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When a liquid drop makes initial contact with any surface, an unbalanced surface tension force drives the contact line, causing spreading. For Newtonian liquids, either liquid inertia or viscosity dictates these early regimes of spreading, albeit with different power-law behaviors of the evolution of the dynamic spreading radius. In this work, we investigate the early regimes of spreading for yield-stress liquids. We conducted spreading experiments with hydrogels and blood with varying degrees of yield stress. We observe that for yield-stress liquids, the early regime of spreading is primarily dictated by their high shear rate viscosity. For yield-stress liquids with low values of high shear rate viscosity, the spreading dynamics mimics that of Newtonian liquids like water, i.e., an inertia-capillary regime exhibited by a power-law evolution of spreading radius with exponent 1/2. With increasing high shear rate viscosity, we observe that a deceptively similar, although slower, power-law spreading regime is obeyed. The observed regime is in fact a viscous-capillary where viscous dissipation dominates over inertia. The present findings can provide valuable insights into how to efficiently control moving contact lines of biomaterial inks, which often exhibit yield-stress behavior and operate at high print speeds, to achieve desired print resolution.

Hypothesis: Generating multi-layer cargo using conventional methods is challenging. We hypothesize that incorporating a Y-junction compound droplet generator to encase a target core inside a second liquid can circumvent the kinetic energy dependence of the impact-driven liquid-liquid encapsulation technique, enabling minimally restrictive multi-layer encapsulation., Experiments: Stable wrapping is obtained by impinging a compound droplet (generated using Y-junction) on an interfacial layer of another shell-forming liquid floating on a host liquid bath, leading to double-layered encapsulation. The underlying dynamics of the liquid-liquid interfaces are captured using high-speed imaging. To demonstrate the versatility of the technique, we used various liquids as interfacial layers, including magnetoresponsive oil-based ferrofluids. Moreover, we extended the technique to triple-layered encapsulation by overlaying a second interfacial layer atop the first floating interfacial layer., Findings: The encapsulating layer(s) effectively protects the water-soluble inner core (ethylene glycol) inside the water bath. A non-dimensional experimental regime is established for successful encapsulation in terms of the impact kinetic energy, interfacial layer thickness, and the viscosity ratio of the interfacial layer and the outer core liquid. Using selective fluorescent tagging, we confirm the presence of individual shell layers wrapped around the core, which presents a promising pathway to visualize the internal morphology of final encapsulated droplets., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Sushanta K. Mitra reports financial support was provided by Natural Sciences and Engineering Research Council of Canada. Sushanta K. Mitra and Sirshendu Misra report a relationship with SLE Enterprises B.V. that includes: equity or stocks. The authors have a PCT Application No. PCT/IB2024/053830 titled “METHODS OF FORMING AN ENCAPSULATED CORE MATERIAL, AN ENCAPSULATED MATERIAL, COMPOSITIONS USES THEREOF”, filed on April 19, 2024, claiming priority from US Provisional Patent Application No. 63/497,617, dated April 21, 2023. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

Hypothesis: Soft materials, particularly elastomers, are extensively studied, but investigations into purely soft gel contact systems are limited due to their complex dual phases consisting of polymer and free liquids. While Dual Wavelength-Reflection Interference Confocal Microscopy (DW-RICM) is effective for noninvasively visualizing interfaces from a bottom view, it faces challenges in gel studies due to close refractive indices of polymeric networks and free liquids. We hypothesize that modulating the refractive index of soft gels using nanoparticles (NPs) enhances the visualization of contact zone beneath the free surface, providing insights into the configuration of phase-separated free oil within gel-on-gel contact systems., Experiments: Gel-on-gel contact systems were fabricated using immiscible organogels and hydrogels. Titanium dioxide (TiO 2 ) NPs were introduced into the organogel to modulate refractive indices. Given the lack of prior studies on the hidden contact zone between gels, various techniques, including DW-RICM, side-view imaging, and inverted optical microscopy, were employed to observe and validate our findings. Comparative analyses were conducted with elastomer-on-rigid, elastomer-on-gel, and gel-on-rigid contact systems., Findings: Our investigation demonstrated that a minimal amount of TiO 2 NPs effectively delineates the direct contact radius between organogel polymeric networks and hydrogel surfaces. Comparative experiments showed that TiO 2 addition did not alter the gels' mechanical and surface properties but significantly enhanced information on gel contact deformation. This enhanced visualization technique has the potential to advance our understanding of adhesive contacts in gels, providing valuable insights into interface phenomena involving biological soft tissues and cells., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Boxin Zhao reports financial support was provided by Natural Sciences and Engineering Research Council of Canada. Sushanta K. Mitra reports financial support was provided by Natural Sciences and Engineering Research Council of Canada. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

We investigate the interfacial dynamics involved in the impact of a droplet on a liquid-liquid system, which involves the impingement of an immiscible core liquid drop from a vertical separation onto an interfacial shell liquid layer floating on a host liquid bath. The dynamics have been studied for a wide range of impact Weber numbers and two different interfacial shell liquids of varying volumes. The core drop, upon impact, dragged the interfacial liquid into the host liquid, forming an interfacial liquid column with an air cavity inside the host liquid bath. The dynamics is resolved into cavity expansion and rapid contraction, followed by thinning of the interfacial liquid. The interplay of viscous dissipation, interfacial pull, and core drop inertia influenced the necking dynamics. The viscous dissipation increases with the thickness of the interfacial layer, which depends on the volume and lateral spread over the water. The necking dynamics transitioned from inertia-dominated deep seal closure at higher spread, lower interfacial film volumes, and higher Weber numbers into inertia-capillary-dominated deep seal closure with an increase in film volumes, decrease in the spread of the interfacial fluid, or decrease in Weber number and finally into a no-seal closure at high volumes, low spread, and low Weber numbers.

The interaction of graphene with gold nanoparticles is investigated using transmission electron microscopy. We observe gold-nanoparticle-mediated etching of graphene flakes, often leading to hole formation. Further, using a combination of high-angle annular dark field imaging and electron energy loss spectroscopy, we highlight that the catalytic effects of gold nanoparticles on graphene lead to the formation of amorphous carbon layers. From the extracted diffractograms, we observe regions with diffraction halos as well as some regions with a weak tetrahedral motif. Using independently performed Raman measurements, we confirm the presence of tetrahedral amorphous carbon as well as mixed graphitic-amorphous regions. For the amorphous carbon regions with mixed sp 2 -sp 3 states, the Raman G peak is red-shifted to 1564 cm -1 and an I D / I G ratio of 0.63 indicates less than 20% sp 3 content. For the tetrahedral amorphous carbon regions, we observe that the Raman G peak is at 1580 cm -1 , close to that of monolayer graphene. However, there is no Raman D peak, i.e. , I D / I G = 0, which indicates close to 100% sp 3 content. The translation of the Raman G peak location and the I D / I G ratios is on par with the amorphization trajectory analysis of Ferrari and Robertson ( Phys. Rev. B: Condens. Matter Mater. Phys. , 2000, 61 , 14095) and validates the conversion route of graphite to amorphous carbon to tetrahedral amorphous carbon. The presented method provides a promising pathway for creating defect-induced amorphous carbon at room temperature, which has a broader impact on the electronics and semiconductor industries., Competing Interests: The authors declare no competing interests., (This journal is © The Royal Society of Chemistry.)

Raindrops falling on window-panes spread upon contact, whereas hail can cause dents or scratches on the same glass window upon contact. While the former phenomenon resembles classical wetting, the latter is dictated by contact and adhesion theories. The classical Young-Dupre law applies to the wetting of pure liquids on rigid solids, whereas conventional contact mechanics theories account for rigid-on-soft or soft-on-rigid contacts with small deformations in the elastic limit. However, the crossover between adhesion and wetting is yet to be fully resolved. The key lies in the study of soft-on-soft interactions with material properties intermediate between liquids and solids. In this work, we translate adhesion to wetting by experimentally probing the static signature of hydrogels in contact with soft PDMS of varying elasticity of both the components. Consequently, we probe this transition across six orders of magnitude in terms of the characteristic elasto-adhesive parameter of the system. In doing so, we reveal previously unknown phenomenology and a theoretical model which smoothly bridges adhesion of glass spheres with total wetting of pure liquids on any given substrate.

Background: Although well-known for their thromboresistance, bileaflet mechanical heart valves (BMHV) require lifelong anti-thrombotic therapy. This must be associated with a certain level of thrombogenicity. Since both thromboresistance and thrombogenicity are explained by the blood-artificial surface or liquid-solid interactions, the aim of the present study was to explore BMHV thromboresistance from new perspectives. The wettability of BMHV pyrolytic carbon (PyC) occluders was investigated in under-liquid conditions. The submerged BMHV wettability clarifies the mechanisms involved in the thromboresistance., Methods: The PyC occluders of a SJM Regent™ BMHV were previously laser irradiated, to create a surface hierarchical nano-texture, featuring three nano-configurations. Additionally, four PyC occluders of standard BMHV (Carbomedics, SJM Regent ™ , Bicarbon ™ , On-X ® ), were investigated. All occluders were evaluated in under-liquid configuration, with silicon oil used as the working droplet, while water, simulating blood, was used as the surrounding liquid. The under-liquid droplet-substrate wetting interactions were analyzed using contact angle goniometry., Results: All the standard occluders showed very low contact angle, reflecting a pronounced affinity for non-polar molecules. No receding of the contact line could be observed for the untreated occluders. The smallest static contact angle of around 61° could be observed for On-X ® valve (the only valve made of full PyC). The laser-treated occluders strongly repelled oil in underwater conditions. A drastic change in their wetting behaviour was observed depending on the surrounding fluid, displaying a hydrophobic behaviour in the presence of air (as the surrounding medium), and showing instead a hydrophilic nature, when surrounded by water., Conclusions: BMHV "fear" water and blood. The intrinsic affinity of BMHV for nonpolar fluids can be translated into a tendency to repel polar fluids, such as water and blood. The blood-artificial surface interaction in BMHV is minimized. The contact between blood and BMHV surface is drastically reduced by polar-nonpolar Van der Waals forces. The "hydro/bloodphobia" of BMHV is intrinsically related to their chemical composition and their surface energy, thus their material: PyC indeed. Pertaining to thromboresistance, the surface roughness does not play a significant role. Instead, the thromboresistance of BMHV lies in molecular interactions. BMHV wettability can be tuned by altering the surface interface, by means of nanotechnology., (© 2024. The Author(s).)

Understanding the interactions of pathogenic droplets with surfaces is crucial to biomedical applications. In this study, using E. coli as the model microbe, we investigate the impact of a bacteria-laden droplet on different substrates, both bare and antimicrobial. In doing so, we unveil the significance of kinetic energy and spreading parameters of the impacting droplet in determining the microbes' proliferation capabilities. Our results indicate an inverse relationship between the impact Weber number and the bacterial ability to proliferate. We reveal that the mechanical stress generated during impact impedes the capabilities of microbes present inside the droplet to create their progeny. Following an order analysis of the mechanical stress generated, we argue that the impact does not induce lysis-driven cell death of the bacteria; rather, it promotes a stress-driven transition of viable bacteria to a viable-but-non-culturable (VBNC) state. Furthermore, variations in the concentration of particles on the antimicrobial surfaces revealed the role of the post-impact spreading behaviour in dictating bacterial proliferation capabilities. Contrary to the conventional notion, we demonstrate that during the early stages of interaction, a bare substrate may outperform an antibacterial substrate in the inactivation of the bacterial load. Finally, we present an interaction map illustrating the complex relationship between bacterial colony-forming units, bactericide concentration on the surface, and the impact Weber number. We believe that the inferences of the study, highlighting the effect of mechanical stresses on the soft cell wall of microbes, could be a useful design consideration for the development of antimicrobial surfaces.

Recently, 2D materials, such as graphene, have been successfully implemented as artificial conduits of molecular sizes. The extreme precision with which these structures can be fabricated provides an unprecedented framework for the development of highly specific and efficient devices. In this work, we study the electrophoretic transport of Cs + ions in a graphene membrane with effective pore heights of 3.4 Å by conducting molecular dynamics simulations. The entrance of the pore is systematically modified to investigate the effect of pore geometry on ionic conductance. Simulation results suggest a significant correlation between ionic conductance and entrance geometry, with a variation of the conductance up to 100% across the studied cases. To explain the observed correlation, two mechanisms involving an intimate relationship between ion dehydration and edge functional groups are proposed. The present study provides theoretical insights that can aid the design of graphene-based membranes with tunable ionic transport properties.

Sessile hydrogel drops on rigid surfaces exhibit a wetting/contact morphology intermediate between liquid drops and glass spheres. Using density functional theory, we reveal the contact forces acting between a hydrogel and a rigid glass surface. We show that while transitioning from liquid-like to solid-like hydrogels, there exists a critical hydrogel elasticity that enables a switch from attractive-to-repulsive interaction with the underlying rigid glass surface. Our theoretical model is validated by experimental observations of sessile polyacrylamide hydrogels of varying elasticity on glass surfaces. Further, the proposed model successfully approaches Young's law in the pure liquid limit and work of adhesion in the glassy limit. Lastly, we show a modified contact angle relation, taking into account the hydrogel elasticity to explain the features of a distinct hydrogel foot.

The cloaking of the droplet and solid spheres by a thin ferrofluid layer forms a ferrofluid-wetting ridge, enabling the magnet-assisted directional manipulation of droplets and solid spheres on the magneto-responsive slippery surface. Understanding the interplay of various forces governing motion unravels the manipulation mechanism. The transportation characteristics of droplets and solid spheres on such surfaces enable their controlled manipulation in multiple applications. Here, we prepare magneto-responsive slippery surfaces by using superhydrophobic coatings on glass slides, creating a porous network and impregnating them with ferrofluid. Using a permanent magnet (and its translation) in the proximity of these surfaces, we manipulate the motion of liquid drops and solid spheres. Upon dispensing the droplet on the magneto-responsive slippery surface, the droplet is cloaked by a thin ferrofluid layer and forms a ferrofluid wetting ridge. Incorporating the magnetic field creates a magnetic-nanoparticle-rich zone surrounding the ferrofluid ridge. Thereafter, the motion of the magnet gives rise to the movement of the droplet. We found that the interplay of the magnetic force and viscous drag leads to the magnetic manipulation of droplets in a controlled fashion up to a certain magnet speed. Increasing the magnet speed further results in the uncontrolled motion of the droplet, where the droplet cannot follow the magnet trajectory. Moreover, we delineate multifunctional droplet manipulations, such as trapping, pendant droplet manipulation, coalescence, and microchemical reactions, which have wide engineering applications.

Lubricant-impregnated surfaces (LIS) are promising as efficient liquid-repellent surfaces, which comprise a surface lubricant layer stabilized by base solid structures. However, the lubricant layer is susceptible to depletion upon exposure to degrading stimuli, leading to the loss of functionality. Lubricant depletion becomes even more pronounced in exposed outdoor conditions, restricting LIS to short-term lab-scale applications. Thus, the development of scalable and long-term stable LIS suitable for practical outdoor applications remains challenging. In this work, we designed "Lubricated Bicontinuous porous Composites" (LuBiCs) by infusing a silicone oil lubricant into a bicontinuous porous composite matrix of tetrapod-shaped zinc oxide microfillers and poly(dimethylsiloxane). LuBiCs are prepared in the meter scale by a facile drop-casting inspired wet process. The bicontinuous porous feature of the LuBiCs enables capillarity-driven spontaneous lubricant transport throughout the surface without any external driving force. Consequently, the LuBiCs can regain liquid-repellent function upon lubricant depletion via capillary replenishment from a small, connected lubricant reservoir, making them tolerant to lubricant-degrading stimuli (e.g., rain shower, surface wiping, and shearing). As a proof-of-concept, we show that the large-scale "LuBiC roof" retains slippery behavior even after more than 9 months of outdoor exposure.

Encapsulation protects vulnerable cores in an aggressive environment and imparts desirable functionalities to the overall encapsulated cargo, including control of mechanical properties, release kinetics, and targeted delivery. Liquid-liquid encapsulation to create such capsules, where a liquid layer (shell) is used to wrap another liquid (core), is an attractive value proposition for ultrafast encapsulation (∼100 ms). Here, we demonstrate a robust framework for stable liquid-liquid encapsulation. Wrapping is achieved by simple impingement of a target core (in liquid form) on top of an interfacial layer of another shell-forming liquid floating on a host liquid bath. Poly(dimethylsiloxane) (PDMS) is chosen as the shell-forming liquid due to its biocompatibility, physicochemical stability, heat curability, and acceptability as both a drug excipient and food additive. Depending on the kinetic energy of the impinging core droplet, encapsulation is accomplished by either of the two pathways─necking-driven complete interfacial penetration and subsequent generation of encapsulated droplets inside the host bath or trapping inside the interfacial layer. Combining thermodynamic argument with experimental demonstration, we show that the interfacially trapped state, which results in a low kinetic energy of impact, is also an encapsulated state where the core droplet is wholly enclosed inside the floating interfacial layer. Therefore, despite being impact-driven, our method remains kinetic energy independent and minimally restrictive. We describe the underlying interfacial evolution behind encapsulation and experimentally identify a nondimensional regime of occurrence for the two pathways mentioned above. Successful encapsulation by either path offers efficient long-term protection of the encased cores in aggressive surroundings (e.g., protection of honey/maple syrup inside a water bath despite their miscibility). We enable the generation of multifunctional compound droplets via interfacial trapping, where multiple core droplets with different compositions are encapsulated within the same wrapping shell. Further, we demonstrate the practical utility of the interfacially trapped state by showing successful heat-curing of the shell and subsequent extraction of the capsule. The cured capsules are sufficiently robust and remain stable under normal handling.

Controlling the impact process of a droplet impacting a liquid film has remained a wide-open challenge. The existing passive techniques lack precise on-demand control of the impact dynamics of droplets. The present study introduces a magnet-assisted approach to control water droplets' impact dynamics. We show that by incorporating a thin, magnetically active ferrofluid film, the overall droplet impact phenomena of the water droplets could be controlled. It is found that by modifying the distribution of the magnetic nanoparticles (MNPs) present inside the ferrofluid using a permanent magnet, the spreading and retraction behavior of the droplet could be significantly controlled. In addition to that, we also show that by altering the impact Weber number ( We i ), and the magnetic Bond number ( Bo m ), the outcomes of droplet impact could be precisely controlled. We reveal the role of the various forces on the consequential effects of droplet impact with the help of phase maps. Without the magnetic field, we discovered that the droplet impact on ferrofluid film results in no-splitting, jetting, and splashing regimes. On the other hand, the presence of magnetic field results in the no-splitting and jetting regime. However, beyond a critical magnetic field, the ferrofluid film gets transformed into an assembly of spikes. In such scenarios, the droplet impact only results in no-splitting and splashing regimes, while the jetting regime remains absent. The outcome of our study may find potential applications in chemical engineering, material synthesis, and three-dimensional (3D) printing where the control and optimization of the droplet impact process are desirable.

Gliomas are the most commonly occurring primary brain tumor with poor prognosis and high mortality rate. Currently, the diagnostic and monitoring options for glioma mainly revolve around imaging techniques, which often provide limited information and require supervisory expertise. Liquid biopsy is a great alternative or complementary monitoring protocol that can be implemented along with other standard diagnosis protocols. However, standard detection schemes for sampling and monitoring biomarkers in different biological fluids lack the necessary sensitivity and ability for real-time analysis. Lately, biosensor-based diagnostic and monitoring technology has attracted significant attention due to several advantageous features, including high sensitivity and specificity, high-throughput analysis, minimally invasive, and multiplexing ability. In this review article, we have focused our attention on glioma and presented a literature survey summarizing the diagnostic, prognostic, and predictive biomarkers associated with glioma. Further, we discussed different biosensory approaches reported to date for the detection of specific glioma biomarkers. Current biosensors demonstrate high sensitivity and specificity, which can be used for point-of-care devices or liquid biopsies. However, for real clinical applications, these biosensors lack high-throughput and multiplexed analysis, which can be achieved via integration with microfluidic systems. We shared our perspective on the current state-of-the-art different biosensor-based diagnostic and monitoring technologies reported and the future research scopes. To the best of our knowledge, this is the first review focusing on biosensors for glioma detection, and it is anticipated that the review will offer a new pathway for the development of such biosensors and related diagnostic platforms., (© 2023 Author(s).)

Hypothesis: Contact angle goniometry suffers from inherent optical challenges such as scattering and diffraction near the triple contact line (TCL) rendering erroneous results. Alternatively, the cantilever-based direct adhesion measurement was constrained to low-energy surfaces to date due to the inability of the probe droplet to retract (pull-off) from high-energy surfaces completely. The present study revisits the cantilever approach from a fundamental physical perspective and generalizes the approach to render it wettability invariant., Experiments: The adhesive wetting interaction between a probe droplet (attached to a cantilever) with the test substrate is recorded with a high-speed camera. Image processing and subsequent motion analysis enable us to accurately calculate the adhesion force (in the sub-micron range) exhibited by the test substrate., Findings: We experimentally demonstrate the contact line depinning (and not the droplet pull-off) to be the only prerequisite for accurately quantifying the characteristic adhesion force. We alsoreveal that depinning precedes the onset of cantilever retraction due to the inertia effect. Further, we establish that the characteristic adhesion corresponds to zero acceleration of the cantilever and not to its maximum deflection. The inferences of the study will be beneficial in the rational design of direct wetting characterization methods for any substrate., Competing Interests: Declaration of Competing Interest None, (Copyright © 2022 Elsevier Inc. All rights reserved.)

Hypothesis: The conventional solid-solid contact is well studied in the literature. However, a number of practical applications, such as adhesive patches and biomimetic surfaces, require a much deeper understanding of soft contact where there is a distinct time-dependent adhesion behavior due to the dual-phase structure (solids and liquids). To understand this, currently existing solid-solid contact behavior is extrapolated to soft contact, wherein the size-effect of the gel film and the preload are typically neglected. When introducing the finite-size effect and preload, gels could experience distinctive long-term contact dynamics in contact with another material., Experiments: We reconstruct the evolving surface profile of the gel films intercalated between a glass sphere and glass slide using dual wavelength-reflection interference contrast microscopy. The macro-sized glass sphere compresses the gel. The indentation depth is comparable to the gel film thickness, wherein the conventional contact theories are inapplicable., Findings: The gel surface experiences two deformation stages. The natural preload and elastic force develop the contact area in the early state. In the later state, the viscous free molecules of the gel develop the ridge. We discover that the residual surface stress relaxes over 85 hr. Our findings on the long-term gel deformation provide a new perspective on soft adhesion, from developing soft adhesives to understanding biological tissues., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Inc. All rights reserved.)

A liquid drop resting on a soft solid deforms the surface at the three-phase contact line. The surface deformation, also called "wetting ridge", varies in size from nanoscales to microscales, depending on the elasticity and thickness of the soft layer. In this work, we probe how surface elasticity and coating thickness influences normal and tangential surface deformation profiles induced by a sessile liquid drop using dual-wavelength reflection interference contrast microscopy. Furthermore, we experimentally verify the appropriate characteristic length scale, which closely describes the ridge profiles on both thick and thin soft layers for two different surface elasticities.

The liquid droplet spreads over a solid surface to minimize the surface energy when brought in direct contact with the surface. The spreading process is rapid in the early stages, tends to slow down during its progress, and has resulted in peculiarity due to the experimental difficulties in the accurate determination of the contact line radius. In the present numerical study, we found that drop spreading begins with a viscosity-dominated Stokes regime, where contact radius scales as r ∼ t for a wide range of drop liquid viscosities. Subsequent to the Stokes regime, the inertial regime is observed where contact radius scales as r ∼ t 0.5 for low- to medium-viscous droplets, whereas for very high viscous drops, the spreading dynamics is completely dominated by the viscous regime. It is also found that the equilibrium wetting condition does not affect the power-law scaling for the contact radius of the drop. The amplitude of capillary waves induced across the interface of the drop is observed to be sufficiently high to cause necking and ejection of satellite drops from the main drop during its spreading for low-viscous liquids from complete wetting to partial wetting conditions. A regime plot between the Ohnesorge number and advancing contact angle of the substrate is presented to demarcate the regions of damped waves without pinch-off and drop spreading with satellite drops.

The contamination of water and food in agricultural areas, where an enormous volume of pesticides is widely employed to enhance crop production, is a challenging reality. The rapid assessment of these contaminants is fundamental to assure water and food quality and safety, particularly for local community members. This work presents a nonexpensive and easy-operational paper-based testing device for the fast detection of insecticides (carbamates and organophosphates) in water samples. The structural design "dip-and-fold" allows us to carry out the analysis without introducing reagents or samples. The device is prepared using different high-quality papers to support the active acetylcholinesterase (AChE) and the customized chemical formulation for colorimetric detection. The chemical principle is based on the AChE inhibition reaction and Ellman's method. The experiments using standard solutions of carbofuran, propoxur, and chlorpyriphos indicated satisfactory detection at concentrations between 0.1 and 0.0001 mM, and the color results are revealed within 10 min. Therefore, this technique represents a promising alternative for implementing low-cost and efficient water monitoring and management solutions.

This paper describes the fabrication of paper-based microfluidic devices using a novel, inexpensive ink composed of bovine serum albumin (BSA), utilizing BSA's thermal denaturation and aggregation to create a hydrophobic barrier on Whatman® Grade 4 filter paper. A 20% aqueous solution of BSA was inked onto the paper using a pen plotter at moderate speed (5 cm/s) with desired shape and size followed by heating at 80 °C to denature the BSA leading to hydrophobic barriers formation, whereas below 80 °C the barrier layer is prone to collapse. The minimum line gap and line width of ~1 mm and ~1.3 mm were achieved. Finally, a proof-of-concept glucose sensing was shown while addressing the issue of the coffee ring effect using the biopolymer NanoCheck-ATH® from ChitoLytic Inc. The glucose concentration limit of detection (LOD) as low as 0.2 mg/mL was estimated. The developed technique offers ease of fabrication, high reproducibility, cost-effectiveness, and is environmentally friendly., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Polydimethylsiloxane (PDMS) is an important viscoelastic material that finds applications in a large number of engineering systems, particularly lab-on-chip microfluidic devices built with a flexible substrate. Channels made of PDMS, used for transporting analytes, are integral to these applications. The PDMS viscoelastic nature can induce additional hydrodynamic contributions at the soft wall/fluid interface compared to rigid walls. In this research, we investigated the pressure drop within PDMS channels bounded by rigid tubes (cellulose tubes). The bulging effect of the PDMS was limited by the rigid tubes under flowing fluids. The PDMS viscoelasticity was modulated by changing the ratio of the base to the cross-linker from 10:1 to 35:1. We observed that the pressure drop of the flowing fluids within the channel decreased with the increased loss tangent of the PDMS in the examined laminar regime [Reynolds number ( Re ) ∼ 23-58.6 for water and Re ∼ 0.69-8.69 for glycerol solution]. The elastic PDMS 10:1 wall channels followed the classical Hagen Poiseuille's equation, but the PDMS walls with lower cross-linker concentrations and thicker walls decreased pressure drops. The friction factor ( f ) for the PDMS channels with the two working fluids could be approximated as f = 47/ Re . We provide a correlation between the pressure drop and PDMS viscoelasticity based on experimental findings. In the correlation, the loss tangent predominates; the larger the loss tangent, the smaller is the pressure drop. The research findings appear to be unexpected if only considering the energy dissipation of viscoelastic PDMS walls. We attributed the reduction in the pressure drop to a lubricating effect of the viscoelastic PDMS walls in the presence of the working fluids. Our results reveal the importance of the subtle diffusion of the residual oligomers and water from the bulk to the soft wall/fluid interface for the observed pressure drop in soft wall channels.

Experimental investigation of dropwise condensation of low-surface-tension liquids remains prone to error owing to the imaging difficulties caused by the typically low droplet height. Using reflection interference contrast microscopy in confocal mode, we demonstrate a noninvasive framework to accurately capture this condensation dynamics of volatile liquids with low surface tension. The capability of the developed framework is demonstrated in studying the condensation dynamics of acetone, where it accurately describes the growth mechanism of condensed microdroplets with excellent spatiotemporal resolution even for submicron-range drop height and a three-phase contact angle of <5°. From experimentally obtained interferograms, the framework can reconstruct three-dimensional topography of the microdroplets even when the contact line of the droplet is distorted due to strong local pinning. The obtained results exhibit excellent quantitative agreement with several theoretically predicted trends. The proposed protocol overcomes the limitation of conventional techniques (e.g., optical imaging/environmental scanning electron microscopy) and provides an efficient alternative for studying the condensation of low-surface-tension liquids under atmospheric conditions.

Anisotropic microparticles have plenty of applications for their asymmetric structure and precisely modified surface. In our research, the uniform anisotropic microparticles with benzyl chloride group were synthesized successfully via emulsion interfacial polymerization. By varying the degree of cross-linking and the concentration of slightly hydrophilic monomer 4-vinyl benzyl chloride (VBC), several types of microparticles with different concavities and different shapes of microparticles (hemisphere, bowl-like, egg-like, etc.) were obtained. Nanoporous microparticles with a walnut-like heterostructure were achieved with modified hydrophilic seeds with the same strategy. The potential applications of shape-controllable fluorescent microparticles and surface modification of microparticles by thiol-click reaction were explored. The modified microparticles achieved in this study are very useful in labeling, tracing, protein separation, and other biomedical fields.

In the present pandemic time, face masks are found to be the most effective strategy against the spread of the virus within the community. As aerosol-based spreading of the virus is considered as the primary mode of transmission, the interaction of masks with incoming droplets needs to be understood thoroughly for an effective usage among the public. In the present work, we explore the interactions of the droplets over the most commonly used 3-ply surgical masks. A detailed study of the wetting signature, adhesion, and impact dynamics of water droplets and microbe-laden droplets is carried out for both sides of the mask. We found that the interfacial characteristics of the incoming droplets with the mask are very similar for the front and the back side of the mask. Further, in an anticipated attempt to reduce the adhesion, we have tested masks with a superhydrophobic coating. It is found that a superhydrophobic coating may not be the best choice for a regular mask as it can give rise to a number of smaller daughter droplets that can linger in air for longer times and can contribute to the transmission of potential viral loads.

Adhesion behavior of microbial pathogens on commonly encountered surfaces is one of the most pertinent questions now. We present the characterization of bacteria-laden droplets and quantify the adhesion forces on highly repellent surfaces with the help of a simple experimental setup. Comparing the force signature measured directly using an in-house capillary deflection-based droplet force apparatus, we report an anomalous adhesion behavior of live bacteria ( E. coli )-laden droplets on repellent surfaces, which stands in stark contrast to the observed adhesion signature when the doping agent is changed to inert microparticles or the same bacteria in an incapacitated state. We showed that the regular contact angle measurements using optical goniometry is unable to differentiate between the live bacteria and the dead ones (including microparticles) and thus delineate its limitations and the complementary nature of the adhesion measurements in understanding the fundamental interfacial interaction of living organisms on solid surfaces.

With the recent advancements in the development and application of repellent surfaces, both in air and under liquid medium, accurate characterization of repellence behavior is critical in understanding the mechanism behind many observed phenomena and to exploit them for novel applications. Conventionally, the repellence behavior of a surface is characterized by the optical measurement of the dynamic contact angle of the target (to be repelled) liquid on the test surface. However, as already established in the literature, optical measurements are prone to appreciable error, especially for repellent surfaces with high contact angles. Here, we present an alternative, more accurate force-based characterization method of both friction and adhesion forces of microparticle-laden aqueous droplets over various repellent surfaces, where the force signature is captured by probing the surface with a droplet of the test liquid mounted at the tip of a flexible cantilever and then tracking the deflection of the tip of the cantilever as the probe droplet interacts with the surface. A systematic investigation of the response of repellent surfaces toward droplets with different microparticle concentrations reveals the dependency and sensitivity of measured adhesion and friction signature toward particle concentration. A comparison with the theoretical estimate from optical goniometry highlights the deviation of the theoretical data from experimentally measured values and further substantiates the need for such a force-based approach for accurate characterization of repellence behavior.

Soft contacts present different tribological responses compared to stiff materials, especially when soft materials exhibit viscoelastic behaviour, as viscoelastic materials have intermediate mechanical properties between viscous liquids and elastic solids. In this work, we investigated the influence of viscoelasticity of soft materials on sliding friction in dry and lubricated conditions. To achieve this, soft tribopairs with varying viscoelasticity were obtained by tuning the weight ratios of polydimethylsiloxane (PDMS) base and curing agent. The real-time friction force and preload were observed over multiple conditions, with systematic control of lubricant viscosity, preload, and sliding velocity. Tribopairs with a higher proportion of viscous character had more oscilliations in the friction force. They also presented a higher friction coefficient due to the increased contribution of viscoelastic hysteresis losses on friction. Through regression analysis, the models of the friction coefficient were found, which are in good agreement with experimental results. From the models, we found that in both dry and lubricated conditions, viscoelasticity of tribopairs, indicated as the loss modulus or loss tangent, plays a key role in determining the friction coefficient. This influence is particularly significant for dry contacts due to the direct interactions between surfaces of tribopairs. This study provides empirical proof and a focused analysis on the role of viscoelasticity in tribological contacts.

Two-dimensional (2D) materials, such as graphene (GE), hold great potential to be employed as the fundamental building blocks of novel nanofluidic devices for a wide range of applications. Recent advances in experimental techniques are materializing such prospects by enabling the assembly of 2D material-based fluidic channels with heights as small as few Ångströms. Here, we conduct molecular dynamics simulations to probe the effect of the relative misalignment between the walls of the GE fluidic channel with Ångströms height on the resistance to water transport through the channel. Two types of misalignments are studied, namely, translational and rotational misalignments. Our results show that the relative misalignment of the GE lattices can lead to a substantial reduction in the friction between water and the channel walls. Moreover, a dependence of the friction on the degree of misalignment and flow direction is found for the cases with translational misalignment. In contrast, the resistance exerted by the channels with rotational misalignment is found to be independent of the rotation angle (θ) for 0° < θ < 60° but always lower than the perfectly aligned case. We associate such lowering of the resistance to water transport to the corrugation and the anisotropy in the corresponding potential energy landscape associated with each degree of misalignment. The findings, therefore, point to an unprecedented possibility of significantly enhancing the water transport in Ångströms height GE channels by engineering the misalignments of the GE channel walls.

In contrast to microdroplet condensation with high contact angles, the one with low contact angles remains unclear. In this study, we investigated dynamics of microdroplet condensation of low-surface-tension liquids on two flat substrate surfaces by using reflection interference confocal microscopy. Spontaneous migration toward relatively larger droplets was first observed for the microdroplets nucleated on the hydrophilic quartz surface. The moving microdroplets showed a contact angle hysteresis of ∼0.5°, which is much lower than the values observed on typical flat substrates and is within the range observed on slippery lubricant-infused porous surfaces. Because the microdroplets on the hydrophobic polydimethylsiloxane surface did not move, we concluded that the ultrathin precursor film is formed only on the hydrophilic surface, which reduces a resistive force to migration. Also, reduced size of droplets promotes the thermocapillary motion, which is induced by a gradient in local temperature inside a small microdroplet arising due to the difference in size of adjacent droplets.

When a liquid drop sits on an atomically thin layer of a 2D van der Waals (vdW) solid (like graphene) supported by a hydrophilic material, it is possible that the drop demonstrates an equilibrium contact angle that is influenced by this underlying hydrophilic material and hence is different from that observed on the bulk 2D material (e.g., graphite) surface. Such a behavior is known as the wetting translucency effect. While the wetting translucency effect of graphene has been extensively studied, the wetting translucency of hexagonal boron nitride (hBN) remains largely unexplored despite significant similarities in structural properties between these materials. In this study, we probe the wetting translucency of hBN. For this purpose, we conduct molecular dynamics simulations of water droplets and water films on hBN layers supported on a gold-like hydrophilic substrate. Our results show that for a substrate coated by monolayer hBN ("coated substrate"), depending on the contact distance between underlying substrate and hBN, an increase in the hydrophilicity of the underlying surface causes a monotonic increase in the overall adhesion energy between water and the coated substrate and a monotonic decrease in the contact angle of a drop on the coated substrate. For an increasing number of stacked hBN layers, the wettability of coated substrate becomes independent of the wettability of the underlying solid. Accordingly, our results confirm a distinct wetting translucency nature of hBN very similar to that observed in graphene.

Nanostructured van der Waals (vdW) layered materials hold great potential for achieving smart surfaces with controllable wettability. Inspired by this possibility, we conduct Molecular Dynamics (MD) simulations of the wetting of nanostructured hexagonal boron nitride (hBN) surfaces. The nanostructure consists of periodically placed nanopillars made of hBN nanoribbons. We demonstrate that the polarity effect of the nanoribbon edges triggers wetting anisotropy of the nanoribbons: the vertical edges of the nanoribbons demonstrate a different wetting behavior as compared to the flat surfaces of the nanoribbons. Depending on the nature of the edge of the nanoribbon (armchair or zigzag), these vertical edges can be more hydrophilic for the zigzag edges or more hydrophobic for the armchair edges than the flat part. Such differences ensure that the nanostructured hBN surfaces become more hydrophilic (hydrophobic) as compared to the flat non-nanostructured hBN surfaces for cases where the edges of the nanoribbon are more hydrophilic (hydrophobic) than the flat part. Overall, the present study develops a most remarkable design space where by introducing nanopillars/nanoribbons on hBN and by merely changing the nature of the edges of these nanopillars, one can ensure atomistically thin coating of hBN with a wide range of wettability.

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., (Copyright © 2019 Elsevier Inc. All rights reserved.)

The cross-stream motion of viscoelastic droplets in viscoelastic fluids has received little attention since the classical study of migration of drops in a second order fluid. In this work, going beyond the existing classical theory, we experimentally elucidate the effect of drop-to-medium viscosity ratio k and elasticity ratio ξ on wall and center migration of viscoelastic droplets in a Poiseuille flow of a viscoelastic medium (PVP) at low Reynolds numbers (Re ≪ 1). We observed a contrasting migration behavior of Newtonian and viscoelastic droplets having the same viscosity ratios and propose the presence of a lift force F VD due to the viscoelasticity of the droplet phase. We use analytical scaling and empirical modelling to show that the force F VD scales with a prefactor that depends upon the Weissenberg number Wi D and drop-to-medium viscosity ratio k and elasticity ratio ξ. Further, we utilize the proposed force for sorting of viscoelastic and Newtonian droplets.

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).

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.

We present experimental data to characterize the spreading of a liquid drop on a substrate kept submerged in another liquid medium. They reveal that drop spreading always begins in a regime dominated by drop viscosity where the spreading radius scales as r ∼ t with a nonuniversal prefactor. This initial viscous regime either lasts in its entirety or switches to an intermediate inertial regime where the spreading radius grows with time following the well-established inertial scaling of r ∼ t(1/2). This latter case depends on the characteristic viscous length scale of the problem. In either case, the final stage of spreading, close to equilibrium, follows Tanner's law. Further experiments performed on the same substrate kept in ambient air reveal a similar trend, albeit with limited spatiotemporal resolution, showing the universal nature of the spreading behavior. It is also found that, for early times of spreading, the process is similar to coalescence of two freely suspended liquid drops, making the presence of the substrate and consequently the three-phase contact line insignificant.

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.

A reconstruction methodology based on different-phase-neighbor (DPN) pixel swapping and multigrid hierarchical annealing is presented. The method performs reconstructions by starting at a coarse image and successively refining it. The DPN information is used at each refinement stage to freeze interior pixels of preformed structures. This preserves the large-scale structures in refined images and also reduces the number of pixels to be swapped, thereby resulting in a decrease in the necessary computational time to reach a solution. Compared to conventional single-grid simulated annealing, this method was found to reduce the required computation time to achieve a reconstruction by around a factor of 70-90, with the potential of even higher speedups for larger reconstructions. The method is able to perform medium sized (up to 300(3) voxels) three-dimensional reconstructions with multiple correlation functions in 36-47 h.

We have derived a modified one-dimensional lubrication equation to describe the early coalescence behavior of a symmetric sessile drop coalescence for under-liquid substrates, which takes into account the viscosities of both the drop and the surrounding medium. We found a time scale, which governs the process, and there exists a crossover time between the universal scaling of the bridge height growth h^{*}∼t^{*} (valid for both under-liquid and air) and a much slower bridge growth h^{*}∼t^{*}^{0.24} occurring at a later time. It is also found that the drop coalescence bridge profile has a self-similarity, which breaks up much earlier for under-liquid substrates as opposed to symmetric coalescence in air.

In this paper, we provide measurement of charge of asphaltene nanoaggregates in air using electrostatic force microscopy. We obtain the average surface charge density of the nanoaggregates as 43.7 nC/cm(2). Among the different aspects of asphaltene, one of the least known is its charge and the effect of solvent and compositional variability (of asphaltene) in dictating this charge. For aqueous systems, asphaltene charge demonstrates a strong dependence on the pH and the salt concentration, indicating that a possible ionization of the surface groups leads to this charging. On the contrary, for asphaltene in nonpolar media (e.g., toluene and heptane), it is believed that asphaltene native charge is central in dictating this charging. This native charge is the solvent-independent charge or the asphaltene charge in air. Our measurements, therefore, provide the first direct quantification (i.e., a quantification of charge not from the measurement of the asphaltene mobilities, which in turn requires specification of the nonuniform asphaltene size distribution) of this asphaltene native charge by conducting the measurements in air. Similar measurements in a solvent may introduce a solvent-dependent value, thereby forbidding not only the exact quantification of this native charge but also the understanding of the specific role of the solvent. This measurement, therefore, will provide a useful starting point to quantify the mechanism of asphaltene charging in nonpolar solvents with important ramifications in deciphering the role of asphaltene in transport and handling of crude and heavy oils.

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.

Simulations have been performed using the free-energy binary-liquid lattice Boltzmann method with sufficient resolution that the critical capillary number for coalescence was determined for collisions between droplets in simple shear with a small initial offset in the shear gradient direction. The simulations were used to study the behavior of the interacting interfaces and the film between them during collisions over a wide range of capillary numbers with emphasis on near-critical conditions. From these three-dimensional simulations with deforming interfaces, several features of the evolution of the film between the drops were observed. The critical film thickness was determined to be similar to the interface thickness, a power law described the dependence of the minimum film thickness on the capillary number in collisions without coalescece, and an inflection point was found in the dynamics of the minimum distance between drops that eventually coalesce. The rotation of the film and the flow in it were also studied, and a reversal in the flow was found to occur before coalescence. The mobility of the phase field was therefore important in the continued thinning of the film at the points of minimum thickness after the flow reversal. A comparison of the critical capillary number and critical film thickness in the simulations with the values for experiments in confined simple shear indicated that the effective physical radius of the simulated droplets was on the order of several micrometers. The results are significant for simulations of droplet interactions and emulsion flows in complex geometries and turbulence because they demonstrate the necessary scale of the computations and how parameters, such as the interface thickness and phase field mobility, should be selected for accurate results.

We provide an analytical model to describe the filling dynamics of horizontal cylindrical capillaries having charged walls. The presence of surface charge leads to two distinct effects: It leads to a retarding electrical force on the liquid column and also causes a reduced viscous drag force because of decreased velocity gradients at the wall. Both these effects essentially stem from the spontaneous formation of an electric double layer (EDL) and the resulting streaming potential caused by the net capillary-flow-driven advection of ionic species within the EDL. Our results demonstrate that filling of charged capillaries also exhibits the well-known linear and Washburn regimes witnessed for uncharged capillaries, although the filling rate is always lower than that of the uncharged capillary. We attribute this to a competitive success of the lowering of the driving forces (because of electroviscous effects), in comparison to the effect of weaker drag forces. We further reveal that the time at which the transition between the linear and the Washburn regime occurs may become significantly altered with the introduction of surface charges, thereby altering the resultant capillary dynamics in a rather intricate manner.

A reconstruction methodology based on threshold energy based energy minimization (TA) and different-phase-neighbor (DPN)-based pixel swapping is presented. The TA method uses an energy threshold rather than probabilities as an acceptance criteria for annealing steps. The DPN-based pixel selection method gives priority to pixels which are segregated from clusters instead of random selection. An in-house solver has been developed to obtain two-dimensional reconstructions of heterogeneous two-phase mediums. Compared to conventional simulated annealing with random pixel swapping, the proposed method was found to achieve an optimal structure with up to an order of magnitude reduction in energy. When selecting a threshold tolerance value, the proposed method showed a 50% improvement in convergence time compared to conventional simulated annealing with random pixel swapping. The improved algorithm is used to study the effect of multiple correlation functions during the reconstruction. It was found that a combination of two-point correlation function and lineal path function for both phases results in most accurate reconstructions.

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.

We use atomic force microscopy to observe the structural changes in petroleum-asphaltene aggregates in air as a function of temperature. The aggregates are obtained by evaporating a toluene solution containing asphaltene. Increase in temperature leads to transition from self-assembled fractal structures to substantially larger mobile "liquid-like" domains that show distinct tendencies of substrate repulsion and self-coalescence. This new aggregation dynamics of asphaltene can be explained by temperature-induced transition of asphaltene from pure amorphous to liquid crystalline phase. Observation of this new phenomenon for asphaltene will have wide implications for asphaltene handling and separation.

In this paper, we unravel new scaling regimes for streaming potential and electroviscous effects in a nanocapillary with thick overlapping Electric Double Layers (EDLs). We observe that the streaming potential, for a given value of the capillary zeta (ζ) potential, varies with the EDL thickness and a dimensionless parameter R, quantifying the conduction current. Depending on the value of R, variation of the streaming potential with the EDL thickness demonstrates distinct scaling regimes: one can witness a Quadratic Regime where the streaming potential varies as the square of the EDL thickness, a Weak Regime where the streaming potential shows a weaker variation with the EDL thickness, and a Saturation Regime where the streaming potential ceases to vary with the EDL thickness. Effective viscosity, characterizing the electroviscous effect, obeys the variation of the streaming potential for smaller EDL thickness values; however, for larger EDL thickness the electroosmotic flow profile dictates the electroviscous effect, with insignificant contribution of the streaming potential., (Copyright © 2013 Elsevier B.V. All rights reserved.)

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.

In this paper, we provide a theory to pinpoint the role of electric double layer (EDL) interactions in governing the contact angle of an electrolyte drop on a charged solid in air or a bubble on a charged surface within an electrolyte solution. The EDL interactions are analytically solved by representing the three phase contact line as a wedge edge, with the wedge being formed by the solid-liquid and the air-liquid interfaces, and calculating the corresponding Maxwell stresses. We demonstrate that the EDL effects induce an "electrowetting-like" behavior, resulting in a lowering of the contact angle. As a specific example, we use this model to analyze the effect of added salt on preformed surface nanobubbles, and find, in contrast to what has been reported earlier, that even for most moderate conditions, added salt may have remarkable effect in altering the contact angle in preformed surface nanobubbles.