In this research, a multilayer spiral microchannel heat exchanger was employed as a cooling core, and the effects of altering the number of parallel layers on the thermal transport performance were studied. Physical devices and numerical models via FLUENT with between two and twenty layers were tested. Additional layers were shown to improve the thermal transport efficiency, with a minimal impact on required pumping power, up to a maximum of 16 layers. Taking into consideration the fabrication cost and manufacturing time, the 10-layer device was selected as the optimal cooling core, with an observed thermal performance saturation occurring around Re = 500, De = 150. The combination of up to six parallel cooling cores form a novel multicore heat exchanger system, and thus the effect of varying the number of cores was also analysed, finding a consistent improvement with additional cores. In this way, the system can be tuned for any application by varying the number of cores employed, up to a maximum of 330 W of cooling with six. Different materials were also compared, namely PDMS, PMMA and copper. Under low flow rate conditions (roughly Re ≤ 200, De ≤ 60), polymer cooling cores were found to achieve comparable or even superior heat transfer to copper cores. Whilst some differences in readings were noted between the numerical and laboratory results, likely due to the fabrication processes, the simple model outlined here can be used to predict the trends of cooling cores, permitting further development of the microfluidic design.

This paper presents two optimized designs of a commonly-used fluid distribution manifold having one entrance and six exits. Numerical simulations were carried out to optimize the dimensions and mechanisms of these proposed designs for the sake of enhancing the uniformity of fluid distribution amongst the exits and reducing the formation of dead zones inside the manifold cavities. Particularly, to make the fluid distribution amongst exits more uniform, this study explored the relationship between entrance diameter and exit diameter. Furthermore, in order to reduce dead zone formations inside the manifold whilst still maintaining uniform fluid distribution, a conical cavity was designed. After that, blockers were designed to replace some exits, permitting a variable number of fluid distribution manifold exits, depending on the specific application. Both designs were found to be able to improve flow uniformity and dead zone reduction compared to the original commonly-used fluid distribution manifold, with the central-feeding distributor performing slightly better than the lateral-feeding distributor overall. From the perspective of manufacturing, each of these two fluid manifolds was made of two pieces with glue and rubber O ring used respectively as the bond between separate pieces. Preliminary experiments with these devices suggest similar results to those from the numerical studies. Based on real application requirements and limitations, the different fluid manifold designs with tunable dimensions can be utilized in various mechanical or biochemical devices to distribute fluid equally amongst several parallel components.

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilize the enhanced fluid mixing characteristics of Dean vortices and thus transferring heat efficiently. Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimizing the heat exchanger design based on the parameters of maximizing heat transfer while minimizing pressure drop and unit cost. A range of cross-sectional geometries for the curved channels was compared, showing significantly higher Nusselt numbers than equivalent straight channels throughout and finding superior performance factors for square, circular, and symmetrical trapezoidal profiles. Due to the difficulty and expense in manufacturing circular microchannels, the relatively simple to fabricate square and symmetrical trapezoidal channels are put forward as the most advantageous designs. The variation of Nusselt number over the length of the channel for a range of different curvatures (and hence Dean numbers) is also examined, showing significantly higher heat transfer occurring in strongly curved channels, especially in areas where the generated Dean vortices are strongest. The variation in Nusselt number was found to form the shape of an "arc." In this way, a relationship between the Dean number and the Nusselt number is characterized and discussed, leading to suggestions regarding optimal microfluidic heat transfer design.

The heat transfer performance of two roll-to-roll microchannel heat exchangers with square cross section and side length ranging from 0.2mm to 0.5mm were investigated via numerical studies. In order to assess the heat transfer enhancement, equivalent straight channel heat exchangers were also researched numerically as comparisons. For the roll-to-roll devices, numerical studies demonstrated that there were two reasons for heat transfer enhancement. First, when the average Dean number of the fluid was greater than approximately 10, Dean vortices started to form within the roll-to-roll microchannels, enhancing the convective heat transfer between channels. Second, the compact roll-to-roll structure of the heat exchangers increased the area of heat transfer compared with straight microchannel equivalents, and thus promoted the conductive heat transfer. Numerical simulations noted both higher Nusselt numbers and higher thermal performance factors (TPF) for roll-to-roll microchannel heat exchangers compared with equivalent straight channels and were employed to optimize both the microchannel cross section dimensions and the wall thickness between channels. In addition, the swirling strength and the heat transfer area were also calculated to characterize the convective and conductive heat transfer, respectively, allowing for a comparison between two rollto- roll microchannel heat exchanger designs.

This work presents the simple and rapid fabrication of a polymer-based microfluidic prototype manufactured by rolling up thin films of polymer. The thin films were fabricated via a casting method and rolled up around a center core with the aid of plasma activation to create a three-dimensional (3D) spiral microchannel, hence reducing the time and cost of manufacture. In this work, rolled-up devices with single or dual fluidic networks fabricated from a single or two films were demonstrated for heat sink or heat exchanger applications, respectively. The experimental results show good heat transfer in the rolled-up system at various flow rates for both heat sink and heat exchanger devices, without any leakages. The rolled-up microfluidic system creates multiple curved channels, allowing for the generation of Dean vortices, which in turn lead to an enhancement of heat and mass transfer and prevention of fouling formation. These benefits enable the devices to be employed for many diverse applications, such as heat-transfer devices, micromixers, and sorters. To our knowledge, this work would be the first report on a microfluidic prototype of 3D spiral microchannel made from rolled-up polymeric thin film. This novel fabrication approach may represent the first step towards the development of a pioneering prototype for roll-to-roll processing, permitting the mass production of polymer-based microchannels from single or multiple thin films.

In this work, we proposed the new design of two spiral networks interconnected with an expansion chamber to create vortex and disruption of the laminar boundary of intermingled streams which can efficiently boost mass transport in the microchannel. Confocal microscope is used to observe the mixing and fluid motion in the microchannel. We observed the evolution of Dean vortices along the spiral inlet channel and the disruption of stream boundary at the expansion chamber which are further pulled along the outlet spiral channel resulting in higher mixing efficiency compared to that without chamber. Laminar flow of two fluids still maintained at the end of the normal spiral networks but the perfect mixing can be achieved at a given flow rates (Re of 15 to 45) from our design. Furthermore, uniform mixing can be obtained even at the spiral channel with shorter channel length. Unlike other complex designs, the design of the expansion chamber does not increase the pressure drop of the microchannel system and its dimension is larger than that of the main channel allowing to be fabricated by using conventional fabrication approach.

Polymer-based heat exchangers are advantageous for corrosion resistance, light weight, and low costs.Microchannel heat exchangers are promising for material saving and high heat transfer capacity.Curved channels can promote the secondary fluid flow to enhance the effectiveness of heat transfer.Roll to roll technology makes it possible to roll thin-film layers up to form curved channels inside the layers.Combining all the strengths mentioned above, this research comes up with a polymer-based roll-to-roll microchannel heat exchanger with three different designs.Through numerical simulations, this study analyses the heat transfer characteristics of those three heat exchanger designs.Dean numbers and Nusselt numbers are calculated to evaluate the formation of Dean Vortices and the effectiveness of heat transfer.Simulation results demonstrate that the formation of Dean Vortices induced through the roll-to-roll microchannel structure helps to improve the heat transfer compared with equivalent straight-channel heat exchangers.In addition, this study calculates not only the Nusselt numbers but also the Thermal Performance Factors to take into account the high pressure drops in the microchannel.Finally, the efficiencies of heat transfer of the three heat exchanger designs are compared with each other.

There exist numerous methods of experimentally testing designs for heat sinks in the laboratory, especially for microscale fluidic devices, which can lead to a problem for comparison between new studies and those in the literature. To explore this issue, laboratory-based experiments on the heat transfer and flow impedance properties of a sample microchannel heat sink were repeated over a varying range of equipment. Three types of heat source (hot plate, film heater and copper block with cartridge heaters), two types of piping (polymer and metal), and the presence or absence of manifolds were investigated and the differences in heat sink performance were noted.Overall, especially in terms of achieving consistent, repeatable results, it was found that the arrangement of copper block heater, metal piping and the inclusion of manifolds was superior for this particular microchannel device. Hence, it is suggested that future testing of heat sinks and heat exchanger devices employ a similar arrangement of equipment for greater accuracy and comparability. In particular, the plastic tubing and hot plate configurations were found to have relatively poor consistency when testing the heat sink, and the film heater produced non-uniform heating, even over a small surface area.

Mass and heat transfer are well studied fundamental engineering principles with established relationships between them. Owing to experimental errors and uncertainties, heat transfer studies have been primarily conducted using numerical tools and analytical models. By studying mass transfer behavior, heat transfer characteristics can be obtained through existing correlation. Through this study we aim to understand the mixing performance in various curvature based designs namely a spiral channel, serpentine, saw tooth, square curve, U shaped and simple curved channel, all with a square cross section of hydraulic diameter 600μm. This was conducted through both numerical and experimental investigation over a Reynolds number range of 10-200 for both cases. From this work it can be concluded that a spiral channel is able to generate superior mixing performance in comparison to other curvature designs due to an increase in Dean strength along the channel length from inlet to the outlet. Spiral channels have also been found to be more advantageous due to their low pressure drop and reduced footprint area in comparison to the other curvature designs making it more favorable for microchip device integration. This experiment based investigation would also enable us to tailor micromixer and heat sink designs based on application.

Mixing in a microfluidic system is challenging due to dominant diffusion effects at a microscale (low Reynolds number). In this work, we report the improvement of mixing performance in spiral microchannels of varying cross-sectional geometry and hydraulic diameter. The formation of secondary flow interactions in spiral channels, known as Dean vortices, aid the primary diffusion process. The evolution of these Dean vortices was experimentally visualized along the length of the microchannel by confocal microscopy, and then compared to numerical studies. The cross-sectional geometries of the spiral channels, especially in the case of irregular shapes such as the semi-circular and trapezoidal profiles, were found to be an important factor in tuning the strength of Dean vortices, which in turn dictate the mixing performance, as opposed to diffusion which is more prominent at lower Re. This experiment-based finding has been validated via the evaluation of swirling strength of the working fluid, obtained using a numerical study. The results thus obtained show a mixing performance greater than 90% above a Reynolds number of 20 for most spiral channel designs, making this system suitable for high throughput operation with reduced pressure drop. This work is the first to experimentally and numerically demonstrate, within this operating range (20 < Re < 277), the impact on mixing performance in curved microchannels of varying cross-sectional geometries of constant cross-sectional area, and of varying hydraulic diameters for square shaped channels. The capability of these channels to operate at a moderately high Re with enhanced mixing performance and reduced pressure drop would be of great use in large-scale industrial operations, such as complex integrated micro-reactors wherein pressure drop plays a key role.

Two factors are of great importance to the overall performance and efficiency of a fluid distributor: uniform distribution and dead zone volume inside the cavity. From the perspective of improving uniform distribution and reducing dead zones, this research optimized the design of a commonly-used fluid distributor with one side inlet, eight outlets, and a cylinder cavity through numerical simulation. For equal-distribution, results demonstrated that after the inlet diameter was fixed, the distribution became equal continuously with the decrease of the outlet diameter. In addition, compared with one-side-inlet fluid distributor, two-side-inlet fluid distributor and middle-inlet fluid distributor promoted the uniformity of fluid distribution greatly. This study also reported that fluid distributor with an outlet angle of around 30° increased equal-distribution mostly compared with other angles. For dead zones inside the distributor cavity, the conical cavity reduced dead zones significantly compared with the fluid distributor with cylinder cavity. However, for the design of a fluid distributor, it is necessary to make a compromise between improving equal-distribution and reducing dead zones, therefore two optimized fluid distributors combining configurations proposed above were designed. Numerical simulation results illustrated that new fluid distributor designs promoted distribution among outlets more uniformly and reduced dead zones inside the distributor cavity tremendously.

Mechanical properties of the extracellular matrix (ECM) have been observed to influence the behavior of cells. Investigations on such an influence commonly rely on using soluble cues to alter the global intrinsic ECM properties in order to study the subsequent response of cells. This article presents an electromagnetic system for inducing a localized force gradient in an ECM, and reports the experimentally observed effect of such a force gradient on in vitro angiogenic sprouting of human microvascular endothelial cells (HMVECs). This force gradient is realized through the induction of magnetic forces on the superparamagnetic microparticle–embedded ECM (sECM). Both analytical and statistically meaningful experimental results demonstrate the effectiveness of this approach in influencing the behavior of a targeted HMVEC sprout without affecting that of other sprouts nearby. These results suggest the possibility of selectively controlling the in vitro behavior of cells by the induction of a localized force gradient in the ECM.

In order to improve upon a conventional straight microchannel heat sink, a range of curved, angular and wavy microchannels were designed in order to increase fluid mixing via the occurrence of secondary flow interactions, in particular Dean vortices, hence augmenting heat transport. Both numerical models conducted in FLUENT and laboratory experiments were employed to investigate the heat transfer enhancement of a range of geometries (single curved, wavy, sawtooth, U-turn and square-wave). In both studies, every channel demonstrated significantly higher Nusselt Numbers and Thermal Performance Factors (TPF) than an equivalent straight channel, despite an increase in pressure drop. The relative order of the channels in terms of TPF was the same for both experiments and numerical simulations, with the exception of the U-turn channel which performed better in the former. However, experimental TPF results were found to be 15 - 20% of those from the simulation - these differences are associated with the relative simplicity of the numerical model and additional non-linear impacts in the experiments. Overall, wavy channels were found to have superior performance, especially over angular channels with sharp turns, thus it is suggested that wavy microchannels are the most advantageous designs for the development of heat sinks, especially in terms of minimising pressure drop whilst still making use of the enhanced heat transfer properties of Dean vortices. Finally, for a given wavy channel, an optimal input flow rate condition is also determined.

Developments in micro-technology have seen vast improvements in the design and the thermal performance of heat sinks and heat exchangers, particularly in the case of spiral microfluidic devices which deals with the flow of liquids inside curved micrometer-sized channels. The current research deals with a specially designed curved microfluidic channel used to employ the fluid mixing characteristics of Dean vortices and thus transfer heat more efficiently. This curved microfluidic channel is deployed as a spiral channel to create an effective heat sink and a heat exchanger. The novel micro heat exchanger is built by integrating two or more of the specially designed microfluidic heat sink layers. For the ease of fabricating the microchannels, these devices are polymer-based. In this paper, the thermal performance of the spiral microfluidic devices is analyzed numerically and experimentally using a range of flow rates where Thermal Performance Factor is used to find a balanced point between heat transfer and pressure drop. The spiral heat exchange device proves to be an effective thermal transport system with the introduction of curved channels in the devices where the presence of Dean vortices in the system is observed, especially at lower flow rates. It can be observed that by increasing the number of layers, the thermal performance is greatly improved. This is due to the higher surface area with increasing number of layers, as well as a parallel flow structure through the layers. These results serve as a design parameter for developing microchannel-based heat transfer devices that can achieve high efficiency of heat and mass transfer. Further heat sink and heat exchanger design improvements are discussed.

Microchannel heat exchangers have become widely employed in modern systems, found within aerospace applications, waste heat recovery, water treatment processes, air conditioning, biomedical treatments and various industrial process applications. The microchannels increase the ratio of heat transfer surface to volume, thus improving the heat transfer performance significantly whilst reducing the overall weight and size. Moreover, by utilizing secondary flow from Dean Vortices induced by curved microfluidic channels, the fluid flow and heat transfer performance can be enhanced even further beyond conventional straight channels. However, since pressure drops found in microchannels are often quite high, channel lengths must be kept relatively short to balance the friction loss and energy consumption. Due to this, the developing region length at the microchannel entrance area has a greater impact than for macroscale channels, in terms of hydrodynamic and thermal performance over the remaining full developed region. The thermo-hydraulic design for heat transfer microchannel surfaces is strongly dependent on several dimensionless performance indicators, namely Nusselt number 'Nu' for heat transfer, and Poiseuille number 'Po', which is the product of Fanning friction factor 'f' and Reynolds number 'Re'. These parameters are used to characterize and optimize the performance of microchannel surfaces and heat exchangers in general, also can be used to determine both the thermal and hydraulic developing region lengths at the channel entrance area. Whilst many such studies exist for theoretical analysis and experimental verifications, currently there is little literature on the developing region lengths and impacts researched through the method of Computational Fluid Dynamics (CFD). As such, this paper identifies and explores via quantitative analysis the hydraulic and thermal performance changes created by the relevant developing region lengths at the entrance area of spiral microchannels, as well as determinations and comparisons of these effects over straight channels. The numerical results, generated via COMSOL Multiphysics and contrasted with previous literature on the subject, also compared with the effect of the developing region on the effectiveness and efficiency of both spiral and straight microchannels, finding an improved heat transfer performance but an increased impact of hydraulic friction as well for spiral channels against straight counterpart. Furthermore, significant differences between thermal developing region length and hydraulic developing region length can be observed throughout, which illustrates high challenge and the need for compromise in microchannel design. In this way, implications for the configuration and design of industrial microchannels and micro heat exchangers are self-evident. All the key factors given in this paper are dimensionless, and thus the generated results can be utilized for a variety of flow conditions. Hence, this work should permit an increased understanding for and boost the curved microchannel and micro heat exchanger designs subsequently, through reducing the required numbers of tests and experiments and expediting the development for similar applications followed.

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently.Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. A range of cross-sectional geometries for the curved channels were compared, showing significantly higher Nusselt Numbers than equivalent straight channels throughout, and finding superior performance factors for square, circular and symmetrical trapezoidal profiles. Due the difficulty and expense in manufacturing circular microchannels, the relatively simple to fabricate square and symmetrical trapezoidal channels are put forward as the most advantageous designs. These results take into account both constant wall temperature and constant heat flux conditions. For a given set of channel dimensions, an optimal input flow rate condition is also determined.

Advancements in the field of microfluidics has led to an increasing interest to study laminar flow in microchannel and its potential applications. Understanding mixing at a microscale can be useful in various biological, heating and industrial applications due to the space and time reduction that micro mixing permits. This work aims to study mixing enhancement due to curved microchannel and the influence of varying microchannel cross sectional shape through numerical and experimental investigations. Unlike prior studies which use channel dimensions in the lower microscale range, this work has been conducted on channels with dimensions in the higher end of micrometer range. Using a cross sectional hydraulic diameter of 600 μm enables introduction of flow into the curved channel at a Reynolds Number ranging from 0.15 to 75, the findings of which show considerable improvement in the mixing performance as compared to that of equivalent straight channels, due to the development of secondary flows known as Dean Vortices.

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently.Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. These studies examined the variation of Nusselt Number over the length of the channel, for a range of different curvatures (and hence Dean numbers). The results showed significantly higher heat transfer occurring in curved channels, especially in areas where the generated Dean vortices are strongest, with the variation in Nusselt Number forming the shape of an 'arc'. In this way, a relationship between the Dean Number and the Nusselt Number is characterised and discussed, leading to suggestions regarding optimal microfluidic heat transfer design.

Artificial microvasculature, particularly as part of the blood-brain barrier, has a high benefit for pharmacological drug discovery and uptake regulation. We demonstrate the fabrication of tubular structures with patterns of holes, which are capable of mimicking microvasculatures. By using photolithography, the dimensions of the cylindrical scaffolds can be precisely tuned as well as the alignment and size of holes. Overlapping holes can be tailored to create diverse three-dimensional configurations, for example, periodic nanoscaled apertures. The porous tubes, which can be made from diverse materials for differential functionalization, are biocompatible and can be modified to be biodegradable in the culture medium. As a proof of concept, endothelial cells (ECs) as well as astrocytes were cultured on these scaffolds. They form monolayers along the scaffolds, are guided by the array of holes and express tight junctions. Nanoscaled filaments of cells on these scaffolds were visualized by scanning electron microscopy (SEM). This work provides the basic concept mainly for an in vitro model of microvasculature which could also be possibly implanted in vivo due to its biodegradability.

We present here a novel microchamber sealing valve that is self-actuated by a pressure change during the temperature change in the thermal activation of reactions. Actuation of our valve requires only the use of the same heating device as employed for the reactions. A thermoplastic UV-curable polymer is used as a device material; the polymer allows realization of the temperature-driven valve actuation as well as the fabrication of multi-layered devices. The self-actuated valve achieves effective sealing of the microchamber for the polymerase chain reaction (PCR) even at 90 °C, which is essential for developing highly parallel PCR array devices without the need for complicated peripherals to control the valve operation.