Efficient particle and droplet manipulation utilizing the combined thermal buoyancy convection and temperature-enhanced rotating induced-charge electroosmotic flow

Light-Responsive Materials in Droplet Manipulation for Biochemical Applications

Miniaturized droplets, characterized by well-controlled microenvironments and capability for parallel processing, have significantly advanced the studies on enzymatic evolution, molecular diagnostics, and single-cell analysis. However, manipulation of small-sized droplets, including moving, merging, and trapping of the targeted droplets for complex biochemical assays and subsequent analysis, is not trivial and remains technically demanding. Among various techniques, light-driven methods stand out as a promising candidate for droplet manipulation in a facile and flexible manner, given the features of contactless interaction, high spatiotemporal resolution, and biocompatibility. This review therefore compiles an in-depth discussion of the governing mechanisms underpinning light-driven droplet manipulation. Besides, light-responsive materials, representing the core of light-matter interaction and the key character converting light into different forms of energy, are particularly assessed in this review. Recent advancements in light-responsive materials and the most notable applications are comprehensively archived and evaluated. Continuous innovations and rational engineering of light-responsive materials are expected to propel the development of light-driven droplet manipulation, equip droplets with enhanced functionality, and broaden the applications of droplets for biochemical studies and routine biochemical investigations.

A portable microfluidic device for thermally controlled granular sample manipulation

Effective granular sample manipulation with a portable and visualizable microfluidic device is significant for lots of applications, such as point-of-care testing and cargo delivery. Herein, we report a portable microfluidic device for controlled particle focusing, migration and double-emulsion droplet release via thermal fields. The device mainly contains a microfluidic chip, a microcontroller with a DC voltage control unit, a built-in microscope with a video transmission unit and a smartphone. Five microheaters located at the bottom of the microfluidic chip are used to unevenly heat fluids and then induce thermal buoyancy flow and a thermocapillary effect, and the experiments can be conveniently visualized through a smartphone, which provides convenient sample detection in outdoor environments. To demonstrate the feasibility and multifunctionality of this device, the focusing manipulation of multiple particles is first analyzed by using silica particles and yeast cells as experimental samples. We can directly observe the particle focusing states on the screen of a smartphone, and the particle focusing efficiency can be flexibly tuned by changing the control voltage of the microheater. Then the study focus is transferred to single-particle migration. By changing the voltage combinations applied on four strip microheaters, the single particle can migrate at predetermined trajectory and speed, showing attractiveness for those applications needing sample transportation. Finally, we manipulate the special three-phase flow system of double-emulsion drops in thermal fields. Under the combined effect of the thermocapillary effect and increased instability, the shell of double-emulsion droplets gradually thins and finally breaks, resulting in the release of samples in inner cores. The core release speed can also be flexibly adjusted by changing the control voltage of the microheater. These three experiments successfully demonstrate the effectiveness and multifunctionality of this thermally actuated microfluidic device on granular manipulation. Therefore, this portable microfluidic device will be promising for lots of applications, such as analytical detection, microrobot actuation and cargo release.

Microfluidic systems for particle capture and release: A review

Microfluidic technology has emerged as a promising tool in various applications, including biosensing, disease diagnosis, and environmental monitoring. One of the notable features of microfluidic devices is their ability to selectively capture and release specific cells, biomolecules, bacteria, and particles. Compared to traditional bulk analysis instruments, microfluidic capture-and-release platforms offer several advantages, such as contactless operation, label-free detection, high accuracy, good sensitivity, and minimal reagent requirements. However, despite significant efforts dedicated to developing innovative capture mechanisms in the past, the release and recovery efficiency of trapped particles have often been overlooked. Many previous studies have focused primarily on particle capture techniques and their efficiency, disregarding the crucial role of successful particle release for subsequent analysis. In reality, the ability to effectively release trapped particles is particularly essential to ensure ongoing, high-throughput analysis. To address this gap, this review aims to highlight the importance of both capture and release mechanisms in microfluidic systems and assess their effectiveness. The methods are classified into two categories: those based on physical principles and those using biochemical approaches. Furthermore, the review offers a comprehensive summary of recent applications of microfluidic platforms specifically designed for particle capture and release. It outlines the designs and performance of these devices, highlighting their advantages and limitations in various target applications and purposes. Finally, the review concludes with discussions on the current challenges faced in the field and presents potential future directions.

Flexible on-chip droplet generation, switching and splitting via controllable hydrodynamics

Flexible droplet preparation and manipulation are significant for lots applications such as immunoreaction and monocellular culture. Herein, we present a novel method for effective on-chip droplet generation, splitting and switching via controllable hydrodynamics. The microchannel of the designed chip has 6 inlets and 3 three outlets. The water solution is injected from a specific inlet (inlet d), and the other 5 inlets are used to inject oil fluids. Under the shearing effect of immiscible oils, the water phase breaks into dispersed droplets first, and the generated droplets can be further split into daughter drops or switched into side outlets from the middle outlet. To investigate the hydrodynamic droplet manipulation behaviors, a two-dimensional simulation model based on phase-field method is established. Utilizing the computational model, we systematically analyze the influences of the flow rates of continuous and dispersed fluids and the manipulation modes on droplet generation, splitting and switching. The numerical results indicate that the droplets can be generated with controlled sizes. For instance, at Q_d = 5 μL/min and Q_c1,2 = 5 μL/min, the droplet diameter decreases from 89.2 μm to 49.2 μm as Q_s1,2 gradually rises from 15 μL/min to 40 μL/min. Moreover, the prepared droplets can realize on-demand splitting and switching. When Q_d, Q_c1,2, and Qs_1,2 are fixed at 5 μL/min, 5 μL/min and 25 μL/min, respectively, the generated droplet is split into different proportional daughter drops with the rising of Q_s3 (or Q_s4) at first, and finally it is switched into the side-outlets when Q_s3 (or Q_s4) is higher than 80 μL/min. Therefore, this proposed droplet manipulation approach will be promising for various applications, and the numerical simulations can provide useful guidelines on the design and operation of droplet-based microfluidic systems.

Rational Design and Numerical Analysis of a Hybrid Floating cIDE Separator for Continuous Dielectrophoretic Separation of Microparticles at High Throughput

Dielectrophoresis (DEP) enables continuous and label-free separation of (bio)microparticles with high sensitivity and selectivity, whereas the low throughput issue greatly confines its clinical application. Herein, we report a novel design of the DEP separator embedded with cylindrical interdigitated electrodes that incorporate hybrid floating electrode layout for (bio)microparticle separation at favorable throughput. To better predict microparticle trajectory in the scaled-up DEP platform, a theoretical model based on coupling of electrostatic, fluid and temperature fields is established, in which the effects of Joule heating-induced electrothermal and buoyancy flows on particles are considered. Size-based fractionation of polystyrene microspheres and dielectric properties-based isolation of MDA-MB-231 from blood cells are numerically realized, respectively, by the proposed separator with sample throughputs up to 2.6 mL/min. Notably, the induced flows can promote DEP discrimination of heterogeneous cells. This work provides a reference on tailoring design of enlarged DEP platforms for highly efficient separation of (bio)samples at high throughput.

On-Chip Arbitrary Manipulation of Single Particles by Acoustic Resonator Array

Effective and arbitrary manipulation of particles in liquid has attracted substantial interest. Acoustic tweezers, a new and promising tool, exhibit high biocompatibility, universality, and precision but lack arbitrariness. In this work, we report a gigahertz (GHz) bulk acoustic streaming tweezer (AST)-based micro-manipulation platform capable of efficiently translating acoustic energy to fluid kinetic energy, creating a controllable, quick-response, and stable flow field and precisely, arbitrarily, and universally manipulating a single particle to move like a microrobot. Through controlling the radio frequency signals applied on these resonators, the intensity and direction of the acoustic streaming flow can be quickly and arbitrarily adjusted. Consequently, the particle dispersed at the bottom can be arbitrarily and steadily driven along the predesigned route to the target position by the acoustic streaming drag force (ASF). We utilized four resonators cooperated as a work group to manipulate single SiO₂ particles to complete nearly uniform linear motions and U-shaped motions, as well as playing billiards and exploring a maze, demonstrating the enormous potential of this GHz AST-based single-particle manipulation platform for separation, assembly, sensing, enriching, transporting, and so forth.

Flexible fabrication of lipophilic-hydrophilic micromotors by off-chip photopolymerization of three-phase immiscible flow induced Janus droplet templates

Self-propelled microparticles are promising for lots of applications ranging from analytical detection to water treatment. Herein, we present an effective approach to fabricate lipophilic-hydrophilic micromotors via the photocuring of three-phase immiscible flow induced droplet templates. In the microfluidic system, two immiscible inner fluids, the lipophilic 1, 6-Hexanediol diacrylate (HDDA), and the hydrophilic poly (ethylene glycol) diacrylate (PEGDA), are simultaneously injected into a theta-shaped cylindrical capillary from two separate inlets, and they are emulsified into Janus drops when encountering the outer immiscible silicone oil. Because of the immiscible feature of droplet templates, off-chip photopolymerization strategy has been used, which can significantly decrease the blocking chance of microdevice. And also, the lipophilic-hydrophilic structure of droplets is convenient for the loading of cargos with different characteristics. More importantly, the size and configuration of droplet templates can be flexibly regulated by changing the flow rates of three different phases. Accordingly, multifunctional micromotors can be fabricated by adding different nanoparticles and materials into the HDDA or PEGDA phase first and then photocuring the droplets. Taking the bubble-propelled micromotors for example, we prepare microswimmers by loading Ag, TiO₂ and Fe₃O₄ nanoparticles into the PEGDA phase. The swimming behaviors of micromotors in H₂O₂ solution are systematically investigated, finding that the proportion of PEGDA phase and the concentration of H₂O₂ both positively affect the moving speed. Furthermore, the applicability of motor particles on water treatment is successfully demonstrated by using neutral red solution as the model pollutant. And the micromotors can be recycled using magnets after the catalytic degradation process. Therefore, this micromotor generation technique and this kind of micromotor can be attractive for many applications.

Review of nonlinear electrokinetic flows in insulator-based dielectrophoresis: From induced charge to Joule heating effects

Insulator-based dielectrophoresis (iDEP) has been increasingly used for particle manipulation in various microfluidic applications. It exploits insulating structures to constrict and/or curve electric field lines to generate field gradients for particle dielectrophoresis. However, the presence of these insulators, especially those with sharp edges, causes two nonlinear electrokinetic flows, which, if sufficiently strong, may disturb the otherwise linear electrokinetic motion of particles and affect the iDEP performance. One is induced charge electroosmotic (ICEO) flow because of the polarization of the insulators, and the other is electrothermal flow because of the amplified Joule heating in the fluid around the insulators. Both flows vary nonlinearly with the applied electric field (either DC or AC) and exhibit in the form of fluid vortices, which have been utilized to promote some applications while being suppressed in others. The effectiveness of iDEP benefits from a comprehensive understanding of the nonlinear electrokinetic flows, which is complicated by the involvement of the entire iDEP device into electric polarization and thermal diffusion. This article is aimed to review the works on both the fundamentals and applications of ICEO and electrothermal flows in iDEP microdevices. A personal perspective of some future research directions in the field is also given.

Directed Concentrating of Micro-/Nanoparticles via Near-Infrared Laser Generated Plasmonic Microbubbles

Directed concentrating of micro- and nanoparticles via laser-generated plasmonic microbubbles in a liquid environment is an emerging technology. For effective heating, visible light has been primarily employed in existing demonstrations. In this paper, we demonstrate a new plasmonic platform based on nanoporous gold disk (NPGD) array. Thanks to the highly tunable localized surface plasmon resonance of the NPGD array, microbubbles of controlled size can be generated by near-infrared (NIR) light. Using NIR light provides several key advantages over visible light in less interference with standard microscopy and fluorescence imaging, preventing fluorescence photobleaching, less susceptible to absorption and scattering in turbid biological media, and much reduced photochemistry, phototoxicity, and so forth. The large surface-to-volume ratio of NPGD further facilitates the heat transfer from these gold nanoheaters to the surroundings. While the microbubble is formed, the surrounding liquid circulates and direct microparticles randomly dispersed in the liquid to the bottom NPGD surface, which can be made to yield a unique collection of 3D hollow dome microstructures with bubbles larger than 5 μm. Such capability can also be employed in concentrating suspended colloidal nanoparticles at desirable sites and with the preferred configuration enhancing the sensor performance. Specifically, the interaction among concentrated nanoparticles and their interactions with the underlying substrate have been investigated for the first time. These collections have been characterized using optical microscopy, scanning electron microscopy, hyperspectral localized surface plasmon resonance imaging, and hyperspectral Raman imaging. In addition to various micro- and nanoparticles, the plasmonic microbubbles are also shown to collect biological cells and extracellular nanovesicles such as exosomes. By using a spatial light modulator to project the laser in arbitrary patterns, parallel concentrating can be achieved to fabricate an array of clusters.

Numerical Investigation of Nanostructure Orientation on Electroosmotic Flow

Electroosmotic flow (EOF) is fluid flow induced by an applied electric field, which has been widely employed in various micro-/nanofluidic applications. Past investigations have revealed that the presence of nanostructures in microchannel reduces EOF. Hitherto, the angle-dependent behavior of nanoline structures on EOF has not yet been studied in detail and its understanding is lacking. Numerical analyses of the effect of nanoline orientation angle θ on EOF to reveal the associated mechanisms were conducted in this investigation. When θ increases from 5° to 90° (from parallel to perpendicular to the flow direction), the average EOF velocity decreases exponentially due to the increase in distortion of the applied electric field distribution at the structured surface, as a result of the increased apparent nanolines per unit microchannel length. With increasing nanoline width W, the decrease of average EOF velocity is fairly linear, attributed to the simultaneous narrowing of nanoline ridge (high local fluid velocity region). While increasing nanoline depth D results in a monotonic decrease of the average EOF velocity. This reduction stabilizes for aspect ratio D/W > 0.5 as the electric field distribution distortion within the nanoline trench remains nearly constant. This investigation reveals that the effects on EOF of nanolines, and by extrapolation for any nanostructures, may be directly attributed to their effects on the distortion of the applied electric field distribution within a microchannel.

Area cooling enables thermal positioning and manipulation of single cells

Contactless particle manipulation based on a thermal field has shown great potential for biological, medical, and materials science applications. However, thermal diffusion from a high-temperature area causes thermal damage to bio-samples. Besides, the permanent bonding of a sample chamber onto microheater substrates requires that the thermal field devices be non-disposable. These limitations impede use of the thermal manipulation approach. Here, a novel manipulation platform is proposed that combines microheaters and an area cooling system to produce enough force to steer sedimentary particles or cells and to limit the thermal diffusion. It uses the one-time fabricated motherboard and an exchangeable sample chamber that provides disposable use. Sedimentary objects can be steered to the bottom center of the thermal field by combined thermal convection and thermophoresis. Single particle or cell manipulation is realized by applying multiple microheaters in the platform. Results of a cell viability test confirmed the method's compatibility in biology fields. With its advantages of biocompatibility for live cells, operability for different sizes of particles and flexibility of platform fabrication, this novel manipulation platform has a high potential to become a powerful tool for biology research.

Numerical Simulation and Experimental Validation of Liquid Metal Droplet Formation in a Co-Flowing Capillary Microfluidic Device

A two-phase flow axisymmetric numerical model was proposed to understand liquid metal droplet formation in a co-flowing capillary microfluidics device based on a phase field model. The droplet detachment processes were observed in the experiment and are in good agreement with the simulation method. The effects of the viscosities and flowrates of the continuous phase fluid, interfacial tension as well as the wetting property of the metallic needle against the bulk liquid metal on the droplet formation and production rate were numerically investigated. It was found that the droplet diameter decreased with the increment of the viscosities and flowrates of the outer phase carrier fluid. The dispersed phase fluid with high interfacial tension tended to prolong the time for equilibrium between the viscous drag force and interfacial tension on the liquid–liquid fluid surface, delaying the droplet to be pinched off from the capillary orifice and causing large droplet diameter. Finally, the wetting performance of the metallic needle against the liquid metal was explored. The result indicate that the droplet diameter became less dependent on the contact angle while the size distribution of the liquid metal droplet was affected by their wetting performance. A more hydrophilic wetting performance were expected to prepare liquid metal droplet with more monodispersity. The numerical model and simulation results provide the feasibility of predicting the droplet formation with a high surface tension in a glass capillary microfluidic device.