External Flow Control Research Articles

OptoAssay-Light-controlled dynamic bioassay using optogenetic switches.

Circumventing the limitations of current bioassays, we introduce a light-controlled assay, OptoAssay, toward wash- and pump-free point-of-care diagnostics. Extending the capabilities of standard bioassays with light-dependent and reversible interaction of optogenetic switches, OptoAssays enable a bidirectional movement of assay components, only by changing the wavelength of light. Demonstrating exceptional versatility, the OptoAssay showcases its efficacy on various substrates, delivering a dynamic bioassay format. The applicability of the OptoAssay is successfully demonstrated by the calibration of a competitive model assay, resulting in a superior limit of detection of 8 pg ml-1, which is beyond those of conventional ELISA tests. In the future, combined with smartphones, OptoAssays could obviate the need for external flow control systems such as pumps or valves and signal readout devices, enabling on-site analysis in resource-limited settings.

Flow mechanism of a highly loaded low pressure turbine cascade with integrated-optimized end wall contouring and root lean

The end wall loss of modern highly loaded low pressure turbine (LPT) has been greatly increased, due to the enhanced secondary flow loss and boundary layer separation loss. Thus, it is of great significance to develop effective flow control strategies to improve the end wall flow condition and aerodynamic performance of modern LPT. This research carried out a numerical investigation on the coupled flow control strategy, which combined non-axisymmetric end wall contouring (NEC) and root tangential lean (RTL), based on a highly loaded LPT cascade (Zweifel = 1.59). Meanwhile, the optimization process was used to get the optimal design parameters of the coupled method NEC&RTL. The results indicate that the optimal coupled configuration can reduce the total pressure loss coefficient by 12.68% and the non-dimensional secondary kinetic energy by 23.91%. Compared with the reference cascade without modification, the coupled method is found to improve the end wall flow conditions: the passage vortex is weakened both in size and strength, mainly attributed to the smaller cross-passage pressure gradient resulting from NEC; the closed separation bubble near end wall and the three-dimensional separation flow before trailing edge are eliminated, due to the great downward pressure gradient near end wall resulting from RTL; and the counter vortex is eliminated and the slender back flow is weakened under the additional coupling flow control effect of NEC&RTL. Therefore, the coupled flow control method can not only highlight the advantages of the independent methods, but also induce external flow control superiorities, demonstrating the application prospect of the coupled flow control strategy on the highly loaded LPT.

Impact of Stratification Mechanisms on Turbulent Characteristics of Stable Open-Channel Flows

Abstract Flow over a surface can be stratified by imposing a fixed mean vertical temperature (density) gradient profile throughout or via cooling at the surface. These distinct mechanisms can act simultaneously to establish a stable stratification in a flow. Here, we perform a series of direct numerical simulations of open-channel flows to study adaptation of a neutrally stratified turbulent flow under the combined or independent action of the aforementioned mechanisms. We force the fully developed flow with a constant mass flow rate. This flow forcing technique enables us to keep the bulk Reynolds number constant throughout our investigation and avoid complications arising from the acceleration of the bulk flow if a constant pressure gradient approach were to be adopted to force the flow instead. When both stratification mechanisms are active, the dimensionless stratification perturbation number emerges as an external flow control parameter, in addition to the Reynolds, Froude, and Prandtl numbers. We demonstrate that significant deviations from the Monin–Obukhov similarity formulation are possible when both types of stratification mechanisms are active within an otherwise weakly stable flow, even when the flux Richardson number is well below 0.2. An extended version of the similarity theory due to Zilitinkevich and Calanca shows promise in predicting the dimensionless shear for cases where both types of stratification mechanisms are active, but the extended theory is less accurate for gradients of scalar. The degree of deviation from neutral dimensionless shear as a function of the vertical coordinate emerges as a qualitative measure of the strength of stable stratification for all the cases investigated in this study.

A new microchannel capillary flow assay (MCFA) platform with lyophilized chemiluminescence reagents for a smartphone-based POCT detecting malaria

There has been a considerable development in microfluidic based immunodiagnostics over the past few years which has greatly favored the growth of novel point-of-care-testing (POCT). However, the realization of an inexpensive, low-power POCT needs cheap and disposable microfluidic devices that can perform autonomously with minimum user intervention. This work, for the first time, reports the development of a new microchannel capillary flow assay (MCFA) platform that can perform chemiluminescence based ELISA with lyophilized chemiluminescent reagents. This new MCFA platform exploits the ultra-high sensitivity of chemiluminescent detection while eliminating the shortcomings associated with liquid reagent handling, control of assay sequence and user intervention. The functionally designed microchannels along with adequate hydrophilicity produce a sequential flow of assay reagents and autonomously performs the ultra-high sensitive chemiluminescence based ELISA for the detection of malaria biomarker such as PfHRP2. The MCFA platform with no external flow control and simple chemiluminescence detection can easily communicate with smartphone via USB-OTG port using a custom-designed optical detector. The use of the smartphone for display, data transfer, storage and analysis, as well as the source of power allows the development of a smartphone based POCT analyzer for disease diagnostics. This paper reports a limit of detection (LOD) of 8 ng/mL by the smartphone analyzer which is sensitive enough to detect active malarial infection. The MCFA platform developed with the smartphone analyzer can be easily customized for different biomarkers, so a hand-held POCT for various infectious diseases can be envisaged with full networking capability at low cost.

On-chip high density droplet-on-template (DOT) array

In this report, we present a new method for generating a high-density (2D) droplet array using double-layered polydimethylsiloxane (PDMS) templates containing honeycomb microwells. Without external flow control, a droplet-on-template (DOT) was created by utilizing capillary forces associated with the interfacial tension between the aqueous and oil phases. The DOT process involved three simple steps: (1) vacuum-assisted filling of microwells; (2) excess water removal; and (3) covering the droplet array with oil. To demonstrate the concept of the DOT, we generated spherical water droplets 147, 191, 238, 326 and 405 μm in diameter from corresponding microwells with lengths of 200, 300, 400, 600 and 800 μm, respectively and a height of 76 μm (up to ~10,000 droplets on a template 25 × 25 mm). Two important factors, including the aspect ratio (height-to-length ratio) of the microwell and the interfacial tension of the two phases, were investigated to understand how those factors affect the shape of the droplets (‘sphere’ or ‘dome’). All the droplets were spherical up to an aspect ratio of 0.55. The droplets were dome-shaped for aspect ratios above 0.82. For a 1 mM sodium dodecyl sulfate (SDS) solution, the use of mineral oil (which had the highest interfacial tension studied) produced spherical droplets, but dome-shaped droplets were produced by corn oil and oleic acid.

Energy efficient hotspot-targeted embedded liquid cooling of electronics

Large data centers today already account for nearly 1.31% of total electricity consumption with cooling responsible for roughly 33% of that energy consumption. This energy intensive cooling problem is exacerbated by the presence of hotspots in multicore microprocessors due to excess coolant flow requirement for thermal management. Here we present a novel liquid-cooling concept, for targeted, energy efficient cooling of hotspots through passively optimized microchannel structures etched into the backside of a chip (embedded liquid cooling or ELC architecture). We adopt an experimentally validated and computationally efficient modeling approach to predict the performance of our hotspot-targeted ELC design. The design is optimized for exemplar non-uniform chip power maps using Response Surface Methodology (RSM). For industrially acceptable limits of approximately 0.4bar (40kPa) on pressure drop and one percent of total chip power on pumping power, the optimized designs are computationally evaluated against a base, standard ELC design with uniform channel widths and uniform flow distribution. For an average steady-state heat flux of 150W/cm2 in core areas (hotspots) and 20W/cm2 over remaining chip area (background), the optimized design reduces the maximum chip temperature non-uniformity by 61% to 3.7°C. For a higher average, steady-state hotspot heat flux of 300W/cm2, the maximum temperature non-uniformity is reduced by 54% to 8.7°C. It is shown that the base design requires a prohibitively high level of pumping power (about 2000 fold for 150W/cm2 case and 600 fold for 300W/cm2 case) to match the thermal performance of the optimized, hotspot-targeting designs. The pumping power requirement for optimized designs is only 0.23% and 0.17% of the total chip power for 150W/cm2 and 300W/cm2 hotspot heat flux respectively. Moreover, the optimized designs distribute the coolant flow without any external flow control devices and the performance is only marginally affected by the manifold geometry used to supply the coolant to the microchannel heat transfer structure. This also attests to the robustness of the optimized embedded microchannel structures.

Investigation of alternating-flow mixing in microchannels

A number of different approaches to mixing liquids in microscale systems can be found in the literature. In the case of miscible liquids it is desirable to produce mixtures with residual non-uniformity in composition that is below some specified level. Yet very little quantitative information is available concerning the conditions required to produce a given level of mixture uniformity. A theoretical approach to this problem is described. Computational fluid dynamics and simple scaling are used to develop a quantitative understanding of the alternating flow method of mixing using pressure driven flow. In this approach, external flow control is used to produce alternating injection into a single microchannel of two or more solutions to be mixed. The resulting streamwise slugs of solution then mix by the stretching of the slugs into thin striations resulting from shear strain. The most challenging condition for mixing is where the Reynolds number is approaching zero and inertia effects are negligible, a common situation in microchannel flows, particularly where relatively high-viscosity liquids, for example ionic liquids, are involved. The scaling theory demonstrates that an initial time period of rapid mixing of fluid outside the core of the flow, scaling as Pe - 2 / 3 , is followed by a far slower process of mixing in the core region, scaling as Pe - 1 / 2 . An approximate correlation for the deviation from the perfectly mixed state as a function of time is found. This correlation applies over the range of Peclet number, slug length and solution mixture ratio that are of interest. The mixture uniformity produced is shown to be limited by the initial uniformity of each solution over the channel section resulting from the injection process.

Dispersion in capillary electrophoresis with external flow control methods

Recently, methods for controlling the bulk flow of the buffer which are independent of the electrophoretic migration have been demonstrated in capillary electrophoresis (CE) systems. These methods include controlling the electroosmosis by radially directed electric fields, and controlling the bulk flow by siphoning. This paper investigates basic dispersion mechanisms at work by examining the dispersion of dimethyl sulfoxide under various external control methods. It is found that the electroosmotic flow control methods exhibit the same dispersive behavior as the conventional CE system. In particular, the results for pH 7 buffers obey the theoretical expression for the plate height in all configurations. However, in all configurations, pH 2.7 buffers do not follow the theory, and counter to intuition, additional pressure can improve the plate height at pH 2.7. Taking all these methods together, one can develop a technique to appropriately adjust the electrophoretic and bulk flow to optimize resolution.

Tapered land hydrostatic oil bearings

A previous paper by Coombs and Dowsondiscussed the effects of non-parallelism of the sealing lands on the performance of a hydrostatic bearing. This paper is concerned with the advantages of incorporating a taper on part or whole of the bearing land. Analytical solutions to the load, leakage and power loss of such bearings are given, together with comprehensive results which are presented graphically for design purposes. The authors conclude that tapered-land hydrostatic oil bearings have a high inherent stiffness which permits their use without a conventional external flow control as predicted by Hirs, and that for optimum conditions part of the land should remain parallel