Recent Progress of Droplet Microfluidic Emulsification Based Synthesis of Functional Microparticles

The remarkable control function over the functional material formation process enabled by droplet microfluidic emulsification approaches can lead to the efficient and one-step encapsulation of active substances in microparticles, with the microparticle characteristics well regulated. In comparison to the conventional fabrication methods, droplet microfluidic technology can not only construct microparticles with various shapes, but also provide excellent templates, which enrich and expand the application fields of microparticles. For instance, intersection with disciplines in pharmacy, life sciences, and others, modifying the structure of microspheres and appending functional materials can be completed in the preparation of microparticles. The as-prepared polymer particles have great potential in a wide range of applications for chemical analysis, heavy metal adsorption, and detection. This review systematically introduces the devices and basic principles of particle preparation using droplet microfluidic technology and discusses the research of functional microparticle formation with high monodispersity, involving a plethora of types including spherical, nonspherical, and Janus type, as well as core-shell, hole-shell, and controllable multicompartment particles. Moreover, this review paper also exhibits a critical analysis of the current status and existing challenges, and outlook of the future development in the emerging fields has been discussed.

Emerging Droplet Microfluidics

Droplet microfluidics generates and manipulates discrete droplets through immiscible multiphase flows inside microchannels. Due to its remarkable advantages, droplet microfluidics bears significant value in an extremely wide range of area. In this review, we provide a comprehensive and in-depth insight into droplet microfluidics, covering fundamental research from microfluidic chip fabrication and droplet generation to the applications of droplets in bio(chemical) analysis and materials generation. The purpose of this review is to convey the fundamentals of droplet microfluidics, a critical analysis on its current status and challenges, and opinions on its future development. We believe this review will promote communications among biology, chemistry, physics, and materials science.

Reactions in ultra-small droplets by tip-assisted chemistry

The confinement of chemical reactions within small droplets has received much attention in the last few years. This approach has been proved successful for the in-depth study of naturally occurring chemical processes as well as for the synthesis of different sets of nanomaterials with control over their size, shape and properties. Different approaches such as the use of self-contained structures or microfluidic generated droplets have been followed over the years with success. However, novel approaches have emerged during the last years based on the deposition of femtolitre-sized droplets on surfaces using tip-assisted lithographic methods. In this feature article, we review the advances made towards the use of these ultra-small droplets patterned on surfaces as confined nano-reactors.

Fouling in microstructured devices: a review

Microstructured devices are widely used for manufacturing products that benefit from process intensification, with pharmaceutical products or specialties of the chemical industry being prime examples. These devices are ideally used for processing pure fluids. Where particulate or non-pure flows are involved, processes are treated with utmost caution since related fouling and blocking issues present the greatest barrier to operating microstructured devices effectively. Micro process engineering is a relatively new research field and there is limited understanding of fouling in these dimensions and its underlying processes and phenomena. A comprehensive review on fouling in microstructured devices would be helpful in this regard, but is currently lacking. This paper attempts to review recent developments of fouling in micro dimensions for all fouling categories (crystallization, particulate, chemical reaction, corrosion and biological growth fouling) and the sequential events involved (initiation, transport, attachment, removal and aging). Compared to fouling in macro dimensions, an additional sixth category is suggested: clogging by gas bubbles. Most of the reviewed papers present very specific fouling investigations making it difficult to derive general rules and parameter dependencies, and comparative or critical considerations of the studies were difficult. We therefore used a statistical approach to evaluate the research in the field of fouling in microchannels.

Segmented flow reactors for nanocrystal synthesis

In the past decade microreactors have emerged as a compelling technology for the highly controlled synthesis of colloidal nanocrystals, offering multiple advantages over conventional batch synthesis methods (including improved levels of control, reproducibility, and automation). Initial work in the field employed simple continuous phase reactors that manipulate miscible streams of a single reagent phase. Recently, however, there has been increasing interest in segmented flow reactors that use an immiscible fluid to divide the reagent phase into discrete slugs or droplets. Key advantages of segmented flow include the elimination of velocity dispersion (a significant cause of polydispersity) and greatly reduced susceptibility to reactor fouling. In this progress report we review the operation of segmented flow microreactors, their application to the controlled synthesis of nanocrystals, and some of the principal challenges that must be addressed before they can become a mainstream technology for the controlled production of nanomaterials.

Two-phase microfluidic droplet flows of ionic liquids for the synthesis of gold and silver nanoparticles

Droplet-based microfluidic platforms have the potential to provide superior control over mixing as compared to traditional batch reactions. Ionic liquids have advantageous properties for metal nanoparticle synthesis as a result of their low interfacial tension and complexing ability; however, droplet formation of ionic liquids within microfluidic channels in a two-phase system has not yet been attained because of their complex interfacial properties and high viscosities. Here, breakup of an imidazolium-based ionic liquid into droplets in a simple two-phase system has for the first time been achieved and characterized by using a microchannel modified with a thin film fluoropolymer. This microfluidic/ionic liquid droplet system was used to produce small, spherical gold (4.28 ± 0.84 nm) and silver (3.73 ± 0.77 nm) nanoparticles.

Fabrication of advanced particles and particle-based materials assisted by droplet-based microfluidics

Recent advances in the fabrication of complex particles and particle-based materials assisted by droplet-based microfluidics are reviewed. Monodisperse particles with expected internal structures, morphologies, and sizes in the range of nanometers to hundreds of micrometers have received a good deal of attention in recent years. Due to the capability of generating monodisperse emulsions and of executing precise control and operations on the suspended droplets inside the microchannels, droplet-based microfluidic devices have become powerful tools for fabricating complex particles with desired properties. Emulsions and multiple-emulsions generated in the microfluidic devices can be composed of a variety of materials including aqueous solutions, gels, polymers and solutions containing functional nanoparticles. They are ideal microreactors or fine templates for synthesizing advanced particles, such as polymer particles, microcapsules, nanocrystals, and photonic crystal clusters or beads by further chemical or physical operations. These particles are promising materials that may be applicable for many fields, such as photonic materials, drug delivery systems, and bio-analysis. From simple to complex, from spherical to nonspherical, from polymerization and reaction crystallization to self-assembly, this review aims to help readers be aware of the many aspects of this field.

Controllable preparation of microscale tubes with multiphase co-laminar flow in a double co-axial microdevice

This article describes a simple method for the fabrication of microscale polymer tubes. A double co-axial microchannel device was designed and fabricated. Liquid/liquid/liquid multiphase co-laminar flows were realized in a microchannel by choosing working systems. Three kinds of polymeric solutions were selected as the middle phase while a polyethyleneglycol aqueous solution was used as the inner and outer phases in the microfluidic process. The outer and inner phases acted as extractants of the polymer solvent. A stable double core-annular flow was formed by optimizing the composition of the outer and inner phases, and highly uniform tubes were successfully fabricated by the solvent extraction method. Both the outer diameter of the tubes and the wall thickness could be adjusted from 300 microm to 900 microm and from 40 microm to 150 microm by varying the flux of the fluids and the rolling velocity of the collection roller. In addition, titanium dioxide (TiO2) nanoparticles were successfully encapsulated into the polymer tubes with this technique. This technology has the potential to generate hollow fiber membranes for applications in separation and reaction processes.

Controllable preparation of nanoparticles by drops and plugs flow in a microchannel device

Well controlled two-liquid-phase flows in a T-junction microchannel device have been realized. The system of H2SO4 and BaCl2, respectively, in two phases to form BaSO4 nanoparticles was used as a probe to characterize the microscale two-phase flow and transport conditions of a system with interphase mass transfer and chemical reaction. Nanoparticles with narrow size and good dispersibility were produced through drops or plugs flow in the microdevice. As a novel work, the influence of mass transfer and chemical reaction on interfacial tension and flow patterns was discussed based on the experiments. At the same time, the effect of the two-phase flow patterns on the nanoparticle size was also discussed. It was found that the increase of the amount of mass transfer and chemical reaction could change the flow patterns from plugs flow to drops flow. The drop diameter or plug length could be changed in a wide range. Accordingly, a new parameter of mu(0)u(c)/gamma(0)/Q(d) was defined to distinguish the flow patterns. The prepared nanoparticles ranged in size from 10 to 40 nm. Apparently, the particle size decreased with the increase of the drop diameter or plug length. Reasons were discussed based on the mass transfer direction and speed in drops and plugs flow patterns.

Droplet microfluidics

Droplet-based microfluidic systems have been shown to be compatible with many chemical and biological reagents and capable of performing a variety of "digital fluidic" operations that can be rendered programmable and reconfigurable. This platform has dimensional scaling benefits that have enabled controlled and rapid mixing of fluids in the droplet reactors, resulting in decreased reaction times. This, coupled with the precise generation and repeatability of droplet operations, has made the droplet-based microfluidic system a potent high throughput platform for biomedical research and applications. In addition to being used as microreactors ranging from the nano- to femtoliter range; droplet-based systems have also been used to directly synthesize particles and encapsulate many biological entities for biomedicine and biotechnology applications. This review will focus on the various droplet operations, as well as the numerous applications of the system. Due to advantages unique to droplet-based systems, this technology has the potential to provide novel solutions to today's biomedical engineering challenges for advanced diagnostics and therapeutics.