Herein, we present the study of evolution of local pressure profile of oil-water system in a micro-capillary. In order to capture the interfacial phenomenon, we employ the volume of fluid model that is available in the commercial software, Ansys Fluent 2021 R1. The formation of flow morphology at various flow ratios (oil/water) and their corresponding pressure characteristics were analyze. It was observe to display a repetitive pressure profile during each stage of the droplet formation. The simulations showed that the local pressure reaches its peak before ejecting out the droplet. This is followed by a decrease in local pressure due to the retraction of the interface along its inlet. The peak pressure was found to vary with velocity ratio. Alongside, the droplet size was found to decrease, as the value of the velocity ratio was increase. The findings of the present work can be of significance for understanding the mechanism of droplet formation and its inter-competing forces.

The present work deals with the study of oil–water flow inside a double T-junction microchannel employing the volume of fluid (VOF) model available in ANSYS FLUENT™. The simulations were carried out for water fraction, wf = 0.2, with capillary number, Ca = 0.0004−0.0075. A total number of ~29,624 computational cells were generated to achieve grid independent solutions, and a time step size of 10–6–10–5 s was set to achieve converge solutions. The result depicts that the formation of droplets could take place by merging and alternating mode, respectively. The study of local pressure characteristics uncovered the inter-competing nature of local pressure at the inlets leading to the formation of droplets in alternate mode. The mode of droplet formation at Ca < 0.016 (Ca = 0.032−0.16) was observed to be in merging (alternating). Moreover, the flow morphologies with variation in contact angle, junction angle, inlet geometry, and viscosity contrast were explored. The results of the present work can be of significance for generating miniaturized alternating droplets for reaction, synthesis, and detection.

Chapter 13 Bio-Butanol as Biofuels: The Present and Future Scope Seim Timung, stimung@ddn.upes.ac.in Department of Chemical Engineering, University of Petroleum and Energy Studies, Dehradun, IndiaSearch for more papers by this authorHarsimranpreet Singh, Department of Chemical Engineering, University of Petroleum and Energy Studies, Dehradun, IndiaSearch for more papers by this authorAnshika Annu, Department of Chemical Engineering, University of Petroleum and Energy Studies, Dehradun, IndiaSearch for more papers by this author Seim Timung, stimung@ddn.upes.ac.in Department of Chemical Engineering, University of Petroleum and Energy Studies, Dehradun, IndiaSearch for more papers by this authorHarsimranpreet Singh, Department of Chemical Engineering, University of Petroleum and Energy Studies, Dehradun, IndiaSearch for more papers by this authorAnshika Annu, Department of Chemical Engineering, University of Petroleum and Energy Studies, Dehradun, IndiaSearch for more papers by this author Book Editor(s):Krushna Prasad Shadangi, Search for more papers by this author First published: 07 May 2021 https://doi.org/10.1002/9781119793038.ch13 AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Summary The rapid growth in population and urbanization has led to the increased use of fossil fuels. This will definitely result in the depletion of the existing fossil fuels in the near future. Therefore, in order to fulfill the energy demand, it is very important to explore various alternate energy sources. In this regard, biofuels (such as bio-ethanol, bio-butanol, and bio-diesel) which are synthesized from biomass, derived from various carbohydrate sources, show potential promise. Therefore, many researchers have investigated the production of biofuels using various carbohydrate sources. However, bio-butanol offers some added advantages when compared to bio-ethanol and bio-diesel. Due to this, a major focus has been shifted towards production of bio-butanol through acetone-butanolethanol (ABE) fermentation. This chapter provides a brief discussion of the market demands and various other challenges faced during the production of biobutanol. It also focusses on the major advancements in the choice of feedstock, bacterium strains, pretreatment and fermentation techniques. Liquid Biofuels: Fundamentals, Characterization, and Applications RelatedInformation

HYPOTHESIS:Mixing of a chemical trigger of lower surface tension into a microdroplet with relatively higher surface tension can cause a rapid spreading of the droplet on a liquid-sublayer to form a host of metastable liquid morphologies such as sheets, toroids, threads, or droplets. Subsequently, such metastable fluidic objects break into a collection of droplets to form microemulsions. EXPERIMENTS:Introduction of surfactant loaded water or long-chain alcohols into an oleic acid microdroplet stimulate a rapid spreading of the same on a water sublayer, which helps in the formation of a metastable liquid sheet connected to a liquid toroid. Much like slipping films, the liquid sheet dewets the water underlayer through the formation of holes before they grow and coalesce to form liquid ribbons. While such liquid structures eventually break into an array of microdroplets, the liquid toroid expands before undergoing a Plateau-Rayleigh instability to form microdroplets. FINDINGS:A single step self-organization process in which a chemical trigger can convert a microdroplet into a liquid-toroid on a water surface, in absence of any rotational influence. A symmetric to asymmetric transition in toroid morphology is observed due to the changeover of laminar to turbulent flow regimes with the reduction in viscosity of fluid-sublayer or variation in chemical triggers. The toroid cross-section and droplet spacing after the toroid breakup follow a length scale evaluated from a linear stability analysis.

A droplet energy harvester (DEH) composed of aqueous salt solution could generate electrical energy from light when placed on a metal-semiconductor Schottky-junction emulating the principles of electrochemical photovoltaics (ECPV). The maximum potential difference generated was ∼95 mV under sun, which was enhanced by ∼1.5 times after the addition of gold nanoparticles (AuNPs) in the droplet because of the generation of additional charge carriers from the localized surface plasmon resonance (LSPR). Focusing the solar illumination through a bi-convex lens on five such droplets increased the voltage to ∼320 mV with a power density of ∼0.25 mW cm-2. When the DEH was converted to a microfluidic energy harvester (MEH) by flowing the AuNP laden salt solution through a microchannel integrated with an array of Schottky-junction electrodes, at an optimal flow rate, another two-fold increase in the power density was observed. In the MEH, because the ECPV aided by the LSPR converted the solar energy into electrical energy, the streaming potential (SP) generated across the electrodes because of the fluid flow converted the mechanical energy into electrical energy. Increase in the number of electrode pairs improved the voltage generation, which suggested that the MEH had potential for microscale-very-large-scale-integration (μ-VLSI). The combined effects of ECPV, LSPR, and SP in the MEH could show an efficiency ∼2.5%, which was one of the highest ones reported, for Schottky-junction energy harvesters. This study shows some simple and efficient pathways to harvest high-density electrical power using microchannels and droplets from the naturally abundant solar or hydroelectric (hydel) energy resources.

Numerical simulations supplemented by experiments together uncovered that strategic integration of discrete electric fields in a non-invasive manner could substantially miniaturize the droplets into smaller parts in a pressure driven oil-water flow inside microchannels. The Maxwell's stress generated from the electric field at the oil-water interface could deform, stretch, neck, pin, and disintegrate a droplet into many miniaturized daughter droplets, which eventually ushered a one-step method to form water-in-oil microemulsion employing microchannels. The interplay between electrostatic, inertial, capillary, and viscous forces led to various pathways of droplet breaking, namely, fission, cascade, or Rayleigh modes. While a localized electric field in the fission mode could split a droplet into a number of daughter droplets of smaller size, the cascade or the Rayleigh mode led to the formation of an array of miniaturized droplets when multiple electrodes generating different field intensities were ingeniously assembled around the microchannel. The droplets size and frequency could be tuned by varying the field intensity, channel diameter, electrode locations, interfacial tension, and flow ratio. The proposed methodology shows a simple methodology to transform a microdroplet into an array of miniaturized ones inside a straight microchannel for enhanced mass, energy, and momentum transfer, and higher throughput.

We report a facile and noninvasive way to disintegrate a microdroplet into a string of further miniaturized ones under the influence of an external electrohydrodynamic field inside a microchannel. The deformation and breakup of the droplet was engendered by the Maxwell's stress originating from the accumulation of induced and free charges at the oil–water interface. While at smaller field intensities, for example less than 1 MV/m, the droplet deformed into a plug, at relatively higher field intensities, e.g. ∼1.16 MV/m, a pair of droplets having opposite surface charge was formed. The charged droplets showed an interesting periodic bridging and breakup during their translation motion across the channel. For even higher field intensities, for example more than 1.2 MV/m, the entire droplet underwent dielectrophoresis toward one of the electrodes before experiencing a strong attractive force from the other electrode to deform into a shape of a Taylor cone. With progress in time, mimicking the electrospraying phenomenon, the cone tip periodically ejected a string of miniaturized water droplets to form a microemulsion inside the channel. The frequency and size of the droplet ejection could be tuned by varying the applied field intensity. A water droplet of ∼214 μm diameter could continuously eject droplets of size ∼10 μm or even smaller to form a microemulsion inside the channel.

We report the capillary and frictional force mediated transitions of morphologies of an oil-water flow inside a microchannel using experiments and computational fluid dynamic simulations. A number of steady and time-periodic flow patterns were reported with the variations in the interfacial tension, exchange of inlets, flow ratio, and viscosity ratio of the phases. Transitions from slug to plug to droplet to stratified flow patterns were obtained by tuning the interfacial tension. Progressive reduction in the interfacial tension transformed big slugs into smaller plugs, plugs into droplets, and droplets into a stratified flow pattern. Interestingly, the simulations uncovered a non-monotonic and nonlinear reduction in pressure drop with the decrease in interfacial tension. The change in the pressure drop was correlated to the variation in the slug, plug, or droplet frequency of water at the outlet. The variations in the pressure drop were also associated with the transition from dripping to jetting of water droplet ejection near the channel inlet. Apart from the interfacial tension, the viscosity stratification across the phases was also found to play an important role in converting the slug flow patterns into smaller plugs or droplets. The study also reports the parametric space in which the droplet flow patterns could be obtained inside a microchannel tuning the flow and viscosity ratios of the phases alongside the interfacial tension. The reported transitions of flow patterns and the pressure drop characteristics can be of significance in improving the efficiency of future microfluidic devices.

An externally applied alternating current (AC) electrostatic field can deform the interface of a pair of weakly conducting liquids to engender droplet flow patterns inside the ‘T’ shaped microchannels.

The present study attempts to develop a flow pattern indicator for gas–liquid flow in microchannel with the help of artificial neural network (ANN). Out of many neural networks present in literature, probabilistic neural network (PNN) has been chosen for the present study due to its speed in operation and accuracy in pattern recognition. The inbuilt code in MATLAB R2008a has been used to develop the PNN. During training, superficial velocity of gas and liquid phase, channel diameter, angle of inclination and fluid properties such as density, viscosity and surface tension have been considered as the governing parameters of the flow pattern. Data has been collected from the literature for air–water and nitrogen–water flow through different circular microchannel diameters (0.53, 0.25, 0.100 and 0.050 mm for nitrogen–water and 0.53, 0.22 mm for air–water). For the convenience of the study, the flow patterns available in literature have been classified into six categories namely; bubbly, slug, annular, churn, liquid ring and liquid lump flow. Single PNN model is unable to predict the flow pattern for the whole range (0.53 mm–0.050 mm) of microchannel diameter. That is why two separate PNN models has been developed to predict the flow patterns of gas–liquid flow through different channel diameter, one for diameter ranging from 0.53 mm to 0.22 mm and another for 0.100 mm–0.05 mm. The predicted map and their transition boundaries have been compared with the corresponding experimental data and have been found to be in good agreement. Whereas accuracy in prediction of transition boundary obtained from available analytical models used for conventional channel is less for all diameter of channel as compared to the present work. The percentage accuracy of PNN (∼94% for 0.53 mm ID and ∼73% for 0.100 mm ID channel) has also been found to be higher than the model based on Weber number (∼86% for 0.53 mm ID and ∼36% for 0.05 mm ID channel).

Introduction: The study of multiphase flows inside the microfluidic devices have received much attention recently because of its variety of application related to heat and mass transfer, mixing, microreaction, and emulsification [1]. Many researchers have concentrated on the influence of flow rate of the fluids, fluid properties such as surface or interfacial tensions, contact angle and viscosity of the liquids on the interfacial morphologies and their transitions [2]. The results suggest that a host of interesting flow patterns can be achieved by controlling the fluid properties (interfacial tension, viscosity) and flow conditions (flow rate). Use of COMSOL Multiphysics: In the present study, we explore the pathways to control the flow morphologies of a liquid-liquid multiphase flow employing an external electrostatic field. The oil-water multiphase system is modelled employing the commercial software COMSOL MULTIPHYSICS. The electrohydrodynamics (EHD) of the multiphase system is numerically resolved employing Navier-Stokes equations of motion coupled with the Maxwell stresses together with enforcing the appropriate boundary conditions.

The present study shows a strategy to transform larger two-phase flow structures into the smaller ones by incorporating simple tuning of microchannel geometry. The two-phase flow is modelled employing the in-built phase-field model which is commercially available with COMSOL Multiphysics software. With the help of a series of numerical simulations, we show that larger plug flow can be transformed into smaller droplets by introducing an orifice at the T-junction of the microfluidic channel. The size of the droplets can be tuned by varying the diameter of the orifice. In addition, the frequency of droplets can also be varied by changing the diameter and position of the orifice along the length of the microfluidic channel. The results shown in this study can be helpful in the design of micro-emulsifiers and microreactors which demands a collection of miniaturized droplets dispersed inside a continuous medium.