We use micro particle image velocimetry (microPIV) and fluorescence microscopy techniques to characterize microscale segmented gas-liquid flow at low superficial velocities relevant for chemical reactions with residence times of up to several minutes. Different gas-liquid microfluidic channel networks of rectangular cross section are fabricated in poly(dimethylsiloxane) (PDMS) using soft lithography techniques. The recirculation motion in the liquid segments associated with gas-liquid flows as well as the symmetry characteristics of the recirculations are quantified for straight and meandering channel networks. Even minor surface roughness effects and the compressibility of the gas phase induce loss of symmetry and enhance mixing across the centerline in straight channels. Mixing is further accelerated in meandering channels by the periodic switching of recirculation patterns across the channel center. We demonstrate a new, piezoelectrically activated flow injection technique for determining residence time distributions (RTDs) of fluid elements in multiphase microfluidic systems. The results confirm a narrowed liquid phase RTD in segmented flows in comparison to their single-phase counterparts. The enhanced mixing and narrow RTD characteristics of segmented gas-liquid flows are applied to liquid mixing and in sol-gel synthesis of colloidal nanoparticles.

We demonstrate the design, fabrication, and operation of microfluidic chemical reactors for the synthesis of colloidal silica particles. Two reactor configurations are examined: laminar flow reactors and segmented flow reactors. We analyze particle sizes and size distributions and examine their change with varying linear flow velocity and mean residence time. Laminar flow reactors are affected by axial dispersion at high linear velocities, thus leading to wide particle size distributions under these conditions. Gas is used to create a segmented flow, consisting liquid plugs separated by inert gas bubbles. The internal recirculation created in the liquid plugs generates mixing, which eliminates the axial dispersion effects associated with laminar flow reactors and produces a narrow size distribution of silica nanoparticles.

A droplet-based microfluidic method for the preparation of anisotropic gold nanocrystal dispersions is presented. Gold nanoparticle seeds and growth reagents are dispensed into monodisperse picoliter droplets within a microchannel. Confinement within small droplets prevents contact between the growing nanocrystals and the microchannel walls. The critical factors in translating macroscale flask-based methods to a flow-based microfluidic method are highlighted and approaches are demonstrated to flexibly fine tune nanoparticle shapes into three broad classes: spheres/spheroids, rods, and extended sharp-edged structures, thus varying the optical resonances in the visible-near-infrared (NIR) spectral range.

Covalent organic frameworks (COFs) have recently emerged as a new class of crystalline porous materials with many potential applications. The development of facile and effective synthetic methods of COFs is highly desirable for their large-scale applications. Herein, we demonstrate the room temperature batch synthesis of three classical two-dimensional (2D) COFs with various types of linkage, namely, COF-LZU1 (imine-linked), TpPa-1 (enamine-linked), and N 3 -COF (azinelinked). These obtained COFs exhibit good crystallinity and high porosity comparable to their counterparts synthesized solvothermally at higher temperatures. The facile formation of these COFs under such mild synthetic conditions can be attributed to (1) high solubility of monomers and (2) the strong π−π stacking interactions between monomers and π-systems of oligomers during the initial and the subsequent error-correction crystallization process. Based on this conclusion, two new iminelinked COFs named NUS-14 and NUS-15 were successfully synthesized with good crystallinity under ambient conditions. Moreover, continuous flow synthesis has been demonstrated in COF-LZU1 with a production rate of 41 mg h −1 at an extremely high space-time yield (STY) of 703 kg m −3 day −1 . This study represents the first example of synthesizing COFs by continuous processes, which sheds light on the scaled-up synthesis of these promising materials.

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This paper looks at the design, fabrication and characterization of stackable microfluidic emulsion generators, with coefficients of variation as low as ~6% and with production rates as high as ~1 L h(-1). This work reports the highest throughput reported in the literature for a microfluidic device with simultaneous operation of liquid-liquid droplet generators. The device was achieved by stacking several layers of 128 flow-focusing droplet generators, organized in a circular array. These layers are interconnected via through-holes and fed with designated fractal distribution networks. The proposed layers were milled on poly(methylmethacrylate) (PMMA) sheets and the stack was thermo-compression bonded to create a three-dimensional device with a high density of generators and an integrated hydraulic manifold. The effect of stacking multiple layers was studied and the results show that fabrication accuracy has a greater impact on the dispersity of the emulsion than the addition of more layers to the stack. Particle crystallization of drugs was also demonstrated as a possible application of this technology in industry.

Core-shell colloidal materials with tailored structural, electronic, photonic and chemical properties have a wide range of applications including coatings, pigments, electronics, catalysis, separations and diagnostics.[1] Typical particle cores are polymeric (e.g., polystyrene), [2] inorganic [3] (e.g., silica) or metallic [4] (e.g., gold) in size ranges between 2 nm to 10 lm. Titania-silica core-shell particles in the sub-micron size range are of particular interest for several applications, including catalysis, [5] pigments (as whiteners) [6] and imaging materials. [7] Microfluidic synthesis schemes, when combined with segmented flow for fast mixing and well defined residence time distribution RTD), produce cores with precisely controlled and narrow size distribution. [8][9][10][11][12] Here we extend this approach to a multi-step addition microfluidic system for growth of well defined shell coatings without complications of secondary nucleation. Photolithography-based microfabrication enables microfluidic designs that provide uniform addition through periodically spaced side inlets along the length of synthesis channel. This procedure is functionally equivalent to slow dropwise addition in conventional batch synthesis with the added advantages of control and continuous processing inherent to microfluidic synthesis. We specifically demonstrate coating colloidal silica core particles with titania layers of tunable thickness in a one-step, continuous flow process through controlled hydrolysis of titanium tetraethoxide (TEOT). Several methods have been reported for titania-coated-silica synthesis, but many of these batch processes suffer from difficulties in controlling overcoat thickness, avoiding secondary nucleation and aggregation, and maintaining a narrow particle size distribution. Silica particles were coated with titania layers of varying thicknesses by aging titanyl sulfate (TiOSO 4 ) in the presence of silica particles. [6,13,14] Thick, irregular layers were obtained via a multi-step coating process that involved repeated filtration and redispersion cycles. Titanium n-butoxide hydrolysis in tetrahydrofuran yielded monolayer coatings on silica particles.[5] Thicker coatings (< 7 nm) were obtained by the hydrolysis of titanium n-butoxide in ethanol. [15,16] Titanium tetraethoxide (TEOT) was also used as a precursor to obtain thick (> 50 nm) coatings. [17,18] Other methods to synthesize titania-silica particles include the use of polyelectrolyte-coated silica spheres as templates for sol-gel reactions, [19] and the deposition of alternating coatings of cationic polyelectrolytes and anionic titania nanosheets on the surface of silica.[20]The titanium alkoxide precursors used in sol-gel coating are highly sensitive to the presence of water, readily hydrolyzing and condensing, [21] which makes it difficult to control thickness and uniformity of the coating process as well as to avoid nucleation of secondary titania particles and aggregation of cores. In order to circumvent these complications, low alkoxide and...

Emulsion-based crystallization to produce spherical crystalline agglomerates (SAs) is an attractive route to control crystal size during downstream processing of active pharmaceutical ingredients (APIs). However, conventional methods of emulsification in stirred vessels pose several problems that limit the utility of emulsion-based crystallization. In this paper, we use capillary microfluidics to generate monodisperse water-in-oil emulsions. Capillary microfluidics, in conjunction with evaporative crystallization on a flat heated surface, enables controllable production of uniformly sized SAs of glycine in the 35–150 μm size range. We report detailed characterization of particle size, size distribution, structure, and polymorphic form. Further, online high-speed stereomicroscopic observations reveal several clearly demarcated stages in the dynamics of glycine crystallization from emulsion droplets. Rapid droplet shrinkage is followed by crystal nucleation within individual droplets. Once a nucleus is formed within a droplet, crystal growth is very rapid (<0.1 s) and occurs linearly along radially advancing fronts at speeds of up to 1 mm/s, similar to spherulitic crystal growth from impure melts. The spherulitic aggregate thus formed ages to yield the final SA morphology. Overall crystallization times are on the order of minutes, as compared to hours in conventional batch processes. We discuss these phenomena and their implications for the development of more generalized processes applicable to a variety of drug molecules. This work paves the way for microfluidics-enabled continuous spherical crystallization processes.