Quantifying Contributions of Different Repulsion to Film Drainage Time during the Bubble-Solid Surface Attachment and Implications for the Flotation of Fine Particles.

The flotation recovery of fine particles faces serious challenges due to the lack of kinetic energy required for supporting their radial displacement and attachment with bubbles. Generally, the hydrodynamic resistance and repulsive disjoining pressure successively inhibit the liquid outflow intervening between the bubble and solid surfaces. To quantitatively characterize the influence of the main repulsion on film thinning time, experiments have been designed in three different aqueous systems. Bubble surface mobility closely associated with hydrodynamic resistance was determined by the rising bubble technique, and the DLVO theory was employed to confirm the evolution of electrostatic repulsion. The film drainage process was then measured based on the high-speed microscopic interferometry. Furthermore, the influence of the main repulsion on bubble-solid surface interactions was examined by flotation recovery. Results show that the earlier buildup of hydrodynamic force ran through the whole film thinning process, and under immobile conditions, the central region gradually became dominant in film thinning due to the very limited fluid flow at the thinnest rim position. Therefore, to achieve the identical film thickness (∼100 nm), the large hydrodynamic resistance could prolong the film thinning time by about 1 order of magnitude, compared with that induced by electrostatic repulsion, which accounts for the increased flotation recovery by 10% using mobile bubbles. This study not only enhances the understanding of how typical repulsive forces work in film drainage dynamics but also opens up an avenue for enhancing flotation and avoiding wasting resources by modulating bubble surface mobility and thus micro/nanoscale fluid flow.

Nanoscale Transport during Liquid Film Thinning Inhibits Bubble Coalescing Behavior in Electrolyte Solutions.

The long-standing puzzle of why two colliding bubbles in an electrolyte solution do not coalesce immediately upon contact is resolved. The water film between the bubbles needs to be drained out first before its rupture, i.e., coalescence. Experiments reveal clearly that the film thinning exhibits a rather sudden slowdown (around 30-50 nm), which is orders of magnitude smaller than similar experiments involving surfactants. A critical step in explaining this phenomenon is to realize that the solute concentration is different in bulk and at the surface. During thinning, this will generate an electrolyte concentration difference in film solution along the interacting region, which in turn causes a Marangoni stress to resist film thinning. We develop a film drainage model that explains the experimentally observed phenomena well. The underlying physical mechanism, that confused the scientific community for decades, is now finally revealed.

Cellulose-based slippery covalently attached liquid surfaces for synergistic rain and solar energy harvesting.

For solar cell-triboelectric nanogenerator (TENG) integration, the design of the solid substrate for the TENG device becomes one of the challenges. The TENG needs to have superior contact electrification properties and be transparent so as to ensure light transmittance. Here, by spontaneous polymerization of dichlorodimethylsilane in the absence of any toxic solvent, we have fabricated a controllable liquid-like polydimethylsiloxane brush, featuring hydrophobicity, long-term stability, robustness, and UV resistance. A drop of liquid slides off at tilt angles below 5° and there is dynamic contact angle hysteresis of no more than 10° that can provide strong self-cleaning ability to the solid substrate. This recipe is also applicable to surfaces composed of hydroxyl group-rich cellulose-based surfaces, such as flexible cellulose acetate film (CAF). Importantly, PDMS@CAF, a flexible, transparent, and self-cleaning TENG device with a light transmission rate of 99% or more, was prepared using a conductive polymer film of PH 1000. The hybrid energy harvesting system formed by the combination of this transparent TENG equipped with solar cells is promising for harvesting energy from the environment in different weather conditions.

Effect of sodium citrate on asphaltene film at the oil-water interface.

HYPOTHESIS: Sodium citrate (Na3Cit) has been proven to improve the oil sands extraction recovery, but its mechanism is still unclear. Here we hypothesize that the presence of Na3Cit affects the asphaltene behaviour at the oil-water interface, which enhances oil-water separation and, thereby, heavy oil recovery. EXPERIMENTS: Na3Cit-asphaltene interaction was first investigated on their interfacial shear rheology at one heptol-water interface. Na3Cit-asphaltene interaction was further revealed by measuring the interaction forces between two heptol-water interfaces using the atomic force microscopy droplet technique combined with the Stokes-Reynolds-Young-Laplace (SRYL) model. Interfacial properties were further illustrated through interfacial tension, zeta potential, Langmuir trough, and FE-SEM. FINDINGS: Na3Cit was found to weaken the strength of the asphaltene film at the heptol-water interface. Moreover, Na3Cit could diminish the adhesion forces observed between two asphaltene-in-heptol droplets in high salinity solutions. Besides, Na3Cit also made the asphaltene-in-heptol droplet more negatively charged. These results collectively suggest that Na3Cit-asphaltene interaction results in a looser and more elastic asphaltene interfacial network with the slow formation and reduces the adhesion between two interfaces, all of which are most likely the consequence of increased electrostatic repulsion between asphaltene interfacial nanoaggregates. Our study provided new understandings of Na3Cit-asphaltene interactions at the interface.

Water Film Drainage between a Very Viscous Oil Drop and a Mica Surface.

We investigate thin film drainage between a viscous oil drop and a mica surface, clearly illustrating the competing effects of Laplace pressure and viscous normal stress (τ_{v}) in the drop. τ_{v} dominates the initial stage of drainage, leading to dimple formation (h_{d}) at a smaller critical thickness with an increase in the drop viscosity (the dimple is the inversion of curvature of the drop in the film region). Surface forces and interfacial tension control the last stage of film drainage. A scaling analysis shows that h_{d} is a function of the drop size R and the capillary numbers of the film (Ca_{f}) and drop (Ca_{d}), which we estimate by h_{d}=0.5Rsqrt[Ca_{f}/(1+2Ca_{d})]. This equation clearly indicates that the drop viscosity needs to be considered when Ca_{d}>0.1. These results have implications for industrial systems where very viscous liquids are involved, for example, in 3D printing and heavy oil extraction process.

Inward Flow of Intervening Liquid Films Driven by the Marangoni Effect during Bubble-Solid Collisions in Ethyl Alcohol-NaCl Aqueous Solutions.

The drainage dynamics of confined thin liquid films between an air bubble and a freshly cleaved mica surface were investigated in ethyl alcohol aqueous solutions. Focus was given to the holding stage, in which an unexpected increase in the thickness of a few hundred nanometers at the center of the film was captured by interferometry in ethyl alcohol-500 mM NaCl aqueous solutions. Such an increase in film thickness occurred when the ethyl alcohol concentration exceeded the critical value at a bubble approach velocity of 100 µm/s. For a given ethyl alcohol concentration, the increase in thickness at the center of the film did not happen when the bubble approach velocity was decreased to 10 µm/s. Compared to the cases in ethyl alcohol-500 mM NaCl solutions, no increase in thickness at the center of the film was observed in ethyl alcohol-water solutions under the same ethyl alcohol concentration and bubble approach velocity. The phenomenon of the increasing thickness at the center of the film was attributed to the net inward flow in the film, resulting from competition between the inward Marangoni flow and the outward drainage flow that was hindered by the narrow channel at the barrier rim of the film under a high electrolyte concentration. The inward Marangoni flow was achieved by a concentration gradient of ethyl alcohol between the film and the bulk solution resulting from the mobile air-liquid interface in the initial approaching period.

Role of Surfactants Based on Fatty Acids in the Wetting Behavior of Solid-Oil-Aqueous Solution Systems.

Surfactants based on fatty acids have attracted extensive attention thanks to their eco-friendly and pH-responsive features. Here, we studied two fatty acid-based surfactants that were paired with the same organic counterion but distinguished by their aliphatic chain lengths (monoethanolamine-oleic acid (MEA-OA) and monoethanolamine-lauric acid (MEA-LA)). Both surfactants exhibited the ability to lower the oil-water interfacial tension but lost their interfacial activity in a low-pH environment. We experimentally investigated their influence on the receding and spreading of oil droplets on solid surfaces. It was found that the interfacial tension reduction could decrease the static contact angle of the aqueous phase and hindered displacement dynamics during the oil droplet receding. Meanwhile, the interfacial activity was more likely to suppress the initiation of the oil droplet spreading due to the more stable thin-film forming prior to the spreading process. Nevertheless, the experimental results also exhibited that MEA-OA was more effective than MEA-LA in suppressing the receding dynamics and the spreading initiation even when they were characterized by similar interfacial tension values. Such an interesting observation could be attributed to the more considerable Marangoni flow in the solution of MEA-OA whose molecules have longer aliphatic chains. The insight from this study is expected to improve the knowledge on the molecular design for more efficient applications of fatty acid-based surfactants.

Probing Specific Adsorption of Electrolytes at Kaolinite-Aqueous Interfaces by Atomic Force Microscopy.

Adsorption of electrolytes (ions) at solid-liquid interfaces alters the physical and chemical properties of materials and hence plays a critical role in manufacturing and processing of nanomaterials featuring large surface or interfacial areas of desired structures and morphology. Many experiments and theoretical calculations using various electrical double layer (EDL) models have been conducted to understand how and where ions adsorb at charged surfaces in a liquid. However, conclusions from previous research remain inconclusive because of model-dependent approaches to studying ion adsorption at diverse solid-liquid interfaces. In this study, atomic force microscopy is used to image in liquids the surface lattice structure of two kaolinite basal planes in the presence and absence of monovalent and divalent cations. Distinct adsorption of ions through different mechanisms (such as electrostatic attraction and specific adsorption) is identified through atomic resolution imaging without the assumption of an EDL structure.

Spreading and receding of oil droplets on silanized glass surfaces in water: Role of three-phase contact line flow direction in spontaneous displacement.

HYPOTHESIS: The spontaneous displacement of both spreading and receding droplets on surfaces are extensively involved in numerous technical applications. We hypothesize that the spreading and receding displacement behaviors could be interpreted differently due to opposite flow directions at the three-phase contact line. EXPERIMENTS: We performed two groups of displacement experiments using different initial setups of oil droplets on silanized glass surfaces in aqueous surroundings. FINDINGS: The different initial configurations mostly resulted in oil displacement in opposite directions: either spreading or receding of the oil droplet. Different static states were observed at the end of the spreading and receding processes on surfaces with the same wettability due to the contact angle hysteresis. The dynamic displacement was analyzed using the hydrodynamic and molecular kinetic models, which showed distinct applicabilities for the data description of the spreading and receding possesses. The model analysis further indicated the different nature of these possesses, in particular, the resistance to displacement dynamics, which was illustrated by the interpretation of the microscopic slip length and contact line friction in the respective models. This study can shed light on the fundamental role of the displacement direction in the spontaneous liquid-liquid displacement.

Control of nanostructures through pH-dependent self-assembly of nanoplatelets.

HYPOTHESIS: The self-assembly of platy clay particles is influenced by anisotropic surface charging properties of the particles. The Stern potentials of particle surfaces are controlled by solution pH. Consequently, the energy-favorable particle association varies with pH, leading to different self-assembly structures. Therefore, it should be possible to form desired nanostructure of final clay product through self-assembly of clay particles by controlling the surface charge through adjusting solution pH. EXPERIMENTS: Three different surfaces of kaolinite clay nanoparticles were selectively exposed for Stern potential determination using an atomic force microscope. Aggregation structures through particle self-assembly were predicted based on the analysis of interaction energies between various types of surfaces of kaolinite particles. The structures were visually confirmed using the freeze-dried scanning electron microscopy technique. FINDINGS: By reducing pH of a concentrated kaolinite suspension from 8 to 5 and 3, the dispersed kaolinite particles were self-assembled to a well-stacked configuration and card-house structure, respectively. Current study demonstrates that the pH-dependent surface properties of platy kaolinite nanoparticles can be successfully used to understand the macroscopic behavior (rheology) of kaolinite nanoparticle suspensions and design nanostructures of clay products (catalysts and sorbents). The pH-dependent self-assembly is also applicable to other platy particles of anisotropic surface (charging or wettability) properties.

Role of surfactants in spontaneous displacement of high viscosity oil droplets from solid surfaces in aqueous solutions.

HYPOTHESIS: Displacement of oil droplets receding from solid surfaces in aqueous solutions plays a critical role in many household activities and industrial operations. Surfactants are often involved in these activities to control the displacement process. We hypothesize that the influence of surfactants on the displacement process of oil is highly dependent on the type and dosage of surfactants, with the mechanisms being elucidated by the analysis using appropriate dynamic wetting models. EXPERIMENTS: We systematically investigated the spontaneous displacement of a high viscosity oil on curved hydrophilic glass surfaces in aqueous solutions of anionic sodium dodecylbenzene sulfonate, cationic hexadecyl trimethyl ammonium bromide, and nonionic TritonTM X-100 over a wide range of concentrations. FINDINGS: The rather different oil displacement behaviors were observed with different surfactant additions. The displacement dynamics of the receding oil droplet was found to be inhibited by surfactant additions and followed two distinct models quantitatively: the power-law model describing the temporal evolution of early-stage displacement, and the molecular kinetic model describing the dependence of the three-phase contact line displacement velocity on the dynamic contact angle at the late stage of oil displacement. The model-based data analysis provided insights on the role of surfactants in controlling the oil displacement dynamics.

Interaction Between the Cyclopentane Hydrate Particle and Water Droplet in Hydrocarbon Oil.

The presence of immiscible water drops in bulk hydrocarbon is likely to bridge hydrate particles to cause hydrate agglomeration, leading to potential pipeline blockage. This can become a major challenge for flow assurance in offshore petroleum transportation. To avoid hydrate aggregation, the attachment between hydrate and water drops should be avoided. In this study, we used our home-designed integrated thin film drainage apparatus to investigate the interactions between a hydrate particle and a water drop inside model oil (i.e., mixture of cyclopentane and toluene with a volumetric ratio of 1:1). Our experiments showed that asphaltenes, a natural component in crude oil, were an effective inhibitor for the attachment between water drops and hydrate particles. Without asphaltenes in the system, the water drop adhered to the hydrate particle immediately after the two surfaces contacted. By adding 0.03 g/L asphaltenes into the oil phase, the attachment was delayed by 0.7 s when the applied preload force was set to around 0.05 mN. By increasing the asphaltenes addition to 0.05 g/L, the attachment between the hydrate and water drop was prevented even when the contact time lasted up to 25 s. This phenomenon could be explained by the adsorption of an asphaltenes layer along the interface between the aqueous drop and hydrocarbon. Measurements of the dynamic interfacial tension and crumping ratio confirmed the presence of the adsorption layer. The addition of 0.6 mol/L NaCl or 0.3 mol/L CaCl2 in the aqueous drop could further enhance the strength of the adsorption layer. Results of this research provide understanding of the benefits of asphaltenes and salt in preventing hydrate agglomeration.

Bubbles with tunable mobility of surfaces in ethanol-NaCl aqueous solutions.

The mobility of bubble surfaces in aqueous solutions can be hindered by even a trace amount of contaminants at the air-liquid interface. Adding ethanol into an aqueous solution is expected to modify the activity of the contaminants and hence their level at the air-liquid interface, which can possibly tune the mobility of the bubble surface. The mobility of the bubble surface was characterized by measuring dynamic interactions between a millimeter-size air bubble and a flat mica surface in ethanol-NaCl aqueous solutions using the newly developed dynamic force apparatus and quantified by the Stokes-Reynolds-Young-Laplace model. For a given bubble approach velocity, the hydrodynamic boundary condition at the air-liquid interface deviated from a tangentially immobile boundary at a critical ethanol concentration and converged gradually to fully mobile boundary with further increasing the ethanol concentration. Increasing the bubble approach velocity was found to reduce the critical ethanol concentration that was needed to change the mobility of the bubble surface. By adding surfactants to the system of high ethanol concentration and fully mobile air-liquid interface, the boundary condition became immobile when the amount of surfactant reached a critical concentration. This study introduced a method to control the hydrodynamic boundary condition at the air-liquid interface in mixed liquids.

Coalescence or Bounce? How Surfactant Adsorption in Milliseconds Affects Bubble Collision.

The coalescence between two colliding bubbles in ultraclean water can be 3 or 4 orders of magnitude faster than coalescence in contaminated solutions. This surprising result can be mostly explained by the mobile or immobile boundary conditions at the air-water interface. In this work, we employ a rising bubble technique to study bubble collisions in aqueous solutions with up to 2 mM surfactant. The experimental results clearly show that freshly generated bubbles can coalesce within milliseconds if they collide right after generation. However, once the bubbles reside in the bulk for tens of milliseconds, the coalescence is heavily hindered. Considering these results, we conclude that a clean air-water interface, rather than clean water, is required to achieve the mobile boundary condition that allows quick coalescence. These findings provide fundamental understanding for further improvements in bubble generation that will benefit industrial processes such as mineral flotation, oil extraction, and wastewater treatment.

Coalescence of Bubbles with Mobile Interfaces in Water.

The fluid flow inside a thin liquid film can be dramatically modified by the hydrodynamic boundary condition at the interfaces. Aqueous systems can be easily contaminated by trace amounts of impurities, rendering the air-liquid interface immobile, thereby significantly resisting the fluid flow. Using high speed interferometry, rapid thinning of the liquid film, on the order of the collision speed, was observed between two fast approaching air bubbles in water, indicating negligible resistance and a fully mobile boundary condition at the air-water interface. By adding trace amounts of surfactants that changed the interfacial tension by 10^{-4} N/m, a transition from mobile to immobile was observed. This provides a fundamental explanation why the bubble coalescence time can vary by over 3 orders of magnitude.

Unraveling Interaction Mechanisms between Molybdenite and a Dodecane Oil Droplet Using Atomic Force Microscopy.

Molybdenite (MoS2) is a mineral that has drawn great interest because of its potential application in various fields. To facilitate the flotation of molybdenite, the mineral pulp is commonly treated with nonpolar oil additives to promote hydrophobicity and to form an oil bridge between ultrafine molybdenite particles for agglomeration. In this study, dodecane was chosen as a model oil to investigate the flotation mechanisms of molybdenite with nonpolar oil. The interaction forces between a micrometer-sized dodecane droplet and the molybdenite basal plane in various electrolyte solutions were directly measured by the atomic force microscope droplet probe technique. The effects of added salts, ionic strength, and solution pH on interaction forces were evaluated by considering van der Waals, electrical double-layer (EDL), and hydrophobic forces. The experimentally measured force curves were found to agree well with the Reynolds lubrication model and the augmented Young-Laplace equation. The results show that the competition between repulsive EDL forces and attractive hydrophobic forces was directly responsible for oil-molybdenite attachment behavior. High pH and low salinity (<24 mM NaCl) led to strong repulsive EDL forces, which stabilized the interaction and prevented the attachment of oil to molybdenite. Both low pH and high salinity facilitated the attachment of oil to molybdenite through the depression of EDL force, allowing attractive hydrophobic force to dominate. The hydrophobic attraction was quantified with an exponential decay length of 1.0 ± 0.1 nm. Furthermore, calcium ions decreased the magnitude of the surface potentials of both oil and molybdenite more than that seen with the same ionic strength of sodium ions, suggesting the suppressed EDL repulsion. This study provides quantitative information about the surface forces between oil and the molybdenite basal plane and an improved understanding of the fundamental interaction mechanisms governing molybdenite recovery by mineral flotation.

Spontaneous Displacement of High Viscosity Micrometer Size Oil Droplets from a Curved Solid in Aqueous Solutions.

Spontaneous displacement of high viscosity (∼103 Pa·s) micrometer size oil droplets from a curved solid in aqueous solutions was investigated. For high viscosity oils, the dynamic droplet shape was found to deviate significantly from a spherical cap shape due to the considerable viscous force in the oil phase. The displacement dynamics of high viscosity droplets were analyzed using molecular kinetic and hydrodynamic models. The molecular kinetic model was found to describe the dynamic displacement well for the droplets of small departure from the spherical cap shape, while the hydrodynamic model is more applicable to the droplets of higher three-phase contact line displacement velocities and hence larger deviation of the droplets from the spherical cap shape.

Probing Boundary Conditions at Hydrophobic Solid-Water Interfaces by Dynamic Film Drainage Measurement.

A newly developed dynamic force apparatus was used to determine hydrodynamic boundary conditions of a liquid on a hydrophobic silica surface. For a given approach velocity of bubble to solid surfaces in an electrolyte solution, a reduced dimple formation and faster film drainage were observed by increasing the hydrophobicity of silica surfaces, indicating a significant change in hydrodynamic boundary conditions of water molecules from an immobile to a mobile water-hydrophobic silica interface. By comparing the measured film profiles with the predictions from the Stokes-Reynolds-Young-Laplace model, the slippage boundary condition of water on the hydrophobic silica surface of surface nanoroughness was quantified. Increasing the surface hydrophobicity was found to increase the mobility of water in the thin liquid film, promoting faster drainage of the liquid. For a given hydrophobicity of solids, the mobility of water occurred only above a critical bubble approach velocity and increased with increasing bubble approach velocity. In contrast, similar experiments with hydrophobized mica surfaces showed no-slip boundary condition of water at the molecularly smooth hydrophobic surface. The results collectively suggest that the observed mobility of water with more than 100 nm in thickness on the studied hydrophobic silica surfaces was due to the nanoroughness of hydrophobic surfaces. Such finding sheds light on one possible way of reducing the friction of water on hydrophobic solid surfaces by creating nanostructured surface of nanoroughness.

Dynamic Interaction between a Millimeter-Sized Bubble and Surface Microbubbles in Water.

The coalescence between microbubbles and millimeter-sized bubbles is an elementary process in various industrial applications such as froth flotation and wastewater treatment. Fundamental understanding of the coalescence behavior between two colliding bubbles requires knowledge of water drainage from the thin liquid film between the deformable air-water surfaces, a simple phenomenon with high complexity in physics because of the interplay of surface forces, hydrodynamic drainage, and surface rheology. In this work, we performed simultaneous measurements of the interaction force and spatial thin-film thickness during the collision between a millimeter-sized bubble (radius 1.2 mm) and surface microbubbles (radii between 30 and 700 µm) using our recently developed dynamic force apparatus. The interaction force during the collision agrees well with the prediction from the Stokes-Reynolds-Young-Laplace model with the tangentially immobile boundary condition at the air-liquid interface. However, the measured coalescence times for different bubble sizes are shorter than the model predictions, possibly caused by a rapid drainage behavior along with the loss of symmetry of the thin liquid film. In dozens of experimental runs, the bubbles coalesced at a critical film thickness of 25 ± 15 nm, which agrees reasonably well with the predicted rupture thickness using attractive van der Waals interaction force. These results suggest that the nonsymmetric drainage process, rather than the rupture thickness, contributes to the scattering of the experimental coalescence time between two fast-colliding air bubbles. Furthermore, our results suggest that smaller surface bubbles (30-100 µm) are more effective for the attachment onto a large bubble as the coalescence time decreases considerably when the microbubbles are smaller than 100 µm.

Coalescence Dynamics of Mobile and Immobile Fluid Interfaces.

Coalescence dynamics between deformable bubbles and droplets can be dramatically affected by the mobility of the interfaces with fully tangentially mobile bubble-liquid or droplet-liquid interfaces expected to accelerate the coalescence by orders of magnitude. However, there is a lack of systematic experimental investigations that quantify this effect. By using high speed camera imaging we examine the free rise and coalescence of small air-bubbles (100 to 1300 µm in diameter) with a liquid interface. A perfluorocarbon liquid, PP11, is used as a model liquid to investigate coalescence dynamics between fully mobile and immobile deformable interfaces. The mobility of the bubble surface was determined by measuring the terminal rise velocity of small bubbles rising at Reynolds numbers, Re, less than 0.1 and the mobility of free PP11 surface by measuring the deceleration kinetics of the small bubble toward the interface. Induction or film drainage times of a bubble at the mobile PP11-air surface were found to be more than 2 orders of magnitude shorter compared to the case of bubble and an immobile PP11-water interface. A theoretical model is used to illustrate the effect of hydrodynamics and interfacial mobility on the induction time or film drainage time. The results of this study are expected to stimulate the development of a comprehensive theoretical model for coalescence dynamics between two fully or partially mobile fluid interfaces.