When one solution of reactant A is displacing another miscible solution of reactant B, a miscible product C can be generated in the contact zone if a simple A + B → C chemical reaction takes place. Depending on the relative effect of A, B and C on the viscosity, different viscous fingering (VF) instabilities can be observed. In this context, a linear stability analysis of this reaction-diffusion-convection problem under the quasi-steady-state approximation is performed to classify the various possible instability scenarios. To do so, we determine the criteria for an instability, obtain dispersion curves both at initial contact time using an analytical implicit solution and at later times via numerical stability analysis. Along with recovering known results for non-reactive systems where the displacement of a more viscous fluid by a less viscous one leads to a VF instability, it is found that in the presence of a chemical reaction, injecting a more viscous fluid into a less viscous fluid can also lead to instabilities. This occurs when the chemical reaction leads to the build up of non-monotonic viscosity profiles. Various instability scenarios are classified in a parameter plane spanned by R b and R c representing the log-mobility ratios of the viscosities of the B and C solution respectively with respect to that of the injected solution of A. A parametric study of the influence on stability of the Damköhler number and of the time elapsed after contact of the two reactive solutions is also conducted.

A novel flow sensor is presented to measure the flow rate within microchannels in a real-time, noncontact and nonintrusive manner. The microfluidic device is made of a fluidic microchannel sealed with a thin polymer layer interfacing the fluidics and microwave electronics. Deformation of the thin circular membrane alters the permittivity and conductivity over the sensitive zone of the microwave resonator device and enables high-resolution detection of flow rate in microfluidic channels using noncontact microwave as a standalone system. The flow sensor has the linear response in the range of 0-150 µl/min for the optimal sensor performance. The highest sensitivity is detected to be 0.5 µl/min for the membrane with the diameter of 3 mm and the thickness of 100 µm. The sensor is reproducible with the error of 0.1% for the flow rate of 10 µl/min. Furthermore, the sensor functioned very stable for 20 hrs performance within the cell culture incubator in 37 °C and 5% CO 2 environment for detecting the flow rate of the culture medium. This sensor does not need any contact with the liquid and is highly compatible with several applications in energy and biomedical engineering, and particularly for microfluidic-based lab-on-chips, micro-bioreactors and organ-on-chips platforms.Microfluidic techniques have been extensively used for efficient manipulation of fluid flow in microscale for biomedical research and analytical chemistry. The control of flow in microfluidic networks is crucial for cell sorting, cell collection, flow mixing, cell adhesion and culture, droplet manipulation and flow driving 1 . Moreover, the flow rate needs to be accurately quantified to determine the concentration of cells 2 , and production of hollow microspheres . A slight change in flow rate may lead to a size variation in the products. To precisely handle fluids at the microscale, the real-time detection of flow rate in microfluidic environment is essential and urgently needed though challenging.Organ-on-a-chip (OOC) technology, in particular, aims to build biomimetic in vitro physiological micro-organs to compliment animal models in biological systems and benefit the pharmaceutical industry for drug discovery 7,8 . Many groups including ours have developed OOC platforms made of microbioreactors and integrated sensors for long-term and real-time monitoring the microenvironment, screening the status of miniaturized organs, and characterizing the response of micro-tissues to drugs [9][10][11][12][13] . The real-time measurement of heat transfer 14 , differential pressure 15 , pH and oxygen 11 and biomarkers 10 are central to biomimetic performance of OOC systems. Miniaturized biosensors provide favorable features like low-cost reagents consumption, decreased processing time, reduced sample volume, laminar flow to cells, parallel detection for multiple samples as well as portability 12,13,16 . However, the OOC systems still need on-chip integrated flow sensors compatible with their fabrication processes and functions 17. The OOC platforms require the...

with a large specific surface area featuring both hydrophilic and hydrophobic characters. [4] The hydrophobic nature of GO nanosheets originates from the basal plane, i.e., carbon rings, while hydrophilicity is imparted to GO by the surface and edge functional groups, e.g., hydroxyl, carboxylic, and epoxy groups. Due to this dual nature, GO nanosheets self-assemble at oil/water (O/W) interfaces, forming nanometer-thick barriers separating water and oil. Thus, by tuning the carbon to oxygen ratio, GO can be assembled at liquid-liquid interfaces to generate hierarchical structures with defined functionalities. [5,6] The promise of GO for numerous applications has increased interest in its interfacial behavior.The assembly of GO at liquid-liquid interfaces has been investigated primarily by dynamic interfacial tension (IFT). Previously, we [7] showed that GO assembles at the O/W interfaces, forming, predominantly a tessellated, nanosheet barrier reducing the interfacial surface energy between the liquids. The low bending modulus of GO enables the assemblies to conform to the curvature of the interface. Similarly, Kim et al. [1] investigated the activity of GO nanosheets at air-liquid, liquid-liquid, and liquid-solid interfaces, showing that, despite the stable dispersion of GO in water, GO segregates to the interfaces to reduce the interfacial tension. Imperiali et al. [8] reported on GO film formation at air/W interfaces, performing compression/expansion experiments in a Langmuir trough. They found that GO assembles at the surface and, upon compression, maitains a single layer thickness, resisting overlap due to attractive lateral forces. However, the influence of GO on the viscoelastic properties of the O/W interfaces has not been thoroughly investigated, which is critical for applications, including emulsification, enhanced oil recovery (EOR), and all-liquid 2D and 3D printing. We recently demonstrated, for example, the importance of the interfacial rheology of O/W on the stabilization of Pickering emulsions. [9] It was shown that the interfacial rheology plays a decisive role in emulsion formation, [9,10] controlling the emulsion morphology and stability. [11,12] In all-liquid 3D printing, reducing the interfacial tension to retard Plateau Rayleigh (PR) instabilities and stabilize the interface [13] is essential. Several nanomaterials have been recently proposed for sculpting liquids. [14] For instance, the printability Tailoring the oil/water (O/W) interface is a prerequisite for structuring these two immiscible liquids into prescribed architectures, i.e., liquid-in-liquid printing, which is an emerging area in material science. Here, assemblies of graphene oxide (GO) at O/W and air/W interfaces are characterized using a wide range of interfacial rheological techniques. It is shown that the GO nanosheets assemble at the interface, even at extremely low concentrations as low as 0.04 vol%, significantly increasing the elasticity at O/W or air/W interfaces. This is attributed to the combined hydrophobic and hydrop...

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Methane (CH 4 ) wettability of shale is a key parameter which determines pore and reservoir-scale fluid distributions, CH 4 reserves estimation, and ultimate recovery efficiency from shale gas reservoirs. Clay minerals usually fill the pore spaces or are adsorbed on the surface of shale rock, thus influencing CH 4 wettability. However, a systematic investigation of the influence of clay on CH 4 shale wettability is lacking. Herein, we investigated the role of clay, pressure, temperature, and salinity on CH 4 wettability of clay-coated quartz (i.e., a well-defined model system for shales). Results indicated that the advancing and receding water contact angles for clean, kaolinite-coated, and montmorillonite-coated quartz increased with pressure. However, the effect of temperature on wettability is complex, and thus the advancing water contact angle for clean quartz increased with temperature while an opposite trend was found for clay-coated quartz. At low temperature (i.e., 300 K), clay coating dewetted the quartz surface, while at elevated temperature (i.e., 323 K), clay coating increased the hydrophilicity of the quartz surface. Furthermore, kaolinite clay particles demonstrated a stronger influence on quartz wettability than montmorillonite particles, both at high and low temperatures. In addition, higher NaCl salinity led to higher advancing water contact angles for the aforementioned three solid surfaces. The effect of salinity on CH 4 wettability is thus intensified in the presence of clays. These insights will thus improve the accuracy of CH 4 reserve estimates and aid methane recovery schemes.

The stability of a horizontal interface between a solution of reactant $A$ on top of another solution of reactant $B$ is analysed. A chemical product $C$ is generated at the interface as a result of a bimolecular chemical reaction $A+ B\ensuremath{\rightarrow} C$. In general, all chemical components are assumed to have different densities and viscosities, and a transverse velocity is introduced parallel to the interface between the reactants. Although the transverse flow is known for its stabilizing effect in viscously unstable non-reactive systems in the presence of an injection velocity, it is shown here that it can actually destabilize an initially stable reactive front. An expression for the critical transverse velocity beyond which an initially stable interface is destabilized is derived in the case of an initial sharp interface for reactants of the same viscosity. The analysis is extended to a diffused profile, and purely buoyancy-driven flows are analysed first in the absence of viscosity contrast and then in the presence of transverse flows and viscosity contrast. Various possible density fingering scenarios are determined based on the relative contribution of each chemical component to the density profile. It is found that the chemical reaction can destabilize a buoyancy-stable initial interface by generating a non-monotonic density profile. Unlike the viscous fingering of a reactive interface, a symmetry in the stability characteristics with respect to density increase or decrease by chemical reaction product is observed in the case of chemically buoyancy-driven flows for identical reactants.

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