Nanoparticle self-structuring in a nanofluid film spreading on a solid surface

On the role of disjoining pressure in nanofluid-assisted enhanced oil recovery: a mini-review

Nanofluid application in enhanced oil recovery (EOR) recently emerged and garnered significant attention within the field. Nanofluids possess unique properties of reducing oil-water interfacial tension, stabilizing emulsions, altering rock surface wettability, and enhancing disjoining pressure between crude oil and rock surfaces, therefore have potential for the oil recovery process. This review provides an in-depth exploration of various aspects related to nanofluids in EOR. Different types of nanofluids are presented with their preparation methods and representative properties. More importantly, the disjoining pressure, a key physical concept in nanofluid-assisted EOR, is introduced and discussed in terms of the mechanism of oil displacement and measurement methods. Further understanding the role of disjoining pressure in nanofluid-assisted oil displacement is necessary for the development and application of effective nanofluids for EOR.

Copper Quantum Dot/Polyacrylamide Composite Nanospheres: Spreading on Quartz Flake Surfaces and Displacing Crude Oil in Microchannel Chips

Polyacrylamide, silica, and other nanoparticles have all been realized in the field of enhanced oil recovery. Researchers often explore the mechanisms of spreading behavior and simulated displacement to develop more efficient types of nanoparticles. In this study, copper quantum dots were introduced into a acrylamide copolymerization system to obtain composite nanospheres and its structure, topographic, and application performance were characterized. The results show that the composite nanospheres have a particle size of around 25 nm, are uniformly loaded with copper particles, and have good temperature resistance. The spreading ability on the quartz flake surfaces and displacement effect in microchannels of composite nanospheres, acrylamide copolymer nanospheres, and copper quantum dots were compared by nanofluid spreading experiments and microchannel chip oil displacement experiments. The results indicate that the composite nanospheres can effectively reduce the water contact angle, promote the spreading of aqueous phase, and accelerate the oil droplet removal process; the accelerating effect is stronger than other samples. Its oil displacement effect is also the strongest, and it is minimized by the influence of channel size, temperature, and dispersing medium, with better stratigraphic adaptability. This work supports the practical application of copper quantum dot/polyacrylamide composite nanospheres in the oilfield.

DLVO surface forces in liquid films and statistical mechanics of colloidal oscillatory structural forces in dispersion stability

This paper focuses on the theory of the dispersion stability considering two models. In the classical DLVO model of surface forces, the interactions between two particles consist of two terms: the London-van der Waals attractive interaction and the electrostatic repulsive interaction in the frame of the Debye-Hückel theory. The solvent, the aqueous solution of the electrolyte, was considered the continuous phase. The film stability criteria are P_γ > Π and dP_γ/dh > 0. Henderson and Lozada-Cassou (HC) applied the statistical mechanics approach to calculate the film free energy to predict the dispersion stability by considering two large hard spheres as colloidal particles immersed in a fluid of dispersed small particles (the solvent). HC applied the radial distribution function g(r) to calculate the free oscillatory structural energy using W(r) = - kT ln g(r). HC's theoretical approach was also applied to the particle collective interactions in the film and explains the stability of film formed from complex fluids (e.g., micellar and colloidal dispersions). The differences between the solvation oscillatory layering forces and colloidal oscillatory structural forces are discussed. The application of the DLVO model to the dispersion stability is critically reviewed. The role of nanobubbles in the dispersion stability is discussed.

Pore-Scale Study of Wettability Alteration and Fluid Flow in Propped Fractures of Ultra-Tight Carbonates

The in situ change in oil flow behavior inside propped fractures due to wettability alteration of proppant grains and fracture surfaces was thoroughly investigated for the first time in this study. A series of microscale flow experiments were performed in mixed-wet fractured and propped miniature ultra-tight carbonate cores where the effect of wettability on oil bridging and fracture oil layer integrity was probed during oil production. During the initial production, proppant wettability changed toward an intermediate-wet state (contact angle (CA) = 96°) while that of fracture surfaces became strongly oil-wet (CA = 139°). Consequently, the fracture oil layer grew in size on both fracture surfaces and imbibed into the proppant pack through piston-like displacement and pore body filling until oil bridges were formed during oil injection. However, subsequent waterflooding induced thinning and rupturing of those bridges due to the accompanying reduction in the threshold capillary pressure of the proppant at higher aging times. The in situ chemical treatment of the proppant by a cationic surfactant (dodecyl tri-methyl ammonium bromide) could reverse its wettability toward weakly water-wet state (CA = 78°) after oil solubilization from the sand grains followed by substitutive surfactant adsorption. Surfactant injection also impacted the wettability of the fracture surface due to oil solubilization, reducing its mean contact angle down to an intermediate range (CA = 99°). As a result, the following oil production cycle yielded a smaller fracture oil layer. The surfactant effect on proppant wettability lasted for 2 weeks while its effect on fracture wettability lasted for more than 6 weeks. Similar flow cycles were performed with an anionic nanoparticle (graphene quantum dot) with hydrogen bonding ability. The nanoparticle solution yielded a quick reduction of the proppant and fracture surface contact angles to nearly 77° and 115°, respectively. Proppant wettability alteration occurred because the nanoparticles self-assembled at the three-point contact region between adsorbed oil and quartz surfaces, leading to oil solubilization in intermediate-wet regions while oil-wet regions remained unchanged. Therefore, re-introducing oil into the fracture instantaneously re-instated the initial wettability state of proppant grains (CA = 88°), deeming the nanoparticle solution ineffective. This study revealed that oil production through hydraulic fractures can be enhanced by monitoring the wettability of the proppant pack. If the production has a high water cut, it is beneficial to use chemical agents that reduce the proppant contact angles to a weakly water-wet state in order to preserve the hydraulic conductivity of the oil layer.

Multiphase displacement manipulated by micro/nanoparticle suspensions in porous media via microfluidic experiments: From interface science to multiphase flow patterns

Multiphase displacement in porous media can be adjusted by micro/nanoparticle suspensions, which is widespread in many scientific and industrial contexts. Direct visualization of suspension flow dynamics and corresponding multiphase patterns is crucial to understanding displacement mechanisms and eventually optimizing these processes in geological, biological, chemical, and other engineering systems. However, suspension flow inside the opaque realistic porous media makes direct observation challenging. The advances in microfluidic experiments have provided us with alternative methods to observe suspension influence on the interface and multiphase flow behaviors at high temporal and spatial resolutions. Macroscale processes are controlled by microscale interfacial behaviors, which are affected by multiple physical factors, such as particle adsorption, capillarity, and hydrodynamics. These properties exerted on the suspension flow in porous media may lead to interesting interfacial phenomena and new displacement consequences. As an underpinning science, understanding and controlling the suspension transport process from interface to flow patterns in porous media is critical for a lower operating cost to improve resource production while reducing harmful emissions and other environmental impacts. This review summarizes the basic properties of different micro/nanoparticle suspensions and the state-of-the-art microfluidic techniques for displacement research activities in porous media. Various suspension transport behaviors and displacement mechanisms explored by microfluidic experiments are comprehensively reviewed. This review is expected to boost both experimental and theoretical understanding of suspension transport and interfacial interaction processes in porous media. It also brings forward the challenges and opportunities for future research in controlling complex fluid flow in porous media for diverse applications.

Nanofluid Structural Forces Alter Solid Wetting, Enhancing Oil Recovery

Nanofluids have attracted significant research interest for their promising application in enhanced oil recovery. One striking feature leading to the outstanding efficiency of nanofluids in enhanced oil recovery is the structure of nanoparticles, which induces oscillatory structural forces in the confined space between fluid–fluid interfaces or air–liquid and liquid–solid interfaces. To promote the understanding of the oscillatory structural forces and their application in enhanced oil recovery, we reviewed the origin and theory of the oscillatory structural forces, factors affecting their magnitude, and the experimental techniques demonstrating their impacts on enhanced oil recovery. We also reviewed the methods, where the benefits of nanofluids in enhanced oil recovery provided by the oscillatory structural forces are directly manifested. The oscillatory structural forces promote the wetting and spreading of nanofluids on solid surfaces, which ultimately enhances the separation of oil from the reservoir. Some imbibition tests demonstrated as much as 50% increased oil recovery, compared to the cases where the oscillatory structural forces were absent.

Wetting Dynamics of Nanoparticle Dispersions: From Fully Spreading to Non-sticking and the Deposition of Nanoparticle-Laden Surface Droplets

Controlled transport of liquid droplets on solid surfaces is critical in many practical applications, such as self-cleaning surfaces, coating, drug delivery, and agriculture. Non-adhesive liquid drops levitate on solid surfaces; therefore, they are highly mobile and directed toward desired locations by external stimuli. Although research on liquid-repellent surfaces has proliferated, the existing methods are still limited to creating surface roughness or coating the liquid droplets. Here, we create non-contact aqueous drops on hydrophilic surfaces in an oleic environment and use them to deposit submicrometer droplets encapsulating nanoparticles on solid surfaces. A glass surface is buried under an oil phase that contains a high concentration of Span 80 surfactants, and a drop of silica nanoparticle dispersion is released on the solid surface. We study the effect of surfactant concentration in oil and nanoparticle concentration in water on wetting dynamics and report a plethora of droplet spreading regimes from fully wetting to non-wetting. We find a threshold Span 80 concentration above which surfactant assemblies are formed on the solid and prevent the direct contact of the drop with the surface. At the same time, water-in-oil emulsions are generated at the drop-oil interface. The drop moves and leaves a trace of emulsions with encapsulated nanoparticles on the solid. We demonstrate the possibility of local surface coating with hydrophilic nanoparticles in a hydrophobic medium. The developed methodology in this study is a generic approach facilitating the droplet patterning in numerous applications, from pharmaceutical polymetric carriers to the formulation of cosmetics, insecticides, and biomedical diagnoses.

The foam film's stepwise thinning phenomenon and role of oscillatory forces

As a foam film formed from complex fluids thins, the particles under the film confinement self-organize into layers. Reflected light was used to monitor the rate of layer-by-layer thinning and the layers' thickness. The microscopic and macroscopic films thin using the same stepwise manner (stratify), via layers or stripes with equal thicknesses. The roles of the film area (size) and film capillary pressure on the film stepwise thinning were studied. A micron-sized dot with a thickness one layer less than that of the surrounding film area is observed. The dot expands into a spot when the film reaches the critical area. The 2D dot-spot exhibits a threshold process. The spot expands and the film's stepwise thinning begins. When the film area is reduced, the spot stops expanding and begins to reduce in size. The film slowly recovers its original thickness in a stepwise manner, one layer at a time. It was demonstrated that the film area is the governing factor in the film stepwise thinning rather than the film capillary pressure. A particle dislocation-diffusion-osmotic pressure model is proposed to explain the mechanism of the film stepwise thinning phenomenon via dot-spot formation. The model explains all the features of the foam stepwise thinning phenomenon, including the reversibility of the film's stepwise thinning. For the first time for a film with a thickness less than three layers, a 2D in-layer hexagonal particle entropy structural transition was observed and theoretically predicted by the analysis of the Radial Distribution Function (RDF).

Multilayer-Stabilized Water-in-Water Emulsions

Water-in-water (W/W) emulsions are of interest for various applications due to their inherent biocompatibility, ultralow interfacial tensions, and large interface thickness. However, it is still challenging to prepare stable W/W emulsions with tailored phase architectures compared to oil-in-water (O/W) and water-in-oil (W/O) emulsions. Here, we report a multilayer-stabilized W/W emulsion composed of poly(ethylene glycol)/dextran in the presence of DNA strands. The W/W emulsions present onion-ring-like structures, which are interpreted by a nanofluid film model. Emulsion behavior, e.g., stability, interface tension, etc., can be controlled by the type of DNA (single or double strands), DNA concentration, and volume fraction of dispersed phase. Our findings could broaden the preparation of novel emulsions for potential applications in emulsion polymerization, new media of homogeneous catalysis, and DNA transportation of water-in-water media.

Magnetophoretic Control of Diamagnetic Particles Inside an Evaporating Droplet

The present study reports the magnetophoresis of diamagnetic particles in an evaporating ferrofluid droplet. Both solid and ring magnet arrangements are used to investigate the effect of magnetic field distribution. The distance of the magnet from the droplet is varied to study the effect of magnetic field strength. The magnetic field distribution is computed using COMSOL multiphysics software. Magnetometer measurements have been carried out to validate the simulation results. The motion of particles and the drying pattern of evaporating ferrofluid droplets are visualized using the confocal microscopy technique. Both bright-field and fluorescence imagings have been carried out to observe the differential deposition of the fluorescent particle (microparticle) and magnetic nanoparticles in the absence and presence of a magnetic field. The velocity of diamagnetic particles as a function of magnetic field distribution and strength has been studied using the micro-PIV technique. In the absence of the magnetic field, a ring-shaped deposition pattern is observed. The mixture of microparticles (diamagnetic) and nanoparticles (magnetic) is deposited between the outer and inner edges of the ring. The diamagnetic particles occupy the inner and outer edges of the ring. Magnetic particles travel toward the higher magnetic field zone and diamagnetic particles move toward the smaller magnetic field zone when a magnetic field is applied by a solid magnet placed over the droplet. This can be attributed to the negative magnetic force originating from the difference between the susceptibility of magnetic and nonmagnetic particles. The negative magnetic force on the microparticle increases as the magnetic field intensity increases, causing the microparticle to convect faster toward the contact line. The deposition behavior can be reversed or suppressed using a ring magnet in place of a solid magnet. In this case, the negative magnetic force is stronger at the contact line region of the droplet and decreases as it approaches the center region of the droplet. The deposition behavior of diamagnetic particle depends on the balance between the Marangoni force and the magnetophoretic force. Overall, the present study demonstrates the capability of the controlled deposition of diamagnetic polystyrene particles by suitable arrangement of the solid and ring magnet.

Direct Pore-Level Visualization and Verification of In Situ Oil-in-Water Pickering Emulsification during Polymeric Nanogel Flooding for EOR in a Transparent Three-Dimensional Micromodel

Different from inorganic nanoparticles, nanosized cross-linked polymeric nanoparticles (nanogels) have been demonstrated to generate more stable Pickering emulsions under harsh conditions for a long term owing to their inherent high hydrophilicity and surface energy. In both core and pore scales, the emulsions are found to be able to form in situ during the nanofluid flooding process for an enhanced oil recovery (EOR) process. Due to the limitation of direct visualization in core scale or deficient pore geometries built by two-dimensional micromodels, the in situ emulsification by nanofluids and emulsion transport are still not being well understood. In this work, we use a three-dimensional transparent porous medium to directly visualize the in situ emulsification during the nanogel flooding process for EOR after water flooding. By synthesizing the nanogel with a fluorescent dye, we find the nanogels adsorbed on the oil-water interface to lower the total interfacial energy and emulsify the large oil droplets into small Pickering oil-in-water emulsions. A potential mechanism for in situ emulsification by nanogels is proposed and discussed. After nanogel flooding, the emulsions trapped in pore throats and those in the effluents are all found encapsulated by the nanogels. After nanogel flooding under different flow rates, the sphericity and diameter changes of remaining oil droplets are quantitatively compared and analyzed using grouped boxplots. It is concluded that in situ emulsification happens during nanogel injection due to the reduction of interfacial tension, which helps to increase the oil recovery rate under different flow rates and pore geometries.

Thermosensitive Core-Shell Fe3O4@poly(N-isopropylacrylamide) Nanogels for Enhanced Oil Recovery

An investigation on the application of thermosensitive core-shell Fe₃O₄@PNIPAM nanogels in enhanced oil recovery was successfully performed. Here, the unique core-shell architecture was fabricated by conducting the polymerization at the surface of 3-butenoic acid-functionalized Fe₃O₄ nanoparticles and characterized using X-ray diffraction (XRD), ¹H NMR, vibration sample magnetometer (VSM), and high-resolution transmission electron microscopy (HR-TEM). According to the results, this core-shell structure was beneficial for achieving the desired high viscosity and low nanofluid mobility ratio at high temperatures, which is essential for enhanced oil recovery (EOR) application. The results demonstrated that the nanogels exhibited a unique temperature-dependent flow behavior due to the PNIPAM shell's ability to transform from a hydrated to a dehydrated state above its low critical solution temperature (LCST). At such conditions, the nanogels exhibited a significantly low mobility ratio (M = 0.86), resulting in an even displacement front during EOR and leads to higher oil production. Based on the result obtained from sand pack flooding, about 25.75% of an additional secondary oil recovery could be produced when the nanofluid was injected at a temperature of 45 °C. However, a further increase in the flooding temperature could result in a slight reduction in oil recovery due to the precipitation of some of the severely aggregated nanogels at high temperatures.

Research on the mechanisms of polyacrylamide nanospheres with different size distributions in enhanced oil recovery

Crosslinked polyacrylamide microspheres are widely used as in-depth flooding agents in petroleum development due to their unique properties of thickening, salt-resistance, high-temperature resistance, low cost, etc. To solve the problem of their injections in heterogeneous reservoirs, polyacrylamide nanospheres were synthesized. However, mechanisms of polymer nanospheres in enhanced oil recovery were not investigated comprehensively. In this study, we synthesized polymer nanospheres with different size distributions and studied their mechanisms in enhancing the oil recovery. First, the effects of polyacrylamide nanospheres in enhanced oil recovery of heterogeneous sand-packed tubes was explored by sand-packed tube oil displacement experiments. Second, the rheological properties of polyacrylamide nanosphere dispersion were explored using a rheometer. Third, through the visual microchannel experiment, the mechanism of polymer nanosphere emulsion on the removal of the residual oil film was explored. Finally, through the crude oil removal experiment, it was found that polymer nanospheres with a particle size of about 54 nm can cooperate with surfactants to accelerate the removal of oil droplets.

In this study, we synthesized polyacrylamide nanospheres with different size distributions and found that the oil displacement mechanisms of polyacrylamide nanospheres are composed of three aspects.

Gas-Wetting Alteration by Fluorochemicals and Its Application for Enhancing Gas Recovery in Gas-Condensate Reservoirs: A Review

Gas-wetting alteration is a versatile and effective approach for alleviating liquid-blockage that occurs when the wellbore pressure of a gas-condensate reservoir drops below the dew point. Fluorochemicals are of growing interest in gas-wetting alteration because of their high density of fluorine groups and thermal stability, which can change the reservoir wettability into more favorable conditions for liquids. This review aims to integrate the overlapping research between the current knowledge in organic chemistry and enhanced oil and gas recovery. The difference between wettability alteration and gas-wetting alteration is illustrated, and the methods used to evaluate gas-wetting are summarized. Recent advances in the applications of fluorochemicals for gas-wetting alteration are highlighted. The mechanisms of self-assembling adsorption layers formed by fluorochemicals with different surface morphologies are also reviewed. The factors that affect the gas-wetting performance of fluorochemicals are summarized. Meanwhile, the impacts of gas-wetting alteration on the migration of fluids in the pore throat are elaborated. Furthermore, the Wenzel and Cassie-Baxter theories are often used to describe the wettability model, but they are limited in reflecting the wetting regime of the gas-wetting surface; therefore, a wettability model for gas-wetting is discussed. Considering the promising prospects of gas-wetting alteration, this study is expected to provide insights into the relevance of gas-wetting, surface morphology and fluorochemicals, further exploring the mechanism of flow efficiency improvement of fluids in unconventional oil and gas reservoirs.

Viscoelastic interfaces comprising of cellulose nanocrystals and lauroyl ethyl arginate for enhanced foam stability

Stable aqueous foams composed of oppositely charged nanoparticles and surfactants have recently gained attention. We studied the draining of thin liquid films and the foam stability of aqueous mixtures of food grade cellulose nanocrystals (CNCs) and an oppositely charged surfactant - lauroyl ethyl arginate (LAE). Dynamic fluid film interferometry experiments with the bubble approaching the air/solution interface revealed a two-fold increase of the initial bubble film thickness and a maximum in drainage time at the optimal stoichiometry of LAE and CNC. The temporal evolution of the fluid film shape indicated a large contribution of structural forces to the film stability. The results of single liquid film drainage time and coalescence time experiments were partially correlated with bulk foam stability. With a further increase of LAE concentration, aggregation-induced foam destruction was observed. In the presence of a cationic surfactant, anisotropic and initially hydrophilic cellulose nanocrystals became partially hydrophobized and self-assembled at the interface. Cellulose nanocrystal shape anisotropy and wetting behaviour which have their origins in OH-exposed and buried crystalline planes are the sources of capillary interactions that promote CNC aggregation at planar and curved liquid/air interfaces. Dilatational and shear interfacial rheology experiments confirmed the formation of a highly elastic surfactant-nanoparticle interfacial layer. To the best of our knowledge, this is the first report on foaming properties for this system with fast adsorption kinetics influenced by CNC.

Surface-Functionalized Superparamagnetic Nanoparticles (SPNs) for Enhanced Oil Recovery: Effects of Surface Modifiers and Their Architectures

Superparamagnetic nanoparticles (SPNs) have been considered as one of the most studied nanomaterials for subsurface applications, including in enhanced oil recovery (EOR), due to their unique physicochemical properties. However, a comprehensive understanding of the effect of surface functionalization on the ability of the nanoparticles to improve secondary and tertiary oil recoveries remains unclear. Therefore, investigations on the application of bare and surface-functionalized SPNs in EOR using a sand pack were carried out in this study. Here, the as-prepared SPNs were functionalized using oleic acid (OA) and polyacrylamide (PAM) to obtain several types of nanostructure architectures such as OA-SPN, core-shell SPN@PAM, and SPN-PAM. Based on the result, it is found that both the viscosity and mobility of the nanofluids were significantly affected by not only the concentration of the nanoparticles but also the type and architecture of the surface modifier, which dictated particle hydrophilicity. According to the sand pack tests, the nanofluid containing SPN-PAM was able to recover as much as 19.28% of additional oil in a relatively low concentration (0.9% w/v). The high oil recovery enhancement was presumably due to the ability of suspended SPN-PAM to act as a mobility control and wettability alteration agent and facilitate the formation of a Pickering emulsion and disjoining pressure.

Structure and stability of nanofluid films wetting solids: An overview

When an air bubble or an oil droplet in a nanofluid (liquid containing dispersed nanoparticles) approaches a solid surface, a nanofluid film is formed between the bubble or drop and a solid substrate. The nanoparticles confined in the film surfaces tend to self-layer and the film thins in a stepwise manner. The wetting behavior and film stability criteria valid for the classical molecularly thin films cannot be applied to nanofilm. Here we present an overview of the structure and stability of multilayer nanofilms wetting solid surfaces. We first present a brief review of the classical concept of molecular films wetting solid, and then we discuss the nanofluid film structure evolution as determined by the in-layer radial distribution function versus nanofilm's number of layers. The role of the particle volume fraction, size and polydispersity on the layering phenomenon is highlighted. The stability of the nanofilm, that is its layer-by-layer thinning is elucidated by the presence of particle voids or dislocations. We calculated the free energy of the nanofilm on a solid surface based on nanofilm osmotic pressure. We independently verified it by the direct measurement of the nanofilm-meniscus contact angle using reflected light interferometry. Finally, we present some practical applications of a wetting aqueous film for oily soil removal from a solid surface and the nanofilm displacing an oil phase from a capillary as in an enhanced oil recovery operation.

Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery

During the past half-century, the prefix “nano” attached to several words, such as “technology”, “motors”, “device”, and so on has denoted cutting-edge research fields and topics at the forefront of classical scientific disciplines. Possible application fields have been frequently evoked, even if real-life examples are still difficult to find. The present review analyzes how nanotechnology is utilized in enhanced oil recovery (EOR) processes so as to increase the efficiency of mature oilfields. Nanotechnology in EOR is classified into three categories: nanoparticles/nanofluids, nanoemulsions, and nanocatalysts. The advantages at the nanoscale are also described and discussed, including an overview of manufacturing methods as well as the concerns about their possible environmental impacts. Clearly, nanotechnology has the potential to boost EOR techniques, although there are still many questions and drawbacks to be tackled.

Field-Assisted Contact Line Motion in Thin Films

The balance of intermolecular and surface forces plays a critical role in the transport phenomena near the contact line region of an extended meniscus in several technologically important processes. Externally applied fields can alter the equilibrium and stability of the meniscus with concomitant effects on its shape and spreading characteristics and may even lead to an oscillation. This feature article provides a detailed account of the present and past efforts in exploring the behavior of curved thin liquid films subjected to mild thermal perturbations, heat input, and electrical and magnetic fields for pure as well as colloidal suspensions, including the effects of particle charge and polarity. The shape-dependent intermolecular force field has been evaluated in situ by a nonobtrusive optical technique utilizing the interference phenomena and subsequent image processing. The critical role of disjoining pressure is identified along with the determination of the Hamaker constant. The spatial and temporal variations of the capillary forces are evaluated for the advancing and receding menisci. The Maxwell-stress-induced enhanced spreading during electrowetting, at relatively low voltages, and that due to the application of a magnetic field are discussed with respect to their distinctly different characteristics and application potentials. The use of the augmented Young-Laplace equation elicited additional insights into the fundamental physics for flow in ultrathin liquid films.

Externally Triggered Oscillatory Structural Forces

This paper addresses triggering of oscillatory structural forces via temperature variation across an aqueous dispersion of thermoresponsive poly( N-isopropylacrylamide) (PNIPAM) nanogels confined between silica surfaces. Oscillatory structural forces are a well-known phenomenon in colloidal science, caused by interactions between molecules or colloids. Modulation of these forces usually requires changing the internal parameters of the dispersion, such as ionic strength, particle concentration, and surface charge, or changing the properties of the confining walls, such as surface roughness, potential, or elasticity. All of these parameters are usually fixed and can only be changed via exchange of the sample or the complete experimental setup. Here, a new approach is presented, combining the characteristics of smart materials with the properties of nanoparticles, using negatively charged PNIPAM nanogels. Aqueous dispersions of these nanogels express no oscillatory structural forces in the initial state (20 °C), below the volume phase transition temperature (32 °C). Heating (60 °C) reduces the nanogel size and leads to a more negative ζ-potential, which triggers the onset of oscillatory structural forces.

Discrete and Continuum Models for the Salt in Crowded Environments of Suspended Charged Particles

Electrostatic forces greatly affect the overall dynamics and diffusional activities of suspended charged particles in crowded environments. Accordingly, the concentration of counter- or co-ions in a fluid-''the salt"-determines the range, strength, and order of electrostatic interactions between particles. This environment fosters engineering routes for controlling directed assembly of particles at both the micro- and nanoscale. Here, we analyzed two computational modeling schemes that considered salt within suspensions of charged particles, or polyelectrolytes: discrete and continuum. Electrostatic interactions were included through a Green's function formalism, where the confined fundamental solution for Poisson's equation is resolved by the general geometry Ewald-like method. For the discrete model, the salt was considered as regularized point-charges with a specific valence and size, while concentration fields were defined for each ionic species for the continuum model. These considerations were evolved using Brownian dynamics of the suspended charged particles and the discrete salt ions, while a convection-diffusion transport equation, including the Nernst-Planck diffusion mechanism, accounted for the dynamics of the concentration fields. The salt/particle models were considered as suspensions under slit-confinement conditions for creating crowded "macro-ions", where density distributions and radial distribution functions were used to compare and differentiate computational models. Importantly, our analysis shows that disparate length scales or increased system size presented by the salt and suspended particles are best dealt with using concentration fields to model the ions. These findings were then validated by novel simulations of a semipermeable polyelectrolyte membrane, at the mesoscale, from which ionic channels emerged and enable ion conduction.

Estimation of structural film viscosity based on the bubble rise method in a nanofluid

When a single bubble moves at a very low capillary number (10^-7) through a liquid with dispersed nanoparticles (nanofluid) inside a vertical tube/capillary, a film is formed between the bubble surface and the tube wall and the nanoparticles self-layer inside the confined film. We measured the film thickness using reflected light interferometry. We calculated the film structural energy isotherm vs. the film thickness from the film-meniscus contact angle measurements using the reflected light interferometric method. Based on the experimental measurement of the film thickness and the calculated values of the film structural energy barrier, we estimated the structural film viscosity vs. the film thickness using the Frenkel approach. Because of the nanoparticle film self-layering phenomenon, we observed a gradual increase in the film viscosity with the decreasing film thickness. However, we observed a significant increase in the film viscosity accompanied by a step-wise decrease in the bubble velocity when the film thickness decreased from 3 to 2 particle layers due to the structural transition in the film.

Oil lenses on the air-water surface and the validity of Neumann's rule

Many studies have focused on the mechanisms of oil spreading over the air-water surface, oil lens formation, and lens dynamics: Franklin et al.(1774), Rayleigh (1890), Neumann and Wangerin (1894), Hardy (1912), Lyons (1930), Langmuir (1933), Miller (1941), Zisman (1941), Pujado and Scriven (1972), Seeto et al. (1983), and Takamura et al. (2012). Despite all of these studies, the phenomenon of the oil lens's air-water surface equilibrium is still under discussion. Here, we highlight an accurate method to study the oil lens's three-phase-contact angle by reflected light interferometry, using both common (CRLI) and differential reflected light interferometry (DRLI) to verify Neumann's rule (the vectorial sum of the three tensions is zero). For non-spreading oils, the validity of Neumann's rule is confirmed for small lenses when the role of the oil film tension around the lens's meniscus is taken into consideration. Neumann's rule was also validated when the monolayer surface pressure isotherm was taken into consideration for oil spreading on the air-water surface. The periodic monolayer surface pressure oscillation of the oil phase monolayer created by the air-evaporating biphilic oil was monitored with time. The monolayer's surface pressure periodic oscillation was attributed to the instability of the aqueous film covering the oil drop phase. The knowledge gained from this study will benefit the fundamental understanding of the oil lens's air-water surface equilibrium and oil spill mechanisms, thereby promoting better methods for the prevention and clean-up of oil spills.

Step-Wise Velocity of an Air Bubble Rising in a Vertical Tube Filled with a Liquid Dispersion of Nanoparticles

The motion of air bubbles in tubes filled with aqueous suspensions of nanoparticles (nanofluids) is of practical interest for bubble jets, lab-on-a-chip, and transporting media. Therefore, the focus of this study is the dynamics of air bubbles rising in a tube in a nanofluid. Many authors experimentally and analytically proposed that the velocity of rising air bubbles is constant for long air bubbles suspended in a vertical tube in common liquids (e.g. an aqueous glycerol solution) when the capillary number is larger than 10^-4. For the first time, we report here a systematic study of an air bubble rising in a vertical tube in a nanofluid (e.g. an aqueous silica dioxide nanoparticle suspension, nominal particle size, 19 nm). We varied the bubble length scaled by the diameter of the tubes (L/D), the concentration of the nanofluid (10 and 12.5 v %), and the tube diameter (0.45, 0.47, and 0.50 cm). The presence of the nanoparticles creates a significant change in the bubble velocity compared with the bubble rising in the common liquid with the same bulk viscosity. We observed a novel phenomenon of a step-wise increase in the air bubble rising velocity versus bubble length for small capillary numbers less than 10^-7. This step-wise velocity increase versus the bubble length was not observed in a common fluid. The step-wise velocity increase is attributed to the nanoparticle self-layering phenomenon in the film adjacent to the tube wall. To elucidate the role of the nanoparticle film self-layering on the bubble rising velocity, the effect of the capillary number, the tube diameter (e.g. the capillary pressure), and nanofilm viscosity are investigated. We propose a model that takes into consideration the nanoparticle layering in the film confinement to explain the step-wise velocity phenomenon versus the length of the bubble. The oscillatory film interaction energy isotherm is calculated and the Frenkel approach is used to estimate the film viscosity.

Escherichia coli removal from model substrates: Underlying mechanism based on nanofluid structural forces

Understanding the interactions between bacteria and solid surfaces that result in bacterial adhesion and removal is of immense importance for reducing foodborne illness outbreaks. A nanofluid formulation comprised of a sodium dodecyl sulfate (SDS) micellar aqueous solution in the presence of an organic acid (as a pH controller) was used to test the E. coli K12 removal from two substrates, polyvinylchloride (PVC) and partially hydrophobic glass. We investigated the bacterial removal efficacy based on the combined effect of the nanofluid's structural forces and bacterial isoelectric point. To quantify the bacteria-PVC coverage, we used fluorescence microscope. The Langmuir isotherm at the low volume fraction was applied to estimate the adsorption energy of E. coli K12. We obtained a value of about 2.5±0.2kT. This value compared favorably with the value of 2.1kT reported previously for E. coli NCTC 9002 (Vanloosdrecht et al., 1989). We applied the dynamic light scattering method to estimate the radius of the gyration of E. coli K12. The radius of the gyration was used to estimate the limit of surface area covered by the bacterium and compared it to the surface area measured from the image taken with fluorescence microscope. We found that they are in good agreement with each other. We modeled the nanofluid oscillatory structural energy against the E. coli K12 adsorption energy by applying the statistical mechanics approach. Based on the model prediction, the oscillatory interaction energy was estimated at the vertex between a bacterium and the substrate (i.e., the wedge film's interaction energy at one particle layer). The evaluated film's repulsive energy due to the oscillatory structural forces (OSF) was about 15.6±4.4kT of the 0.02M SMNF (the SDS micellar nanofluid formulation) and several times higher than the bacterial adsorption energy, 2.5±0.2kT. The OSF of the 0.06M SMNF was measured by AFM (the oscillatory decay force curve). The period and number of oscillations versus distance was annualized and used to obtain information for the effective size of the nanoparticles and nanofluid's effective volume fraction. These findings suggest that the OSF is capable of bacteria/microorganism removal from contaminated substrates.

Effect of a Single Nanoparticle on the Contact Line Motion

In this paper, we use a single nanoparticle (NP) to achieve active control of the droplet contact line. When the droplet is out of equilibrium, the resulting excess free energy provides the driving force for the depinning of the contact line and the NP. There are three ways to increase the energy barriers to be surmounted and to realize the pinning of the contact line, namely, the enhancement of the interactions between the NP and the substrate, the increase in substrate hydrophilicity, and the reduction in the NP hydrophilicity. On this basis, we obtained three styles of contact line motion including complete slipping, alternate pinning-depinning, and complete pinning and theoretically interpreted them. The basic theory presented in this paper can be applied to explain and regulate the dynamics of the contact line involved in many physical processes such as evaporation and spreading.

Stratification of a Foam Film Formed from a Nonionic Micellar Solution: Experiments and Modeling

Thin liquid films containing surfactant micelles or other nanocolloidal particles are considered to be the key structural elements of foams containing gas and liquid. We report here the experimental results and theoretical modeling for the phenomenon of the stratification (stepwise thinning) of a foam film formed from a nonionic micellar solution. The film stratification phenomenon was experimentally observed by reflected light microinterferometry. We observed that the stepwise layer-by-layer decrease of the film thickness is due to the appearance and growth of a dark spot of one layer less than the film thickness in the film. The dark spot expansion is driven by the diffusion of the dislocation (or vacancy) in the micellar lattice. The vacancies from the meniscus diffuse and condense into the dark spot, leading to its expansion inside the film. We experimentally observed the expansion of the dark spot at various film thicknesses (i.e., the number of micellar layers) and at different film sizes. We also measured the contact angle between the film and the meniscus; we used the data to estimate the structural film interaction energy barrier and the apparent diffusion coefficient. We used the two-dimensional diffusion model to model the dynamics of the dark spot expansion with consideration to the apparent diffusion coefficient and the film size. The model predictions are in good agreement with the experimental observations. On the basis of this model, we carried out a parametric study depicting the effects of the film thickness (or the number of micellar layers) and film area on the rate of the dark spot expansion. We also generalized the model previously proposed by Kralchevsky et al. [ Langmuir 1990 , 6 , 1180 - 1189 ], incorporating the effects of the film size, film thickness, and apparent diffusion coefficient to predict the dark spot expansion rate.

Hydrodynamic confinement and capillary alignment of gold nanorods

Controlling the alignment and orientation of nanorods on various surfaces poses major challenges. In this work, we investigate hydrodynamic confinement and capillary alignment of gold nanorod assembly on chemically stripe-patterned substrates. The surface patterns consist of alternating hydrophilic and hydrophobic micrometer wide stripes; a macroscopic wettability gradient enables controlling the dynamics of deposited suspension droplets. We show that drying of residual liquid on the hydrophilic stripes gives rise to spatially localized deposition and alignment of the nanorods. Moreover, a universal relation between the extent of order within the single layers of nanoparticles and the lateral dimension of the deposits is presented and discussed.