Innovations in applications and prospects of non-isocyanate polyurethane bioplastics.

Currently, conventional plastics are necessary for a variety of aspects of modern daily life, including applications in the fields of healthcare, technology, and construction. However, they could also contain potentially hazardous compounds like isocyanates, whose degradation has a negative impact on both the environment and human health. Therefore, researchers are exploring alternatives to plastic which is sustainable and environmentally friendly without compromising its mechanical and physical features. This review study highlights the production of highly eco-friendly bioplastic as an efficient alternative to non-biodegradable conventional plastic. Bioplastics are produced from various renewable biomass sources such as plant debris, fatty acids, and oils. Poly-addition of di-isocyanates and polyols is a technique employed over decades to produce polyurethanes (PUs) bioplastics from renewable biomass feedstock. The toxicity of isocyanates is a major concern with the above-mentioned approach. Novel green synthetic approaches for polyurethanes without using isocyanates have been attracting greater interest in recent years to overcome the toxicity of isocyanate-containing raw materials. The polyaddition of cyclic carbonates (CCs) and polyfunctional amines appears to be the most promising method to obtain non-isocyanate polyurethanes (NIPUs). This method results in the creation of polymeric materials with distinctive and adaptable features with the elimination of harmful compounds. Consequently, non-isocyanate polyurethanes represent a new class of green polymeric materials. In this review study, we have discussed the possibility of creating novel NIPUs from renewable feedstocks in the context of the growing demand for efficient and ecologically friendly plastic products.

Molecular Dynamic Insights into the Distinct Solvation Structures of Aromatic and Aliphatic Compounds in Monoethanolamine-Based Deep Eutectic Solvents.

Deep eutectic solvents (DESs) are developing as an alternate medium for aromatic extraction, especially benzene and thiophene from aliphatic hydrocarbon mixtures. In this work, molecular dynamics (MD) simulations were first used to investigate the solvation structure of benzene, thiophene, and n-hexane in monoethanolamine-based DESs. It reveals the liquid structures in the adjacent neighbor shells, which is a function of electron-withdrawing sulfur attached to thiophene and the π-electron cloud of benzene. The intermolecular forces between aromatic, aliphatic, and DES components are analyzed in van der Waals and hydrogen bond interactions. The chloride ions serve as a charge carrier bridge between choline and monoethanolamine precursors. The solvation of benzene, thiophene, and n-hexane in the DESs depends on volume expansion and minor solvent structural changes. Density functional theory results provided information on the mechanism of short-range interactions between organic solutes and studied DES. It aids in understanding the structural orientations of a DES with the addition of solutes, essential to the formation of DES. The solvation shell structure and characteristics were investigated in tandem with the possibility of benzene and thiophene clustering. The 1H NMR and 2D 1H-1H-NOESY were used to investigate the intermolecular interactions between benzene, thiophene, and n-hexane with monoethanolamine-based solvents. It concludes that high-ordered DES1 is more inclined to higher solubility than lower-ordered ones with a higher molar ratio of monoethanolamine. The solvation was reduced because the entropy gain was not maximized in the lower ordered DESs.

Molecular and Spectroscopic Insights into a Metal Salt-Based Deep Eutectic Solvent: A Combined Quantum Theory of Atoms in Molecules, Noncovalent Interaction, and Density Functional Theory Study.

Deep eutectic solvents (DESs) based on metal halide salts are highly catalytic, low toxic, reusable, cost-effective, and have higher thermal stability than their analogue ionic liquids (ILs). In this work, we have reported the formation mechanism of metal salt-based DESs at the molecular level along with their charge-transfer analysis and thermodynamics associated with their formation using density functional theory. The DES systems analyzed in the present work were choline chloride and tin(II)chloride (DES1) and choline chloride and zinc(II)chloride (DES2), both in a molar ratio of 1:2, respectively. An excellent correlation is obtained between the theoretically calculated IR spectra of the DES systems and the previously reported experimental findings for the formation of the complex systems. The DESs were found to be stable systems due to traditional hydrogen bonding and electrostatic interactions resulting in the ionic species [Sn2Cl5]- and [Zn2Cl5]- and are elucidated with the help of electronic structure calculations. CHELPG partial charge analysis and natural bond orbital analysis suggest a charge transfer from Cl- (chloride) to Ch+ (choline) and metal salts in the DES structures. The atom-in-molecules and noncovalent interaction (NCI) analysis suggest a strong electrostatic interaction within the DES2 system as compared to DES1. Higher stability and reactivity are observed in the DES2 system based on the frontier molecular orbital analysis. Our analysis offers important insights into the formation mechanism of these economic IL analogues.

Synthesis and characterization of xylan-gelatin cross-linked reusable hydrogel for the adsorption of methylene blue.

Xylan and gelatin-based hydrogels are prepared in different molar ratios using ethylene glycol diglycidyl ether cross-linker. The hydrogels are characterized by Fourier transform infrared spectroscopy, morphology, thermal analysis, and swelling ratio. The rheological experiment shows the gels are cross-linked successfully by revealing the viscoelastic nature. The xylan-gelatin gel synthesized in a 5:1 molar ratio (hence XG5) has higher storage modulus, gelation temperature, and time among the synthesized gels. The adsorption behaviour of the synthesized gels is studied for the removal of methylene blue, by varying adsorbate concentration, pH, and temperature. Among the synthesized hydrogels, XG5 shows the highest adsorption capacity of 26.04 mg g-1 at pH = 5.84 and 25 °C. The kinetics of the adsorption process follows the pseudo-second-order model, and monolayer adsorption is adequately represented by the Langmuir isotherm model. The adsorption process is established as spontaneous, exothermic, and physisorption from the thermodynamic parameters.

A priori prediction of complex liquid-liquid-liquid equilibria in organic systems using a continuum solvation model.

Liquid-liquid-liquid equilibria (LLLE) is usually observed in many industrial processes primarily linked to enhanced oil recovery techniques. However their measurements are complex and so are their computations. An inherently predictive tool is often useful for elucidating their distribution ratios and phase compositions. In the present work, the phase behavior of nine ternary and two quaternary LLLE systems were predicted employing the quantum chemical based COnductor like Screening MOdel-Segment Activity Coefficient (COSMO-SAC) model. The methodology namely, Rachford-Rice LLLE (RRL3E) algorithm and Henley-Rosen LLLE (HRL3E) algorithm were used to predict the triphasic compositions in each system. In the RRL3E approach, the triphasic systems were assumed into two co-existing biphasic liquid-liquid equilibria systems, whereas in the HRL3E approach, all three phases were considered to be in equilibrium with each other simultaneously. Apart from predicting the local compositions, the HRL3E algorithm was also used to predict the individual phase splits and phase fractions of the LLLE region. Average overall root mean square deviation (rmsd (%)) values considering all 42 datasets and corresponding to 414 data points were recorded as 4.65% and 4.83% using the RRL3E and HRL3E algorithms respectively. Further, the RRL3E algorithm was extended to correlate the LLLE data for all systems using the Genetic Algorithm (GA) based NRTL (GA-NRTL) and UNIQUAC (GA-UNIQUAC) models.

Molecular Dynamics Insights and Water Stability of Hydrophobic Deep Eutectic Solvents Aided Extraction of Nitenpyram from an Aqueous Environment.

In recent times, deep eutectic solvents (DES) have received attention as an extractive media for separations. In this work, the water stability of eight menthol-based DESs and two tetrabutylammonium chloride (N4444Cl) based DESs with organic acid-based hydrogen bond donors (HBD) at a temperature of 298.15 K and atmospheric pressure were studied. dl-Menthol and N4444Cl were considered as the hydrogen bond acceptors (HBA). Molecular dynamics simulation (MD) was used as a tool to examine the distribution of molecules of DES and water in either phase. The intermolecular nonbonded interaction among the species of the systems was analyzed with radial distribution function, interaction energy, and hydrogen-bonding analysis to understand the stability of DESs in an aqueous medium. The results showed that the strong hydrogen bond plays a crucial role in the water stability of the DES. The degree of hydrogen bonding in HBD-water in terms of HBDs obtained by MD simulation can be presented in the order of acetic acid > levulinic acid > butanoic acid > pyruvic acid > hexanoic acid > octanoic acid > decanoic acid > dodecanoic acid. The strength of the hydrogen bond was attributed to the structure of solvents and the alkyl chain length of the HBD group. Overall, the order of stability of DES in water based on a "relative stability factor" was found as dl-menthol:acetic acid (1:1) < dl-menthol: levulinic acid (1:1) < dl-menthol:butanoic acid (1:1) < dl-menthol:pyruvic acid (1:2) < dl-menthol:hexanoic acid (1:1) < dl-menthol:octanoic acid (1:1) < dl-menthol: decanoic acid (1:1) < dl-menthol:dodecanoic acid (2:1). The transfer of molecules in the system from the aqueous phase to the DES rich phase was analyzed with the help of mean-square displacement and diffusion-coefficients. dl-Menthol and organic acids starting from octanoic acid and higher ones can be used in aqueous systems as solvents. Finally, dl-menthol:octanoic acid (1:1) -based DES was used to benchmark and predict the extraction efficiency of a pesticide (nitenpyram) from an aqueous feed. Hydrogen bond analysis demonstrated higher interactions of nitenpyram with dl-menthol and octanoic acid as compared to water. The MD simulation of the ternary system consisting of DES, water, and nitenpyram showed encouraging results, and gave an excellent agreement with experimental literature data in terms of extraction efficiency (∼42 to 46.7%) and distribution ratio (0.72).

Chemically crosslinked xylan-ß-Cyclodextrin hydrogel for the in vitro delivery of curcumin and 5-Fluorouracil.

Novel hydrogels were synthesized from xylan and ß-Cyclodextrin using Ethylene Glycol Diglycidyl Ether as a crosslinker in alkaline medium at different molar ratio. The physical characterization of hydrogels was carried out by the swelling study whereas the chemical characterization was performed in Fourier Transform Infrared Spectroscopy. The morphological analysis revealed the porous structure of hydrogel and the rheological study summarised the flow behavior and gelation characteristics of the hydrogels. Curcumin and 5-Fluorouracil (5-FU) were used as the model drugs to be loaded in hydrogel and subsequent studies involving the in vitro release in phosphate buffer saline (PBS, pH = 7.4). The hydrogels showed drug loading of 98% of 5-FU and 26% of curcumin. Furthermore, the gels showed the highest cumulative release of 56% 5-FU and 37% curcumin after 24 h. The kinetics of drug release was then analyzed by various kinetic models.

Development of microcrystalline cellulose based hydrogels for the in vitro delivery of Cephalexin.

Three hydrogels namely, microcrystalline cellulose (MCC), microcrystalline cellulose-carboxymethyl cellulose (MCC-CMC) and microcrystalline cellulose-xylan (MCC-xylan) are synthesized using ethylene glycol diglycidyl ether as crosslinker. For the chemical characterization, FT-IR spectroscopy is adopted, whereas gel fraction and swelling ratio are used for the physical characterization of the hydrogels. Coarse morphology of hydrogels is further visualized by microscopic observation. The rheological characterization proves that MCC-CMC gel withstands higher strain to resist permanent deformation than the other two gels. The hydrogels are used for the loading and in vitro release of Cephalexin. The in vitro delivery is carried out in various simulated body fluids such as phosphate buffer saline (PBS), artificial intestinal fluid (AIF) and artificial gastric fluid (AGF). MCC-CMC is observed to deliver Cephalexin individually 15% in AGF, 86% in AIF, 98% in PBS and 98% in consecutive buffers (AGF followed by AIF and PBS).

Palmitic-Acid-Based Hydrophobic Deep Eutectic Solvents for the Extraction of Lower Alcohols from Aqueous Media: Liquid-Liquid Equilibria Measurements, Validation and Process Economics.

A new, natural, hydrophobic deep eutectic solvent (NADES) based on DL-menthol and palmitic acid is adopted for the extraction of alcohols from aqueous phase. DL-menthol is used as a hydrogen bond acceptor and palmitic acid, being a natural organic acid, as a hydrogen bond donor. The synthesis is carried out by the addition of DL-menthol and palmitic acid in a defined molar ratio. Physical properties of NADES along with water stability are then measured. Liquid-liquid equilibria (LLE) of lower alcohols, namely, DES (1) + lower alcohols (ethanol/1-propanol/1-butanol) (2) + water (3) are carried out at p = 1 atm and T = 298.15 K. LLE results show type-I phase behavior, where alcohol is preferentially attracted toward DES. The tie lines are then regressed via nonrandom two liquid and universal quasichemical models, which give root mean square deviation (RMSD) in the range of 0.29-0.35% and 0.39-0.75%, respectively. Finally, the quantum-chemical-based conductor-like screening model-segment activity coefficient is used to predict the tie lines, which gives an RMSD of 2.1-5.2%. A hybrid extractive distillation flowsheet is then used for scale up, process economics, and solvent recovery aspects in ASPEN using DES as a "pseudocomponent."

Carboxymethyl Cellulose-Xylan Hydrogel: Synthesis, Characterization, and in Vitro Release of Vitamin B12.

The current work reports the synthesis of carboxymethyl cellulose (CMC) and xylan-based homopolymerized as well as copolymerized hydrogels using an ethylene glycol diglycidyl ether cross-linker in alkaline medium. The hydrogels are physically characterized by the swelling ratio and gel fraction. The morphological observation of hydrogels reveals the porous structure for the copolymerized gels. The rheological behavior of the gels elaborates that the copolymerized CMC-xylan gel synthesized in a 1:1 molar ratio has superior strain-bearing ability and possesses the shortest gelation temperature and time. Vitamin B12 here is used as the model vitamin to be loaded in the hydrogels and subsequent studies involving the in vitro release in artificial gastric fluid (AGF, pH = 1.2), artificial intestinal fluid (AIF, pH = 6.8), and phosphate-buffered saline (PBS, pH = 7.4). The synthesized gels show a cumulative release of 19-28% in AGF, 80-88% in AIF, and 93-98% in PBS, independently. Further, the highest cumulative release of 93-99% is recorded for all gels when in vitro release is performed in successive buffers, that is, first in AGF, followed by AIF and PBS.

Molecular Dynamic Simulations for the Extraction of Quinoline from Heptane in the Presence of a Low-Cost Phosphonium-Based Deep Eutectic Solvent.

The present study aims at the extraction of a polyaromatic hydrocarbon from fuel oils using a novel low-cost deep eutectic solvent (DES). The DES is prepared by mixing the hydrogen bond acceptor (HBA; methyltriphenylphosphonium bromide, MTPB) and hydrogen bond donor (HBD; ethylene glycol) at a molar ratio of 1:4. The liquid-liquid equilibrium is then measured at ambient condition. The classical molecular dynamic (MD) simulation technique is then employed to investigate and compare the experimental phase behavior of a DES-quinoline-heptane ternary system. For performing the MD simulations, two experimental feed points are considered which gave high selectivity and distribution coefficient values. The interaction energies of different species and the structural properties such as radial distribution functions, average number of hydrogen bonds, and spatial distribution functions (SDFs) are then computed. It is found that the cation within the HBA, namely, MTP, possesses favorable interactions with quinoline when compared to HBD or anion (Br). MTP here acts as a HBA and contributes to the hydrogen bonding with quinoline, which results in higher experimental selectivity values. The calculations of SDFs further reveal the fact that the DES molecules are evenly distributed around the active sites of the quinoline molecule, whereas heptane molecules are found to be distributed around the nonactive sites of the aromatic ring.

Alkali Metal Ion Partitioning with Calix[4]arene-benzo-crown-6 Ionophore in Acidic Medium: Insights from Experiments, Statistical Mechanical Framework, and Molecular Dynamics Simulations.

The current work reports the experimental and predicted interfacial behavior of metal ion extraction from aqueous phase-diluent system using a newly synthesized calix-benzo-crown-6 (CBCBGA) ionophore. Conductor-like screening model for real solvents was used to predict the selectivity at infinite dilution for the metal ion complexes in both aqueous and diluent phases. The selectivity for Cs+-CBCBGA extraction was found to be higher than that of other metal ions, namely, K+, Na+, and Rb+. This was confirmed by the experimental distribution coefficients obtained in the diluents system at 3 M HNO3 along with 0.01 M CBCBGA/organic solvents. The high selectivity of Cs+-CBCBGA complex over other complexes (K+, Rb+, and Na+) in nitrobenzene was also confirmed and validated by the highest occupied molecular orbital-lowest unoccupied molecular orbital energy gap (i.e., 0.13114 > 0.12411 > 0.11719 > 0.11561 eV) and interaction energy (i.e., -68.25 > -57.11 > -55.52 > -52.37 kcal/mol). The interaction and free energies of the extraction were found to increase with the dielectric constant of the organic solvents, namely, nitrobenzene > o-nitrophenyl hexyl ether > 1-octanol > chloroform. Overall, a higher selectivity of Cs+ ion over that of other metal ions (K+, Na+, and Rb+) was obtained for the newly synthesized CBCBGA ionophore in a radioactive waste solution.

Thermophysical and Forced Convection Studies on (Alumina + Menthol)-Based Deep Eutectic Solvents for Their Use as a Heat Transfer Fluid.

The current work reports the thermophysical and flow measurements of novel thermal solvents based on deep eutectic solvents (DESs) and alumina-based nanoparticle-dispersed deep eutectic solvents (NDDESs) for its use as a potential solar energy storage medium. The DESs were synthesized using a hydrogen bond donor (i.e., oleic acid) and a hydrogen bond acceptor (i.e., dl-menthol) by using the COSMO-SAC-predicted equimolar ratio at a temperature of 350.15 K. Thereafter, NDDESs or nanofluids were formed by dispersing different volume fractions (0.001, 0.005, 0.0075, and 0.01) of Al2O3 nanoparticles in the DESs. The optimum volume fraction (0.005) of Al2O3 nanoparticles was selected through their thermophysical properties (density, viscosity, thermal conductivity, and specific heat capacity) and its agglomeration or stability behavior. As expected, NDDESs with a 0.005 volume fraction gave a higher enhancement in thermal conductivity, viscosity, heat capacity, and density as compared to DESs. To evaluate the heat transfer coefficient, forced convection experiments were conducted in a circular test section for both DESs and NDDESs under laminar conditions (Re = 124, 186, and 250). The enhancement of the local heat transfer coefficient was found to be higher when compared to their thermophysical properties. This was due to the nanoparticle migration resulting in a non-uniform distribution of both thermal conductivity and viscosity fields, which was inherently found to reduce the thermal boundary layer thickness. In the final section, the heat transfer coefficient and the Nusselt number were also validated with COMSOL Multiphysics simulations.

Interfacial Behavior of Cs+, K+, Na+, and Rb+ Extraction in the Presence of Dibenzo-18-Crown-6 from the Nitrobenzene-Water Biphasic System: Experimental, Quantum Chemical, and Molecular Dynamic Studies.

Extraction of metal ions (i.e., Cs+, K+, Na+, and Rb+) in the presence of ionophore such as dibenzo-18-crown-6 (DB18C6) from the nitrobenzene-water biphasic system is reported by COSMO-RS (conductor-like screening model for real solvents) predictions, molecular dynamics simulation, along with experimental validation. The predicted values of selectivity as obtained for the Na+-DB18C6 complex were 4.571, 4.877, and 4.947 at 298.15, 308.15, and 318.15 K, respectively. This was then confirmed by the experimental distribution coefficient (D) as obtained in the diluent systems along with by varying the metal ion to crown ether ligand (M-L) mole ratios: 10:1 (0.1 M M+ and 0.01 M DB18C6), 1:1 (0.01 M M+ and 0.01 M DB18C6), and 1:10 (0.001 M M+ and 0.01 M DB18C6). The experimentally determined values of D Na (i.e., 0.059, 0.060, and 0.056) were found to be very large as compared to the values of D Cs (i.e., 0.001, 0.010, and 0.024) in the nitrobenzene phase. It indicates an excellent extraction ability of DB18C6 for Na+. The rate of phase separation for the Cs+NO3 - system was slow as compared to other metal ion systems. The binding energies, free energies, and nonbonded interaction energies of the complexed metal ion in solution were calculated with both explicit and implicit solvent models. A higher interaction energy between Na+-DB18C6 complex and nitrobenzene was observed (i.e., -289.92 in the explicit model and -143.12 kcal/mol in the implicit model) when compared with other metal ions (i.e., Cs+, K+, and Rb+).

Ionic Liquid Facilitated Dehydrogenation of tert-Butylamine Borane.

The current work reports ionic liquid (IL) facilitated dehydrogenation of tert-butylamine borane (TBAB) at 90 and 105 °C. For the screening of potential IL solvent, solubility predictions of TBAB in ILs were performed by the conductor-like screening model segment activity coefficient (COSMO-SAC) model. The COSMO-SAC model predicted a logarithmic infinite dilution activity coefficient of -6.66 and -7.31 for TBAB in 1-butyl-3-methylimidazolium acetate [BMIM][OAc] and trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate [TDTHP][Phosph], respectively. Hydrogen (1.95 equiv) was seen to release from TBAB/[BMIM][OAc] at 105 °C, whereas TBAB/[TDTHP][Phosph] produced 1.63 equiv of hydrogen after 360 min of dehydrogenation. The proton nuclear magnetic resonance (1H NMR) characterization of TBAB/IL systems revealed the structural integrity of ILs during dehydrogenation. Further characterization through the boron NMR (11B NMR) technique disclosed the time-resolved formation and stability of the starting compound, intermediate boron moieties, and product distribution. The 11B NMR characterization also revealed the fact that the TBAB/[TDTHP][Phosph] mixture dehydrogenates via bimolecular addition of TBAB by forming borohydride anion (-BH4 -). It was seen to oligomerize with the subsequent addition of TBAB in the oligomer chain. For the TBAB/[BMIM][OAc] system, the 11B NMR characterization could not identify the borohydride anion but confirmed a faster formation of the B=N moiety when compared to the TBAB/[TDTHP][Phosph] system. On the basis of the NMR characterization, IL-facilitated dehydrogenation mechanism of TBAB is proposed.

COSMO-RS-Based Screening of Antisolvents for the Separation of Sugars from Ionic Liquids: Experimental and Molecular Dynamic Simulations.

The use of ionic liquids (ILs) in the biorefinery process has been increasing for the past few decades. In biorefinery, the separation process with respect to sugars needs to be evaluated for an efficient process design. Therefore, the present work aims to investigate the separation of sugars and ILs by means of a precipitation process using an antisolvent method. For this purpose, both theoretical and experimental studies were conducted. Initially, the conductor-like screening model for real solvents model was employed to screen the suitable antisolvents for the separation of sugars from the ILs. From the screening study, dichloromethane (DCM) and 1,2-dichloroethane were found to be the better antisolvents for the separation process. With the selected antisolvents, precipitation experiments were conducted for the mixtures involving four different sugars and three ILs at different experimental conditions. The process variables such as different antisolvents, sugars, ILs, antisolvent-IL molar ratios, and temperatures were examined in terms of their effect on sugar removal and IL recovery. DCM was found to be the most suitable antisolvent in this study with 90-99% of sugar removal and 80-98% of IL recovery. Further, molecular dynamics simulations were adopted to understand the structural properties of carbohydrates with ILs and antisolvents via interaction energies, hydrogen bonding, and coordination numbers. It was observed that the interaction energy between the sugars and IL plays a critical role in the removal of sugar. Higher the interaction energy between the sugars and IL, lower is the sugar removal.

Hydrolysis of bamboo biomass by subcritical water treatment.

The aim of present study was to obtain total reducing sugars (TRS) from bamboo under subcritical water (SCW) treatment in a batch reactor at the temperature ranging from 170 °C to 220 °C and 40 min hydrolysis time. Experiments were performed to investigate the effects of temperature and time on TRS yield. The maximum TRS yield (42.21%) was obtained at lower temperature (180 °C), however longer reaction time (25 min). X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM) analysis were used to characterise treated and untreated bamboo samples. The XRD profile revealed that crystallinity of bamboo increased to 71.90% with increase in temperature up to 210 °C and decreased thereafter to 70.92%. The first-order reaction kinetic model was used to fit the experimental data to obtain rate constants. From the Arrhenius plot, activation energy and pre-exponential factor at 25 min time were found to be 17.97 kJ mol(-1) and 0.154 min(-1), respectively.

Instability modes of a two-layer Newtonian plane Couette flow past a porous medium.

We explore the salient features of the different instability modes of a pressure-driven two-layer plane Couette flow confined between a moving wall and a Darcy-Brinkman porous layer. A linear stability analysis of the conservation laws leads to an Orr-Sommerfeld system, which is solved numerically with appropriate boundary conditions to identify the time and length scales of the instability modes. The study reveals that the movement of the confining wall together with the slippage at the porous-liquid interface originating from the flow inside the porous layer can stimulate a pair of finite-wave-number shear modes in addition to the long-wave interfacial mode of instability. The shear modes dominate the interfacial mode, especially when the frictional influence at the liquid layers is smaller due to the movement of the confining plate or due to the larger slippage at the porous-liquid interface. The shear modes are found to be present under all combinations of the viscosity µ(r) and thickness h(r) ratios of the liquid layers. This is in stark contrast to the two-layer flow confined between nonporous plates where the interfacial (shear) mode is observed only when µ(r)>h(r)(2) (µ(r)

Instabilities of a free bilayer flowing on an inclined porous medium.

The instabilities of a free bilayer flowing on an inclined Darcy-Brinkman porous layer have been explored. The bilayer is composed of a pair of immiscible liquid films with a deformable liquid-liquid interface and a liquid-air free surface. An Orr-Sommerfeld analysis of the governing equations and boundary conditions uncovers that this configuration can be unstable by a pair of long-wave interfacial modes at the free surface and at the interface together with a couple of finite wave-number shear modes originating from the inertial influences at the liquid layers. In particular, one of the shear modes originates beyond a threshold flow rate owing to the slippage at the porous-liquid interface and is found to be the dominant one even when the porous medium is moderately thin, porous, and permeable. The strength of the porous media mediated mode (a) grows with increase in porosity, (b) grows and then remains invariant with increase in thickness, and (c) initially grows and then decays with increase in the permeability of the porous layer. Further, the presence of a lower layer with smaller viscosity and a thicker upper layer is found to facilitate the growth of this newly identified porous media mode. Importantly, beyond a threshold upper to lower thickness and viscosity ratios and the angle of inclination the porous media mode dominates over all the other interfacial or shear modes, highlighting its importance in the bilayer flows down an inclined porous medium. The study showcases the importance of a porous layer in destabilizing a free bilayer flow down an inclined plane, which can be of importance to improve mixing, emulsification, and heat and mass transfer characteristics in the microscale devices.