The current work involves the synthesis of novel composite nanofluid (CNF) and the evaluation of their thermal performance as coolant in a rectangular microchannel heat sink (RMCHS). The research work combines numerical simulations and experimental validation. The RMCHS, designed with 18 channels of 400-micron width, facilitates the laminar flow of synthesized CNF, containing Silver (Ag) – Graphene oxide (GO) and multi-walled carbon nanotubes (CNT). In this study, the flow characteristics and thermal performance of the RMCHS were analyzed using Computational Fluid Dynamics (CFD) and compared with the experimental results. The synthesized CNF was evaluated for its stability and morphology, and its thermophysical properties were appraised. The performance of CNF in the removal of heat from the sink was studied and compared with the conventional hybrid nanofluids (HNF), against key parameters such as convective heat transfer coefficient (HTC), the base temperature, and thermal resistance. The findings reveal CNF to be more effective in heat dissipation with a 24% enhanced HTC when compared to water, as against the 14% obtained for HNF. The base temperature was reduced by 10°C with CNF, whereas HNF led to a 4°C reduction, both compared to water. The thermal resistance, was significantly lower for CNF (0.0101 K/W), by almost 40% than the HNF (0.0163 K/W). These results highlight the superior heat dissipation potential of the CNF, making it an assuring choice for use in improved microchannel designs to address electronic chip cooling.

In this study, we develop a comprehensive two-phase model to analyze the dynamics of bacterial swarming on porous substrates. The two distinct phases under consideration are the cell and aqueous phases. We use the thin-film approximation, as the characteristic height of the swarm is significantly lower than its characteristic radius. Our model incorporates surfactant generation by microorganisms, drag forces between the cell and aqueous phases, osmotic influx, and Marangoni stresses. The disjoining pressure is included to account for substrate wettability, and a precursor film is used to address the contact line singularity. Several morphologies of bacterial swarms, such as arrested, circular, modulated, branching, droplet, fingering, and dendrite, have been observed experimentally. The model developed is capable of predicting all these shapes for realistic parameter values. An increase in the wettability of the substrate leads to faster expansion, while increased surface tension helps redistribute biomass radially. The role of biomass growth and surfactant production rate, surfactant diffusivity, and osmotic influx on the morphology of bacterial swarms are explained.

Recycling and recovery of metals from e-waste help develop a circular economy. Current methods of recovering metals from e-waste employ strong inorganic acids and generate harmful byproducts and effluent streams. Besides, the metals are recovered in the form of salts, requiring further purification. Green solvents such as deep eutectic solvents (DES) are capable of dissolving metals. Here, we focus on using a hydrophobic thymol-capric acid DES to study the dissolution and recovery of metallic copper through controlled studies. The extraction efficiency of dissolved Cu metal from DES into an aqueous solution of trisodium citrate (TSC) was 98.4

This work focuses primarily on the two-step synthesis of ternary nanofluids consisting of silver (Ag), graphene oxide (GO), and multi-walled carbon nanotubes (MWCNT) in volume fractions ranging from 0.005 to 0.03, their stability and structural (morphological) analysis, and appraisal of their thermophysical properties such as thermal conductivity, viscosity, density and specific heat capacity in the temperature range from 20 to 80 degrees C. The thermal conductivity and viscosity were found to be 0.7845 W/m.K and 0.8718 cP at 30 degrees C for 3 vol %. The work also involved the optimization and validation of these thermophysical parameters using Artificial Neural Network (ANN). The ANN was constituted and tested with the application of Levenberg-Marquardt (LM) algorithm. The titular network size has been inherently optimized as per the relative error to enhance the model's ability to predict thermal conductivity and viscosity. The Levenberg-Marquardt feed-forward network possessing the optimal network design, 6-1 (hidden layer nodes-Output layer nodes) for thermal conductivity and 2-4 for viscosity have been identified as the best training approach. The ANN results exhibit the coefficient of regression (R2) to be significant at 0.99736 and 0.99725 for thermal conductivity and viscosity respectively, and the upper limit of relative error was negligible. The same data set subjected to the standard fitting model gave the R2 of 0.9931 and 0.9944 with mean square error of 0.0064 and 0.0170, respectively.

The development of rapid, sensitive, and affordable antimicrobial susceptibility testing (AST) is essential for controlling antibiotic overuse, thereby creating a critical checkpoint for the emerging antimicrobial resistance threat. Here, we introduce a novel method of electrochemical monitoring of bacterial growth in a diluted low-conductivity nutrient medium for rapid susceptibility testing using impedance spectroscopy. The method works on the change in charge transfer resistance exhibited by bacteria in response to antibiotics. The proposed Electrochemical Microfluidic device (ε-µD) employs low-cost carbon screen-printed electrodes and uses a simple microfluidic geometry. We explored the utilisation of a diluted nutrient medium as an electrolyte since it provides a higher charge transfer baseline signal for better sensitivity and supports the growth of the bacteria required for detection. The method enables sensitive detection of bacteria even at a low density of 84/mm2 in three hours of incubation time. For proof of concept, bacteria such as Escherichia coli and Bacillus subtilis were used, and the efficacy of the ampicillin and tetracycline drugs were tested. The experiments were done with the spiked urine samples, which correlated well with the controlled sample. The proposed system enhances the accessibility and affordability of rapid susceptibility testing, enabling its widespread use.

A novel electrochemical molecularly imprinted composite (MIC)-based sensor for detection of triclosan was developed. MIC was synthesized from o-phenylenediamine (o–PD), -COOH functionalized multiwalled carbon nanotube (cf-MWCNT), and triclosan by cyclic voltammetry on molybdenum nanoparticle (Mo-NP) embedded cf-MWCNT (Mo-cf-MWCNT) coated glassy carbon (GC) electrode, following removal of surface triclosan to form MIC/Mo-cf-MWCNT/GC. In our earlier work, two novel electrodes MIC/cf-MWCNT/GC and MIC/GC were fabricated. The presence of cf-MWCNT coating substrate on GC in MIC/cf-MWCNT/GC had improved the sensing performance than MIC/GC since presence of this substrate had decreased the electrochemical band gap (Eg) and increased Debye length (λd), Gibb’s free energy of adsorption (− ΔGads), electrochemical surface area (Ae), and surface redox site concentration (C*). Therefore, further improvement in sensing performance can be carried out by utilizing Mo-NP in the cf-MWCNT coating substrate using MIC to be the sensing material. This novel electrode (MIC/Mo-cf-MWCNT/GC) provided a limit of detection (LOD) of 900 ppt of triclosan, which was lower than the LOD achieved by using MIC/cf-MWCNT/GC (10 ppb) and MIC/GC (40 ppb). Adsorption isotherm was constructed for MIC/Mo-cf-MWCNT/GC delivering − ΔGads value of 59.049 kJ/mol indicating stronger chemisorption. To understand the role of Mo-cf-MWCNT in detection of triclosan, cyclic voltammetry, electrochemical impedance spectroscopy, and electrochemical band gap studies were conducted. This MIC/Mo-cf-MWCNT/GC showed good selectivity towards triclosan in presence of interfering ions.

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.

Tuning the parameters related to the sensing mechanism, a novel sensing electrode was fabricated to construct a sensor for triclosan detection at ultra-low level. A novel molecularly imprinted composite (Au@MIC) was synthesized from o-phenylenediamine (o–PD), gold nanoparticle (Au-NP)–embedded -COOH-functionalized multiwalled carbon nanotube (Au-cf-MWCNT), and triclosan as template by cyclic voltammetry (CV) on Au-cf-MWCNT–coated glassy carbon (GC) electrode (Au-cf-MWCNT/GC), following removal of surface triclosan. This novel electrode (Au@MIC/Au-cf-MWCNT/GC) provided a limit of detection (LOD) of 50 ppt of triclosan, which was lower than the LOD achieved by using MIC/cf-MWCNT/GC, mentioned in our earlier work. The sensor showed a good linearity in the range from 75 ppt to 5 ppb on a logarithmic scale. Langmuir adsorption isotherm gave the best fit for Au@MIC/Au-cf-MWCNT/GC with -ΔGads value of 65.925 kJ/mol indicating stronger chemisorption. The -ΔGads value was higher than that achieved for MIC/cf-MWCNT/GC. To understand the role of Au-cf-MWCNT in detection of triclosan, CV, electrochemical impedance spectroscopy, and electrochemical band gap studies were conducted. This electrode was able to withstand several ions of similar structure of triclosan showing good selectivity.

We investigate the growth kinetics of bacterial biofilms on porous substrates. A three-phase model is developed, which accounts explicitly for the cell phase, extracellular matrix (ECM), and nutrient-rich aqueous phase. We use the thin-film approximation as the characteristic height of the biofilm is much smaller than its characteristic radius. We use the 2D axisymmetric model to capture biofilm growth on a porous agar substrate. Our model accounts for osmotic flux and predicts the spatiotemporal variations of the volume fractions of the different phases and the nutrient concentrations in the biofilm and the substrate. An increase in surface tension helps redistribute biomass radially. Our model captures the behavior of different kinds of biofilms: films characterized by low (yeast) and high (bacterial) ECM content. The predictions of our model are quantitatively validated with the experimental data from the literature. Our model provides insights on the role of different parameters on biofilm growth, which can be used to develop strategies to prevent or accelerate biofilm formation on surfaces.

Selectivity and sensitivity are the two key parameters for construction of a sensor. In this work, a novel electrochemical sensor based on molecularly-imprinted composites synthesized from o-phenylenediamine (o-PD) and multiwalled carbon nanotube (MWCNT) to detect triclosan is reported. Two different sensors were developed MIC/GC and MIC/ cf -MWCNT/GC. To fabricate MIC/GC, molecularly imprinted composite (MIC) was synthesized by cyclic voltammetry using o-PD, COOH-functionalized MWCNT ( cf -MWCNT) and triclosan on glassy carbon (GC) electrode, following removal of surface triclosan. MIC/ cf -MWCNT/GC was fabricated by synthesizing MIC on cf -MWCNT coated GC. Template removal was performed using NaOH solution. MIC/GC could detect triclosan till 40 ppb while using MIC/ cf -MWCNT/GC, 10 ppb of limit of detection (LOD) was achieved. Adsorption isotherms were constructed for both the films. Langmuir adsorption isotherm gave the best fit for MIC/ cf -MWCNT/GC with -ΔG ads value of 54.952 kJ mol −1 indicating stronger chemisorption. To understand the role of cf -MWCNT in detection of triclosan, electrochemical band gap studies, electrochemical impedance spectroscopy, and cyclic voltammetry studies were conducted. Both the sensors were found to be efficient for detection of triclosan in the presence of interfering ions.

In this work, we focus on an autocatalytic reaction-diffusion model and carry out multiple scale weakly nonlinear analysis. A cubic and a quadratic autocatalytic reaction system is analysed. We develop a framework to identify the critical surfaces in parameter space across which the nature of the Turing bifurcation changes from supercritical to subcritical. These are verified by direct numerical simulations of the system. Using weakly nonlinear analysis, we derive equations up to the fifth order that governs the amplitude of the spatial patterns. The limit point of the bifurcating solution is captured accurately by extending the analysis to the fifth order for the case of subcritical bifurcation. The numerical solutions are in good agreement with the predictions of the weakly nonlinear analysis for supercritical bifurcations. We show that when multiple steady states arise Turing patterns can coexist with another spatially uniform steady states. Furthermore, we show that our framework can be extended to get different patterns like squares and hexagons in a two-dimensional domain. We show that the shape of Turing patterns is influenced by the domain size. This shows that the geometry can influence the kind of patterns formed in natural systems. This study will aid the experimentalist identify operating conditions where Turing patterns can be obtained.

A microfluidic paper-based analytical device (μPAD) for detection of biopesticide - azadirachtin (Aza).

In this work, we focus on an autocatalytic reaction-diffusion model and carry out multiple-scale weakly nonlinear analysis. A cubic and a quadratic autocatalytic reaction system is analyzed. We develop a framework to identify the critical surfaces in parameter space across which the nature of the Turing bifurcation changes from supercritical to subcritical. These are verified by numerical simulations of the system using Chebyshev spectral collocation method. Using weakly nonlinear analysis, we derive the amplitude equations up to the fifth order. This helps determine accurately the limit point of the bifurcating solution. The numerical solutions are in good agreement with the predictions of the weakly nonlinear analysis for supercritical bifurcations. We show that when multiple steady states arise, Turing patterns can coexist with other spatially uniform steady states. Furthermore, we show that our framework can be extended to get different patterns, such as squares, in a two-dimensional domain. This study will help identify operating conditions where Turing patterns can be obtained for reaction-diffusion 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.

This paper focuses on recovering tin, lead, and copper (Sn, Pb, Cu) from low-grade printed circuit boards (PCBs). The metals are recovered by series of dissolution, filtration, and electrowinning steps. During electrowinning, copper is recovered as Cu sheets at the cathode, and Pb is recovered as lead dioxide (PbO2) at the anode. Process intensification is achieved through the simultaneous recovery of these two metals in a single electrowinning step. The unreacted nitrate salts along with nitric acid from the electrowinning step is recycled back for dissolution making the process sustainable. Dissolution experiments with PCBs helped determine the concentration of Cu and Pb that can be expected in the feed solution of the electrowinning step. Electrowinning experiments were performed with synthetic solutions at these concentrations to determine the optimum conditions. Electrowinning experiments with PCB leachate were carried out to determine interference effects of other metals. To establish the robustness and scalability, multi-cell experiments were performed using synthetic and PCB solutions. The results of these experiments were used to establish mass balance for the process with recycle streams to treat 100 kg of PCBs per batch. The proposed mass balance is based on data collected from experiments for each of the steps.

Undesired side reactions can significantly impact the efficiency and economic viability of chemical processes. Droplet-based microfluidics(DBM) offers precise control over reaction conditions by isolating reactants in distinct droplets. Each droplet acts as microreactor with high surface-to-volume ratio. This paper comprehensively investigates the role of two reactant contact modes that give rise to droplet flow. The first contact-mode, Conjugate Mass Transfer Mode(CMTM), is characterized by mass transfer of a reactant from Continuous phase(CP) to Dispersed phase(DP). In second mode, Single-Sphere Mode(SSM), both reactants coexist within the dispersed phase, and there is no effect of external mass transfer on performance. The Hadamard-Rybczynski flow field is used to obtain insights into the system behavior. Two reaction networks, Parallel and Series-Parallel reactions, are considered, which arise from side reactions that accompany the primary reaction.This work highlights the importance of choosing an appropriate reactant contact-mode when side reactions occur. The role of two experimentally controllable parameters, diffusivity ratio(Dr) and feed concentration ratio(M), is analyzed to identify the desirable contact mode. Notably, our study showed that, for Parallel reactions, CMTM has a superior performance compared to SSM under some conditions. Whereas for Series-Parallel reactions, SSM exhibits better performance when equal concentrations of reactants are employed.

Rapid on-site monitoring of nitrite as a diagnostic biomarker has drawn considerable attention in recent years. However, conventional detection techniques face limitations in terms of portability and ease of usage. In this work microfluidic paper based colorimetric sensor (µPCS) is developed for detection of nitrite from simulated saliva sample using Griess reagent. We discovered that the chemical nature of the filter papers has significant effect on the analytical sensitivity of nitrite detection. Among different paper substrate tested, the glass microfiber filter (GMF) paper exhibits exceptional detection sensitivity. The µPCS was fabricated using GMF as the substrate material through a lamination process. We have fabricated a portable light box using 3D printing and coupled it with µPCS device for image acquisition. This helps to maintain uniform lighting and avoid external interference during image acquisition. Under optimal conditions, a linear change in response with respect to nitrite concentration was observed for 0.5–200 µM with a limit of detection of 3.22 µM and a limit of quantification of 10.72 µM. The proposed method was validated with UV–Vis spectrophotometer. The device developed in this work showed good accuracy (recovery of 103–113

Gold nanoparticles (AuNPs) have diagnostic and therapeutic applications as they are biocompatible and can be surface-functionalized. The use of organic solvents in the synthesis of AuNPs hampers their applications in the medicinal field. The large-scale production of nanoparticles requires their simultaneous synthesis and separation. Self-assembly of nanoparticles at the fluid-fluid interface facilitates their separation from the bulk and eliminates a downstream processing step. In this work, we exploit this in an aqueous two-phase system (ATPS) to synthesize and separate stable AuNPs. The ATPS was based on polyethylene glycol (PEG) and trisodium citrate dihydrate (citrate) as both these compounds can reduce Au ions. After the synthesis of nanoparticles, using one of the solutes, a complementary solution containing the other solute is added to form a two-phase system to facilitate self-assembly at the interface. The nanoparticles synthesized in different phases are characterized using UV-visible spectroscopy, scanning electron microscopy, and transmission electron microscopy. The AuNPs synthesized using the citrate solution are found to be unstable. Particles synthesized using the ATPS with PEG-600 are trapped at the interface while those using PEG-6000 remain in the bulk. Continuous synthesis and separation of nanoparticles in slug flow in a millichannel are demonstrated as a first step for large-scale controlled synthesis.

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.

In this work, adsorption of triclosan (TCS) on nylon 66 membrane is explored to develop a preconcentration and sensing platform. Nylon 66 membrane exhibits superior sorption ability even for trace amounts of TCS (10 μg/L). Investigating the surface adsorption chemistry by XPS analysis revealed the formation of a hydrogen bond between the hydroxyl group of TCS and the amide group of nylon 66. In the absence of TCS, the amphiprotic water molecule forms a multilayer OH group on the membrane surface. However, TCS showed preferential adsorption on the membrane-replacing water molecule due to its higher hydrophobic partition coefficient. We validated the effective preconcentration of TCS on the membrane using LC-MS analysis. Performing colorimetry directly on the TCS-enriched membrane surface showed a visible color change for concentrations as low as 10 μg/L. The relative blue intensity was found to vary linearly over a concentration range of 10-100 μg/L, and we achieved a detection limit of 7 μg/L for a 5 mL sample. This method utilizes easy-to-use resources which drastically reduce the cost and complexity of analysis.