Mixing within micro- and millichannels is a pivotal element across various applications, ranging from chemical synthesis to biomedical diagnostics and environmental monitoring. The inherent low Reynolds number flow in these channels often results in a parabolic velocity profile, leading to a broad residence time distribution. Achieving efficient mixing at such small scales presents unique challenges and opportunities. This review encompasses various techniques and strategies to evaluate and enhance mixing efficiency in these confined environments. It explores the significance of mixing in micro- and millichannels, highlighting its relevance for enhanced reaction kinetics, homogeneity in mixed fluids, and analytical accuracy. We discuss various mixing methodologies that have been employed to get a narrower residence time distribution. The role of channel geometry, flow conditions, and mixing mechanisms in influencing the mixing performance are also discussed. Various emerging technologies and advancements in microfluidic devices and tools specifically designed to enhance mixing efficiency are highlighted. We emphasize the potential applications of micro- and millichannels in fields of nanoparticle synthesis, which can be utilized for biological applications. Additionally, the prospects of machine learning and artificial intelligence are offered toward incorporating better mixing to achieve precise control over nanoparticle synthesis, ultimately enhancing the potential for applications in these miniature fluidic systems.

Polyethylene glycol-6000 (PEG-6000) and trisodium citrate dihydrate (TSC) form an aqueous two-phase system (ATPS). This ATPS has been explored for synthesis and separation of silver nanoparticles (AgNPs). However, synthesis of stable nanoparticles using high TSC concentration corresponding to generic ATPS composition is not possible as high ionic strength of TSC causes aggregation and precipitation of silver nuclei. In the current study, this problem was overcome by synthesizing AgNPs at low concentrations of TSC. A two-step process was used for simultaneous synthesis and separation of AgNPs. Once stable nanoparticles were synthesised at low TSC concentration, the solution was taken to biphasic ATPS composition by mixing higher concentrations of TSC and PEG-6000 solutions to ensure particle entrapment. A silver to TSC (Ag+:TSC) molar ratio ≥ 67 × 10–3 that corresponds to lower TSC concentrations resulted in the formation of stable silver nanoparticles. The synthesis was carried out in batch and continuous modes. Spherical particles of average size 9.1 ± 3.4 nm were obtained in batch mode while in the continuous mode, the size was 10.6 ± 2.5 nm. Addition of polyvinylpyrrolidone (PVP) helped in sterically stabilizing the particles. Spontaneous formation and stabilization of AgNPs were observed within 15 min of reaction time as opposed to several hours reported in literature. The adopted synthesis route thus, is an effective green process to synthesise stable nanoparticles of silver which can be used in biological applications.

The morphological evolution of organic crystals during crystallization depends on the face-specific growth rates. Classical growth rate models relate the face-specific growth rates to the crystal lattice, energy of stable facets, growth mechanism, and supersaturation. The complexities of these models have increased over time to account accurately for solution conditions, the structure of growth units, and their attachment rates. Such advanced growth rate models require several layers of computations to obtain attachment energies of facets, nucleation rates, kink density, and attachment rates. Among these, the most intensive and time-consuming computation is for attachment rates, which require molecular dynamic simulations. This substantially increases the overall computation time to predict the absolute growth rate for even one crystallization condition. Since it is nearly impossible to iterate such a growth rate model, optimization schemes cannot be implemented to identify solution conditions that favor specific crystal growth. To reduce the computational time for attachment rate calculations, we implement a group contribution method (GCM) that relates the properties of functional groups in a molecule to their attachment rates to the crystal lattice, thereby rapidly estimating the growth rates of organic crystals. The process of molecular attachment involves partial desolvation of a solvated molecule, referred to as a transition state, followed by total desolvation via spontaneous attachment to a crystal facet. The first step in GCM is to identify the equilibrium states of fully solvated and partially desolvated solute molecules. The degree of supersaturation dictates the extent of this equilibrium and, thereby, the activation barrier for the growth of crystals, according to transition state theory. Identifying this equilibrium phenomenon allows for capturing the functional-group-specific interactions that depend on molecular motion, which could be related to operating conditions such as temperature and pressure. The stochastic optimization technique with Monte-Carlo sampling allows an efficient optimization problem solution to obtain the group interaction parameters. The GCM approach is first validated for the estimation of growth rates of glutamic acid and L-histidine, and then extended to predict growth rates of alanine and glycine rapidly. The optimized parameters and GCM scheme can be used to estimate growth rates in other crystallization systems.

Sunset Yellow (SSY) is a neutral dye commonly used in the textile, cosmetic, pharmaceutical and food industries. This is found in the effluent streams of these industries. In this study, the extraction of this dye is analyzed using a polymer-salt aqueous two-phase system (ATPS). Classical extraction processes are based on contacting an organic phase with an aqueous phase. Our focus is on developing a green process where the use of an organic solvent is avoided. Polyethylene glycol of molecular weights 600 and 6000 are chosen as two candidates for the polymer phase and sodium sulfate as the salt phase. The propensity of the dye to selectively transfer into the polymer-rich phase is exploited to study the extraction process. The binodal curves are experimentally obtained using the cloud point method. The composition of salt and polymer was chosen so as to ensure that the volume of the polymer-rich phase is relatively low in the two-phase system. This choice results in obtaining a high concen-tration of the dye in the polymer-rich phase from a dilute solution of dye in the salt-rich phase. This is a spontaneous energy-efficient process which avoids the use of any other energy-intensive processes such as evaporation. The dye extraction is studied experimentally in batch mode where 75% extraction occurs in 2 h when the mass transfer is only by diffusion. To accelerate the process a milli channel is used under stratified flow conditions where the length scales are decreased, and 77% extraction is achieved in around 3 min. ATPS is characterized by low interfacial tension and obtaining slug flows in a microchannel is a challenge. We show that by using a milli channel we can lower the capillary number and obtain slug flow at higher flow rates. The internal circulations present in the slugs help in an accelerated mass transfer and 85% extraction is achieved in the PEG6000-salt system in 3 min.

Liquid-liquid segmented flows in microchannels have been extensively investigated in the context of nanoparticle synthesis. The enhanced mixing in the slugs results in monodispersed particles. Earlier studies have focused on Organic-Aqueous Systems (OAS). The nanoparticles synthesized in the presence of organic solutions have limited applications. An alternative green route for the synthesis can be developed using an Aqueous Two-Phase System (ATPS). These systems are characterized by interfacial tensions, which are two orders of magnitude lower than typical organic aqueous systems. In this work, flow patterns and hydrodynamics of ATPS are investigated as a first step. Polyethylene glycol -trisodium citrate system was chosen as ATPS. The objective of this work is to see if any new physics arises in an ATPS system. The low interfacial tension results in high Capillary numbers (Ca >> 3) in a microfluidic system. Consequently, the flow observed here is parallel or core-annular. However, in a millichannel, the capillary number becomes lower (Ca << 1) for an ATPS system. In this work, experiments were carried out in a millichannel to span different flow patterns. The pattern formation was analyzed and classified into three categories, i.e., slug flow (interfacial tension dominated), transition flow, and core annular flow (inertia dominated). Flow regime maps based on the Reynolds number, Capillary number, and Weber number of each phase were found to be qualitatively similar to those of OAS. Simulations were performed for various interfacial tension values. An interfacial tension value of 1.25x10-4 N/m was found to yield slug sizes which fitted well with the experimental data. Film thickness was measured experimentally and with simulations compared favorably with the correlations available in the literature for OAS.

Several hydrometallurgical routes have been proposed in the literature to treat printed circuit boards (PCBs) from waste electronic equipment. These employ different chemical reagents for the recovery and separation of metals as metals or their salts. The recovery of multiple chemical reagents (unreacted acid, neutralising agent, metal salts) in a process requires several additional downstream steps affecting economic feasibility. There is a need to develop a safe, eco-friendly, and economically feasible process to recover metals from PCBs. In this work, we achieve this using a hydrometallurgical process for treating PCB with nitric acid by extracting metals sequentially. The focus is on tin, lead, and copper, which are present in significant quantities in PCBs. The process is scalable and is based on exploiting the physio-chemical interactions between the different metals and the acid. Tin and lead present in the solder are selectively dissolved at low acid concentrations. Tin comes out in the form of colloidal particles of metastannic acid. The low solubility of lead nitrate in concentrated nitric acid is exploited by evaporating the solution containing unreacted nitric acid and lead nitrate. In the concentrated acid, lead nitrate crystals are precipitated out, and the concentrated nitric acid is recycled for dissolution. The different nitrogen oxide gases generated are absorbed in water and recycled. Copper present in copper tracks was recovered by reacting with a higher concentration of acid in a downstream step. The copper nitrate obtained was separated from nitric acid by extracting nitric acid using Tri-Butyl Phosphate. Tin, lead, and copper are sequentially extracted as tin oxide (s), lead nitrate (s), and copper nitrate (aq.) with an efficiency of 77%–97%, 51%–85%, and 100%, respectively.

The physio-chemical properties of metallic nanoparticles are different from their corresponding bulk material. Synthesizing stable zero valent copper nanoparticles is a challenge since they get oxidized easily. This paper discusses both the batch and continuous synthesis of copper nanoparticles using a polyol process in the absence of an inert atmosphere. The nanoparticles were synthesized using copper amine complex as a precursor, ascorbic acid as a reducing agent, and polyvinyl pyrrolidone as a capping agent. UV-Vis spectra confirmed that particles from a continuous synthesis had better Localized Surface Plasmon Resonance (LSPR) peak than those from batch synthesis. At 120 °C, nanoparticles in the continuous process could be synthesized at a residence time of 1 min in contrast to the batch reactor, which needed a reaction time of 4 min. The nanoparticles synthesized were of size 1.5–6 nm. Those synthesized in continuous mode were stable for 10 days as compared to those synthesized in batch mode.