Friction Stir Welding (FSW) is a solid-state joining process that involves the stirring of material heated to a plasticized state by frictional heat generated by rotating tool on the workpiece, which leads to the welding of the material. The material flow behaviour during FSW is a key area of study as it is critical to the quality and strength of the welds. Simulation and prediction of material flow during the Friction Stir Welding process give valuable insights into the process that is often difficult to achieve experimentally, therefore the Computational Fluid Dynamics (CFD) has been increasingly used for material flow analysis of the FSW process. This paper reviews the latest studies that use CFD analysis of material flow in FSW and influence of tool geometry, welding parameters, material properties and heat generation. It presents a detailed comparison of CFD findings from different studies and highlights its contributions, findings, and limitations. Finally, the paper discusses the future work directions in CFD simulations for material flow during FSW.

Spray pyrolysis is a versatile technique utilized for producing nanostructured powders and thin films of various materials such as metal oxides, metal chalcogenides, alloys, and composites. Even though the method was first recognized to produce nanostructured powder materials, it was explored extensively later for thin film deposition, as spray pyrolysis deposition. It is a facile method which yields homogeneous, scalable, and reproducible thin films of high quality. The deposition method can be classified depending on the types of atomization methods used to produce the spray or based on the energy source of the precursor for deposition. The properties of the thin films vary with the deposition process and parameters. In this chapter, we introduce the spray pyrolysis deposition method and its basic experimental set-up, followed by a brief description of its classification. Further, the process parameters and their effect on the characteristics of the films are discussed. Finally, the applications of the deposited films for energy storage, conversion, gas sensing, and catalysis are reviewed.

Machining of hardened tool steels concerns severe unstable surface integrity, tool wear, and complex thermo-mechanical interfaces, presenting challenges for sustainable manufacturing. This work aims to investigate the effect of nano-aluminium oxide (Al2O3) minimum quantity lubrication (MQL) on the machinability of heat-treated high-speed tool steel (YXR-7) and to evaluate the ability of data-driven models in predicting machining responses under limited experimental conditions. A total of 81 controlled milling experiments were performed to estimate surface roughness (Ra), material removal rate (MRR), and tool wear rate (TWR) under dry and nano-MQL conditions. Experimental findings confirmed that nano-MQL decreased surface roughness by 65.61% and tool wear rate by nearly 56% compared to dry machining, while maintaining higher productivity. FESEM (Field emission scanning electron microscopy) observations showed an evolution from severe adhesive–diffusive wear in dry cutting to moderately mild abrasive–oxidative wear with tribofilm formation under nano-lubrication. XRD (X-ray diffractometry) analysis confirmed the presence of oxide and various carbide phases, supporting the experience of stress-assisted tribo-chemical relations. To investigate predictive performance, Multiple Linear Regression (MLR), Extreme Gradient Boosting (XGBoost), Feedforward Neural Network (FNN), and Gaussian Process Regression (GPR) models were utilised by cross-validation. For the available dataset, simpler regression models exhibited stable generalisation behaviour, while more complex nonlinear models established sensitivity to data scale. The integrated experimental and modelling approach provides insight into lubrication-driven wear mechanisms and offers practical guidance for improving process efficiency and sustainability in hard milling applications.

Recent advances in machine learning have revolutionized numerous research domains by extracting the hidden features and properties of complex systems, which are not otherwise possible using conventional ways. One such development can be seen in designing smart materials, which intersects the ability of microfluidics and metamaterials with machine learning to achieve unprecedented abilities. Microfluidics involves generating and manipulating fluids in the form of liquid streams or droplets from microliter to femtoliter regimes. However, analysis of such fluid flows is always tiresome and challenging due to the complexity involved in the integration and detection of various chemical or biological processes. On the other hand, acoustic metamaterials manipulate acoustic waves to achieve unparalleled properties, which is not possible using natural materials. Nonetheless, the design of such metamaterials relies on the expertise of specialists or on analytical models that require an enormous number of expensive function evaluations, making this method extremely complex and time-consuming. These complexities and exorbitant function evaluations of both fluidic and metamaterial systems embark on the need for the support of computational tools that can identify, process, and quantify the large amounts of intricacy, thus machine learning techniques. This review discusses the shortcomings of microfluidics and acoustic metamaterials, which are overcome by neoteric machine learning approaches for building smart materials. The following review ends by providing the importance and future perspective of integrating machine learning and optimization approaches with microfluidic-based acoustic metamaterials to build smart and efficient intelligent next-generation materials.

Microfluidics has achieved a paradigm-shifting advancement in life sciences, automation, thermal management, and various other engineering streams. In recent years, a considerable amount of research has been conducted on the use of microfluidics in designing novel systems and fabricating next-generation smart materials that are capable of outperforming historical barriers and achieving unprecedented qualities. One such innovative development is the integration of fluidics into building artificially structured smart materials called acoustic metamaterials to achieve active tunability for a real-time controllable manipulation of acoustic waves. Leveraging the capability of microfluidics to automate the manipulation of liquid droplets, fluid streams, or bubbles in a required arrangement has revolutionised the development of actively tunable fluidics-integrated acoustic metamaterials for widescale applications. This review first discusses the prominent microfluidic actuation mechanisms used in the literature to develop fluidics-integrated smart acoustic metamaterials, and then it details integrated metamaterial design and extraordinary applications such as active acoustic wave manipulation or building tunable acoustic holograms etc. The following review concludes by providing the importance and future perspective of integrating microfluidic techniques with novel metamaterial designs, paving the way for innovative futuristic applications.

Droplet manipulation using digital microfluidics is extensively being researched for various biological and chemical sensor applications. Among the various methods, liquid dielectrophoresis (L-DEP) offers precise droplet manipulation using high-frequency electric fields. L-DEP also aids in generating temperature inside the droplet. Even though droplet heating using L-DEP promises various potential capabilities in microfluidic sensor development, droplet actuation at high voltages [greater than 400 V peak voltage (Vp)] remains a concern. In this manuscript, the parameters, such as dielectric material and dielectric thickness, which are responsible for droplet heating, are investigated numerically through simulations. By keeping the dielectric thickness constant, the relation between temperature rise and frequency for various Vp was simulated for seven different dielectrics, mainly Si3N4, ZnO, and alumina. A temperature rise of 100 °C was generated using Vp = 200 V at 200 kHz using Si3N4 as the dielectric, which proves the capability of using this technique at lower voltages. However, the complexity in fabrication hinders its usage in microfluidic applications. Thus, we investigated low dielectric strength materials, such as ZnO and alumina. We observed that despite the dielectric film being porous, due to synthesis, the effect of the porosity of these films in droplet heating is found to be minimal. Finally, the variation of temperature rise inside the droplet with varying dielectric film thickness for various kHz frequencies by keeping the Vp is studied. This study is crucial in developing droplet thermal sensors, which could replicate the functions of microheaters for various microfluidic applications.

The rheological properties of synovia are of interest because of their importance in the lubrication of knee joints. The synovial fluid has excellent lubrication properties at both low and high loads as well as shear (the friction coefficient is in the order of 0.01–0.02). The synovial fluid (SF) serves as a joint lubricant, a shock absorber, and a cause of sustenance for articular cartilage. The most prevalent kind of arthritis, osteoarthritis (OA), is a progressive joint condition categorized by the loss of articular cartilage. Changes in the rheological characteristics of the synovial fluid might result from damage to the articular cartilage in an osteoarthritic joint. The major aims of osteoarthritis (OA) treatment are to alleviate pain and preserve joint motion. Viscosupplementation, or intra-articular injections of hyaluronic acid (HA), is one of the non-operative actions for osteoarthritis (OA). It functions as a shock absorber for joint loads and a lubricant to allow bones to glide over one another. Polyvinylpyrrolidone (PVP) hyaluronate and moderate molecular weight PVP were found to be the closest to natural synovial fluid in artificial synovial fluid (ASF). Artificial articular lubricants were used in this treatment to help the joints function better. The present work proposes the usefulness of the PVP solution as a synovial mimic fluid and highlights the need to realistically assess the rheological characteristics of synovial fluid. Using viscosupplements that mimic healthy synovial fluid is a sensible strategy since several viscosupplements used in osteoarthritis treatment help restore the synovial fluid to its natural composition and feature.

Hypothesis: Surface acoustic waves (SAW) propagating along a solid surface can significantly affect the dynamics of droplet impact. Although droplet impact in presence of SAW has been attempted recently, here, we investigate the effects of surface wettability, droplet size, impact velocity, and SAW power on the impact and spreading dynamics along with post-impact oscillation dynamics of a drop. Experiments: Here, we study droplet impact on a surface exposed to traveling SAW produced using an interdigitated electrode patterned on a piezoelectric substrate. The effects of Weber number (We), surface wettability, and SAW power on the impact and spreading dynamics and post-impact oscillation dynamics are studied. Findings: Our study unravels that the interplay between capillary and viscous forces, and inertia forces arising due to pre-impact kinetic energy and SAW-induced bulk acoustic streaming underpins the phenomena. Remarkably, we find that the effect of SAW on droplet impact dynamics is predominant in the case of a hydrophilic (HPL) substrate at a higher SAW power and smaller We and hydrophobic (HPB) substrate irrespective of SAW power. Our study reveals that the maximum droplet spreading diameter increases with SAW power at smaller We for an HPL surface whereas it is independent of SAW power at higher We. Post-impact oscillation of a droplet over an HPL surface is found to be overdamped with a smaller amplitude compared to an HPB substrate, and a faster decay in oscillation amplitude is observed in the case of an HPB surface and higher We. Our study provides an improved understanding of droplet impact on a surface exposed to SAW that may find relevance in various practical applications.

Deicing has significant relevance in various applications such as transportation, energy production, and telecommunication. The use of surface acoustic waves (SAWs) is an attractive option for deicing as it offers several advantages such as localized heating, in situ control, low power, and system integration for highly efficient deicing. Here, we report an understanding of the dynamics of deicing of microlitre volume water droplets (1 to 30 μL) exposed to low power (0.3 W) SAW actuation using an interdigitated electrode on a piezoelectric (LiNbO3) substrate. We study the time variation of the volume of liquid water from the onset of SAW actuation to complete deicing, which takes 2.5 to 35 s depending on the droplet volume. The deicing phenomenon is attributed to acoustothermal heating which is found to be greatly influenced by the loss of ice adhesion with the substrate and the acoustic streaming within the liquid water. Acoustothermal heating inside the droplet is characterized by the temperature distribution inside the droplet using infrared thermography, and acoustic streaming is observed using dye-based optical microscopy. A rapid enhancement in deicing is observed upon the detachment of ice from the substrate and the onset of acoustic streaming, marked by a sudden increase in the liquid water volume, droplet temperature, and heat transfer coefficient. The deicing time is found to increase linearly with droplet volume as observed from experiments and further verified using a theoretical model. Our study provides an improved understanding of the recently introduced SAW-based deicing technique that may open up the avenue for a suitable alternative to standard deicing protocols.

Microscale droplet generation and manipulation have widespread applications in numerous fields, from biochemical assays to printing and additive manufacturing. There are several techniques for droplet handling. Most techniques, however, can generate and work with only a limited range of droplet sizes. Furthermore, there are constraints regarding the workable variety of fluid properties (e.g., viscosity, surface tension, mass loading, etc.). Recent works have focused on developing techniques to overcome these limitations. This feature article discusses advances in this area that cover a wide range of droplet sizes from subpicoliter to microliter.

We report the fabrication and experimental study of a flexible bi-layer microfluidic device for blood oxygenation, mimicking the thin alveolar exchange barrier constituting a lung. A facile technique is employed to fabricate the device by sandwiching a thin polymeric membrane as the gas exchange layer between two flexible microchannels. A numerical model coupling the mass, momentum, and species transport equations, is used to simulate oxygen diffusion between the blood and oxygen channels across the gas exchange membrane. The oxygen saturation is experimentally measured at different locations in the blood channel along the flow direction and compared against the simulation results, which show a very good agreement. The effect of blood and oxygen flow rates, channel height, and membrane thickness on the variations in oxygen concentration in the blood and oxygen channels and the diffusion membrane are studied. The outcome of the present study may find relevance in the development of organ-on-chip devices for blood oxygenation.

Microfluidics as a field has a plethora of applications in several fields. From heat transfer to biomedical applications, microfluidic techniques are used to deliver solutions. In the present chapter, we look into the basics of microfluidic techniques used to manipulate tiny volumes of fluids. Further, a detailed discussion on acoustofluidics, lab/organ-on-chip, biosensing, and cell manipulation follows. Section 2.4 focuses on the use of bulk and surface acoustic waves to manipulate particles and cells. Section 2.5 sheds light on the use of microfluidic chips mimicking an organ or its basic process and how the same is used to study the effect of drugs on the organs. Section 2.6 focuses on using microfluidic techniques for disease detection and prognosis monitoring. The part on Cell manipulation cuts through various active and passive techniques for cell trapping, focusing, and sorting.

Fluid manipulation using surface acoustic waves (SAW) has been utilized as a promising technique in the field of microfluidics due to its numerous advantages, over other active techniques, such as low power requirement, facile fabrication methods, and non-invasive nature. Even though SAW-based generation of micron-sized droplets through atomization has been studied, the role of substrate wettability on the characteristics of the transferred droplets has not been explored to date. Here, we study the generation and effective transfer of micron-sized droplets using SAW onto wettable substrates whose water contact angles vary from 5° to 145°. The characteristics of transferred droplets after impacting the wettable substrates are characterized in terms of the contact line diameter and polydispersity index. A theoretical model is formulated to predict the initial average size of the transferred droplets on the wettable substrates of different contact angles. The variation of polydispersity and number density with contact angle is explained by considering droplet coalescence and bouncing. The relevance of the technique in biological assays is demonstrated by transferring droplets of streptavidin protein samples onto a substrate.

Actuation of sub-millimeter sized droplets has important implications in heat transfer, microfluidics, and self-cleaning surfaces. Here, we report the motion of satellite droplets generated around a primary droplet actuated using liquid dielectrophoresis (L-DEP). Satellite droplets spanning the electrode gap grow due to the electric field induced enhanced merging of droplets. These satellite droplets attain a dumbbell shape due to the applied field. Interestingly, satellite droplets follow the primary drop interface maintaining a constant gap between themselves and the primary drop. The motion of satellite droplets was captured using high-speed imaging, and the role of temperature gradient was verified by measuring temperature distribution using an IR camera. Through observations and simulations, we qualitatively believe that this behavior arises from (a) repulsive electrostatic and temperature gradient forces; and (b) attractive vapor concentration gradient forces. Simulations were used to estimate the direction of individual forces. Interfacial charges from the applied electric field give rise to the electrostatic repulsion. During L-DEP, the primary droplet heats due to induced current. Thermal conduction through the substrate sets up a temperate gradient, which adds to the repulsion forces between the droplets. Evaporation of the primary droplet leads to the vapor concentration gradient, responsible for the attraction force between the droplets.

Ice accumulation on mechanical structures pose series of safety challenges in daily life. Therefore, it is important to detect the presence of ice and subsequently remove it, thereby hindering its accumulation. We demonstrate an electrode design which can function concurrently as an ice detection and de-icing system. A dielectric coated interdigitated device was fabricated. Ice was generated using a Peltier cooler. By measuring the impedance change of the device with frequency under different conditions (dry and wet), ice was detected. Extent of ice formation can be estimated from the impedance values. Using the same interdigitated electrode device, de-icing was also established by applying a high frequency (50 kHz) and high voltage (200 Vrms) AC signal. The output power of the system was calculated using an equivalent circuit model which also showed that the de-icing system delivers negligible output power in the absence of ice making it energy efficient. For performing de-icing on curved structures, the technique was demonstrated using flexible substrates where interdigitated electrodes were printed on flex polyamide substrates. [2019-0254]

One of the most vital problems in fuel cell research is the synthesis of low cost, active and stable electro-catalyst for anodic and cathodic reactions. In this work, palladium nanostructures with a coral-like morphology (PdNC) is deposited on reduced graphene oxide (rGO) by electrochemical method. The nanocoral exhibits superior activity towards methanol oxidation (MOR) and oxygen reduction reactions (ORR) in alkaline media. To improve the efficiency, Pt is deposited on the surface by galvanic replacement. Presence of Pt on palladium nanocoral (PdNC-Pt) leads to an enhanced catalytic activity for both the electrode reactions. A mass activity of 1700 mA mg− 1 at 1.04 V for MOR and a current density of −5.72 mA cm− 2 and an onset potential of 0.86 V vs RHE is observed for ORR. Both the electrode reactions result in higher current than the commercial Pd/C and Pt/C catalysts. A remarkable improvement in durability is seen for ORR after depositing Pt.

We demonstrate a novel concept to heat a drop. Temperature rise occurs when high frequency (10-50 kHz) AC actuation voltage (270 Vrms-460 Vrms) is applied on the dielectric coated coplanar electrodes. The temperature rise (up to 40 °C) inside the drop provides calibrated high temperatures for enabling chemical reactions on Lab-on a-Chip devices. Droplet spreading along the electrode gap happens as this actuation regime also coincides with dielectrophoresis. The proposed technique does not require separate microheaters on the chip as electrodes designed for droplet actuation can be used to achieve localized heating.

Generation and transfer of micron-sized droplets are of significant interest for several applications such as electronics cooling, lab-on-chip, microscale printing, etc. We report generation of microscale droplet streams when a larger droplet is exposed to high frequency (>10 kHz) electric fields using dielectric covered coplanar electrodes. Generation of these droplet streams is found to be a function of the actuation frequency and voltage. Our experiments rule out electrospray as the relevant mechanism. Experiments further reveal that the droplet streams are formed due to (1) Evaporation of the larger droplet due to Joule heating; and (2) Enhanced localized condensation on nucleation sites originating from the leakage currents at high electric fields. The condensed droplets are dragged along with the convection current of the larger droplet, leading to the observation of streams. A computational model is used to solve for the convection velocities which is compared with the experimental high-speed videos. The stream velocity is found to increase with the applied field, which is attributed to the smaller size of the condensed droplet at higher electric fields. Capability to generate microscale droplets and transfer them to other substrates is desired in current microfluidic platforms for various applications. This study reports a viable technique and presents critical information for its design.

We demonstrate a new technique of heating aqueous droplets on conventional EWOD electrodes by using high-frequency high-voltage AC signals. At high actuation frequencies (10–1000 kHz), the droplet temperature rises due to Joule heating from the ohmic currents inside the drop. Using this direct heating technique, we were able to achieve temperatures of 93–94 °C, which is significant for several biochemical applications. The technique is studied extensively using experiments and modelling. Several performance parameters of this heating technique were compared with a standard microheater through experiments and simulation. For the presented technique, the substrate near the droplet was cooler in comparison to the microheater. This will reduce parasitic heating of nearby droplets. A comprehensive study regarding the optimization of the geometrical parameters and the capability to heat solutions to higher temperatures using lower voltage and higher frequency were also performed using simulations. As conventional EWOD electrodes are used for heating the liquid, separate microheaters are not required. This significantly simplifies design and allows us to heat any droplet at any location on the chip. This on demand reconfigurability of droplet heating is the primary benefit of this technique. To establish the abilities of our suggested method, two biochemical experiments were demonstrated.

On-chip cooling poses a potential challenge in semiconductor industry. Electronic devices generate more heat when scaled down and thus heat needs to be dissipated effectively. For cooling local hot spots on electronic chips, Electrowetting on Dielectric (EWOD) based droplet cooling has been adapted. But here the performance of droplet evaporation is limited by the area of cooling at a particular time. Therefore, the technology needs efficient control over the droplets in order to maximize the cooling rate. In the present work, a large hot spot area (15mm2) has been cooled using a single deionized water drop by inducing non-uniform electric field. Water droplet on two coplanar electrodes, upon applying AC voltage, experiences non-uniform electric field and spreads along the electrodes. This phenomenon, known as Liquid Dielectrophoresis (L-DEP), enables the drop to spread. Cr/Au coplanar electrodes have been patterned with 100μm gap along with a back side aligned Ti/Pt heater in the gap between the coplanar electrodes. Heat flux of 100W/cm2 is applied to the heater by constant power mode and effective evaporation of the drop provides 76% of the cooling of the heated area. The power consumption for the droplet spreading is calculated as 30.2μW moving with an average velocity of 2.8 mm/s.

This paper reports a new phenomenon for jetting of droplets and demonstrates its use for generation and transfer of monodispersed droplets in sub-picoliter volumes. This technique obtains jetting at lower voltages (combination of 470Vac and -250Vdc), even for high surface tension liquids. The reported technique is fast in achieving a dense transfer of micro-droplets (14,000/mm2) in less than 10s. Compared to other microfluidic techniques, the new technique uses simpler fabrication and does not require bulky components (e.g. pumps). The technique is extremely easy and economical to scale making it suitable for portable applications.