Efficient Fluid Mixing Research Articles

Gas-rigid-flexible compound blade coupling enhanced experimental study on chaotic mixing of multiphase flow

Efficient fluid mixing is essential for process intensification. This study proposes a new method in which gas-rigid-flexible composite blades are coupled to enhance chaotic mixing in multiphase flow systems. The rigidity and flexibility of the blades were adjusted by intermittent gas injection, which increased the effectiveness of mixing of the liquid-liquid two-phase fluid. This study investigates the influence of different process parameters on the mixing efficiency and quantifies the chaotic characteristics of fluid mixing through pressure-time series analysis of multiscale entropy and the 0–1 test. A high-speed camera recorded the bubble movement in the flow field, while particle image velocimetry (PIV) revealed the enhancement of the properties of the flow field in the system due to the suspended motion of the particles. Using suitable process parameters, gas-rigid-flexible composite blade coupling significantly enhanced the mixing effect, where the mixing time of the G-RFCP system was reduced by 1.42 times compared to that of the CP system. Bubble motion, deformation, and rupture enhanced the mechanical agitation, increasing the intensity of the turbulence and chaotic behaviour. Flow-field analysis indicated a three-fold increase in the vorticity and a 1.04-fold increase in the velocity difference for the G-RFCP system compared with those of the CP system. This study provides theoretical and experimental foundations for understanding chaotic mixing in liquid-liquid two-phase fluids.

Mass transfer and mixing performance in jet-to-counterflow micromixer

As an important component of the fine organic microreaction system, micromixers can realize efficient mixing of different fluids within a short distance. In spite of great progress in micromixer design for high mixing performance over the last decade, micromixers are limited by the narrowness of channels, making it impossible to operate at high throughputs. Here, we propose a novel jet-to-counterflow (JTC) micromixer, which is based on the offset fluctuation jet mechanism to build a jet-to-counterflow zone and an annular liquid film zone to promote the rapid and efficient mixing of fluids. Micro-PIV and dye visualization experiments are used to investigate the internal flow behaviors and mixing performance of the JTC micromixer. The results show that in the “jet-to-counterflow mixing zone”, the jet undergoes the transition of three flow regimes i.e. “no offset jet − stable offset jet − fluctuating offset jet” with the increase of Re under the effects of wall effect and fluid inertia. When Re = 100, the fluctuating offset jet enhances the internal perturbation, which promotes the mixing index of the micromixer within 3 mm up to 78 %. In addition, the fluctuating offset jet intensifies the disturbance of the fluid flow in the annular liquid film zone, which further enhances the fluid mixing. A mixing efficiency of 97 % is achieved in mixing experiments over flow rates ranging from 0.075 – 37.66 mL/min−1. Our results demonstrate that fluctuating offset jet can be used to enhance microfluidic internal perturbation and mixing, which provide new insights into the design of novel micromixers.

Periodic Flows in Microfluidics.

Microfluidics, the science and technology of manipulating fluids in microscale channels, offers numerous advantages, such as low energy consumption, compact device size, precise control, fast reaction, and enhanced portability. These benefits have led to applications in biomedical assays, disease diagnostics, drug discovery, neuroscience, and so on. Fluid flow within microfluidic channels is typically in the laminar flow region, which is characterized by low Reynolds numbers but brings the challenge of efficient mixing of fluids. Periodic flows are time-dependent fluid flows, featuring repetitive patterns that can significantly improve fluid mixing and extend the effective length of microchannels for submicron and nanoparticle manipulation. Besides, periodic flow is crucial in organ-on-a-chip (OoC) for accurately modeling physiological processes, advancing disease understanding, drug development, and personalized medicine. Various techniques for generating periodic flows have been reported, including syringe pumps, peristalsis, and actuation based on electric, magnetic, acoustic, mechanical, pneumatic, and fluidic forces, yet comprehensive reviews on this topic remain limited. This paper aims to provide a comprehensive review of periodic flows in microfluidics, from fundamental mechanisms to generation techniques and applications. The challenges and future perspectives are also discussed to exploit the potential of periodic flows in microfluidics.

Fabrication of Patterned Magnetic Particles in Microchannels and Their Application in Micromixers.

Due to the extremely low Reynolds number, the mixing of substances in laminar flow within microfluidic channels primarily relies on slow intermolecular diffusion, whereas various rapid reaction and detection requirements in lab-on-a-chip applications often necessitate the efficient mixing of fluids within short distances. This paper presents a magnetic pillar-shaped particle fabrication device capable of producing particles with planar shapes, which are then utilized to achieve the rapid mixing of multiple fluids within microchannels. During the particle fabrication process, a degassed PDMS chip provides self-priming capabilities, drawing in a UV-curable adhesive-containing magnetic powder and distributing it into distinct microwell structures. Subsequently, an external magnetic field is applied, and the chip is exposed to UV light, enabling the mass production of particles with specific magnetic properties through photo-curing. Without the need for external pumping, this chip-based device can fabricate hundreds of magnetic particles in less than 10 min. In contrast to most particle fabrication methods, the degassed PDMS approach enables self-priming and precise dispensing, allowing for precise control over particle shape and size. The fabricated dual-layer magnetic particles, featuring fan-shaped blades and disk-like structures, are placed within micromixing channels. By manipulating the magnetic field, the particles are driven into motion, altering the flow patterns to achieve fluid mixing. Under conditions where the Reynolds number in the chip ranges from 0.1 to 0.9, the mixing index for substances in aqueous solutions exceeds 0.9. In addition, experimental analyses of mixing efficiency for fluids with different viscosities, including 25 wt% and 50 wt% glycerol, reveal mixing indices exceeding 0.85, demonstrating the broad applicability of micromixers based on the rapid rotation of magnetic particles.

A Novel DC Electroosmotic Micromixer Based on Helical Vortices

This work introduces a novel direct current electroosmosis (DCEO) micromixer designed for rapid and efficient fluid mixing. This micromixer demonstrates excellent capability, achieving approximately 98.5% mixing efficiency within a one-second timespan and 99.8% efficiency within two seconds, all within a simple channel of only 1000 µm in length. A distinctive feature of this micromixer is its ability to generate robust and stable helical vortices by applying a controlled DC electric field. Unlike complex, intricate microfluidic designs, this work proposes a simple yet effective approach to fluid mixing, making it a versatile tool suitable for various applications. In addition, through simple modifications to the driving signal configuration and channel geometry, the mixing efficiency can be further enhanced to 99.3% in one second.

Versatile hybrid technique for passive straight micromixer manufacturing by combining pulsed laser ablation, stereolithographic 3D printing and computational fluid dynamics.

There is a need to develop new and versatile fabrication methods to achieve efficient mixing of fluids in microfluidic channels using microstructures. This work presents a new technique that combines stereolithography (SLA) and pulsed laser ablation (PLA) to manufacture a straight micromixer for uniform mixing of two samples. Computational fluid dynamics (CFD) simulation is performed to deeply understand the physical mechanisms of the process. The results suggest that this new optical technique holds the potential to become a versatile hybrid technique for manufacturing remarkable mixing microfluidic devices.

Micromixing within microfluidic devices: Fundamentals, design, and fabrication

As one of the hot spots in the field of microfluidic chip research, micromixers have been widely used in chemistry, biology, and medicine due to their small size, fast response time, and low reagent consumption. However, at low Reynolds numbers, the fluid motion relies mainly on the diffusive motion of molecules under laminar flow conditions. The detrimental effect of laminar flow leads to difficulties in achieving rapid and efficient mixing of fluids in microchannels. Therefore, it is necessary to enhance fluid mixing by employing some external means. In this paper, the classification and mixing principles of passive (T-type, Y-type, obstructed, serpentine, three-dimensional) and active (acoustic, electric, pressure, thermal, magnetic field) micromixers are reviewed based on the presence or absence of external forces in the micromixers, and some experiments and applications of each type of micromixer are briefly discussed. Finally, the future development trends of micromixers are summarized.

Numerical investigation on intensified mixing performance with modified dual impeller

Process intensification is important for achieving efficient fluid mixing. In this study, the process intensification mechanism of mixing was investigated based on the evolution of sub-steady flow pattern. The results showed that the modified dual impeller (Mod-DI) could eliminate the isolation zone and symmetry of the flow pattern, which always appears in the traditional dual impeller (Trad-DI) system. Furthermore, the mixing time (tm) and dynamic equilibrium time (tde) of the Mod-DI system were significantly shorter than those of the Trad-DI system, which could be attributed to the enhancement of fluid flow fluctuations caused by the loss of stability of sub-steady flow field. In the presence of baffles, tm and tde could be further reduced. The shortening of time was at the expense of increasing power consumption. Overall, the intensification mechanism of the Mod-DI is mainly since the formation of sub-steady flow field, which enhances the instability of the fluid flow.

Thermo-hydraulic investigation of open micro prism pin fin heat sink having varying prism sides

The current trend of miniaturization in many electronic devices, like computer processors, high-speed servers, MEMS devices, etc., has shown augmented performance. Still, thermal management is a significant concern for them. The present work performed a numerical analysis on the microchannel heat sink's open micro prism pin fin configuration (MCHS). The influence of prism pin fin sides (3 to 9) has been studied for copper substrate and working fluid water (single phase fluid flow). Finite volume-based commercial code has been opted to perform the numerical simulations. The present configurations simulated different Reynolds numbers (50–300) under the heat flux of 500 kW/m2. Observation revealed that the four-side arrangements have the best performance due to prominent secondary flow leading to efficient fluid mixing, whereas three-side configurations have the worst performance. Further, an increase in pin fin sides from 5 to 9 has almost negligible variation in thermal and hydraulic characteristics because of similar fluid flow characteristics. The four-side micro prism pin fin configuration has been further analysed by varying the prism radius from 0.175 mm to 0.325 mm. It was observed that a prism radius between 0.250 mm and 0.275 mm led to optimum overall performance due to more stable secondary fluid flow.

Straight micromixer manufacturing combining stereolithography and pulsed laser ablation and simulation - INVITED

Achieving efficient mixing of fluids is a great challenge in microfluidics that has been addressed using microstructures. In this work, Stereolithography (SLA) and Pulsed Laser Ablation (PLA) were combined to manufacture a straight micromixer for uniform mixing of fluids. Computational Fluid Dynamics (CFD) simulation was performed to test the device. The results suggest that the combination of these optical technologies can be an effective method for fabricating microfluidic devices with great mixing capabilities.

Acoustofluidic bubble-driven micromixers for the rational engineering of multifunctional ZnO nanoarray

Acoustofluidics that combines acoustics with microfluidics represents an emerging and promising technology for the efficient mixing of fluids and synthesis of functional nanomaterials. However, it remains a great challenge to the controllable synthesis of three-dimensional functional nanostructures in a confined space. Herein, we introduce a robust acoustofluidic bubble-driven micromixer for the controllable synthesis of functional nanoarray inside the glass capillary toward efficient photodegradation and metal ion enrichment. The acoustofluidic micromixer functions with microbubbles-induced acoustic streaming from arrow-patterned microchannels. The morphology, density, and size of ZnO nanorods and nanosheets array can be precisely adjusted by reactant type, seeding phase, and reaction time. ZnO nanorod array exhibits approximately 90% degradation efficiency of R6G even after ten cycles, which are significantly better than ZnO nanosheet array and blank control groups. Enrichment tests of heavy metal ions show that ZnO nanoarray performs better enrichment performance of Cu2+ than that of Ag+. In addition, both photodegradation and enrichment capabilities can be well regulated by tuning the flow rates (i.e., the residence time). These findings provide important guidelines for the rational design of functional nanomaterials and shed new light on the fabrication of advanced lab-on-a-chip devices.

Fast fluid heating by adaptive flow reorientation

Transport of scalar quantities such as e.g. heat or chemical species in laminar flows is key to many industrial activities and stirring of the fluid by flow reorientation is a common way to enhance this process. However, “How best to stir?” remains a major challenge. The present study aims to contribute to existing solutions by the development of a dedicated flow-control strategy for the fast heating of a cold fluid via a hot boundary in a representative case study. In-depth analysis of the dynamics of heating in fluid flows serves as foundation for the control strategy and exposes fluid deformation as the “thermal actuator” via which the flow affects the heat transfer. This link is non-trivial, though, in that fluid deformation may both enhance and diminish local heat exchange between fluid parcels and a fundamental “conflict” between local heat transfer and thermal homogenisation tends to restrict the beneficial impact of flow to short-lived episodes. Moreover, the impact of fluid deformation on the global fluid heating is primarily confined to the direct proximity of the moving boundary that drives the flow. These insights imply that incorporation of the thermal behaviour is essential for effective flow-based enhancement strategies and efficient fluid mixing, the conventional approach adopted in industry for this purpose, is potentially sub-optimal. The notion that global heating encompasses two concurrent processes, i.e. increasing energy content (“energising”) and thermal homogenisation, yields the relevant metrics for the global dynamics and thus enables formulation of the control problem as the minimisation of a dedicated cost function. This facilitates step-wise determination of the “best” flow reorientation from predicted future evolutions of actual intermediate states and thereby paves the way to (real-time) regulation of scalar transport by flow control in practical applications. Performance analyses reveal that this “adaptive flow reorientation” significantly accelerates the fluid heating throughout the considered parameter space and thus is superior over conventional periodic schemes (designed for efficient fluid mixing) both in terms of consistency and effectiveness. The controller in fact breaks with conventions by, first, never selecting these periodic schemes and, second, achieving the same superior performance for all flow conditions irrespective of whether said mixing occurs. The controller typically achieves this superiority by thermal plumes that extend from the hot wall into the cold(er) interior and are driven by two alternating and counter-rotating circulations.

AC electroosmosis micromixing on a lab-on-a-foil electric microfluidic device

Efficient mixing of fluids in lab-on-a-chip devices is very important for many biomedical and biochemical applications. Lab-on-a-foil as a novel concept provides a method for fast prototyping or mass production of microfluidic devices based on thin and flexible films materials. In this article, electroosmosis micromixing is conducted in a lab-on-a-foil microfluidic device. With the electroosmotic flow (EOF), an efficient micromixing is realized inside a microchannel by tooth-shaped planar electrodes. The mixing performance is evaluated based on intensity measurement, and frequency sweeping is used to identify optimal performance. Furthermore, according to local intensity profiles, the EOF pattern is analyzed to provide a deep understanding on the influence of frequency and flow rate. The amplitude of voltage and the number of pairs of electrode tooth are also investigated to find the optimal conditions of the device. To the best of our knowledge, this is the first demonstration of the AC EOF in a lab-on-a-foil electric device and the exploration of EOF pattern vertically and horizontally in the microchannels. This study provides a method to optimize mixing performance in EOF-based micromixer. Furthermore, the fabrication method cast the potential for mass production of low-cost flexible electric microfluidic devices.

Effect of Thermal Energy and Ultrasonication on Mixing Efficiency in Passive Micromixers

Micromixing is a key process in microfluidics technology. However, rapid and efficient fluid mixing is difficult to achieve inside the microchannels due to unfavourable laminar flow. Active micromixers employing ultrasound and thermal energy are effective in enhancing the micromixing process; however, integration of these energy sources within the devices is a non-trivial task. In this study, ultrasound and thermal energy have been extraneously applied at the upstream of the micromixer to significantly reduce fabrication complexity. A novel Dean micromixer was laser-fabricated to passively increase mixing performance and compared with T- and Y-micromixers at Reynolds numbers between 5 to 100. The micromixers had a relatively higher mixing index at lower Reynolds number, attributed to higher residence time. Dean micromixer exhibits higher mixing performance (about 27% better) than T- and Y-micromixers for 40 ≤ Re ≤ 100. Influence of ultrasound and heat on mixing is more significant at 5 ≤ Re ≤ 20 due to the prolonged mechanical effects. It can be observed that mixing index increases by about 6% to 10% once the temperature of the sonicated fluids increases from 30 °C to 60 °C. The proposed method is potentially useful as direct contact of the inductive energy sources may cause unwanted substrate damage and structural deformation especially for applications in biological analysis and chemical synthesis.

Mixing mechanism of a straight channel micromixer based on light-actuated oscillating electroosmosis in low-frequency sinusoidal AC electric field

The traditional electroosmotic microfluidic mixers generally involve microelectrodes embedded on the inner wall of microchannel, which increases the difficulty of microchip processing and reduces the functional flexibility inevitably. In this paper, we report the development of a novel microfluidic mixer with straight microchannel based on the light-actuated oscillating electroosmosis in low-frequency sinusoidal AC electric field. The mixing performance is investigated by solving the coupled system of flow field, electric field and concentration field using finite element method. The formation mechanism of a pair of rotating vortices at both edges of the optical virtual electrode and the mixing mechanism of fluid samples are revealed. The effect of several key parameters including external applied potential, electric field frequency and inlet mean velocity on the mixing performance is also explored. The current technique provides a new method for efficient mixing of fluids in modern microfluidic analysis systems, and could potentially promote the comprehensive integration of functions, simplifying structure and reducing manufacturing costs of microchip.

Enhancement of Fluid Mixing with U-Shaped Channels on a Rotating Disc.

In this study, centrifugal microfluidics with a simple geometry of U-shaped structure was designed, fabricated and analyzed to attain rapid and efficient fluid mixing. Visualization experiments together with numerical simulations were carried out to investigate the mixing behavior for the microfluidics with single, double and triple U-shaped structures, where each of the U-structures consisted of four consecutive 90° bends. It is found that the U-shaped structure markedly enhances mixing by transverse secondary flow that is originated from the Coriolis-induced vortices and further intensified by the Dean force generated as the stream turns along the 90° bends. The secondary flow becomes stronger with increasing rotational speed and with more U-shaped structures, hence higher mixing performance. The mixing efficiency measured for the three types of mixers shows a sharp increase with increasing rotational speed in the lower range. As the rotational speed further increases, nearly complete mixing can be achieved at 600 rpm for the triple-U mixer and at 720 rpm for the double-U mixer, while a maximum efficiency level of 83–86% is reached for the single-U mixer. The simulation results that reveal detailed characteristics of the flow and concentration fields are in good agreement with the experiments.

High-throughput conventional and stealth cationic liposome synthesis using a chaotic advection-based microfluidic device combined with a centrifugal vacuum concentrator

Microfluidics is an emerging technology that allows efficient mixing of fluids on the nanoliter scale and has recently been employed for the production of lipid-based systems. In this study, we investigated the manufacturing of conventional (EPC/DOTAP/DOPE) and stealth (EPC/DOTAP/DOPE/DSPE-PEG(2000)) cationic liposomes (CLs) in a high-throughput chaotic advection-based microfluidic device (CA-MD) using water and ethanol. We assessed the effects of total flow rate (TFR), flow rate ratio (FRR), and flow configuration on CL formation. We compared the results with properties of CLs produced on a diffusion-based microfluidic device (D-MD). FRR (aqueous/solvent ratio) values were found to have a strong correlation with vesicle formation and values close to 1 led to the production of conventional and stealth CLs with diameter and polydispersity indices close to 200 nm and 0.2, respectively. Both liposomes are unilamellar, and stealth CL production could minimize micelle formation and operate at high flow velocities without altering characteristics of CLs. These findings were supported with dynamic light scattering (DLS), cryo-transmission electron microscopy (Cryo-TEM), and synchrotron small angle X-ray scattering (SAXS) techniques. The high solvent content in liposomes after microfluidic synthesis (FRR = 1) was reduced by employing centrifugal vacuum concentrators (CVC) at low pressure and 43 °C as an alternative distillation process for successful ethanol removal without changing the structural properties of these nanoaggregates. Finally, both CLs were able to transfect pDNA into PC-3 cancer cells. The CA-MD coupled to a CVC technique produced conventional and stealth CLs with high productivity, which makes it readily applicable for industrial production.

Rayleigh-Taylor convective dissolution in confined porous media

RT instability and convection in confined porous media is examined via DNS. Interactions between gravity and diffusion produce efficient fluid mixing, and a superlinear asymptotic growth of the mixing rate is observed and modeled for the first time. Potential geophysical applications are envisioned.

A switched system approach to optimize mixing of fluids

In this paper, we discuss the problem of efficient fluid mixing which is tackled by means of (approximate) dynamic programming from a switched systems perspective. In current practice, typically pre-determined periodic mixing protocols are used. As we will show in this paper, feedback control can be used to improve mixing significantly. To make this control problem tractable, temporal and spatial discretization is used by means of the cell-mapping method on the original infinite-dimensional fluid models. This translates the original control problem into the design of a (sub)optimal switching law that determines discrete mixing actions for a discrete-time switched linear system. Exploiting this switched systems perspective, a novel feedback law for mixing fluids is proposed inspired by suboptimal rollout policies in dynamic programming contexts. By design, this feedback law for mixing guarantees a performance improvement over any given (open-loop) periodic mixing protocol. This new design methodology is validated by means of simulations for the benchmark journal bearing flow showing improved performance with respect to periodic mixing strategies.

Direct numerical simulation of water–ethanol flows in a T-mixer

The efficient mixing of fluids is key in many applications, such as chemical reactions and nanoparticle precipitation. Detailed experimental measurements of the mixing dynamics are however difficult to obtain, and so predictive numerical tools are helpful in designing and optimizing many processes. If two different fluids are considered, the viscosity and density of the mixture depend often nonlinearly on the composition, which makes the modeling of the mixing process particularly challenging. Hence water-water mixtures in simple geometries such as T-mixers have been intensively investigated, but little is known about the dynamics of more complex mixtures, especially in the turbulent regime. We here present a numerical method allowing the accurate simulation of two-fluid mixtures. Using a high-performance implementation of this method we perform direct numerical simulations resolving the spatial and temporal dynamics of water–ethanol flows for Reynolds numbers from 100 to 2000. The flows states encountered during turbulence transition and their mixing properties are discussed in detail and compared to water-water mixtures.