Experimental investigations of liquid flow in rib-patterned microchannels with different surface wettability

The effects of rib-patterned surfaces and surface wettability on liquid flow in microchannels were experimentally investigated in this study. Microchannels were fabricated on single-crystal silicon wafers by photolithographic and wet-etching techniques. Rib structures were patterned in the silicon microchannel, and the surface was chemically treated by trichlorosilane to create hydrophobic condition. Experiments with water as the working fluid were performed with these microchannels over a wide range of Reynolds numbers between 110 and 1914. The results for the rib-patterned microchannels showed that the friction factor with the hydraulic diameter based on the rib-to-upper-wall height was lower than that predicted from incompressible theory with the same height. The friction factor-Reynolds number products for the hydrophobic condition increased as Reynolds number increased in the laminar flow regime. The experimental results were also compared with the predictive expressions from the literature, and it was found that the experimental data for the small rib/cavity geometry was in good agreement with those in the literature.     

Experimental and numerical investigation of capillary flow in SU8 and PDMS microchannels with integrated pillars

Microfluidic channels with integrated pillars are fabricated on SU8 and PDMS substrates to understand the capillary flow. Microscope in conjunction with high-speed camera is used to capture the meniscus front movement through these channels for ethanol and isopropyl alcohol, respectively. In parallel, numerical simulations are conducted, using volume of fluid method, to predict the capillary flow through the microchannels with different pillar diameter to height ratio, ranging from 2.19 to 8.75 and pillar diameter to pitch ratio, ranging from 1.44 to 2.6. The pillar size (diameter, pitch and height) and the physical properties of the fluid (surface tension and viscosity) are found to have significant influence on the capillary phenomena in the microchannel. The meniscus displacement is non-uniform due to the presence of pillars and the non-uniformity in meniscus displacement is observed to increase with decrease in pitch to diameter ratio. The surface area to volume ratio is observed to play major roles in the velocity of the capillary meniscus of the devices. The filling speed is observed to change more dramatically under different pillar heights upto 120 μm and the change is slow with further increase in the pillar height. The details pertaining to the fluid distribution (meniscus front shapes) are obtained from the numerical results as well as from experiments. Numerical predictions for meniscus front shapes agree well with the experimental observations for both SU8 and PDMS microchannels. It is observed that the filling time obtained experimentally matches very well with the simulated filling time. The presence of pillars creates uniform meniscus front in the microchannel for both ethanol and isopropyl alcohol. Generalized plots in terms of dimensionless variables are also presented to predict the performance parameters for the design of these microfluidic devices. The flow is observed to have a very low Capillary number, which signifies the relative importance of surface tension to viscous effects in the present study.     

Capillary flow in microchannels

The surface of microchannels, especially polymer channels, often needs to be treated to acquire specific properties. This study investigated the capillary flow and the interface behavior in several glass capillaries and fabricated microchannels using a photographic technique and image analysis. The effect of air plasma treatment on the characteristics of capillary flow in three types of microfluidic chips, and the longevity of the acquired surface properties were also studied. It was observed that the dynamic contact angles in microchannels were significantly larger than those measured from a flat substrate and the angle varied with channel size. This suggests that dynamic contact angle measured in situ must be used in the theoretical calculation of capillary flow speed, especially for microfabricated microchannels since the surface properties are likely to be different from the native material. This study also revealed that plasma treatment could induce different interface patterns in the PDMS channels from those in the glass and PC channels. The PDMS channel walls could acquire different level of hydrophilicity during the plasma treatment, and the recovery to hydrophobicity is also non-homogeneous.     

Slip flow in non-circular microchannels

Microscale fluid dynamics has received intensive interest due to the emergence of Micro-Electro-Mechanical Systems (MEMS) technology. When the mean free path of the gas is comparable to the channel’s characteristic dimension, the continuum assumption is no longer valid and a velocity slip may occur at the duct walls. Non-circular cross sections are common channel shapes that can be produced by microfabrication. The non-circular microchannels have extensive practical applications in MEMS. Slip flow in non-circular microchannels has been examined and a simple model is proposed to predict the friction factor and Reynolds product fRe for slip flow in most non-circular microchannels. Through the selection of a characteristic length scale, the square root of cross-sectional area, the effect of duct shape has been minimized. The developed model has an accuracy of 10% for most common duct shapes. The developed model may be used to predict mass flow rate and pressure distribution of slip flow in non-circular microchannels.     

Surface tension driven flow for open microchannels with different turning angles

Present study examines the flow characteristics of open microchannels with sharp turns by experimental and numerical methods. For the open channel system in microscale, the flow is mainly driven by surface tension at atmospheric pressure. The open channels are of various aspect ratios of depth-to-width, ranging from 0.75 to 3, and of turning angles from 45° to 135°. It is found that the turning angle and the aspect ratio of depth-to-width play major roles in the velocity of liquid front advancing, the meniscus of liquid–gas interface shape, and head loss of flow due to turning. Besides, the radius of curvature of the liquid front is reduced as the liquid front travels downstream and over the turning elbow. The loss coefficient remains the same for turning angles less than 75°, whereas it is increased further and is even more pronounced for turning angles larger than 105°. Numerical predications based on conservation laws agree with the experimental observations, and the flow characteristics are well described for open channel in microscale, as the aspect ratio is greater than or near to 1.5.     

Gaseous slip flow in long microchannels

An analytic and experimental investigation into gaseous flow with slight rarefaction through long microchannels is undertaken. A two-dimensional (2-D) analysis of the Navier-Stokes equations with a first-order slip-velocity boundary condition demonstrates that both compressibility and rarefied effects are present in long microchannels. By undertaking a perturbation expansion in ϵ, the height-to-length ratio of the channel, and using the ideal gas equation of state, it is shown that the zeroth-order analytic solution for the streamwise mass flow corresponds well with the experimental results. Also, the effect of slip upon the pressure distribution is derived, and it is obtained that this slip velocity leads directly to a wall-normal migration of mass. The fabrication of wafer-bonded microchannels that possess well-controlled surface structure is described, and a means for accurately measuring the mass how through the channels is presented. Experimental results obtained with this mass-flow measurement technique for streamwise helium mass flow through microchannels 52.25-μm wide, 1.33-μm deep, and 7500-μm long for a pressure range of 1.6-4.2 atmospheres (outlet pressures at atmospheric) are presented and shown to compare favorably with the analysis     

Experimental characterization of the temperature dependence of zeta potential and its effect on electroosmotic flow velocity in microchannels

This study attempts to characterize the influence of temperature on zeta potential for a number of commonly used buffers in both poly(dimethylsiloxane) (PDMS):glass and PDMS:PDMS microchannels. The study is motivated by the apparent inability of the Smoluchowski equation for electroosmotic flow (EOF) velocity, U = [ε ₀ ε _r ζ/μ]E _z , to accurately predict EOF velocities at elevated temperatures. Error can result if zeta potential (ζ) is taken to be constant, even if permittivity (ε) and viscosity (μ) are treated as temperature-dependent variables. In some cases, velocity may be underestimated by more than 30%. In this study, the time-interval current-monitoring method was used to measure zeta potential. A hotplate maintained precise channel temperatures and applied electric field strengths were selected so that Joule heating was negligible. Results show that in some solutions (e.g., KCl, TBE), the zeta potential can exhibit a strong dependence on temperature, changing by as much as 50% over a span of 60°C. This influence was found to increase with solution concentration. Other buffers (e.g., TE, Na₂CO₃/NaHCO₃) were stable over all measured temperatures.     

An experimental investigation of two-phase coating flow within microchannels: the effect of coating fluid rheology

The study of multiphase flow within micro-scale geometries has garnered much attention in recent history. One system of interest is the flow of a low viscosity fluid thread, labeled as the coring fluid, through a much higher viscosity liquid, labeled the coating fluid. On the macro-scale, this process has shown great utility in the plastics industry, but it has yet to be completely characterized on the micro-scale. Detailed here is a set of experiments performed within square microchannels, of nominal area 250 μm by 250 μm, where the coring fluid was Newtonian and the coating fluid was either Newtonian or viscoelastic. Visual data were collected and subsequently analyzed to determine the thickness of coating fluid remaining on the walls of the microchannel after the coring fluid front passed. The results for flow through a Newtonian coating fluid show similarity to results for flow in macro-scale capillaries; the thickness of the coating fluid, after initial penetration by the coring fluid thread, follows the same dependence upon the velocity of the thread front as seen for the macro-scale. The case of a viscoelastic coating fluid in a microchannel, however, shows interesting differences to the macro-scale results. Most notably, the surface of the coring fluid thread was highly unstable and the entire thread migrated toward one wall.     

Electroosmotic flow in microchannels with prismatic elements

Fundamental understanding of liquid flow through microchannels with 3D prismatic elements is important to the design and operation of lab-on-a-chip devices. In this paper, we studied experimentally and theoretically the electroosmotic flow (EOF) in slit microchannels with rectangular 3D prismatic elements fabricated on the bottom channel wall. The average electroosmotic velocity measured by the current-monitoring technique was found lower than that in a smooth microchannel. This velocity reduction becomes larger in microchannel with larger but less number of the prisms even though the space taken by the prisms are identical. The velocity distribution and streamlines on two typical horizontal planes in the microchannel are measured and visualized by a particle-based technique. These experimental observations are in good agreement with the numerical simulation. The comparison of streamlines near the prisms in the pressure-driven flow with that in the EOF showed that the EOF was more sensitive to the local geometry.     

Effect of finite reservoir size on electroosmotic flow in microchannels

In electrokinetically driven microfluidic applications, reservoirs are indispensable and have finite sizes. During operation processes, as the liquid level in reservoirs keeps changing as time elapses, a backpressure is generated. Thus, the flow in microfluidic channels actually exhibits a combination of the electroosmotic flow and the time-dependent induced backpressure-driven flow. In this paper, a model is presented to describe the effect of the finite reservoir size on electroosmotic flow in a rectangular microchannel. Important parameters that describe the effect of finite reservoir size on flow characteristics are discussed. A new concept termed as “effective pumping period” is introduced to characterize the reservoir size effect. The proposed model identifies the mechanisms of the finite-reservoir size effects and is verified by experiment using the micro-PIV technique. The results reported in this study can be used for facilitating the design of microfluidic devices.     

Numerical study on the cell motility interacting with the chemical flow in microchannels

Beneficial to the steady control of flow properties and concentration gradient profiles, quantification of cell chemotaxis based on microfluidic devices could achieve on the scale of a single cell. However, normal experimental studies assumed that the concentration field was not affected by the existing cell or by the impact of the cell motion. The present paper systematically simulated the interactions of the cell translational and rotational movements with its chemical gradient flow by both 2D and 3D models. The influences of the chemical flow Peclet number, cell’s translational velocity, cell’s rotational velocity and direction on the sensed chemical gradient were investigated. Results showed that both the cell’s translational and rotational movements disturbed its surrounding chemical distribution and affect chemotactic speed and direction later on. Rotating cell brings in flow circulation with it, and consequently the sensed chemical gradient dramatically deviates from the original direction. The cell’s two contrary rotational directions lead to contrary results. 2D model with circular cell is practically feasible due to simplicity, while 3D model with spherical cell is closer to reality. Numerical comparison showed that the 2D model can be used to analyze the cell’s chemotactic tendency, but it also amplifies the cell’s perturbation and then separation to its surrounding chemical flow. Finally, a single cell’s interactive chemotaxis in a micro chamber was simulated based on experimental measured chemotactic coefficient. The interactive chemotactic cell kept moving slightly upstream instead of upright crossing the interface. This work may contribute to the development of chemotactic measurement method and precise evaluation on the cell’s chemotactic sensitivity.     

Spontaneous capillary flow in curved,open microchannels

Capillary flows are increasingly used in biotechnology, biology, chemistry, energy and space applications. Motivated by these new developments, designs of capillary channels have become more sophisticated. In particular, capillary microsystems often use winding channels for reasons such as compactness, or mixing. The behavior of capillary microflows in curved channels is still underdeveloped. In this work, we investigate this type of behavior. In the case of suspended capillary flows, is shown that the flow profile in the curved section is approximately analogous to that in a rectilinear section. In the case of open U-grooves where inner corners are present, the importance of the turn sharpness and of the presence of capillary filaments is pointed out. For sharp turns, and/or in the presence of precursor capillary filaments, we found the phenomenon that the inner filament precedes the outer filament in the channel. Analysis of the capillary flow in curved channels is performed experimentally using rectangular U-grooves and suspended channels. The experimental observations are compared to Surface Evolver numerical software results.     

Analysis of laminar-to-turbulent transition for isothermal gas flows in microchannels

In this work the laminar-to-turbulent transition in microchannels of circular cross-section is studied experimentally. In order to single out the effects of relative roughness, compressibility and channel length-to-diameter ratio on the Reynolds number at which transition occurs, experimental runs have been carried out on circular microchannels in fused silica—smooth for all purposes—and in stainless steel (which possess a high surface roughness), with a diameter between 125 and 180 μm and a length of 5–50 cm through which nitrogen flows. For each tube the friction factor has been computed. The values of the critical Reynolds number have been determined plotting the Poiseuille number (i.e., the product of the friction factor, f, times the Reynols number, Re) as a function of the average Mach number between inlet and outlet. The transitional regime was found to start no earlier than at values of the Reynolds number around 1,800–2,000. It has been observed that surface roughness has no effect on the hydraulic resistance in the laminar region for a relative roughness lower than 4.4%, and that friction factor obeys the Poiseuille law, if it is correctly computed taking compressibility into account. It is found that recent correlations for the prediction of the critical Reynolds number in microchannels that link the relative roughness of the microtubes to the critical Reynolds number do not agree with the present results.     

Fiber Bragg grating sensor for two-phase flow in microchannels

A new non-intrusive measurement technique for two-phase flow in microchannels is presented. The development of an evanescent field-based optical fiber Bragg grating (FBG) sensor is described, and experiments coupled with flow visualization demonstrating the performance of this sensor are presented. Two adjacent 1-mm FBGs in etched D-shaped fiber are embedded into the surface of a PDMS microchannel. Experiments are conducted in both droplet and slug flow regimes and high-speed digital video is captured synchronously with the sensor data. The FBGs exhibit an on?Coff type response to the passage droplets which is shown to correlate precisely with the passage of the liquid phase. This correlation enables the measurement of droplet average velocity and size using only the sensor data. In addition to the use of both FBG signals for the purpose of measuring droplet speed and size, it is shown that for droplets larger than the FBG length, a single FBG can be used to estimate the convection velocity and size of fast moving droplets. This sensing method is potentially useful for monitoring two-phase flow in fuel cells and microfluidic applications such as micro-heat exchangers and lab-on-a-chip systems.     

Measuring flow velocity distribution in microchannels using molecular tracers

We demonstrate the capability of a molecular tracer based laser induced fluorescence photobleaching anemometer for measuring fluid velocity profile in microfluidics. To validate the feasibility and accuracy of this measurement system, the velocity profiles of the cylindrical and rectangular microchannels are measured, respectively. We compare our experimental results with theoretical prediction. The theoretical prediction shows a good consistency with practical measurement results. The spatial resolutions are analyzed and validated. Finally, the error source of the presented system is analyzed, which makes it possible to compensate some error quantitatively and guides the application of this system.     

Experimental and numerical investigation on steady and transient flow in the flat-walled micro-diffuser/nozzle

Microsystem Technologies - In this paper, experiments and numerical simulations have been conducted to investigate the flow characteristics in the flat-walled micro-diffuser/nozzle on conditions of...     

Patterning of flow and mixing in rotating radial microchannels

We demonstrate how the speed of mixing under laminar conditions can be appreciably enhanced in concurrent centrifugal flows through straight, low-aspect-ratio microchannels pointing in radial direction in the plane of rotation. The convective mixing is driven by the inhomogeneous distribution of the velocity-dependent Coriolis pseudo force and the interaction of the so-induced transverse currents with the side walls. By investigating the key impact parameters, which are the geometry of the channels and the speed of rotation, it is shown that the contact surface between two laminar flows can be folded to shorten mixing times by up to two orders of magnitude!