Fluidic Resistance Research Articles

A Review on the Effect of Surface Roughness and Liquid Slip on fluid flow in PDMS Microchannels

Microfluidics is about studying flows with characteristic length scales of the orders of microns (typically sub millimeters). The fluid dynamics at the microscale can vary from “macro fluidic” behavior in those factors such as surface tension and energy dissipation.The fluidic resistance will keep on developing to dominate the system. Polydimethylsiloxane (PDMS) is a polymer commonly used for microfluidic chip manufacture and prototyping. PDMS becomes a hydrophobic elastomer after cross linking.Small dimensions make it impossible to produce a microchannel with smooth surface. Hence entrance effect, surface roughness, turbulence and viscous effects have a reasonable effect on fluid flow. This paper aims to look into characterization techniques of surface roughness in a channel and liquid slip phenomenon considering changes in elementary geometry, flow conditions and different roughness elements.

A Novel Approach for Tuning of Fluidic Resistance in Deterministic Lateral Displacement Array for Enhanced Separation of Circulating Tumor Cells

A Novel Approach for Tuning of Fluidic Resistance in Deterministic Lateral Displacement Array for Enhanced Separation of Circulating Tumor Cells

Rapid manufacturing of micro-drilling devices using FFF-type 3D printing technology

Micro-drilling devices with different blade shapes were fabricated with a rapid and facile manufacturing process using three-dimensional (3D) printing technology. The 3D-printed casting mold was utilized to customize the continuous shape of the blades without the need for expensive manufacturing tools. A computational fluid dynamics simulation was performed to estimate the pressure differences (fluidic resistance) around each rotating device in a flowing stream. Three types of blades (i.e., 45°, 0°, and helical type) were manufactured and compared to a device without blades (i.e., plain type). As a result, the device with the 45° blades exhibited the best drilling performance. At a rotational speed of 1000 rpm, the average drilling depth of the device with the 45° blades to penetrate artificial thrombus for 90 s was 3.64 mm, which was ~ 2.4 times longer than that of helical blades (1.51 mm). This study demonstrates the feasibility of using 3D printing to fabricate microscale drilling devices with sharp blades for various applications, such as in vivo microsurgery and clogged water supply tube maintenance.

Early turbulence and pulsatile flows enhance diodicity of Tesla\u2019s macrofluidic valve

Microfluidics has enabled a revolution in the manipulation of small volumes of fluids. Controlling flows at larger scales and faster rates, or macrofluidics, has broad applications but involves the unique complexities of inertial flow physics. We show how such effects are exploited in a device proposed by Nikola Tesla that acts as a diode or valve whose asymmetric internal geometry leads to direction-dependent fluidic resistance. Systematic tests for steady forcing conditions reveal that diodicity turns on abruptly at Reynolds number {rm{Re}}approx 200 and is accompanied by nonlinear pressure-flux scaling and flow instabilities, suggesting a laminar-to-turbulent transition that is triggered at unusually low {rm{Re}}. To assess performance for unsteady forcing, we devise a circuit that functions as an AC-to-DC converter, rectifier, or pump in which diodes transform imposed oscillations into directed flow. Our results confirm Tesla’s conjecture that diodic performance is boosted for pulsatile flows. The connections between diodicity, early turbulence and pulsatility uncovered here can inform applications in fluidic mixing and pumping.

Planar hydrodynamic traps and buried channels for bead and cell trapping and releasing.

We present a novel concept for the controlled trapping and releasing of beads and cells in a PDMS microfluidic channel without obstacles present around the particle or in the channel. The trapping principle relies on a two-level microfluidic configuration: a top main PDMS channel interconnected to a buried glass microchannel using round vias. As the fluidic resistances rule the way the liquid flows inside the channels, particles located in the streamlines passing inside the buried level are immobilized by the round via with a smaller diameter, leaving the object motionless in the upper PDMS channel. The particle is maintained by the difference of pressure established across its interface and acts as an infinite fluidic resistance, virtually cancelling the subsequent buried fluidic path. The pressure is controlled at the outlet of the buried path and three modes of operation of a trap are defined: idle, trapping and releasing. The pressure conditions for each mode are defined based on the hydraulic–electrical circuit equivalence. The trapping of polystyrene beads in a compact array of 522 parallel traps controlled by a single pressure was demonstrated with a trapping efficiency of 94%. Pressure conditions necessary to safely trap cells in holes of different diameters were determined and demonstrated in an array of 25 traps, establishing the design and operation rules for the use of planar hydrodynamic traps for biological assays.

Electropermanent magnet-driven droplet size modulation for two-phase ferromicrofluidics

A new ferrofluidic droplet generator is demonstrated using an electropermanent magnet to change the viscosity of the ferrofluid oil phase in a flow-focusing junction, which enables control of the aqueous droplet size. Electropermanent magnets are capable of fast switching (< 100 $$\upmu$$ s) and have magnetic field strengths comparable to those of permanent magnets. When switched on, the magnetoviscous effect increases the viscosity of the ferrofluid, thus increasing the channel’s fluidic resistance, slowing the flow rate of the oil-based phase and changing the droplet diameter. We have demonstrated on-demand droplet size modulation as large as 44% at speeds fast enough to modulate the size of a single droplet in a stream. Due to compliance in the supply tubing and Polydimethylsiloxane (PDMS) microfluidic channels, the change in droplet size relaxes with a time scale of seconds. An application of droplet size modulation was demonstrated by integrating a passive size-based magnetic droplet sorting stage.

Cross-Sectional Dimension Dependence of Electroosmotic Flow in Fractal Treelike Rectangular Microchannel Network.

The present work theoretically and numerically studies the electroosmotic flow (EOF) within a fractal treelike rectangular microchannel network with uniform channel height. To obtain minimum EOF fluidic resistance, the microchannel cross-sectional dimensions of the fractal network are optimized. It is found that the cross-sectional dimension dependence of EOF fluidic resistance within a symmetric fractal network is only dependent on the channel width when the total channel volume is constant, and the optimal microchannel widths to reach the minimum EOF fluidic resistance satisfy the scaling law of κ = N−1 (where κ is the width ratio of the rectangular channels at two successive branching levels, N is the branching number); however, for the symmetric fractal network with constant total surface area, the optimal cross-sectional dimensions should simultaneously satisfy κ = N−1 and (where H is the channel height, S is the total channel surface area, l0 is the channel length at the original branching level, γ is the channel length ratio at two successive branching levels and m is the total branching level) to obtain the minimum EOF fluidic resistance. The optimal scaling laws established in present work can be used for the optimization design of the fractal rectangular microchannel network for EOF to reach maximum transport efficiency.

Characterizing the impact of thermal gels on isotachophoresis in microfluidic devices.

Thermally reversible Pluronic gels have been employed as separation matrices in microfluidic devices in the analysis of biological macromolecules. The phase of these gels can be tuned between liquid and solid states using temperature to vary fluidic resistance and alter peak resolution. Although separations in thermal gels have been characterized, their effect on isotachophoresis has not. This study used fluorescein as a model analyte to evaluate isotachophoretic preconcentration as a function of thermal polymer concentration and temperature. Results demonstrated that increasing polymer concentration in microfluidic channels increased the apparent analyte concentration. A critical minimum of 10% (w/v) Pluronic was required to achieve efficient preconcentration with maximum focusing occurring in 20 and 25% polymer gels. Temperature of the thermal gel also impacted analyte focusing. Most efficient focusing was achieved at 25°C with diminishing analyte accumulation at higher and lower temperatures. Under optimal conditions, isotachophoretic preconcentration increased an additional threefold simply by including thermal gels in the system. This approach can be readily implemented in other applications to increase detection sensitivity and measure low-concentration analytes within simple microfluidic devices.

The fluidic resistance of an array of obstacles and a method for improving boundaries in deterministic lateral displacement arrays

Deterministic lateral displacement (DLD) is a microfluidic method of separating particles by size. DLD relies on precise flow patterns to deliver high-resolution particle separation. These patterns determine which particles are displaced laterally, and which follow the flow direction. Prior research has demonstrated that the lateral array boundaries can be designed to improve the uniformity of the critical size and hence separation performance. A DLD device with an invariant critical size throughout is yet unknown. In this work, we propose a 3D design approach. We first represent the flow through the DLD as a 2D lattice of resistors. This is used to determine the relative flow resistances at the boundaries that will deliver the correct flux patterns. We then use the lattice Boltzmann method to simulate fluid flow in a 3D unit cell of the DLD and measure the fluidic resistance for a range or typical dimensions. The results of this work are used to create a new equation for fluidic resistance as a function of post size, post height, and post spacing. We use this equation to determine array geometries that should have the appropriate resistances. We then design and simulate (in COMSOL) complete devices and measure fluid fluxes and first flow-lane widths along the boundaries. We find that the first flow-lane widths are much more uniform than in any devices described previously. This work provides the best method for designing periodic boundaries, and enables narrower, shorter, and higher throughput DLD devices.

A simple micro check valve using a photo-patterned hydrogel valve core

Micro check valves play a pivotal role in many microfluidic systems. However, there is still an unmet need for low-cost check valves with good performance and easy integration. In this work, we present a planar micro check valve using a hydrogel valve core fabricated in situ in a microchannel. This valve is characterized by a simple fabrication process and easy integration with other microfluidic devices. Experimental results proved that the check valve performs better than most existing in-plane micro check valves, especially in low-pressure operations, with nearly zero forward cracking pressure and reverse leakage, adjustable forward fluidic resistance, and good repeatability. We further proposed a normally-closed check valve with controllable cracking pressure to meet different requirements of microfluidic devices. Precise cracking pressures could be achieved by regulating the valve spring width. To demonstrate the valve usefulness, we employed these valves to make a finger-actuated micro-mixer and a bio-actuated micro-pump. This study makes the flow control more simplified in integrated microfluidic systems.

The Impact of Surfactants on the Inertial Separation of Bubbles in Microfluidic Electrolyzers

Improving the throughput of electrochemical reactions such as water electrolysis is an ongoing effort. In a membrane-less electrolyzer, product separation can be achieved via controlled two-phase flows within the electrochemical flow cells. In this case, it is important to keep the diameter of the bubbles small while ensuring that they follow an off-center trajectory in the channel. This can be accomplished by either increasing the flow, which in turn increases the power loss due to the fluidic resistance, and/or decreasing the electrical current which means a lower production rate. To avoid these drawbacks, here we show that by adding surfactants to the electrolyte, bubbles are kept smaller due to the bubble coalescence inhibition and faster bubble detachment from the electrodes. We find out that by using surfactants, the required flow rate for efficient product separation and the corresponding pumping power decrease. Moreover, we show that higher throughputs can be achieved at a given flow rate by using surfactants. The surfactant used in this study introduces an undesirable increase in the overpotential. This is compensated at high production rates where the surface coverage of the electrodes by the generated gas bubbles is smaller in the presence of the surfactant.

Scaling of deterministic lateral displacement devices to a single column of bumping obstacles.

We describe a deterministic lateral displacement (DLD) for particle separation with only a single column of bumping features. The bifurcation of fluid streams at obstacles is not set by the "tilt" of columns with respect to macroscopic current flow, but rather by the fluidic resistances for lateral flow at each obstacle. With one column of 14 bumping features and corresponding inlet/outlet channels, the single-column DLD can separate particles with diameters of 4.8 μm and 9.9 μm at 30 μL min-1, with an area of only 0.37 mm × 1.5 mm (0.55 mm2). The large-cell output contains over 99% of the 9.9 μm particles and only 0.2% of the 4.8 m particles. The throughput per area of 54 μL min-1 per mm2 represents a 10× increase over previous selective harvesting reports for microfluidic devices in a similar particle size range.

On-chip mixing of liquids with swap structures written by two-photon polymerization

A fluidic micromixer realized by two-photon polymerization writing is embedded in a predefined microchannel. The micromixer consists of a 3D defined channel system which swaps incoming fluid streams, resulting in laminar alternating streams consisting of the two liquids to be mixed. Due to the shortened diffusion length, mixing times are reduced dramatically to 50 ms or lower. The structure is designed in such a way that the fluidic resistance in each swapper channel is balanced, yielding equal distribution of the alternating streams. The mixer element has a total size of about $$225\times 100\times 50\,\upmu \hbox {m}^{3}$$, fitting exactly in a prefabricated microchannel. The structure is strongly anchored in the channel so that it can handle high flow rates of up to 100 $$\upmu \text {L/min}$$ (corresponding to an average flow velocity of $$400\,\text {mm/s}$$). A benefit of the fabrication method is the short processing time. The writing of one mixer element takes less than $$30\,\text {s}$$, so that a whole wafer can be processed in typically less than one hour.

ELECTROOSMOTIC FLOW IN TREE-LIKE BRANCHING MICROCHANNEL NETWORK

As a kind of microchannel layout with good transport performance, tree-like branching microchannel network has been widely used for microfluidic systems, however, the optimal analysis of the tree-like branching microchannel network for electroosmotic flow (EOF) to reach a minimized fluidic resistance still needs a deep study. In this work, the EOF in tree-like branching microchannel network is theoretically and numerically studied. It is found that there is an optimal structure of the tree-like branching network for the EOF to achieve a minimum fluidic resistance under the size constraint of constant total channel volume. This work found that the optimal channel radii of the tree-like network for EOF to reach a minimum fluidic resistance satisfy the relationship of [Formula: see text], where [Formula: see text] is the radius of the parent channel, [Formula: see text] is the radius of the child channels and [Formula: see text] is the total number of child channels. This formula can be regarded as an extended Murray’s law for EOF and is helpful for the optimization design of tree-like branching microchannel network for EOF to reach maximum transport efficiency under the constant applied driven voltage.

Fluidic resistance control enables high-throughput establishment of mixed-species biofilms.

Bacteria often live in communities of mixed species embedded in a self-produced extracellular matrix of polysaccharides, proteins and DNA, termed biofilms. The BioFlux microfluidic flow system is useful for studying biofilm formation in different media under flow. However, analyzing the architecture and maturation of biofilms under flow requires a proper seeding, which can prove difficult when working with bacteria of different sizes, motile bacteria or aiming for a high number of replicates. Here we developed an efficient protocol that exploits viscosity tuning and seeding indicator dyes to improve seeding and allow for high-throughput examination and visualization of consistent mono- and mixed-species biofilm developments under flow.

Meander Designer: Automatically Generating Meander Channel Designs.

Microfluidics continues to bring innovation to the life sciences. It stimulates progress by enabling new ways of research in biology, chemistry, and biotechnology. However, when designing a microfluidic device, designers have to conduct many tasks by hand—resulting in labor-intensive processes. In particular, when drawing the design of the device, designers have to handle re-occurring entities. Meander channels are one example, which are frequently used in different platforms but always have to fit the respective application and design rules. This work presents an online tool which is capable of automatically generating user-defined, two-dimensional designs of fluidic meander channels facilitating fluidic hydrodynamic resistances. The tool implements specific design rules as it considers the user’s needs and fabrication requirements. The compliance of the meanders generated by the proposed tool is confirmed by fabricating the generated designs and comparing whether the resulting devices indeed realize the desired specification. To this end, two case studies are considered: first, the realization of dedicated fluidic resistances and, second, the realization of dedicated mixing ratios of fluids. The results demonstrate the versatility of the tool regarding application and technology. Overall, the freely accessible tool with its flexibility and simplicity renders manual drawing of meanders obsolete and, hence, allows for a faster, more straightforward design process.

Pressure drop of two-phase liquid-liquid slug flow in square microchannels

Networks of microdevices can be used to form, sort, split and recombine droplets by varying the fluidic resistance of the channels. To facilitate design of such systems we propose two models to calculate pressure drop of liquid-liquid slug flow in square microchannels. By approximating a droplet surrounded by liquid film as annular flow, the moving film model includes the velocity profile in the lubricating layer. In the no-film model, the presence of liquid film is neglected. Pressure drop measurements of water-toluene and water-silicone oil slug flow generated in glass/silicon devices of 200 and 400 μm width serve to validate these two models. We found that the droplet viscosity influences the applicability of the models. Good agreement is obtained for water-toluene slug flow, with the average error of 15–20% between the measured and calculated values for both models, proving that the presence of the lubricating film may be neglected for flows with similar viscosities of both phases. On the contrary, both predictions agree poorly with experiments for silicon oil-water flow which we attribute to the lower than expected liquid film thickness around the silicon oil droplets. Therefore we use the moving film model to estimate the thickness of the lubricating layer which we find to lie between 0.5% and 0.75% of the channel width, lower than the assumed 2%. Finally we compare the performance of our models against two correlations adapted from literature. We thus demonstrate that our approach allows reliable pressure drop prediction for liquids with similar viscosities and delivers important information on flow hydrodynamics.

Development of PZT Actuated Valveless Micropump

A piezoelectrically actuated valveless micropump has been designed and developed. The principle components of this system are piezoelectrically actuated (PZT) metal diaphragms and a complete fluid flow system. The design of this pump mainly focuses on a cross junction, which is generated by a nozzle jet attached to a pump chamber and the intersection of two inlet channels and an outlet channel respectively. During each PZT diaphragm vibration cycle, the junction connecting the inlet and outlet channels with the nozzle jet permits consistencies in fluidic momentum and resistances in order to facilitate complete fluidic path throughout the system, in the absence of any physical valves. The entire micropump structure is fabricated as a plate-by-plate element of polymethyl methacrylate (PMMA) sheets and sandwiched to get required fluidic network as well as the overall device. In order to identify the flow characteristics, and to validate the test results with numerical simulation data, FEM analysis using ANSYS was carried out and an eigenfrequency analysis was performed to the PZT diaphragm using COMSOL Multiphysics. In addition, the control system of the pump was designed and developed to change the applied frequency to the piezoelectric diaphragms. The experimental data revealed that the maximum flow rate is 31.15 mL/min at a frequency of 100 Hz. Our proposed design is not only for a specific application but also useful in a wide range of biomedical applications.

Capillary pumping independent of the liquid surface energy and viscosity

Capillary pumping is an attractive means of liquid actuation because it is a passive mechanism, i.e., it does not rely on an external energy supply during operation. The capillary flow rate generally depends on the liquid sample viscosity and surface energy. This poses a problem for capillary-driven systems that rely on a predictable flow rate and for which the sample viscosity or surface energy are not precisely known. Here, we introduce the capillary pumping of sample liquids with a flow rate that is constant in time and independent of the sample viscosity and sample surface energy. These features are enabled by a design in which a well-characterized pump liquid is capillarily imbibed into the downstream section of the pump and thereby pulls the unknown sample liquid into the upstream pump section. The downstream pump geometry is designed to exert a Laplace pressure and fluidic resistance that are substantially larger than those exerted by the upstream pump geometry on the sample liquid. Hence, the influence of the unknown sample liquid on the flow rate is negligible. We experimentally tested pumps of the new design with a variety of sample liquids, including water, different samples of whole blood, different samples of urine, isopropanol, mineral oil, and glycerol. The capillary filling speeds of these liquids vary by more than a factor 1000 when imbibed to a standard constant cross-section glass capillary. In our new pump design, 20 filling tests involving these liquid samples with vastly different properties resulted in a constant volumetric flow rate in the range of 20.96–24.76 μL/min. We expect this novel capillary design to have immediate applications in lab-on-a-chip systems and diagnostic devices.

High-precision microfluidic pressure control through modulation of dual fluidic resistances

This work presents an approach to the modulation of dual fluidic resistances for long-term, high-speed, and high precision (less than 0.5% steady-state error) control of the inlet pressure of a microfluidic device. This is accomplished through independent controls of dual variable resistances in a fluid network between a pressurized reservoir and a microfluidic device. We show the superior characteristics of the system with dual resistance modulation by experimentally comparing our new model with our previous approach. We demonstrate the performance of the controlled system and address the long-term stability and robustness. This system can be utilized in a variety of applications that require high-precision, high-speed, and long-term controls of microfluidic flows, including chemical synthesis, cell sorting, energy harvesting optofluidics, microbial fuel cells, and multiscale biological investigation of cellular or tissue level.