Tuning silica particle shape at fluid interfaces

Preparation of ethyl cellulose particles with different morphologies through microfluidics

The sizes and shapes of polymer particles determine their performance and application. In this paper, ethyl cellulose particles with different morphologies are generated through extraction and solidification in a microfluidic device with double T-junctions. Droplets of ethyl acetate containing ethyl cellulose are formed first, then, pure water is employed to extract the solvents in the droplets and the ethyl cellulose is solidified to form monodisperse particles. By changing the flow rates of the continuous phase and the dispersed phase and the concentration of ethyl cellulose, red-blood-cell-like, doughnut-like, dimpled and spherical particles are fabricated, and the regime of different particle morphologies is given. The more important is that the physical mechanisms and explanations of the formation of different particle morphologies are clearly disclosed by analyzing the circulation flows outside and inside the droplets. The flow patterns in the microchannel, and the diffusion and solidification properties of the molecules are the key factors that affect the final morphology of particles. Due to the circulation, there are two stagnation points at the front and rear of the droplet, and they are the approximate locations where the dimple in the dimpled particle, the hole in the doughnut-like particle and the two pits in the red-blood-cell-like particles are formed. These analysis and results are useful in flow chemistry, in the fabrication of particle materials, and so on.

Microfluidic Controlled Mass-Transfer and Buckling for Easy Fabrication of Polymeric Helical Fibers

Microfluidic fabrication of helical microfibers is still a big challenge. The reason is that this always includes designing the necessary geometrical channels and chemical conditions to first form a flowing liquid jet, which has to be continually reacting and rapidly evolving in time from viscous liquid to a flexible solid to maintain the helical structure inside the microfluidic channels. In this report, dextran aqueous solution and liquid PEG400 are infused separately into the inner and outer channels of a simple single emulsion microfluidic device, respectively. The formed two phase stream then enters a widening collection tube, where automatically formation of dextran helical fiber happened due to water shifting and widening of the channel cooperatively induced buckling. Various experimental conditions that influence the amplitudes, wavelengths, and diameters of the formed helical fibers are discussed.

Microfluidics-assisted diffusion self-assembly: toward the control of the shape and size of pectin hydrogel microparticles

We demonstrated the generation of pectin hydrogel microparticles having complex shapes either by combining the phenomenon of gelation and water diffusion-induced self-assembly in microfluidic channels (on-chip) or by the deformation of the pregelled droplets outside the channels (off-chip) at a fluid-fluid interface. We proved that by tuning the mode of pectin cross-linking (CaCl2 vs CaCO3) and the degree of shrinking (water content in the dimethyl carbonate (DMC) organic continuous phase) we can control the shape of the final particle. Sphere, doughnut, oblate ellipsoid, or mushroom-type morphologies were thus produced, demonstrating the ability to control the formation of anisotropic biopolymer-based hydrogel microparticles using microfluidics. Shape changes were explained by the redistribution of calcium ions in combination with the local Peclet number experienced by the microdroplets during the on-chip process. Moreover, during the off-chip process, the interplay between elastic and viscous forces for microdroplets entering the CaCl2-DMC interface caused deformation of the pregelled droplets to occur and therefore resulted in the formation of microparticles with a mushroom-like morphology.

Microfluidics assisted generation of innovative polysaccharide hydrogel microparticles

Capillary flow-based approach such as microfluidic devices offer a number of advantages over conventional flow control technology because they ensure highly versatile geometry and can be used to produce monodisperse spherical and non-spherical polymeric microparticles. Based on the principle of a flow-focusing device to emulsify the coflow of aqueous solutions in an organic phase, we were able to produce the following innovative polysaccharide hydrogel microparticles: - Janus hydrogel microparticles made of pectin–pectin (homo Janus) and pectin–alginate (hetero Janus) were produced. The efficiency of separation of the two hemispheres was investigated by confocal scanning laser microscopy (CSLM) of previously labelled biopolymers. The Janus structure was confirmed by subjecting each microparticle hemisphere to specific enzymatic degradation. As a proof of concept, free BSA or BSA grafted with dextran, were encapsulated in each hemisphere of the hetero Janus hydrogel microparticles. While BSA, free or grafted with dextran, was always confined in the alginate hemisphere, a fraction of BSA diffused from the pectin to the alginate hemisphere. Methoxy groups along the pectin chain will be responsible of the decrease of the number of attractive electrostatic interactions occurring between amino groups of BSA and carboxylic groups of pectin. - Pectin hydrogel microparticles of complex shapes were successfully produced by combining on-chip the phenomenon of gelation and water diffusion induced self-assembly, using dimethyl carbonate as continuous phase, or by deformation of the pre-gelled droplets off-chip at a fluid–fluid interface. Sphere, oblate ellipsoid, torus or mushroom-type morphologies were thus obtained. Moreover, it was established that after crossing the interface during their collect, mushroom-type microparticles did not migrate in the calcium or DMC phase but stayed at the liquid–liquid interface. These new and original hydrogel microparticles will open up opportunities for studying relationships between combined enzymatic hydrolysis and active release for Janus particles and relationships between shape and swelling behaviour for anisotropic pectin microparticles.

Microfluidic production of single micrometer-sized hydrogel beads utilizing droplet dissolution in a polar solvent

In this study, a microfluidic process is proposed for preparing monodisperse micrometer-sized hydrogel beads. This process utilizes non-equilibrium aqueous droplets formed in a polar organic solvent. The water-in-oil droplets of the hydrogel precursor rapidly shrunk owing to the dissolution of water molecules into the continuous phase. The shrunken and condensed droplets were then gelled, resulting in the formation of hydrogel microbeads with sizes significantly smaller than the initial droplet size. This study employed methyl acetate as the polar organic solvent, which can dissolve water at 8%. Two types of monodisperse hydrogel beads-Ca-alginate and chitosan-with sizes of 6-10 μm (coefficient of variation < 6%) were successfully produced. In addition, we obtained hydrogel beads with non-spherical morphologies by controlling the degree of droplet shrinkage at the time of gelation and by adjusting the concentration of the gelation agent. Furthermore, the encapsulation and concentration of DNA molecules within the hydrogel beads were demonstrated. The process presented in this study has great potential to produce small and highly concentrated hydrogel beads that are difficult to obtain by using conventional microfluidic processes.