Formation of monodisperse silica microparticles with various shapes and surface morphologies using double emulsion templates

Generation of liquid metal double emulsion droplets using gravity-induced microfluidics

Several microfluidic applications are available for liquid metal droplet generation, but the surface oxidation of liquid metal has placed limitations on its application. Multiphase microfluidics makes it possible to protect the inner droplets by producing the structure of double emulsion droplets. Thus, the generation of liquid metal double emulsion droplets has been developed to prevent the surface oxidation of Galinstan. However, the generation using common methods faces considerable challenges due to the gravity effect introduced from the high density of liquid metal, making it difficult for the shell phase to wrap the inner phase. To overcome this obstacle, we introduce an innovative method – a gravity-induced microfluidic device – to creatively generate controllable liquid metal double emulsion droplets, achieved by altering the measurable inclination angle of the plane. It is found that when the inclination angle ranges from 30° to 45°, the device manages to generate liquid metal double emulsion droplets with perfect double sphere-type configuration. Additionally, the core–shell liquid metal hydrogel capsules present potential applications as multifunctional materials for controlled release systems in drug delivery and biomedical applications. By regulating pH or imposing mechanical force, the hydrogel shell can be dissolved to recover the electrical conductivity of Galinstan for applications in flexible electronics, self-healing conductors, elastomer electronic skin, and tumor therapy.

An innovative method – a gravity-induced microfluidic device – to generate liquid metal double emulsion droplets to prevent the formation of an oxide layer on the liquid metal is introduced.

Synthesis and Surface Engineering of Inorganic Nanomaterials Based on Microfluidic Technology

The controlled synthesis and surface engineering of inorganic nanomaterials hold great promise for the design of functional nanoparticles for a variety of applications, such as drug delivery, bioimaging, biosensing, and catalysis. However, owing to the inadequate and unstable mass/heat transfer, conventional bulk synthesis methods often result in the poor uniformity of nanoparticles, in terms of microstructure, morphology, and physicochemical properties. Microfluidic technologies with advantageous features, such as precise fluid control and rapid microscale mixing, have gathered the widespread attention of the research community for the fabrication and engineering of nanomaterials, which effectively overcome the aforementioned shortcomings of conventional bench methods. This review summarizes the latest research progress in the microfluidic fabrication of different types of inorganic nanomaterials, including silica, metal, metal oxides, metal organic frameworks, and quantum dots. In addition, the surface modification strategies of nonporous and porous inorganic nanoparticles based on microfluidic method are also introduced. We also provide the readers with an insight on the red blocks and prospects of microfluidic approaches, for designing the next generation of inorganic nanomaterials.

Capillary-Based Microfluidics-Coflow, Flow-Focusing, Electro-Coflow, Drops, Jets, and Instabilities

Capillary-based microfluidics is a great technique to produce monodisperse and complex emulsions and particulate suspensions. In this review, the current understanding of drop and jet formation in capillary-based microfluidic devices for two primary flow configurations, coflow and flow-focusing is summarized. The experimental and theoretical description of fluid instabilities is discussed and conditions for controlled drop breakup in different modes of drop generation are provided. Current challenges in drop breakup with low interfacial tension systems and recent progress in overcoming drop size limitations using electro-coflow are addressed. In each scenario, the physical mechanisms for drop breakup are revisited, and simple scaling arguments proposed in the literature are introduced.

Microfluidics for silica biomaterials synthesis: opportunities and challenges

The rational design and controllable synthesis of silica nanomaterials bearing unique physicochemical properties is becoming increasingly important for a variety of biomedical applications from imaging to drug delivery. Microfluidics has recently emerged as a promising platform for nanomaterial synthesis, providing precise control over particle size, shape, porosity, and structure compared to conventional batch synthesis approaches. This review summarizes microfluidics approaches for the synthesis of silica materials as well as the design, fabrication and the emerging roles in the development of new classes of functional biomaterials. We highlight the unprecedented opportunities of using microreactors in biomaterial synthesis, and assess the recent progress of continuous and discrete microreactors and the associated biomedical applications of silica materials. Finally, we discuss the challenges arising from the intrinsic properties of microfluidics reactors for inspiring future research in this field.

Polymerization kinetics of a multi-functional silica precursor studied using a novel Monte Carlo simulation technique

Silica polymerization has been extensively used to synthesize various fascinating materials for industrial and technological applications. The polymerization protocol is modified by altering several parameters (such as the concentration of the precursor, temperature, pH) heuristically to obtain the desired end product. To properly understand the effect of such parameters, knowledge of molecular events occurring during the process of polymerization is essential. In this work, we developed algorithms to capture molecular events such as translation, rotation, and reactions using the reaction ensemble Monte Carlo (REMC) technique. Our algorithms simulate molecular events in accordance with physical time by correctly scaling the movements of a cluster with the monomer, thereby capturing the kinetics of the process. We studied the polymerization of the four coordinated silica (f₄) precursor using our algorithm and observed excellent agreement between simulation results and experimental data. The algorithm was also used to study the polymerization of the three coordinated silica (f₃) precursor and it was found that our simulations capture experimental kinetics well, thereby confirming that the developed algorithms are robust. We studied the effect of the functionality of the precursor on polymerization kinetics and the resulting structure by simulating silica systems having a mixture of two, three and four functional (f₂, f₃, and f₄) silica precursors. We observed that network formation and cluster size decrease with the increase in the concentration of the f₂ precursor. The radius of gyration (R_g) of the system initially increases due to network formation and decreases later due to the collapse of a large cluster. The R_g is directly correlated with the total number of primitive rings present in the system. The molecular level understanding obtained will be useful in the design of tailored silica nanoparticles.

Emulsion-based synthesis of porous silica

We review the use of various types of emulsions as media for synthesis of porous silica particles. The use of high internal phase emulsions, i.e. emulsions with a high ratio of dispersed to continuous phase, is an approach that has attracted considerable attention. Polymerization of the continuous phase followed by removal of the dispersed phase leads to a material with an even distribution of pores if the emulsion droplets are uniform in size. Another route is to use particle stabilized emulsions as template. This will lead to either hydrophilic or hydrophobic porous silica particles depending on whether the templating emulsion is oil-in-water or water-in-oil, respectively. Use of double emulsions as templates is a way to obtain porous particles with hierarchical porosity, usually both macropores and mesopores. Templating amphiphiles, which are often polyoxyethylene-polyoxypropylene-polyoxyethylene block copolymers, are needed in order to obtain materials with highly ordered pore structure. Non-ordered mesoporous silica with small particle size and relatively large pores can be obtained by emulsifying a silica precursor together with an inert solvent in water and allowing the silica to gel within the drops. Hollow silica spheres, i.e. spherical particles with a void in the middle, can be prepared by using emulsion drops as templates around which silica polymerizes. The particles have nanometer-sized pores penetrating the shell.