Encapsulation with an interfacial liquid layer: Robust and efficient liquid-liquid wrapping.

HYPOTHESIS: We developed an impact driven liquid-based encapsulation method by utilizing the fundamental thermodynamic tendency of a suitable three-liquid combination towards formation of a core-shell structure. EXPERIMENTS: Stable wrapping is achieved by impinging a core drop from a vertical separation on an interfacial liquid film floating on a host liquid bath. The resulting interfacial dynamics is captured using a high-speed camera. Several combinations of impact height and interfacial film thickness are investigated for a quantitative description of the phenomena. FINDINGS: The stability and integrity of the liquid encapsulating layer are confirmed both experimentally (by analyzing the under-liquid wetting signature) and theoretically (by equilibrium thermodynamic analysis). Effect of viscous dissipation on the dynamics is explained and a consequent theoretical threshold for minimum allowable drop size is provided. A non-dimensional experimental regime is also constructed for successful encapsulation in terms of impact kinetic energy and interfacial layer thickness. Additionally, the encapsulating layer is shown to protect the core drop even when the core and host liquids are miscible. The demonstrated method is simple to implement yet robust, offers flexibility regarding varying both the size and the material properties of the core and shell liquids and consistently produces stable monodispersed encapsulated drops in an ultrafast manner.

Anomalous Wetting of Underliquid Systems: Oil Drops in Water and Water Drops in Oil.

We have investigated the wetting phenomena of two underliquid systems, i.e., oil (drop) in water medium and water (drop) in oil medium for two different substrates, poly(methyl methacrylate) (PMMA) and glass. We have conducted detailed static (equilibrium) and dynamic contact angle measurements of drops on substrates kept in air, water, and oils of varying densities, viscosities, and surface tensions. We compared the experimentally observed contact angles with those predicted by the conventional wetting theories, namely, Young's equation and the Owens and Wendt approach. The results reported herein showed that experimental values vary in the range of 8-20% with the conventional theoretical model for water (drop) in oil (viscous surrounding medium) on PMMA substrate. However, oil (drop) in water medium on PMMA does not show such an anomaly. By taking into consideration a thin oil film between a water drop and PMMA originating from the surrounding oil medium, the modified Young's equation is proposed here. We found that the percentage difference between experimentally observed contact angles with modified Young's equation is in the range of 0.88-5.88%, which is very less compared to percentage difference with classic Young's equation. For glass substrates, the standard Young's equation does not translate to the underliquid systems whereas the Owens and Wendt theory could not correctly predict the underliquid contact angles. However, the modified Young's equation with thin-film consideration agrees very well with the experimental values and thereby demonstrated the presence of a thin film between a drop and glass substrate originating from the surrounding viscous medium. This present experimental study coupled with detailed theoretical analyses demonstrates the anomalous wetting signature of drops on substrates submerged in surrounding viscous medium, which is very different from the reported studies for drops on substrates kept in air (inviscid medium).

DipTest: A litmus test for E. coli detection in water.

We have developed a new litmus paper test (DipTest) for detecting Escherichia coli (E. coli) in water samples by performing enzymatic reactions directly on the porous paper substrate. The paper strip consists of a long narrow piece of cellulose blotting paper coated with chemoattractant (at bottom edge), wax hydrophobic barrier (at the top edge), and custom formulated chemical reagents (at reaction zone immediately below the wax hydrophobic barrier). When the paper strip is dipped in water, E. coli in the water sample is attracted toward the paper strip due to a chemotaxic mechanism followed by the ascent along the paper strip toward the reaction zone due to a capillary wicking mechanism, and finally the capillary motion is arrested at the top edge of the paper strip by the hydrophobic barrier. The E. coli concentrated at the reaction zone of the paper strip will react with custom formulated chemical reagents to produce a pinkish-red color. Such a color change on the paper strip when dipped into water samples indicates the presence of E. coli contamination in potable water. The performance of the DipTest device has been checked with different known concentrations of E. coli contaminated water samples using different dip and wait times. The DipTest device has also been tested with different interfering bacteria and chemical contaminants. It has been observed that the different interfering contaminants do not have any impact on the DipTest, and it can become a potential solution for screening water samples for E. coli contamination at the point of source.

A hydrogel based rapid test method for detection of Escherichia coli (E. coli) in contaminated water samples.

We have formulated a new chemical composition for rapid detection of Escherichia coli (E. coli) with currently available enzymatic substrates. We have evaluated the performance of the new chemical composition with different kinds of bacteria, and metallic and ionic interferences and optimized the chemical composition for rapid and specific detection of E. coli. We used a novel hydrogel based porous matrix to encapsulate the optimized chemical compounds and incorporated it within a readily available plunger-tube assembly. This overall system allows efficient, field deployable, rapid testing of water samples by simultaneously pre-concentrating and detecting E. coli within one integrated unit. We were able to detect E. coli concentrations of 4 × 10(6) CFU mL(-1) to 4 × 10(5) CFU mL(-1) within 5 min and 4 × 10(4) CFU mL(-1) to 400 CFU mL(-1) within 60 min using the integrated plunger-tube assembly containing the hydrogel matrix.

Under-water superoleophobicity of fish scales.

Recent surge in the development of superhydrophobic/superoleophobic surfaces has been motivated by surfaces like fish scales that have hierarchical structures, which are believed to promote water or oil repellency. In this work, we show that the under-water oil repellency of fish scales is entirely due to the mucus layer formation as part of its defense mechanism, which produces unprecedented contact angle close to 180°. We have identified the distinct chemical signatures that are responsible for such large contact angle, thereby making fish scale behave highly superoleophobic inside the water medium. In absence of the mucus layer, it is found that the contact angle decreases quite dramatically to around 150°, making it less oleophobic, the degree of such oleophobicity can then be contributed to its inherent hierarchical structures. Hence, through this systematic study, for the first time we have conclusively shown the role of the fish's mucus layer to generate superoleophobicity and negate the common notion that hierarchical structure is the only reason for such intrinsic behavior of the fish scales.

Optical biosensors with an integrated Mach-Zehnder Interferometer for detection of Listeria monocytogenes.

In this work, we have demonstrated an efficient optical immunoassay technique for the detection of a food-borne pathogen, Listeria monocytogenes, using a Mach-Zehnder Interferometer (MZI) configuration. We have investigated ten different MZI configurations with angular and Sbend Y-junction geometries. An efficient Hydrofluoric acid (HF) based technique was used for rapid and specific binding of L. monocytogenes to the sensor arm of the MZI biosensor. The MZI biosensor was able to detect L. monocytogenes at concentrations of the order of 10(5) CFU/ml, which is lower than the infection dose for healthy human beings. SEM analysis and light intensity measurements showed the biosensor is highly selective to L. monocytogenes over other microbial species (such as Escherichia coli). Finally, a novel calibration scheme of the MZI biosensor was developed from experimental data that can be used for determining unknown concentrations of L. monocytogenes.

Micro-spot with integrated pillars (MSIP) for detection of dengue virus NS1.

In this paper, we demonstrate an extremely efficient technique of diagnosing dengue virus non-structural protein (NS1) using Micro-Spot with Integrated Pillars (MSIP). Detection using MSIP is performed by employing fluorescence immunoassay specific to dengue virus NS1. MSIPs are chemically modified to ensure efficient covalent binding of antibodies on the micropillars, whereas the enormous increase in the surface area (available for the reaction) induced by the micropillars amplifies the apparent rate, which enhances the signal intensity. Therefore, the detection response of a MSIP, quantified by the intensity of the fluorescence signal, is found to be almost five times magnified than the response of a similar size micro-spot without micropillars. The response of the micropillars also depend on the pillar arrangement, since for identical concentration of dengue NS1 antigen, a stronger intensity signal is obtained for a hexagonal close packed array (staggered) pillar arrangement as compared to a square array arrangement.

Characterization of nanometer-scale porosity in reservoir carbonate rock by focused ion beam-scanning electron microscopy.

Sedimentary carbonate rocks are one of the principal porous structures in natural reservoirs of hydrocarbons such as crude oil and natural gas. Efficient hydrocarbon recovery requires an understanding of the carbonate pore structure, but the nature of sedimentary carbonate rock formation and the toughness of the material make proper analysis difficult. In this study, a novel preparation method was used on a dolomitic carbonate sample, and selected regions were then serially sectioned and imaged by focused ion beam-scanning electron microscopy. The resulting series of images were used to construct detailed three-dimensional representations of the microscopic pore spaces and analyze them quantitatively. We show for the first time the presence of nanometer-scale pores (50-300 nm) inside the solid dolomite matrix. We also show the degree of connectivity of these pores with micron-scale pores (2-5 µm) that were observed to further link with bulk pores outside the matrix.

Reservoir-on-a-chip (ROC): a new paradigm in reservoir engineering.

In this study, we design a microfluidic chip, which represents the pore structure of a naturally occurring oil-bearing reservoir rock. The pore-network has been etched in a silicon substrate and bonded with a glass covering layer to make a complete microfluidic chip, which is termed as 'Reservoir-on-a-chip' (ROC). Here we report, for the first time, the ability to perform traditional waterflooding experiments in a ROC. Oil is kept as the resident phase in the ROC, and waterflooding is performed to displace the oil phase from the network. The flow visualization provides specific information about the presence of the trapped oil phase and the movement of the oil/water interface/meniscus in the network. The recovery curve is extracted based on the measured volume of oil at the outlet of the ROC. We also provide the first indication that this oil-recovery trend realized at chip-level can be correlated to the flooding experiments related to actual reservoir cores. Hence, we have successfully demonstrated that the conceptualized 'Reservoir-on-a-Chip' has the features of a realistic pore-network and in principle is able to perform the necessary flooding experiments that are routinely done in reservoir engineering.

Modeling of dielectrophoretic transport of myoglobin molecules in microchannels.

Myoglobin is one of the premature identifying cardiac markers, whose concentration increases from 90 pgml or less to over 250 ngml in the blood serum of human beings after minor heart attack. Separation, detection, and quantification of myoglobin play a vital role in revealing the cardiac arrest in advance, which is the challenging part of ongoing research. In the present work, one of the electrokinetic approaches, i.e., dielectrophoresis (DEP), is chosen to separate the myoglobin. A mathematical model is developed for simulating dielectrophoretic behavior of a myoglobin molecule in a microchannel to provide a theoretical basis for the above application. This model is based on the introduction of a dielectrophoretic force and a dielectric myoglobin model. A dielectric myoglobin model is developed by approximating the shape of the myoglobin molecule as sphere, oblate, and prolate spheroids. A generalized theoretical expression for the dielectrophoretic force acting on respective shapes of the molecule is derived. The microchannel considered for analysis has an array of parallel rectangular electrodes at the bottom surface. The potential and electric field distributions are calculated using Green's theorem method and finite element method. These results also compared to the Fourier series method, closed form solutions by Morgan et al. [J. Phys. D: Appl. Phys. 34, 1553 (2001)] and Chang et al. [J. Phys. D: Appl. Phys. 36, 3073 (2003)]. It is observed that both Green's theorem based analytical solution and finite element based numerical solution for proposed model are closely matched for electric field and square electric field gradients. The crossover frequency is obtained as 40 MHz for given properties of myoglobin and for all approximated shapes of myoglobin molecule. The effect of conductivity of medium and myoglobin on the crossover frequency is also demonstrated. Further, the effect of hydration layer on the crossover frequency of myoglobin molecules is also presented. Both positive and negative DEP effects on myoglobin molecules are obtained by switching the frequency of applied electric field. The effect of different shapes of myoglobin on DEP force is studied and no significant effect on DEP force is observed. Finally, repulsion of myoglobin molecules from the electrode plane at 1 KHz frequency and 10 V applied voltage is observed. These results provide the ability of applying DEP force for manipulating nanosized biomolecules such as myoglobin.