Current methods for characterising mixing and flow in microchannels

This article reviews existing methods for the characterisation of mixing and flow in microchannels, micromixers and microreactors. In particular, it analyses the current experimental techniques and methods available for characterising mixing and the associated phenomena in single and multiphase flow. The review shows that the majority of the experimental techniques used for characterising mixing and two-phase flow in microchannels employ optical methods, which require optical access to the flow, or off-line measurements. Indeed visual measurements are very important for the fundamental understanding of the physics of these flows and the rapid advances in optical measurement techniques, like confocal scanning laser microscopy and high resolution stereo micro particle image velocimetry, are now making full field data retrieval possible. However, integration of microchannel devices in industrial processes will require on-line measurements for process control that do not necessarily rely on optical techniques. Developments are being made in the areas of non-intrusive sensors, magnetic resonance techniques, ultrasonic spectroscopy and on-line flow through measurement cells. The advances made in these areas will certainly be of increasing interest in the future as microchannels are more frequently employed in continuous flow equipment for industrial applications

Monodisperse ZnO Micro and Nanoparticles Obtained by Micro Segmented Flow Synthesis

Mikro- und Nanopartikel aus Zinkoxyd (ZnO) besitzen bemerkenswerte Eigenschaften für Applikationen im Bereich der Elektronik, Optik und Photonic. Als ein Halbleitermaterial mit großer Bandlücke ist ZnO ebenfalls für die Entwicklung von Sensoren, Light Emitting Diodes (LEDs) und Solarzellen von hohem Interesse. Die Herstellung definierter Materialien mit einheitlicher Morphologie und enger Partikel-Größenverteilung ist hierzu eine wichtige Voraussetzung. Verschiedene Verfahren zur Herstellung entsprechender Partikel sind in der Vergangenheit untersucht worden. Die tropfenbasierte Mikrofluidik bietet die Möglichkeit einer exzellenten Reaktionskontrolle durch die Verwendung eines Tropfens als Reaktionsgefäß. Kurze Mischzeiten, hohe Heiz-/Kühlraten sowie eine definierte Verweilzeit ermöglichen so neben stöchiometrischen Parametern eine exakte Reaktionsführung. Ziel der hier vorliegenden Dissertationsschrift ist die Untersuchung der ZnO-Präzipitation in entsprechenden mikrofluidischen Systemen sowie die Charakterisierung der hergestellten Materialien.ZnO micro and nanoparticles have attracted considerable interest because of their remarkable performance in electronics, optics and photonics. As a wide band gap semiconductor material, ZnO is also a potential candidate for various applications including gas sensing, light emitting devices and solar cells. Although some technologies have been developed to produce well-defined ZnO particles of different shapes and sizes, ZnO particles prepared by micro segmented flow synthesis have been rarely reported. The aim of this work was to develop a microfluidic system based on the micro segmented flow method and to test whether the microfluidic components are suitable for the generation and investigation of ZnO particles with improved homogeneity.In order to optimize the experimental conditions, ZnO particles were first synthesized in batch. The optimized batch conditions were then adapted to two microfluidic arrangements for continuous synthesis of ZnO particles below 100°C. The set-ups included computer-controlled syringe pumps, T-injectors, PTFE tubings and PTFE knot mixers in a thermostat water bath. The ZnO particles were obtained under strong alkaline conditions at elevated temperature in aqueous solution and DMSO solution. Needle-like, flower-like and compact ZnO particles were obtained. In nearly all cases, a strong effect of the flow conditions on the homogeneity of the formed particles was observed. The higher quality of the particles can be attributed to the fast mixing and enhanced heat transfer caused by segment-internal convection.In addition, two other microfluidic set-ups were developed to control the ZnO formation reaction at temperature up to 150°C. A static micromixer was used for mixing the reactants at room temperature. The formation of segmented flow was realized by injection of the reaction mixture into a carrier stream. The particle growth took place in PTFE tube coils inside a thermostat, which allowed to heat up to 150°C. By using this set-up, flower-like, star-like, and spherical ZnO particles were successfully synthesized. The shape and size of the formed particles were strongly dependent on the reactant concentration and the molar ratio of NaOH/Zn(Ac)2. The total residence time for preparation of these particles was only 9.3s, which is very short compared to the most conventional methods.The effect of the solvent on the formation of ZnO particle has also been investigated using this microfluidic set-up. Two different experimental conditions were applied to prepare ZnO particles, where Zn(Ac)2 and NaOH in ethylene glycol (EG) were mixed with water or water/EG mixing solvent to achieve different water contents in the final mixture solution. The formation of homogeneous particles was characterized by SEM and TEM. A stronger dependence of the particle size and shape on the water content was observed. Furthermore, the water content can be used for tuning the optical absorption spectra of the formed ZnO particles. Besides the ZnO microparticles, ZnO nanoparticles with an average diameter around 4-5nm have been synthesized using Zn(Ac)2 and LiOH in ethanol. The prepared nanoparticles exhibited green and blue emission under excitation at 325nm. In order to understand the size-dependent optical properties of ZnO nanoparticles, extended X-ray absorption fine structure (EXAFS) spectroscopy was applied to study their local structure properties and compared with that of ZnO flower-like microparticles. The EXAFS measurements revealed higher vacancies and a higher degree of structural disorders in the nanoparticles than the microparticles. These disorders and vacancies could contribute to the blue shift of the visible emission from ZnO nanopartilces.Due to the potential applications of semiconductor-metal composite particles in diverse areas, the flower-like ZnO microparticles obtained by micro segmented flow synthesis were used to fabricate ZnO/4-MBA/Au composite particles using a simple strategy. The formed composite particles were very homogeneous in shape and size. The surface coverage of Au nanoparticles on ZnO/4-MBA particles can be adjusted by changing the molar ratio of ZnO/4-MBA to Au. In order to study the interaction of 4-MBA molecules with ZnO and Au particles, Raman spectra of ZnO/4-MBA and ZnO/4-MBA/Au particles were analysed.In summary, the segmented flow technique is suitable to generate ZnO particles with controlled size and morphology. Compared to most conventional methods, this technique offers several advantages, and it provides a new insight into material synthesis under environmentally friendly conditions

Continuous Flow Synthesis of Gold Nanoparticles in Microfluidic Systems for SERS and Antimicrobial Applications

In this thesis we focus on the synthesis of gold nanoparticles (Au NPs) which are mainly used for the Surface Enhanced Raman Scattering (SERS) and antimicrobial applications. As the properties of Au NPs are morphology-dependent, synthetic routes with good control are quite important to obtain the Au NPs with best performance during applications. The aim of this thesis was to investigate the adaptation of different synthetic routes of Au NPs from batch to continuous microfluidic systems to overcome some shortcomings in batch and achieve controllable fabrication. Firstly, sub-3 nm citrate-capped gold nanoparticles were synthesized in continuous flow capillary reactors with enhanced nucleation for easier further functionalization compared to thiol-capped Au NPs in similar size range. It is speculated that the negatively charged tubing-water interface, offers heterogeneous nucleation sites for positively charged citrate-gold precursor species, stabilizing the nuclei and inhibiting their growth. As a gaseous reducing agent, carbon monoxide (CO) has a big potential in nanoparticle synthesis as it is very easy to remove from the product. The second synthetic route was the synthesis of Au NPs by CO reduction in microfluidic system. With the aid of segmented microfluidic system, the polydispersity of Au NPs was reduced to 95%, throughput ~ 50 mg/h) synthesis of Au25(Cys)18 nanoclusters (NCs) via CO reduction in segmented microfluidic system. These Au NCs as a crystal violet (CV) photobactericidal enhancement agent was tested against S. aureus, which showed significantly enhanced bactericidal activity of crystal violet encapsulated in a silicon substrate under white light illumination of ~312 lux

Experimental study of helical milling on CFRP (carbon fibre reinforced polymer) for the hole making process

Generate borehole by helical milling process may be used effectively since accurate location of the hole may be secured by means of the feed screw graduations. Fiber delamination which is the main defect occurred during hole making process on carbon fiber reinforced polymer (CFRP) were investigate throughout an experimental study. Effects of thrust force (Fz), delamination factor (Fd) and surface roughness are evaluated. Objective of the experiment are to find best cutting parameter and tool design suitable to performed helical milling operation on CFRP. Two types of end mill with 4 flutes were used and results are evaluated. It was found that tool design 2-1 has higher performance on CFRP

Optical methods to investigate the enhancement factor of an oxygen-sensitive colorimetric reaction using microreactors

Visualization of mass transfer is a powerful tool to improve understanding of local phenomenon. The use of an oxygen-sensitive dye (colorimetric technique1) has showed its relevancy for locally visualizing and characterizing gas-liquid mass transfer at different scales2,3. At present, the occurrence of a possible enhancement of the gas-liquid mass transfer by this reaction has not been yet demonstrated. This paper aims at filling this gap by evaluating the Hatta number Ha and the enhancement factor E associated with the oxygen colorimetric reaction when implementing in milli/micro channels. For that, as data on the kinetic of the colorimetric reaction are seldom in the literature, the reaction characteristic time was firstly estimated by carrying out experiments in a microchannel equipped with a micromixer. The diffusion coefficients of dihydroresorufin and O2 were then determined by implementing two original optical methods in a specific coflow microchannel device, coupled with theoretical modelling. The knowledge of these parameters enabled at last to demonstrate that no enhancement of the gas-liquid mass transfer by this colorimetric reaction existed. Complementary information about the reliability of the colorimetric technique to characterize the gas-liquid mass transfer in milli/micro systems was also give

Continuous Solid Particle Flow in Microreactors for Efficient Chemical Conversion

The incorporation of flowing nanoparticles or microparticles for use in catalysis as well as in the enhancement of mass transfer in microreactors opens a new avenue for chemical process intensification. In this work, the recent application of handling suspended solid particles in microreactors for carrying out efficient chemical conversions is reviewed, including the use of colloidal suspensions, Pickering emulsions, and catalyst slurries. Emphasis is laid on the effect of the presence of solid particles on the microflow characteristics, mass transfer property, and reaction enhancement, especially in multiphase fluid systems as the most frequently used one in microreactors. A future perspective regarding the potential application of such microreactor systems as well as challenges, especially related to stable flow operation and catalyst recycling, is further provided

Continuous Synthesis of Gold Nanoparticles in Micro- and Milli-fluidic Systems

Gold nanomaterials have diverse applications ranging from healthcare and nanomedicine to analytical sciences and catalysis. Microfluidic and millifluidic reactors offer multiple advantages for their synthesis and manufacturing, including controlled or fast mixing, accurate reaction time control and excellent heat transfer. These advantages are demonstrated by reviewing gold nanoparticle synthesis strategies in flow devices. However, there are still challenges to be resolved, such as reactor fouling, particularly if robust manufacturing processes are to be developed to achieve the desired targets in terms of nanoparticle size, size distribution, surface properties, process throughput and robustness. Solutions to these challenges aremore effective through a coordinated approach from chemists, engineers and physicists, which has at its core a qualitative and quantitative understanding of the synthesis processes and reactor operation. This is important as nanoparticle synthesis is complex, encompassing multiple phenomena interacting with each other, often taking place at short timescales. The proposed methodology for the development of reactors and processes is generic and contains various interconnected considerations. It aims to be a starting point towards rigorous design procedures for the robust and reproducible continuous flow synthesis of gold nanoparticles

Precipitation of Nanosized and Nanostructured Powders: Process Intensification and Scale-Out Using a Segmented Flow Tubular Reactor (SFTR)

The successful scale-out and process intensification using a segmented flow tubular reactor (SFTR) for ultrafine CaCO3, BaTiO3, and nanosized ZnO from optimized minibatch (20 mL) conditions is presented. The capacity of the SFTR in process intensification was demonstrated by producing ∼ 5 kg batches of BaTiO3 powders with excellent batch-to-batch reproducibility. The SFTR scale-out or numbering-up capacity was demonstrated for a nanostructured CaCO3 in 500 g batches by scaling-out from one to six segmented flow tubular reactors run in parallel (scale-out/-up ratio of 5000 compared to lab batch experiments). The SFTR was then used to demonstrate its potential for nanosized ZnO powders producing 50 g lots of these nanopowders in a continuous process, a scale-out/-up ratio of 250 compared to lab batch experiments without any loss of powder quality. The SFTR allows a precise control of precipitation conditions, leading to an excellent reproducibility in powder characteristics, and shows great promise as a simple production process of powders and advanced nanomaterials with highly controlled properties