Magnetically manipulated droplet splitting on a 3D-printed device to carry out a complexometric assay

Miniaturizing chemistry and biology using droplets in open systems

Open droplet microfluidic systems manipulate droplets on the picolitre-to-microlitre scale in an open environment. They combine the compartmentalization and control offered by traditional droplet-based microfluidics with the accessibility and ease-of-use of open microfluidics, bringing unique advantages to applications such as combinatorial reactions, droplet analysis and cell culture. Open systems provide direct access to droplets and allow on-demand droplet manipulation within the system without needing pumps or tubes, which makes the systems accessible to biologists without sophisticated setups. Furthermore, these systems can be produced with simple manufacturing and assembly steps that allow for manufacturing at scale and the translation of the method into clinical research. This Review introduces the different types of open droplet microfluidic system, presents the physical concepts leveraged by these systems and highlights key applications.

Applications of magnetic and electromagnetic forces in micro-analytical systems

Novel applications of magnetic fields in analytical chemistry have become a remarkable trend in the last two decades. Various magnetic forces have been employed for the migration, orientation, manipulation, and trapping of microparticles, and new analytical platforms for separating and detecting molecules have been proposed. Magnetic materials such as functional magnetic nanoparticles, magnetic nanocomposites, and specially designed magnetic solids and liquids have also been developed for analytical purposes. Numerous attractive applications of magnetic and electromagnetic forces on magnetic and non-magnetic materials have been studied, but fundamental studies to understand the working principles of magnetic forces have been challenging. These studies will form a new field of magneto-analytical science, which should be developed as an interdisciplinary field. In this review, essential pioneering works and recent attractive developments are presented.

Ferromagnetic Liquid Droplet on a Superhydrophobic Surface for the Transduction of Mechanical Energy to Electricity Based on Electromagnetic Induction

Ferromagnetic liquids undergo reversible magnetization changes upon varying external magnetic field levels. The movement of ferromagnetic liquid droplets across a coil under an external magnetic field holds promise as an energy transducer from mechanical force to electricity; however, it suffers from an adhesive issue between the ferromagnetic liquid and the solid pedestal. We introduce a superhydrophobic support that uses antiwetting surfaces to remarkably reduce adhesion during the movement of ferromagnetic liquid droplets. Maxwell numerical simulation was utilized to analyze the working mechanism and improve further electrical outputs. By controlling the droplet size, the strength of the magnetic bottom and the tilting speed of the test condition, we generated a ferromagnetic liquid droplet-based superhydrophobic magnetoelectric energy transducer (FLD-SMET) that can convert vibrational energy to electricity. When a 100 μL ferromagnetic liquid droplet was used for FLD-SMET under a 13 mT magnetic field, an electrical voltage response of 280 μV and electrical current response of ∼7.5 μA were generated using a shaking machine with a tilting speed of 9.5°/s. We thus show that such a device can serve as a self-powered light buoy floating on a water surface. Our study presents an applied concept for the design of droplet-based energy harvesters to convert surrounding vibrational energy to electricity.

Tuning the Splitting Behavior of Droplet in a Bifurcating Channel through Wettability-Capillarity Interaction

We present a comprehensive computational physics-based study of the influence of surface wettability on the displacement behavior of a droplet in a three-dimensional bifurcating channel. Various surface wettability configurations for the daughter branches are considered to gain insight into the wettability-capillarity interaction. Also, the influence of initial droplet size on the splitting dynamics for different wettability configurations is investigated. Time evolution of the droplet displacement behavior in the bifurcating channel is discussed for different physicochemical parameters including capillary number and wettability. Three distinct flow regimes are identified as the droplet interacts with the bifurcating tip of the channel, namely, splitting, nonsplitting, and oscillating regimes. Furthermore, the occurrence of Rayleigh-Plateau instability in different wettability scenarios is discussed. Additionally, the intricacies associated with the droplet dynamics are elucidated through the temporal evolution of the droplet surface area and mass outflow of the continuous phase. A flow regime map based on the capillary number and wettability contrast of the daughter branches is proposed for a comprehensive description of the droplet dynamics.

Flexible Ferrofluids: Design and Applications

Ferrofluids, also known as ferromagnetic particle suspensions, are materials with an excellent magnetic response, which have attracted increasing interest in both industrial production and scientific research areas. Because of their outstanding features, such as rapid magnetic reaction, flexible flowability, as well as tunable optical and thermal properties, ferrofluids have found applications in various fields, including material science, physics, chemistry, biology, medicine, and engineering. Here, a comprehensive, in-depth insight into the diverse applications of ferrofluids from material fabrication, droplet manipulation, and biomedicine to energy and machinery is provided. Design of ferrofluid-related devices, recent developments, as well as present challenges and future prospects are also outlined.

Facile actuation of aqueous droplets on a superhydrophobic surface using magnetotactic bacteria for digital microfluidic applications

Magnetic actuation provides a low-cost, simple method for droplet manipulation on a digital microfluidic platform. The impetus to move the droplets on a low friction surface can come from internal superparamagnetic particles or paramagnetic salts. Recently, the use of microbes for bio-actuation has been established, where the thrust produced by the microbes can be exploited to exert the force required for droplet movement. This study presents biologically-driven magnetic actuation of droplets on a superhydrophobic surface using magnetotactic bacteria (MTB). MTB-droplets were impelled along various trajectories such as rectangular and figure-of-eight-shaped paths. Droplets were reproducibly actuated with speeds up of to 30 mm s^-1. We demonstrated the ability to sequentially merge and mix multiple droplets by merging a 10 μL MTB droplet with two 4 μL colored droplets. The reorientation of MTB in the droplet enhanced mixing rate of the merged fluids by ∼40% compared with the control experiment where no actuation was used. Biologically-driven magnetic actuation was compared with actuation by superparamagnetic particles and paramagnetic salts, in terms of controllability and speed. MTB droplet was moved with the same average speed as other two methods and showed higher response time as the magnet acceleration increased. Lastly, MTB were used to perform a phosphatase assay using endogenous enzyme. The relative absorbance at 405 nm, indicating the production of the yellow product, increased over time and levels off after 75 min.

Semi-automated on-demand control of individual droplets with a sample application to a drug screening assay

Automated control of individual droplets in microfluidic channels offers tremendous potential for applications requiring high accuracy and minimal user involvement. The feasibility of active droplet control has been previously demonstrated with pressure-driven flow control and visual feedback, but the manual operation required to perform droplet manipulations limited the accuracy, repeatability, and throughput. The present study improves upon the aforementioned challenges with a higher-level algorithm capturing the dynamics of droplet motion for a semi-automated control system. With a simple T junction geometry, droplets can now be automatically and precisely controlled on-demand. Specifically, there is ±10% accuracy for droplet generation, ±1.3% monodispersity for 500 μm long droplets and ±4% accuracy for splitting ratios. On-demand merging, mixing, and sorting are also demonstrated as well as the application of a drug screening assay related to neurodegenerative disorders. Overall, this system serves as a foundation for a fully automated system that does not require valves, embedded electrodes, or complex multi-layer fabrication.

Organic-free, versatile sessile droplet microfluidic device for chemical separation using an aqueous two-phase system

This work presents a novel portable, versatile sessile droplet microfluidic (SDMF) device to perform liquid manipulation operations such as confining, splitting and colorimetric detection. Furthermore, chemical isolations based on an aqueous two-phase system (ATPS) for separating an analyte of choice from a complicated sample matrix can be carried out. ATPS extractions can replace conventional liquid-liquid extractions and take away the need for harmful organic solvents. Superhydrophobic (SH) surfaces were fabricated from a commercially available material, Ultra-Ever Dry® (UED®). On these SH surfaces, surface energy traps (SETs) were produced either by air plasma treatment (simultaneously) or laser micromachining (sequentially) to dock/pin an ATPS containing droplet onto the surface. Splitting of droplets or removing a precise volume of the top phase from a pinned extraction system was achieved with a sandwich-chip approach. For this, an additional SET patterned substrate was placed on top of the droplet and subsequently lifted. This multipurpose platform was used to isolate Cd from a mixture of several other metal ions (i.e. Mn, Ni, Cu, Pb, Fe) for its subsequent interference-free detection. An ATPS consisting of sodium sulfate and polyethylene glycol (PEG) as phase forming components and potassium iodine as extractant allowed separation of cadmium with an extraction efficiency of q(Cd2+) = 98.5%. Using a portable, cost-effective, smartphone-based UV/vis spectrometer, Cd was detected with a LoD of 3.4 ppm. Alternatively, the multipurpose platform can also be used as sampling platform for a benchtop UV/vis spectrometer, where a LoD of 0.53 ppm was obtained. Potential applications of the presented platform include sample preparation and separation that can be achieved by aqueous two-phase extractions, such as proteins, antibodies, DNA, cells, organic molecules and metal ions.

Droplet Mechanical Hand Based on Anisotropic Water Adhesion of Hydrophobic-Superhydrophobic Patterned Surfaces

Superhydrophobic copper surfaces patterned with non-round hydrophobic areas were fabricated by a combination of through-mask chemical oxidation and fluorocarbon film deposition techniques. The anisotropic sliding resistance of droplets on typical non-round hydrophobic patterns such as semicircle, V-shape, and line segment hydrophobic patterns was observed. The dependence of sliding anisotropy on the pattern shape and dimensions was investigated. Results showed that the experimental sliding resistance was in good agreement with the calculated data using a classical drag-resistance model (Furmidge equation). By taking advantage of the anisotropic sliding resistance, these patterned surfaces can be used as droplet mechanical hands to capture, transfer, mix, and release in situ micro droplets by simply moving the surfaces in different directions. A droplet pinned on a non-round hydrophobic pattern can be captured by lifting a surface with another non-round hydrophobic pattern in a large-sliding-resistance direction after touching it, while the captured droplet can be released in situ with nearly no mass loss by horizontally moving the surface in the low-sliding-resistance direction. The lossless droplet manipulations using hydrophobic/superhydrophobic patterned surfaces have advantages of being low in cost and easy to operate and may have great promising applications to high throughput drug screening, molecular detection, and other lab-on-chip devices.

3D-printed miniaturized fluidic tools in chemistry and biology

3D printing (3DP), an additive manufacturing (AM) approach allowing for rapid prototyping and decentralized fabrication on-demand, has become a common method for creating parts or whole devices. The wide scope of the AM extends from organized sectors of construction, ornament, medical, and R&D industries to individual explorers attributed to the low cost, high quality printers along with revolutionary tools and polymers. While progress is being made but big manufacturing challenges are still there. Considering the quickly shifting narrative towards miniaturized analytical systems (MAS) we focus on the development/rapid prototyping and manufacturing of MAS with 3DP, and application dependent challenges in engineering designs and choice of the polymeric materials and provide an exhaustive background to the applications of 3DP in biology and chemistry. This will allow readers to perceive the most important features of AM in creating (i) various individual and modular components, and (ii) complete integrated tools.