We introduce a microfluidic platform that combines hydrodynamic trapping and magnetic switching techniques for precise manipulating magnetic micro-particles and forming particle and cell arrays, in a massively parallel manner. In the first step, hydrodynamic forces transport and then trap the single particles and cells in a large array. Then, magnetic forces transfer them into adjacent low-shear compartments which are used as single cell culture micro-chambers. Furthermore, using this tool, we run single cell assays and perform drug screening as well as mRNA analysis experiments. The flawless magnetic transport in this work is obtained by growing a non-fouling layer on microchannel surfaces. The chip treatment became possible by a novel microfluidic chip design and a new simple self-aligned fabrication method, based on a silicon substrate, magnetic nano thin-films embedded in the microfluidic walls, and a glass lid. The resulting chip is an innovative tool suitable for long-term single cell phenotypic and genotypic studies, with applications in immunology and medicine.

Shape-stabilized Phase Change Materials Based on Carbon Matrix for Thermal Performance Enhancement: A Review

With the growingly serious energy crisis and the dramatically increasing greenhouse gas emission worldwide, thermal energy storage has been attracting extensive attention for its application in re-regeneration energy, which provides a perfect solution to coordinate the mismatch between energy supply and demand. Organic phase change materials are the most widely used materials for thermal energy storage, but the inherent disadvantages drastically impede their further application, such as low thermal conductivity and leakage. Previous research shows that carbon-based scaffold can effectively prevent the leakage during phase transition attributing to the capillary force and the surface tension between the phase change material and the micro-channel surface. The porous structure provides abundant heat transfer areas, and the scaffold itself is perfect heat transfer media. It makes the heat transfer greatly enhanced by combining with carbon-based matrix. This composite shows huge potential for promoting the efficiency of thermal management and energy conversion. This paper gives a state-to-art review about the development of shape-stabilized composite phase change material with carbon-based matrix as the supporting material for organic phase change material including the fabrication and thermal property of the composite phase change material, and the newest application of the composite.

Chemical Etching Treatment of Polydimethylsiloxane for Smoothing Microchannel Surface

Chemical Etching Treatment of Polydimethylsiloxane for Smoothing Microchannel Surface

Molecular dynamic simulation of Argon boiling flow inside smooth and rough microchannels by considering the effects of cubic barriers

This paper presents the effects of barriers with cubic geometry on the boiling flow behavior of Argon flow with 10 million atoms flowing inside smooth and rough microchannels using molecular dynamic simulation (MDS) method. Effective parameters of this research are boundary temperature of 108 K on the walls of microchannels to prepare boiling condition and driving forces as much as 0.002, 0.01, and 0.02 eV/Å, which is enforced respectively on the fluid at the inlet of the microchannel. For ease of investigation in comparison to results by statistical method, the cubic shape of roughness barriers is simulated on the surfaces of a smooth microchannel with the same dimensions. Results showed that the presence of cubic roughness elements increases oscillations in density profiles due to their influences in arrangements of argon atoms and their consequences in the diminishing role of the boiling process to move atoms from lateral regions to empty region in the middle section of the microchannel. Also, the statistical approach demonstrated that the presence of cubic roughness elements reduces average velocity as much as 1.8 to 13.59%. Moreover, applying external forces in a range of 0.01 to 0.02 eV/Angstrom brings so turbulences in the rough microchannel that destroys stability and normal distribution of atoms under boiling force which is not applicable for practical application.