Interaction Mechanisms between Air Bubble and Molybdenite Surface: Impact of Solution Salinity and Polymer Adsorption

Increasing surface hydrophilicity with biopolymers: a combined single bubble collision, QCM-D and AFM study

HYPOTHESIS

Naturally extracted polysaccharides, such as guar gum, are promising candidates for environmentally friendly flotation reagents. It is hypothesized that the kinetics of collision of sub- to millimeter gas bubbles with a hydrophobic graphite surface, and the stability of thin liquid film formed between the bubble and surface is affected by an adsorbed layer of guar gum.

EXPERIMENTS

A combination of gravimetric (quartz crystal microbalance with dissipation) and imaging (atomic force microscopy) techniques was used to investigate the adsorption of guar gum on graphite surface, while high-speed camera imaging allowed for direct observation of the bubble collision process with guar gum-modified graphite surfaces with millisecond resolution.

FINDINGS

Atomic force microscope topography images revealed a guar gum concentration-dependent interconnected network of guar gum molecules adsorbed at graphite surface. These adsorbed molecules at low surface coverage, changed the wettability of the graphite surface, resulting in a film drainage time longer by an order of magnitude, while at higher surface coverage successfully prevented bubble attachment to the graphite surface. Most importantly, the adsorbed layer changed the strength of the bubble's bouncing off the graphite surface. This enhanced bubble bouncing can be correlated with the film drainage time and used to predict a successful bubble-particle attachment.

Stratification and film ripping induced by structural forces in granular micellar thin films

HYPOTHESIS

Interactions across incredibly thin layers of fluids, known as thin films, underpin many important processes involving colloids, such as wetting-dewetting phenomena. Often in these systems, thin films are composed of complex fluids that contain dispersed components, such as spherical micelles, giving rise to oscillatory structural forces due to preferential layering under confinement. Modelling of thin film dynamics involving Derjaguin-Landau-Verwey-Overbeek (DLVO) type forces has been widely reported using the Stokes-Reynolds-Young-Laplace (SRYL) model, and we hypothesize that this theory can be extended to a concentrated micellar system by including an oscillatory structural force term in the disjoining pressure.

EXPERIMENTS

We study the drainage behaviour of thin films comprising sodium dodecyl sulfate (SDS) micelles across a range of concentrations and interaction conditions between an air bubble and a mica disk using a custom-built dual-wave interferometry apparatus.

FINDINGS

Early-stage film behaviour is dominated by hydrodynamics, which can be well reproduced by the SRYL model. However, experimental profiles drain significantly faster than predicted, transitioning into a structural force dominated phase characterised by four types of film ripping instabilities that we term 'waving', 'ridging', 'webbing', and 'hole-sheeting'. These instabilities were mapped according to SDS concentration and approach velocity, providing insight into the interplay between structural forces and hydrodynamic conditions.

Anti-Oil-Adhesion Property of Superhydrophilic/Underwater Superoleophobic Phytic Acid-FeIII Complex Coatings

Crude oil adhesion issues are widespread in the petroleum industry, leading to inefficient production and high maintenance costs. Developing efficient antifouling materials and investigating the microscopic adhesion mechanism are of substantial significance. In the present work, a superhydrophilic/underwater superoleophobic PAFC coating with excellent antifouling properties was constructed by the coordination-driven self-assembly of phytic acid (PA) and FeCl₃ (FC). The atomic force microscope (AFM) droplet probe technique was employed to elucidate the underlying mechanism of the anti-oil-adhesion property of the PAFC coating. Results showed that the PAFC modification achieved the optimum effect at a molar ratio of 1:3 between PA and Fe^III. Applying a (3-aminopropyl)triethoxysilane (APTES) interlayer can effectively improve the performance of the PAFC coating on silica substrates. AFM droplet probe experiments indicated that the adhesion force between submerged micrometer-sized oil droplets and PAFC-modified substrates was significantly weaker than that with the untreated substrate. Meanwhile, the adhesion forces between oil droplets and surfaces were inversely proportional to the contact angle of the oil in water and were enhanced by higher salinity, lower collision velocity, and stronger loading force. The oil injection and wall sticking tests also confirmed the effectiveness of the PAFC modification in resisting the adhesion of crude oil.

Enhanced Flotation Separation of Low-Rank Coal with a Mixed Collector: Experimental and Molecular Dynamics Simulation Study

The efficient flotation separation of low-rank coal is still a difficult problem. The development of a collector is the key to solve the problem. In this study, a kind of mixed collector scheme with a practical value is proposed. The effects of the single collector dodecane (D) and methyl oleate (MO) and mixed collector D-didodecyl dimethyl ammonium bromide (DDAB) and MO-DDAB on flotation separation of low-rank coal were investigated. The flotation test results show that when the molar ratios of the mixed collector D-DDAB and MO-DDAB are 9:1 and the flotation time is 4 min, the cumulative combustible recovery of low-rank coal flotation is 71.49 and 76.73%, respectively, and the cleaned coal ash is 15.26 and 13.03%, respectively. The mixed collector significantly improves the flotation effect of low-rank coal compared with the single collector. According to the analysis results of the contact angle, wetting heat, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy, the hydrophobicity of the low-rank coal surface is enhanced under the action of the mixed collector, and the adsorption between the mixed collector and coal surface is stronger. In addition, molecular dynamics (MD) simulation results show that compared with D or MO, DDAB tends to adsorb on the surface of low-rank coal, and the diffusion coefficients of water molecules on the surface of coal increase. The mixed collector first repels water molecules through its double hydrophobic carbon chain to weaken the binding of the coal surface to water molecules and then uses D or MO to further repel water molecules, thus effectively enhancing the surface hydrophobicity of low-rank coal.

Recent progress on research of molybdenite flotation: A review

Molybdenum is an important alloy element for metallurgical industry because of its high temperature stability. As the major mineral reserve for molybdenum, molybdenite (MoS₂) is commonly found in porphyry copper deposits. Molybdenite is naturally floatable and can be separated from copper sulfide mineral using froth flotation. Properties of molybdenite such as mineralogy, microstructure, surface wettability, zeta potential, etc. can have a great effect on its floatability. Organic and inorganic depressants and surface pre-treatment methods are applied to improve the recovery of molybdenite. Electrochemical potential measurements using different electrodes are used to monitor process conditions and enable processing parameter adjustments to improve flotation circuit performance and reduce operating costs. Cations like Ca²⁺ and Mg²⁺ are reported to have negative effects on the flotation of molybdenite in alkaline solution, and dispersants and oil collectors need to be added to restore the flotation of molybdenite. In addition, effects of gangue minerals, particle size, and oil collectors and surfactants on molybdenite recovery are also discussed in this manuscript.

Surface interaction mechanisms in mineral flotation: Fundamentals, measurements, and perspectives

As non-renewable natural resources, minerals are essential in a broad range of biological and technological applications. The surface interactions of mineral particles with other objects (e.g., solids, bubbles, reagents) in aqueous suspensions play a critical role in mediating many interfacial phenomena involved in mineral flotation. In this work, we have reviewed the fundamentals of surface forces and quantitative surface property-force relationship of minerals, and the advances in the quantitative measurements of interaction forces of mineral-mineral, bubble-mineral and mineral-reagent using nanomechanical tools such as surface forces apparatus (SFA) and atomic force microscope (AFM). The quantitative correlation between surface properties of minerals at the solid/water interface and their surface interaction mechanisms with other objects in complex aqueous media at the nanoscale has been established. The existing challenges in mineral flotation such as characterization of anisotropic crystal plane or heterogeneous surface, low recovery of fine particle flotation, and in-situ electrochemical characterization of collectorless flotation as well as the future work to resolve the challenges based on the understanding and modulation of surface forces of minerals have also been discussed. This review provides useful insights into the fundamental understanding of the intermolecular and surface interaction mechanisms involved in mineral processing, with implications for precisely modulating related interfacial interactions towards the development of highly efficient industrial processes and chemical additives.

Probing the Interactions between Pickering Emulsion Droplets Stabilized with pH-Responsive Nanoparticles

The presence and adsorption of particles at the oil/water interface play a critical role in stabilizing Pickering emulsions and affecting their bulk behavior. For water-in-oil (W/O) and oil-in-water (O/W) Pickering emulsions with pH-responsive nanoparticles, their interaction forces and stabilization mechanisms at the nanoscale have not been reported. Herein, the Pickering emulsions formed by oil/water mixtures under different pH values with bilayer oleic acid-coated Fe₃O₄ nanoparticles (Fe₃O₄@2OA NPs) were characterized using microscopy imaging and zeta potential and interfacial tension (IFT) measurements. The interaction forces between formed emulsion droplets were quantified using an atomic force microscope (AFM) drop probe technique. A W/O emulsion formed at pH 2 and 4 is mainly stabilized by the steric barrier formation of confined particle layers (with Fe₃O₄@2OA NPs and aggregates). At pH 9 and 11, an O/W emulsion is formed, and its stabilization mechanism is mainly due to relatively low IFT, strong electrostatic repulsion due to carboxyl groups, and steric repulsion from confined nanoparticles and aggregates, leading to a stable confined thin water film. Increasing the maximum loading force and dwelling time enhances the confinement of Fe₃O₄@2OA particles and aggregates at the oil/water interface. This work provides useful insights into the interaction and stabilization mechanisms of Pickering emulsions with stimuli-responsive interface-active particles.

Atomic Force Microscopy Study of Non-DLVO Interactions between Drops and Bubbles

The heterointeraction between liquid drops and air bubbles dispersed in another immiscible liquid is studied with the application of the atomic force microscopy (AFM) probe techniques. The tetradecane drops and air bubbles readily coalescence to form a lens-like structure in 100 mM sodium chloride aqueous solution, demonstrating strong hydrophobic (HB) attraction. The interaction range and strength of this hydrophobic attraction between oil drops and air bubbles is investigated by fine control of electrical double layer thicknesses related to specific electrolyte concentrations, and a midrange term in combination with a short-range term is found to present a proper characterization of this hydrophobic attraction. A further step is taken by introducing a triblock copolymer (Pluronic F68) into the aqueous solution, with results indicating that a relatively long-range steric hindrance (SH) furnished by a polymer "brush" surmounts the hydrophobic attraction. Finally, the interaction between a water drop and an air bubble in tetradecane is also measured as a comparison. The repelling action between a hydrophobic body (air bubble) and water drop indicates a strong repulsion. The present results show an interesting understanding of hydrophobic interactions between drops and bubbles, which is of potential application in controlling dispersion stability.

Nanomechanical Insights into Versatile Polydopamine Wet Adhesive Interacting with Liquid-Infused and Solid Slippery Surfaces

Mussel-inspired polydopamine (PDA) can be readily deposited on almost all kinds of substrates and possesses versatile wet adhesion. Meanwhile, slippery surfaces have attracted much attention for their self-cleaning capabilities. It remains unclear how the versatile PDA adhesive would interact with slippery surfaces. In this work, both liquid-infused poly(tetrafluoroethylene) (PTFE) (LI-PTFE) and solid slippery surfaces (i.e., self-assembly of small thiol-terminated organosilane, polysiloxane covalently attached to substrates) were fabricated to investigate their capability to prevent PDA deposition. It was found that PDA particles could be easily deposited on a PTFE membrane and the two types of solid slippery surfaces, which resulted in the alternation of their surface wettability and slippery behavior of water droplets. Adhesion was detected between a PDA-coated silica colloidal probe and the PTFE membrane or solid slippery surfaces through quantitative force measurements using an atomic force microscope (AFM), mainly due to van der Waals (vdW) and hydrophobic interactions, which led to the PDA deposition phenomenon. In contrast, LI-PTFE with a thin liquid lubricant film could effectively prevent PDA deposition, with negligible changes in surface morphology, wettability, and slippery characteristics. Although PDA particles could be loosely attached to the lubricant/water interface for LI-PTFE based on the capillary adhesion measured by AFM, they could be readily removed by gentle rinsing with water, as demonstrated by the ultralow friction over LI-PTFE as compared to PTFE using lateral force microscopy (LFM). Our results indicate that LI-PTFE possesses excellent antifouling and self-cleaning properties even when interacting with the versatile PDA wet adhesives. This work provides new insights into the deposition of PDA on slippery surfaces and their interaction mechanism at the nanoscale, with useful implications for the design and development of novel slippery surfaces.

Effect of Dodecane-Oleic Acid Collector Mixture on the Evolution of Wetting Film between Air Bubble and Low-Rank Coal

The wetting film evolution process is essential for flotation, especially in bubble–particle attachment. A mixed collector has been proved effective in promoting flotation. In this paper, the effect of a mixed collector (MC) composed by n-dodecane (D) and oleic acid (OA) on wetting film evolution was investigated using the extended Derjagin–Landau–Verwey–Overbeek (EDLVO) theory, the Stefan–Reynolds model, induction time, and zeta potential measurement. The hydrophobic force constant between bubble and coal treated by different collectors was analyzed. The results showed that MC was superior in reducing the induction time and increasing the zeta potential. When bubbles interacted with coal treated by MC, they had relatively low interaction energy, high critical film thickness, and high drainage rate. The order of hydrophobic force constant was no reagent < D < OA < MC. It indicated that the hydrophobic interaction between bubbles and coal particles treated by MC was the strongest because of the synergistic effect of D and OA.

Recent Advances in the Quantification and Modulation of Hydrophobic Interactions for Interfacial Applications

Hydrophobic interaction is responsible for a variety of colloidal phenomena, which also plays a key role in achieving the desired characteristics and functionalities for a wide range of interfacial applications. In this feature article, our recent advances in the quantification and modulation of hydrophobic interactions at both solid/water and air/water interfaces in different material systems have been reviewed. On the basis of surface forces apparatus (SFA) measurements of hydrophobic polymers (e.g., polystyrene), a three-regime hydrophobic interaction model that could satisfactorily encompass the hydrophobic interaction with different ranges was proposed. In addition, the atomic force microscope (AFM) coupled with various techniques such as the colloidal probe, the electrochemical process, and force mapping were employed to quantify the hydrophobic interaction from different perspectives. For the hydrophobic interactions involving deformable bubbles, the bubble probe AFM combined with reflection interference contrast microscopy (RICM) was used to simultaneously measure the interaction force and spatiotemporal evolution of the thin film drainage process between air bubbles and hydrophobized mica surfaces in an aqueous medium. The studies on the interactions of air bubbles with self-assembled monolayers (SAMs) demonstrated that the range of hydrophobic interactions does not always increase monotonically with the hydrophobicity of interacting surfaces as characterized by the static water contact angle; viz., surfaces with similar hydrophobicity can exhibit different ranges of hydrophobic interaction, while surfaces with different hydrophobicities can exhibit a similar range of hydrophobic interactions. It is found that the hydrophobic interaction can be modulated by tuning the surface nanoscale structure and chemistry. Moreover, the long-range "hydrophilic" attraction that resembles the hydrophobic interaction was discovered between water droplets and polyelectrolyte surfaces in an oil medium, on the basis of which polyelectrolyte coating materials were designed for oil cleaning, oil/water separation, and demulsification. The interfacial applications, remaining challenges, and future perspectives of hydrophobic interactions are discussed.

Interaction Forces between Water Droplets and Solid Surfaces across Air Films

Wetting of solid surfaces occurs when the intervening air film between a water droplet and a solid surface ruptures. Although this rupturing phenomenon is well known, the underlying mechanism has not yet been well understood. In this work, the rupture of intervening air films is systematically studied by measuring the spatiotemporal thickness profiles of the air films between droplets of deionized water and flat solid surfaces using a synchronized triwavelength reflection interferometry microscope. It has been shown that the critical rupture thickness of the air film (h _c) depends on the surface hydrophobicity of solid surfaces. The h _c value was increased from 50 nm on a hydrophobic surface having an equilibrium water contact angle (θ_w) of 96° to 1.42 μm on a hydrophilic surface having a θ_w of 25°. In addition, an increase in the critical rupture thickness with decreasing surface hydrophobicity was found to be applicable not only to chemically treated quartz surfaces but also to a variety of natural mineral surfaces. By determining the pressure within the air films, we have shown that a strong attractive force is present between water droplets and hydrophilic surfaces, thereby accelerating the draining of air films. The measured forces might be of electrostatic origin, and the forces become less attractive with increasing hydrophobicity of solid surfaces. The present result provides a fundamental insight into the rupture of air films from the perspective of surface forces.

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.

Hetero-difunctional Reagent with Superior Flotation Performance to Chalcopyrite and the Associated Surface Interaction Mechanism

Surface modification by chemical reagents is of profound importance to modulate the surface characteristic and functionality of materials, which has attracted tremendous interest in many research fields and industrial applications, such as froth flotation of minerals. In this work, a new reagent S-[(2-hydroxyamino)-2-oxoethyl]- O-octyl-dithiocarbonate ester (HAOODE) with heterodifunctional ligands was designed and synthesized to improve the flotability of chalcopyrite (CuFeS₂). Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy results showed the co-adsorption of heterodifunctional ligands (i.e., dithiocarbonate and hydroxamate groups) of HAOODE on chalcopyrite via Cu(I)-S and Cu(II)-O covalent bonds. The bubble probe atomic force microscopy (AFM) technique was employed to quantitatively measure the air bubble-chalcopyrite interactions with and without the reagent adsorption. AFM force results revealed that the bubble could be more readily attached to flat chalcopyrite after HAOODE treatment under different hydrodynamic conditions because of the enhanced hydrophobic interaction, with the decay length D₀ increasing from 0.65 to 1.20 nm. The calculated bubble-particle interaction forces also demonstrated the critical influence of HAOODE treatment, hydrodynamic conditions, and bubble size on the interaction behavior and thin-film drainage process in flotation. In froth flotation, HAOODE exhibited superior recovery for chalcopyrite over pH 3-12 and excellent selectivity for chalcopyrite against pyrite (FeS₂) above pH 10.5, as compared to the conventional reagent sodium isobutyl xanthate. This work provides a useful approach to develop effective reagents that could selectively adsorb on desired mineral surfaces through heterodifunctional ligands and to quantitatively evaluate the role of reagent adsorption in the interactions between air bubbles and mineral surfaces at the nano- and microscale. Our results show implications on developing molecular design principles of novel reagents for surface modifications of materials in a wide range of engineering and biological applications.

Attachment, Coalescence, and Spreading of Carbon Dioxide Nanobubbles at Pyrite Surfaces

Recently, it was reported that using CO₂ as a flotation gas increases the flotation of auriferous pyrite from high carbonate gold ores of the Carlin Trend. In this regard, the influence of CO₂ on bubble attachment at fresh pyrite surfaces was measured in the absence of collector using an induction timer, and it was found that nitrogen bubble attachment time was significantly reduced from 30 ms to less than 10 ms in CO₂ saturated solutions. Details of CO₂ bubble attachment at a fresh pyrite surface have been examined by atomic force microscopy (AFM) measurements and molecular dynamics (MD) simulations, and the results used to describe the subsequent attachment of a N₂ bubble. As found from MD simulations, unlike the attached N₂ bubble, which is stable and has a contact angle of about 90°, the CO₂ bubble attaches, and spreads, wetting the fresh pyrite surface and forming a multilayer of CO₂ molecules, corresponding to a contact angle of almost 180°. These MDS results are complemented by in situ AFM images, which show that, after attachment, CO₂ nano-/microbubbles spread to form pancake bubbles at the fresh pyrite surface. In summary, it seems that CO₂ bubbles have a propensity to spread, and whether CO₂ exists as layers of CO₂ molecules (gas pancakes) or as nano-/microbubbles, their presence at the fresh pyrite surface subsequently facilitates film rupture and attachment of millimeter N₂ bubbles and, in this way, improves the flotation of pyrite.

Measurement of Instability of Thin Liquid Films by Synchronized Tri-wavelength Reflection Interferometry Microscope

Film thickness measurement of unstable thin liquid films (TLFs) remains a challenge due to the difficulty in determining the order of fringes prior to the film rupture. In the present work, a synchronized tri-wavelength reflection interferometry microscope (STRIM) was developed and employed to determine the spatiotemporal thickness profiles of the TLFs between air bubbles and various hydrophobic surfaces in 10^-2 M NaCl solutions. Both accuracy and precision of film thickness measurements were found to be better than 3 nm over the range of 0-1 μm. It was found that when the radii of air bubbles were in the range 0.71-0.88 mm, the critical rupture thicknesses of the wetting films formed on hydrophobic quartz surfaces having water contact angles of 95° scattered over a range of 57-335 nm with a medium rupture thickness of 122 nm. For smaller air bubbles with radii of 0.13-0.26 mm, the critical rupture thicknesses were much more narrowly distributed with a medium rupture thickness of 27 nm. The result obtained with the TLFs between two air bubbles, i.e., foam film, showed that the critical rupture thickness was increased from 25 to 40 nm, when the sizes of air bubbles were increased from 220 to 960 μm. Compared to rupture thickness of the foam film, the critical rupture thickness of the TLF between an air bubble and a dodecane droplet was smaller, indicating that the film rupture might be related to the hydrophobicity of interacting surfaces. In addition to attractive surface forces, both wave motions and gas molecules in TLF might be associated with the film rupture.

Intermolecular and surface forces at solid/oil/water/gas interfaces in petroleum production

Many challenging issues are encountered along the petroleum production such as the wettability alteration of reservoir solids due to deposition of petroleum materials, stabilization/destabilization of water-in-oil and oil-in-water emulsions and treatment of tailings water. All these problems are essentially driven by the fundamental intermolecular and surface forces among the different components (i.e., water, oil, solid and gas) in the surrounding complex fluid media, and comprehensive understanding of the interactions among these components will pave the way to the development of advanced materials and technologies for improved petroleum production processes. In this work, we have reviewed the quantitative force measurement methods in different petroleum systems by using nanomechanical techniques including surface forces apparatus (SFA) and atomic force microscope (AFM). Interaction forces between petroleum components (e.g., asphaltenes) and mineral solids in both organic solvents and aqueous solutions are reviewed and correlated to the wettability change of the reservoir solids. The recent key progress in quantifying the surface forces of water-in-oil and oil-in-water emulsion drops using AFM drop probe techniques are discussed. The interaction forces of polymer flocculants and colloidal particles are correlated to the performance of tailings water treatment. The current knowledge gap and future perspectives are also presented.

The wettability of gas bubbles: from macro behavior to nano structures to applications

In recent years, various interfaces related to bubble wettability have been fabricated, which have already been widely applied in various disciplines and fields. Therefore, to better research and understand the wettability of gas bubbles, recent progress with interfaces and wettability of bubbles in aqueous media, including superaerophilicity and superaerophobicity, is summarized. Many biological interfaces which exhibit marvelous characteristics are discussed for reference. Because of the similar behavior between gas bubbles in aqueous media and droplets in air, the two wetting conditions are compared together to better illustrate theories of gas bubble wettability. Based on these theories, effective and available manipulation of gas bubbles' wettability provides a novel idea and method to solve practical problems in various aspects, i.e., superaerophobic electrodes for gas evolution reactions, superaerophilic electrodes for gas compensation reactions, superaerophilic interfaces for directional collection and transportation of gas bubbles, and so on.

Anisotropic Polymer Adsorption on Molybdenite Basal and Edge Surfaces and Interaction Mechanism With Air Bubbles

The anisotropic surface characteristics and interaction mechanisms of molybdenite (MoS₂) basal and edge planes have attracted much research interest in many interfacial processes such as froth flotation. In this work, the adsorption of a polymer depressant [i.e., carboxymethyl cellulose (CMC)] on both MoS₂ basal and edge surfaces as well as their interaction mechanisms with air bubbles have been characterized by atomic force microscope (AFM) imaging and quantitative force measurements. AFM imaging showed that the polymer coverage on the basal plane increased with elevating polymer concentration, with the formation of a compact polymer layer at 100 ppm CMC; however, the polymer adsorption was much weaker on the edge plane. The anisotropy in polymer adsorption on MoS₂ basal and edge surfaces coincided with water contact angle results. Direct force measurements using CMC functionalized AFM tips revealed that the adhesion on the basal plane was about an order of magnitude higher than that on the edge plane, supporting the anisotropic CMC adsorption behaviors. Such adhesion difference could be attributed to their difference in surface hydrophobicity and surface charge, with weakened hydrophobic attraction and strengthened electrostatic repulsion between the polymers and edge plane. Force measurements using a bubble probe AFM showed that air bubble could attach to the basal plane during approach, which could be effectively inhibited after polymer adsorption. The edge surface, due to the negligible polymer adsorption, showed similar interaction behaviors with air bubbles before and after polymer treatment. This work provides useful information on the adsorption of polymers on MoS₂ basal/edge surfaces as well as their interaction mechanism with air bubbles at the nanoscale, with implications for the design and development of effective polymer additives to mediate the bubble attachment on solid particles with anisotropic surface properties in mineral flotation and other engineering processes.

Interaction Mechanism between Molybdenite and Kaolinite in Gypsum Solution Using Kerosene as the Flotation Collector

This paper aims to understand the fundamental interaction mechanism between molybdenite and kaolinite in gypsum solution using kerosene as collector. Micro-flotation tests were conducted to study the effect of gypsum solution on the flotation performance of mixed −74 μm molybdenite and −10 μm kaolinite mineral. The results showed that the recovery of molybdenite decreased from 86% to 74% while the gypsum solution concentration increased from 0 to 800 mg/L, indicating the detrimental effect of kaolinite on molybdenite flotation could be enhanced by gypsum solution. This is mainly caused by the slime coating of kaolinite on molybdenite through dissolved calcium ion of gypsum solution. In order to confirm the slime coating phenomenon, zeta potential distribution, scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements were used to investigate interaction characteristics and mechanisms. The zeta potential distribution results revealed that mixed samples had the value between signal molybdenite and kaolinite samples in gypsum solution, which proved the coating phenomenon of kaolinite on molybdenite. Moreover, the coating phenomenon was becoming more and more obvious with the gypsum solution concentration. The coating phenomenon of kaolinite on molybdenite surface was also directly observed from SEM results. The AFM results provided further evidence for the possibility of slime coating, as the adhesion force increased with the gypsum solution concentration, which means the aggregates of molybdenite and kaolinite were becoming more stable.

The application of atomic force microscopy in mineral flotation

During the past years, atomic force microscopy (AFM) has matured to an indispensable tool to characterize nanomaterials in colloid and interface science. For imaging, a sharp probe mounted near to the end of a cantilever scans over the sample surface providing a high resolution three-dimensional topographic image. In addition, the AFM tip can be used as a force sensor to detect local properties like adhesion, stiffness, charge etc. After the invention of the colloidal probe technique it has also become a major method to measure surface forces. In this review, we highlight the advances in the application of AFM in the field of mineral flotation, such as mineral morphology imaging, water at mineral surface, reagent adsorption, inter-particle force, and bubble-particle interaction. In the coming years, the complementary characterization of chemical composition such as using infrared spectroscopy and Raman spectroscopy for AFM topography imaging and the synchronous measurement of the force and distance involving deformable bubble as a force sensor will further assist the fundamental understanding of flotation mechanism.

Correlation of Surface Adsorption and Oxidation with a Floatability Difference of Galena and Pyrite in High-Alkaline Lime Systems

When it comes to Pb-Zn ores with high amounts of pyrite, the major problem encountered is the low separation efficiency between galena and pyrite. By virtue of high dosage of lime and collector sodium diethyl dithiocarbamate (DDTC), pyrite and zinc minerals are depressed, allowing the galena to be floated. However, there have been significant conflicting reports on the flotation behavior of galena at high pH. In this context, correlation of the surface adsorption and oxidation with the floatability difference of galena and pyrite in high-alkaline lime systems would be a key issue for process optimization. Captive bubble contact angle measurements were performed on freshly polished mineral surfaces in situ exposed to lime solutions of varying pH as a function of immersion time. Furthermore, single mineral microflotation tests were conducted. Both tests indicated that the degree of hydrophobicity on the surfaces of galena and pyrite increased in the presence of DDTC at natural or mild pulp pH. While in a saturated lime solution, at pH 12.5, DDTC only worked for galena, but not for pyrite. Surface chemistry analysis by time-of-flight secondary ion mass spectrometry (Tof-SIMS) confirmed the preference of DDTC on the galena surface at pH 12.5, which contributed to a merit recovery. Further important evidence through measurements of Tof-SIMS, ion chromatography, and high-performance liquid chromatography indicated that in high-alkaline lime systems, the merit floatability of galena could exclude the insignificant contribution of elemental sulfur (S₈) and was dominantly attributed by the strong adsorption of DDTC. In contrast, the poor flotation response of pyrite at high pH was due to the prevailing adsorption of CaOH⁺ species. This study provides an important surface chemistry evidence for a better understanding of the mechanism on the better selectivity in the galena-pyrite separation adopting high-alkaline lime systems.

Probing Anisotropic Surface Properties and Surface Forces of Fluorite Crystals

Fluorite is the most important mineral source for producing fluorine-based chemicals and materials in a wide range of engineering and technological applications. In this work, atomic force microscopy was employed, for the first time, to probe the surface interactions and adhesion energy of model oleic acid (a commonly used surface modification organics for fluorite) molecules on fluorite surfaces with different orientations in both air and aqueous solutions at different pH conditions. Fitted with the Derjaguin-Landau-Verwey-Overbeek theory, the force results during surface approaching demonstrate the anisotropy in the surface charge of different orientations, with the {111} surface exhibiting a higher magnitude of surface charge, which could be attributed to the difference in the atomic composition. The adhesion measured during surface retraction shows that model oleic acid molecules have a stronger adhesion with the {100} surface than with the {111} surface in both air and aqueous solutions. The anisotropic adhesion energy was analyzed in relation to the surface atom (especially calcium) activity, which was supported by the surface free energy results calculated based on a three-probe-liquid method. Each calcium atom on the {100} surface with four dangling bonds is more active than the calcium atom on the {111} surface with only one dangling bond, supported by a larger value of the Lewis acid component for the {100} surface. The model oleic acid molecules present in the ionic form at pH 9 exhibit a higher adhesion energy with fluorite surfaces as compared to their molecular form at pH 6, which was related to the surface activity of different forms. The adhesion energy measured in solution is much lower than that in air, indicating that the solvent exerts an important influence on the interactions of organic molecules with mineral surfaces. The results provide useful information on the fundamental understanding of surface interactions and adhesion energy of organic molecules on mineral surfaces with different orientations, and the methodology can be extended to many other crystal surfaces in various interfacial processes.

Probing the Interaction Mechanism between Air Bubbles and Bitumen Surfaces in Aqueous Media Using Bubble Probe Atomic Force Microscopy

Surface interactions involving deformable air bubbles have attracted tremendous interest in a wide range of engineering applications, such as mineral flotation and bitumen extraction. In this work, for the first time, the interaction forces between air bubbles and bitumen surfaces in complex aqueous media of varying pH, salinity, and salts were directly measured using a bubble probe atomic force microscope (AFM) technique. The AFM topographic imaging reveals that bitumen surface tends to be rougher and form distinct domains at high NaCl concentration or under strongly alkaline environment. The force measurements demonstrate the critical role of ionic strength and solution pH in bubble-bitumen interaction and attachment, which could be well described by a theoretical model based on Reynolds lubrication theory and augmented Young-Laplace equation by including the effect of disjoining pressure. In 1 mM NaCl, the electrical double layer (EDL) repulsion inhibited bubble-bitumen attachment, and such a repulsive effect could be further strengthened with increasing solution pH. In 500 mM NaCl, the hydrophobic attraction could lead to bubble-bitumen attachment, while a high solution pH could weaken the hydrophobic interaction. The addition of calcium ion in 500 mM NaCl could enhance the hydrophobic interaction and facilitate the bubble-bitumen attachment, most likely attributed to the bridging effect between calcium ions and the functional groups (e.g., carboxyl group) of interface-active molecules on bitumen surfaces, thus leading to higher surface roughness and hydrophobic moieties/aggregates on bitumen as confirmed by AFM imaging. Our results provide quantitative information on the interaction mechanism between air bubbles and bitumen surfaces in complex aqueous solutions at the nanoscale, which has useful implications to many related interfacial interactions in industrial processes such as oil production, oil-water separation, and wastewater treatment.

Recent experimental advances for understanding bubble-particle attachment in flotation

Bubble-particle interaction is of great theoretical and practical importance in flotation. Significant progress has been achieved over the past years and the process of bubble-particle collision is reasonably well understood. This, however, is not the case for bubble-particle attachment leading to three-phase contact line formation due to the difficulty in both theoretical analysis and experimental verification. For attachment, surface forces play a major role. They control the thinning and rupture of the liquid film between the bubble and the particle. The coupling between force, bubble deformation and film drainage is critical to understand the underlying mechanism responsible for bubble-particle attachment. In this review we first discuss the advances in macroscopic experimental methods for characterizing bubble-particle attachment such as induction timer and high speed visualization. Then we focus on advances in measuring the force and drainage of thin liquid films between an air bubble and a solid surface at a nanometer scale. Advances, limits, challenges, and future research opportunities are discussed. By combining atomic force microscopy and reflection interference contrast microscopy, the force, bubble deformation, and liquid film drainage can be measured simultaneously. The simultaneous measurement of the interaction force and the spatiotemporal evolution of the confined liquid film hold great promise to shed new light on flotation.

Surface Forces and Interaction Mechanisms of Emulsion Drops and Gas Bubbles in Complex Fluids

The interactions of emulsion drops and gas bubbles in complex fluids play important roles in a wide range of biological and technological applications, such as programmable drug and gene delivery, emulsion and foam formation, and froth flotation of mineral particles. In this feature article, we have reviewed our recent progress on the quantification of surface forces and interaction mechanisms of gas bubbles and emulsion drops in different material systems by using several complementary techniques, including the drop/bubble probe atomic force microscope (AFM), surface forces apparatus (SFA), and four-roll mill fluidic device. These material systems include the bubble-self-assembled monolayer (SAM), bubble-polymer, bubble-superhydrophobic surface, bubble-mineral, water-in-oil and oil-in-water emulsions with interface-active components in oil production, and oil/water wetting on polyelectrolyte surfaces. The bubble probe AFM combined with reflection interference contrast microscopy (RICM) was applied for the first time to simultaneously quantify the interaction forces and spatiotemporal evolution of a confined thin liquid film between gas bubbles and solid surfaces with varying hydrophobicity. The nanomechanical results have provided useful insights into the fundamental interaction mechanisms (e.g., hydrophobic interaction in aqueous media) at gas/water/solid interfaces, the stabilization/destabilization mechanisms of emulsion drops, and oil/water wetting mechanisms on solid surfaces. A long-range hydrophilic attraction was found between water and polyelectrolyte surfaces in oil, with the strongest attraction for polyzwitterions, contributing to their superior water wettability in oil and self-cleaning capability of oil contamination. Some remaining challenges and future research directions are discussed and provided.

The hydrophobic force for bubble–particle attachment in flotation – a brief review

Both exponential decay and power decay laws could be employed to quantitatively describe the hydrophobic force between bubble and particle.