The effect of the microfluidic diodicity on the efficiency of valve-less rectification micropumps using Lattice Boltzmann Method

The efficiency of the valve-less rectification micropump depends primarily on the microfluidic diodicity (the ratio of the backward pressure drop to the forward pressure drop). In this study, different rectifying structures, including the conventional structures (nozzle/diffuser and Tesla structures), were investigated at very low Reynolds numbers (between 0.2 and 60). The rectifying structures were characterized with respect to their design, and a numerical approach was illustrated to calculate the diodicity for the rectifying structures. In this study, the microfluidic diodicity was evaluated numerically for different rectifying structures including half circle, semicircle, heart, triangle, bifurcation, nozzle/diffuser, and Tesla structures. The Lattice Boltzmann Method (LBM) was utilized as a numerical method to simulate the fluid flow in the microscale. The results suggest that at very low Reynolds number flow, rectification and multifunction micropumping may be achievable by using a number of the presented structures. The results for the conventional structures agree with the reported results.     

Polymer microvalve with pre-stressed membranes for tunable flow�Cpressure characteristics

A novel, inexpensive, polymer-based valve approach is presented that offers the combination of a check valve’s rectifying properties with the possibility to actively control the flow rate in the forward (open) direction. An elastic membrane with an incision is clamped between two rigid polymer plates and expands when pressure is applied in the forward direction allowing fluid to pass. In the backward direction, expansion of the membrane is not possible and flow is prevented. Varying the clamping force influences the expansion capability of the membrane due to its elasticity and thus enables control of the flow rate.     

Topology optimization of a no-moving-part valve incorporating Pareto frontier exploration

No-moving-part (NMP) valves, such as Tesla valves, are engineered fluid channels whose flow resistance depends on the flow direction. They have no moving parts and do not deform, but rely on inertial forces of the fluid to preferentially allow flow in one direction while strongly inhibiting flow in the reverse direction. NMP valves have significant advantages over active valves in terms of their reliability and easy manufacturability. Several previous studies have explored optimum designs of NMP valves, and the most widely used indicator of NMP valve performance is diodicity, defined as the ratio of the pressure drop of reverse flow to that of the forward flow. However, higher diodicity does not necessarily imply a lower pressure drop for the forward flow, and if this pressure drop is too high, significant pumping power is required, which makes the NMP valve inefficient for use in pumping applications. Therefore, for the design NMP valves, treating the forward and reverse flow pressure drops independently in a multiobjective formulation is preferable to optimization of the diodicity alone. In this paper, we propose a bi-objective topology optimization method for an optimum design of an NMP valve. One objective function is to minimize the pressure drop in the forward flow, and the other is to maximize the pressure drop in the reverse flow. A numerical example is provided to illustrate the effectiveness of the proposed method.     

A Nuclear Spin Valve: Towards the Read-out of Single Nuclear Spin Qubits

We propose a scheme to read out qubits defined in single nuclear spins—addressing one of the main obstacles on the way to a solid state NMR quantum computer. It is based on a “spin valve” between bulk nuclear spin systems that is highly sensitive to the state of the qubit spin. We suggest a concrete realization of that detector in a Si lattice and show that it can be operated over a broad range of experimental parameters. Transport of spin through the proposed spin valve is analogous to that of charge through an electronic nanostructure, but exhibits distinctive new features.      

Bi-objective topology optimization of asymmetrical fixed-geometry microvalve for non-Newtonian flow

Fixed-geometry microvalves such as Tesla microvalves rely on the inertial forces of the fluid to allow flow in the desired direction while inhibiting flow in undesired direction. In the traditional topology optimization design methods of fixed-geometry microvalves, single objective function is used to minimize the energy dissipation of forward flow. And several previous studies have widely used diodicity to indicate the performance of fixed-geometry microvalves, which is defined as the ratio of the pressure drop of reverse flow to that of the forward flow. However, higher diodicity does not reflect the degree of forward energy dissipation, leading to a significant pumping power is required to drive flow. Therefore, treating the forward flow pressure drop and its performance independently by a bi-objective formulation is preferable to design fixed-geometry microvalve. This paper proposes a bi-objective topology optimization design method and uses the regularization constraint to design asymmetrical fixed-geometry microvalve for non-Newtonian flow. Several numerical examples with different bifurcation angles, Darcy number and weight coefficients of the bi-objective functions are studied and the validity of the topology optimization method presented in this paper is demonstrated.

     

Micromixing via recirculatory flow generated by an oscillatory microplate

Mixing in micro-environment has been explored in a number of studies. This study presents a unique approach of efficient mixing of two heterogeneous streams via two counter-rotating recirculatory streams induced by in-plane resonance of a rectangular microplate actuated via Lorentz force. The generated time-mean flow structure was interrogated for mixing efficacy over a range of excitation voltage, Reynolds number, and Pèclet number, along with numerical analysis to probe the time-mean flow physics. Results show that the recirculatory flow is generated at the plate’s edges due to local flow non-linearity, characteristic of acoustic streaming. The percentage of mixing, at one device length-scale downstream, attains 93% at a low Reynolds number of 0.0037 (based on mean velocity of 0.078 mm/s and channel height of 50 μm) at 8 V excitation. Further characterization via enhanced diffusivity shows a maximum of 80.7-fold increase. Comparison with other active mixers shows the current device achieves mixing in one of the shortest distances. The proposed approach is robust, tunable to attain desired mixing characteristics and essentially independent of the properties of the fluid medium, which should be useful in a number of microfluidic applications requiring fast mixing.     

A fast microfluidic mixer based on acoustically driven sidewall-trapped microbubbles

Due to the low Reynolds number associated with microscale fluid flow, it is difficult to rapidly and homogenously mix two fluids. In this letter, we report a fast and homogenized mixing device through the use of a bubble-based microfluidic structure. This micromixing device worked by trapping air bubbles within the pre-designed grooves on the sidewalls of the channel. When acoustically driven, the membranes (liquid/air interfaces) of these trapped bubbles started to oscillate. The bubble oscillation resulted in a microstreaming phenomenon—strong pressure and velocity fluctuations in the bulk liquid, thus giving rise to fast and homogenized mixing of two side-by-side flowing fluids. The performance of the mixer was characterized by mixing deionized water and ink at different flow rates. The mixing time was measured to be as small as 120 ms.     

Liquid and gas flows in microchannels of varying cross section: a comparative analysis of the flow dynamics and design perspectives

This paper presents a comparative study of the flow of liquid and gases in microchannels of converging and diverging cross sections. Towards this, the static pressure across the microchannels is measured for different flow rates of the two fluids. The study includes both experimental and numerical investigations, thus providing several useful insights into the local information of flow parameters as well. Three different microchannels of varying angles of convergence/divergence (4°, 8° and 12°) are studied to understand the effect of the angle on flow properties such as pressure drop, Poiseuille number and diodicity. A comparison of the forces involved in liquid and gas flows shows their relative significance and effect on the flow structure. A diodic effect corresponds to a difference in the flow resistance in a microchannel of varying cross section, when the flow is subjected alternatively to converging and diverging orientations. In the present experiments, the diodic effect is observed for both liquid and gas as working fluids. The effect of governing parameters—Reynolds number and Knudsen number, on the diodicity is analysed. Based on these results, a comparison of design perspectives that may be useful in the design of converging/diverging microchannels for liquid and gas flows is provided.     

Lateral air cavities for microfluidic pumping with the use of acoustic energy

An acoustically activated micropump is fabricated and demonstrated using a single step lithography process and an off-chip acoustic energy source. Using angled lateral cavities with trapped air bubbles, acoustic energy is used to oscillate the liquid–air interface to create a fluidic driving force. The angled lateral cavity design allows for fluid rectification from the first-order pulsatile flow of the oscillating bubbles. The fluid rectification is achieved through the asymmetrical flow produced by the oscillating interface generating fluid flow away from the lateral cavity interface. Simulation and experimental results are used to develop a pumping mechanism that is capable of driving fluid at pressures of 350 Pa. This pumping system is then integrated into a stand-alone battery operated system to drive fluid from one chip to another.     

Single-phase laminar and turbulent heat transfer in smooth and rough microtubes

An experimental campaign was carried out studying laminar and turbulent heat transfer in uniformly heated smooth glass and rough stainless steel microtubes from 0.5 mm down to 0.12 mm. Heat transfer in turbulent regime proved to be coherent—within experimental accuracy—with the classic Gnielinski correlation for the Nusselt number. For the laminar case, an anomalous drop in Nusselt number for decreasing Reynolds number was observed in the smooth glass tubes. As the stainless steel tubes manifested relatively normal diabatic behaviour in this regime (apart from the evident influence of the thermal development region that increases heat transfer above the thermally fully developed value), the explanation of this unexpected diminution of the Nusselt number must be sought in the dispersion of heat, put in externally through the thin film deposited on the glass tube outer surface, to peripheral attachments to the test section. This distorts the measured energy balance of the experiment, especially as the convective force of the fluid diminishes, resulting in lower Nusselt numbers at lower Reynolds numbers.     

Application of reconfigurable pinhole mask with excimer laser to fabricate microfluidic components

An excimer laser incorporating a reconfigurable intelligent pinhole mask (IPM) is demonstrated for the fabrication of microfluidic geometries on a poly(methyl methacrylate) substrate. Beam reconfiguration techniques are used to overcome some of the drawbacks associated with traditional scanning laser ablation through a static mask. The production of zero lead-in (ZLI) features are described, where the ramp lead-in angle—inherent to scanning laser ablation—is reduced to be in line with the cross-sectional side-wall angle of the microchannel itself. The technique is applied to eliminate under-cutting and ramping at channel junctions—features resulting from scanning ablation through a fixed mask—and produce flat crossing sections, junctions and inlets. The development of a prediction model for microchannel visualisation and refinement prior to the fabrication step is also described. The model includes variables from the IPM, laser, scanning stage and material etch rate allowing quantitative measurement of generated microchannel geometry. One application of the model is the development of microchannel mixing geometry which is analysed using computational fluid dynamic (CFD) techniques. For this purpose, the effect of varying the overall channel geometry on mixing within a microchannel was investigated for flows with low Reynolds numbers. The resulting geometry is found to reduce the distance required for mixing by 50% in comparison to a straight planar channel, thereby enabling smaller device geometries.     

初始条件对滑阀性能影响的数值研究

对流体通过滑阀的阶跃响应进行了数值解析计算.分别在静止流体和稳定流体的初始条件下,给出了压力差为阶跃变化时液流通过滑阀的流线图.讨论了初始条件和压力差对射流角、流量,流量系数和雷诺数等的影响,做出了不同初始条件下,射流角、流量,流量系数及雷诺数与压力差的关系图.数值计算的结果表明,当压力差大于临界值时,由于初始条件的不同,将导致两个迥然不同的射流流动.在初始条件为静止流体的情况下,其射流角、流量,流量系数和雷诺数比初始条件为稳定流体的情况下的对应值小.     

Molecular spin in nano-confined fluidic flows

In this paper, we study the effect of molecular spin on the fluid dynamics of molecular nano-confined fluids using the extended Navier–Stokes equations. We show that the effect of spin is non-negligible for non-steady flows and we then discuss two examples, namely, a zero mean oscillatory flow and an oscillatory lid driven cavity flow. In the discussion of the former, we propose a dimensionless quantity that qualitatively predicts the effect of the spin. From this it is shown that only for sufficiently small system sizes and extremely high frequencies will molecular spin be relevant, depending on the molecular fluid’s rotational inertia and the rotational viscosity. In the lid driven cavity flow we observe that the thermodynamic energy dissipation due to molecular spin undergoes period doubling when increasing the Reynolds number and, under some circumstances, it may be negative.     

Subdynamic asymptotic behavior of microfluidic valves

Decreasing the Reynolds number of microfluidic no-moving-part flow control valves considerably below the usual operating range leads to a distinct "subdynamic" regime of viscosity-dominated flow, usually entered through a clearly defined transition. In this regime, the dynamic effects on which the operation of large-scale no-moving-part fluidic valves is based, cease to be useful, but fluid may be driven through the valve (and any connected load) by an applied pressure difference, maintained by an external pressure regulator. Reynolds number ceases to characterize the valve operation, but the driving pressure effect is usefully characterized by a newly introduced dimensionless number and it is this parameter which determines the valve behavior. This summary paper presents information about the subdynamic regime using data (otherwise difficult to access) obtained for several recently developed flow control valves. The purely subdynamic regime is an extreme. Most present-day microfluidic valves are operated at higher Re, but the paper shows that the laws governing subdynamic flows provide relations useful as an asymptotic reference.     

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.     

A high-shear, low Reynolds number microfluidic rheometer

We present a microfluidic rheometer that uses in situ pressure sensors to measure the viscosity of liquids at low Reynolds number. Viscosity is measured in a long, straight channel using a PDMS-based microfluidic device that consists of a channel layer and a sensing membrane integrated with an array of piezoresistive pressure sensors via plasma surface treatment. The micro-pressure sensor is fabricated using conductive particles/PDMS composites. The sensing membrane maps pressure differences at various locations within the channel in order to measure the fluid shear stress in situ at a prescribed shear rate to estimate the fluid viscosity. We find that the device is capable to measure the viscosity of both Newtonian and non-Newtonian fluids for shear rates up to 10⁴ s^?1 while keeping the Reynolds number well below 1.     

Tangibles for learning: a representational analysis of physical manipulation

Manipulatives—physical learning materials such as cubes or tiles—are prevalent in educational settings across cultures and have generated substantial research into how actions with physical objects may support children’s learning. The ability to integrate digital technology into physical objects—so-called ‘digital manipulatives’—has generated excitement over the potential to create new educational materials. However, without a clear understanding of how actions with physical materials lead to learning, it is difficult to evaluate or inform designs in this area. This paper is intended to contribute to the development of effective tangible technologies for children’s learning by summarising key debates about the representational advantages of manipulatives under two key headings: offloading cognition—where manipulatives may help children by freeing up valuable cognitive resources during problem solving, and conceptual metaphors—where perceptual information or actions with objects have a structural correspondence with more symbolic concepts. The review also indicates possible limitations of physical objects—most importantly that their symbolic significance is only granted by the context in which they are used. These arguments are then discussed in light of tangible designs drawing upon the authors’ current research into tangibles and young children’s understanding of number.     

Local heat transfer process and pressure drop in a micro-channel integrated with arrays of temperature and pressure sensors

One of the most important components in micro-fluidic system is the micro-channel which involves complicated flow and transport process. This study presents micro-scale thermal fluid transport process inside a micro-channel with a height of 37 μm. The channel can be heated on the bottom wall and is integrated with arrays of pressure and temperature sensors which can be used to measure and determine the local heat transfer and pressure drop. A more simplified model with modification of Young’s Modulus from the experimental test is used to design and fabricate the arrays of pressure sensors. Both the pressure sensors and the channel wall use polymer materials which greatly simplify the fabrication process. In addition, the polymer materials have a very low thermal conductivity which significantly reduces the heat loss from the channel to the ambient that the local heat transfer can be accurately measured. The air flow in the micro-channel can readily become compressible even at a very low Reynolds number condition. Therefore, simultaneous measurement of both the local pressure drop and the temperature on the heated wall is required to determine the local heat transfer. Comparison of the local heat transfer for a compressible air flow in micro-channel is made with the theoretical prediction based on incompressible air flow in large-scale channel. The comparison has clarified many of the conflicting results among different works.     

Spatially resolved temperature measurement in microchannels

Non-intrusive local temperature measurement in convective microchannel flows using infrared (IR) thermography is presented. This technique can be used to determine local temperatures of the visualized channel wall or liquid temperature near this wall in IR-transparent heat sinks. The technique is demonstrated on water flow through a silicon (Si) microchannel. A high value of a combined liquid emissivity and substrate overall transmittance coupled with a low uncertainty in estimating this factor is important for quantitative temperature measurement using IR thermography. The test section design, and experimental and data analysis procedures that provide increased sensitivity of the detected intensity to the desired temperature are discussed. Experiments are performed on a 13-mm long, 50 μm wide by 135 μm deep Si microchannel at a constant heat input to the heat sink surface for flow rates between 0.6 and 1.2 g min⁻¹. Uncertainty in fluid temperature varies from a minimum of 0.60°C for a Reynolds number (Re) of 297 to a maximum of 1.33°C for a Re of 251.     

A disposable,integrated loop-mediated isothermal amplification cassette with thermally actuated valves

An inexpensive, disposable, integrated, polymer-based cassette for loop-mediated isothermal amplification (LAMP) of target nucleic acids was designed, fabricated, and tested. The LAMP chamber was equipped with single-use, thermally actuated valves made with a composite consisting of a mixture of PDMS and expandable microspheres. The effect of the composite composition on its expansion was investigated, and the valve’s performance was evaluated. In its closed state, the valve can hold pressures as high as 200 kPa without any significant leakage. Both the LAMP chamber and the valves were actuated with thin film heaters. The utility of the cassette was demonstrated by carrying out LAMP of Escherichia coli DNA target and reverse transcribed loop meditated isothermal amplification (RT-LAMP) of RNA targets. The amplicons were detected in real time with a portable, compact detector. The system was capable of detecting as few as 10 target molecules per sample in well under 1 h. The portable, integrated cassette system described here is particularly suited for applications at the point of care and in resource-poor countries, where funds and trained personnel are in short supply.