Characterization of the interaction between AFM tips and surface nanobubbles

Toward a Comprehensive Understanding of the Anomalously Small Contact Angle of Surface Nanobubbles

Experimental studies have demonstrated that the gas phase contact angle (CA) of a surface nanobubble (SNB) is much smaller than that of a macroscopic gas bubble. This reduced CA plays a crucial role in prolonging the lifetime of SNBs by lowering the bubble pressure and preventing gas molecules from dissolving in the surrounding liquids. Despite extensive efforts to explain the anomalously small CA, a consensus about the underlying reasons is yet to be reached. In this study, we conducted experimental investigations to explore the influence of gas molecules adsorbed at the solid-liquid interface on the CA of SNBs created through the solvent exchange (SE) method and temperature difference (TD). Interestingly, no significant change is observed in the CA of SNBs on highly oriented pyrolytic graphite (HOPG) surfaces. Even for nanobubbles on micro/nano pancakes, the CA only exhibited a slight reduction compared to SNBs on bare HOPG surfaces. These findings suggest that gas adsorption at the immersed solid surface may not be the primary factor contributing to the small CA of the SNBs. Furthermore, the CA of SNBs formed on polystyrene (PS) and octadecyltrichlorosilane (OTS) substrates was also investigated, and a considerable increase in CA was observed. In addition, the effects of other factors including impurity, electric double layer (EDL) line tension, and pinning force upon the CA of SNBs were discussed, and a comprehensive model about multiple factors affecting the CA of SNBs was proposed, which is helpful for understanding the abnormally small CA and the stability of SNBs.

Molecular Dynamics Study of the Microscopic Mechanical Balance at the Three-Phase Contact Line of Interfacial Nanobubble

This study reveals the microscopic mechanical balance at the three-phase contact line (TPCL) of an interfacial nanobubble on a substrate with a wettability pattern using molecular dynamics simulations. The apparent contact angle was compared to that evaluated using Young's equation, in which the interfacial tensions were computed using a mechanical route. The comparison was conducted by changing the wettability of the substrate from hydrophilic to neutral while maintaining a hydrophobic region in the center of the substrate. When the wettability pattern pins the TPCL at the wettability boundary, the contact angle computed by Young's equation is larger than the apparent contact angle because a pinning force exists in the inward direction of the nanobubble. Conversely, on the surfaces where the wettability pattern does not pin the TPCL, the contact angle computed by Young's equation agrees with the apparent contact angle because the pinning force disappears. The distribution of principal stresses around the TPCL, which was visualized for the first time in this study, indicates that large compressive principal stresses exist between the liquid phase and the solid substrate interface, which pin the TPCL at the surface wettability boundary, and that the maximum principal stress occurs in the inward direction of the nanobubbles at the TPCL. The normalized pinning force estimated from the maximum principal stress is equivalent to that measured experimentally.

Adsorption behavior of long-chain perfluoroalkyl substances on hydrophobic surface: A combined molecular characterization and simulation study

Hydrophobic interaction is a prevalent sorption mechanism of poly- and perfluoroalkyl substances (PFAS) in natural and engineered environments. In this study, we combined quartz crystal microbalance with dissipation (QCM-D), atomic force microscope (AFM) with force mapping, and molecular dynamics (MD) simulation to probe the molecular behavior of PFAS at the hydrophobic interface. On a CH₃-terminated self-assembled monolayer (SAM), perfluorononanoic acid (PFNA) showed ∼2-fold higher adsorption than perfluorooctane sulfonate (PFOS) that has the same fluorocarbon tail length but a different head group. Kinetic modeling using the linearized Avrami model suggests that the PFNA/PFOS-surface interaction mechanisms can evolve over time. This is confirmed by AFM force-distance measurements, which shows that while the adsorbed PFNA/PFOS molecules mostly lay flat, a portion of them formed aggregates/hierarchical structures of 1-10 nm in size after lateral diffusion on surface. PFOS showed a higher affinity to aggregate than PFNA. Association with air nanobubbles is observed for PFOS but not PFNA. MD simulations further showed that PFNA has a greater tendency than PFOS to have its tail inserted into the hydrophobic SAM, which can enhance adsorption but limit lateral diffusion, consistent with the relative behavior of PFNA/PFOS in QCM and AFM experiments. This integrative QCM-AFM-MD study reveals that the interfacial behavior of PFAS molecules can be heterogeneous even on a relatively homogeneous surface.

Detection of Nanobubbles on Lubricant-Infused Surfaces Using AFM Meniscus Force Measurements

So far, the presence of nanobubbles on lubricant-infused surfaces (LIS) has been overlooked, because of the difficulty in detecting them in such a complex system. We recently showed that anomalously large interfacial slip measured on LIS is explained by the presence of nanobubbles [Vega-Sánchez, Peppou-Chapman, Zhu and Neto, Nat. Commun., 2022 13, 351]. Crucial to drawing this conclusion was the use of atomic force microscopy (AFM) force-distance spectroscopy (meniscus force measurements) to directly image nanobubbles on LIS. This technique provided vital direct evidence of the spontaneous nucleation of nanobubbles on lubricant-infused hydrophobic surfaces. In this paper, we describe in detail the data collection and analysis of AFM meniscus force measurements on LIS and show how these powerful measurements can quantify both the thickness and distribution of multiple coexisting fluid layers (i.e., gas and oil) over a nanostructured surface. Using this technique, thousands of force curves were automatically analyzed. The results show that the interfacial tension of the nanobubbles is reduced from 52 ± 9 mN m^-1 to 39 ± 4 mN m^-1 by the presence of the silicone oil layer.

Interfacial Micropancakes: Gas or Contaminations?

Micropancake, a flat domain with micrometer-scale lateral size and a few nanometer thickness, is usually accompanied by the generation of interfacial nanobubbles at the liquid/solid surfaces. Unlike the nanobubbles, micropancakes are difficult to be produced efficiently, impeding further investigations of their mysterious properties. Very recently, An et al. even argued that the previously observed micropancakes were most likely the contaminate, not the gas layers. Herein, to reveal the nature of micropancakes with solid evidence, we presented the in situ characterization of micropancakes at a highly oriented pyrolytic graphite (HOPG) surface produced by the ethanol-water exchange or gas-supersaturated water. By washing with deeply degassed water (DW), the dissolution of those micropancakes was clearly observed, indicating that they may very well be composed of gas. In addition, the analysis of the force measurements showed the intrinsic differences between those gaseous micropancakes and the insoluble organic films. The data and results supported the interpretation that the real existence of gas micropancakes at liquid/solid surfaces.

Nanobubbles explain the large slip observed on lubricant-infused surfaces

Lubricant-infused surfaces hold promise to reduce the huge frictional drag that slows down the flow of fluids at microscales. We show that infused Teflon wrinkled surfaces induce an effective slip length 50 times larger than expected based on the presence of the lubricant alone. This effect is particularly striking as it occurs even when the infused lubricant’s viscosity is several times higher than that of the flowing liquid. Crucially, the slip length increases with increasing air content in the water but is much higher than expected even in degassed and plain Milli-Q water. Imaging directly the immersed interface using a mapping technique based on atomic force microscopy meniscus force measurements reveals that the mechanism responsible for this huge slip is the nucleation of surface nanobubbles. Using a numerical model and the height and distribution of these surface nanobubbles, we can quantitatively explain the large fluid slip observed in these surfaces.

Why are lubricant-infused surfaces so effective at reducing drag in microfluidic flow? Here, authors reveal that infused nanostructured Teflon wrinkles induce large interfacial slip due to the spontaneous nucleation of surface nanobubbles, a mechanism likely to occur on most rough infused surfaces.

Elasticity-to-Capillarity Transition in Soft Substrate Deformation

Whereas capillarity controls fluid dynamics at submillimeter scale and elasticity determines the mechanics of rigid solids, their coupling governs elastocapillary deformations on soft solids. Here, we directly probed the deformations on soft substrates induced by sessile nanodroplets. The wetting ridge created around the contact line and the dimple formed underneath the nanodroplet were imaged with a high spatial resolution using atomic force microscopy. The ridge height nonmonotonically depends on the substrate stiffness, and the dimple depth nonlinearly depends on the droplet size. The capillarity of the substrate overcomes the elasticity of the substrate in dominating the deformations when the elastocapillary length is approximately larger than the droplet contact radius, showing an experimental observation of the elasticity-to-capillarity transition. This study provides an experimental approach to investigate nanoscale elastocapillarity, and the insights have the potential to kick-off future work on the fundamentals of solid mechanics.

Nanobubble Labeling and Imaging with a Solvatochromic Fluorophore Nile Red

Herein, we report the use of a polarity-sensitive, solvatochromic fluorophore Nile red to label and probe individual hydrogen nanobubbles on the surface of an indium-tin oxide (ITO) electrode. Nanobubbles are generated from the reduction of water on ITO and fluorescently imaged from the transient adsorption and desorption process of single Nile red molecules at the nanobubble surface. The ability to label and fluorescently image individual nanobubbles with Nile red suggests that the gas/solution interface is hydrophobic in nature. Compared to the short labeling events using rhodamine fluorophores, Nile red-labeled events appear to be longer in duration, suggesting that Nile red has a higher affinity to the bubble surface. The stronger fluorophore-bubble interaction also leads to certain nanobubbles being co-labeled by multiple Nile red molecules, resulting in the observation of super-bright and long-lasting labeling events. Based on these interesting observations, we hypothesize that Nile red molecules may start clustering and form some kind of molecular aggregates when they are co-adsorbed on the same nanobubble surface. The ability to observe super-bright and long-lasting multifluorophore labeling events also allows us to verify the high stability and long lifetime of electrochemically generated surface nanobubbles.

Recent advances for understanding the role of nanobubbles in particles flotation

Traditional froth flotation is the primary method for the separation and upgrading of fine mineral particles. However, it is still difficult for micro-fine and low-quality minerals to effectively separate. It is generally believed that bubble miniaturization is of great significance to improve flotation efficiency. Due to their unique physical and chemical properties, the application of nanobubbles (NBs) in ore flotation and other fields has been widely investigated as an important means to solve the problems of fine particle separation. Therefore, a fundamental understanding of the effect of NBs on flotation is a prerequisite to adapt it for the treatment of fine and low-quality minerals for separation. In this paper, recent advances in the field of nanobubble (NB) formation, preparation and stability are reviewed. In particular, we highlight the latest progress in the role of NBs on particles flotation and focus in particular on the particle-particle and particle-bubble interaction. A discussion of the current knowledge gap and future directions is provided.

Nanobubble-Assisted Nanopatterning Reveals the Existence of Liquid Quasi-Two-Dimensional Foams Pinned to a Water-Immersed Surface

This paper reports on our observation of a quasi-two-dimensional (quasi-2D) liquid nanofoam spontaneously appearing on a submersed solid surface. Unlike common liquid foams existing on top of the liquid, the quasi-2D liquid nanofoam is pinned to a water-immersed solid surface. The foam imaging was performed by a nanobubble imprint technique, which allows recording the positions of the surface nanobubbles by their imprints in a polystyrene film, as described in our previous papers [Tarábková et al. Langmuir 2014, 30, 14522; Tarábková et al., Langmuir 2016, 32, 11221]. Nanobubble imprints are then examined by ex situ atomic force microscopy. Besides randomly distributed nanoprotrusions corresponding to solitary nanobubbles, quasi-periodic arrangements of a tight cellular structure and more spaced round-shaped patterns, corresponding to "dry" and "wet" quasi-2D micro- and nanofoams, respectively, are identified. Although randomly spread solitary nanobubbles can occupy up to 30% of an immersed solid surface, their self-organization in a quasi-2D nanofoam leads to surface gas coverage reaching up to 80%, which implies significantly lowered surface wetting. Existence of a submersed quasi-2D nanofoam thus opens the novel question on the impact of dense surface nanobubble assemblies on heterogeneous processes at the solid-liquid interface.

Force Spectroscopy Revealed a High-Gas-Density State near the Graphite Substrate inside Surface Nanobubbles

The absorption of gas molecules at hydrophobic surfaces may have a special state and play an important role in many processes in interfacial physics, which has been rarely considered in previous theory. In this paper, force spectroscopic experiments were performed by a nanosized AFM probe penetrated into individual surface nanobubbles and contacted with a highly ordered pyrolytic graphite (HOPG) substrate. The results showed that the adhesion force at the gas/solid interface was much smaller than that in air measured with the same AFM probe. The adhesion data were further analyzed by the van der Waals force theory, and the result implied that the gas density near the substrate inside the surface nanobubbles was about 3 orders of magnitude higher than that under the standard pressure and temperature (STP). Our MD simulation indicated that the gas layers near the substrate exhibited a high-density state inside the surface nanobubbles. This high-density state may provide new insight into the understanding of the abnormal stability and contact angle of nanobubbles on hydrophobic surfaces, and have significant impact on their applications.

Long-Term Stability of Surface Nanobubbles in Undersaturated Aqueous Solution

Surface nanobubbles should not be stable for more than a few milliseconds; however they have been shown to persist for days. Pinning of the three-phase contact line of surface nanobubbles has been proposed to explain the discrepancy between the theoretical and experimental results. According to this model, two factors stabilize surface nanobubbles, namely solution oversaturation and surface pinning. Hereby, we investigate experimentally the impact of the solution saturation on the stability of nanobubbles. For this purpose, surface nanobubbles have been nucleated on hydrophobic surfaces by two methods, and then characterized by Atomic Force Microscopy (AFM). Thereafter, the surrounding liquid has been exchanged multiple times with partially degassed water. Two degassing techniques are presented. Both sets of experiments lead to the conclusion that surface nanobubbles are stable in undersaturated conditions for hours. We compare the measured lifetime of nanobubbles to calculations for pinned nanobubbles in undersaturated conditions. The stability of surface nanobubbles in undersaturated solutions observed here is incommensurate with the pinning mechanism as the origin of the long-term stability of surface nanobubbles.

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.

Lateral Force Microscopy of Interfacial Nanobubbles: Friction Reduction and Novel Frictional Behavior

Atomic force microscopy is used to conduct single-asperity friction measurements at a water-graphite interface. Local mapping of the frictional force, which is based on the degree of the cantilever twisting, shows nearly friction-free when a tip scans over a nanobubble. Surprisingly, apart from being gapless, the associated friction loop exhibits a tilt in the cantilever twisting versus the tip's lateral displacement with the slope depending on the loading force. The sign of the slope reverses at around zero loading force. In addition, the measured normal and lateral tip-sample interactions exhibit unison versus tip-sample separation. Theoretical analysis, based on the balance of forces on the tip originated from the capillary force of the nanobubble and the torsion of the cantilever, offers quantitative explanations for both the tilted friction loop and the unison of force curves. The analysis may well apply in a wider context to the lateral force characterization on cap-shaped fluid structures such as liquid droplets on a solid substrate. This study further points to a new direction for friction reduction between solids in a liquid medium.

Micro and nanobubble technologies as a new horizon for water-treatment techniques: A review

This review article organizes the studies conducted on the areas of microbubbles and nanobubbles with a special emphasis on water treatment. The basic definitions of bubble types and their size ranges are also presented based on the explanations of different researchers. The characterization parameters with state-of-the-art measuring and analysis techniques of microbubble and nanobubble technologies are summarized. Some major applications of these technologies in water-treatment processes are reviewed and briefly discussed. Based on the reviews, various potential areas for research and bubble application gaps in water and wastewater treatment technologies are identified for further study. The article is prepared in such a way that it provides a step-by-step acquaintance to the subject matter with the objective of focusing on the application of microbubbles and nanobubbles in water-treatment technology.

The Dynamic Steady State of an Electrochemically Generated Nanobubble

This article describes the dynamic steady state of individual H₂ nanobubbles generated by H⁺ reduction at inlaid and recessed Pt nanodisk electrodes. Electrochemical measurements coupled with finite element simulations allow analysis of the nanobubble geometry at dynamic equilibrium. We demonstrate that a bubble is sustainable at Pt nanodisks due to the balance of nanobubble shrinkage due to H₂ dissolution and growth due to H₂ electrogeneration. Specifically, simulations are used to predict stable geometries of the H₂/Pt/solution three-phase interface and the width of exposed Pt at the disk circumference required to sustain the nanobubble via steady-state H₂ electrogeneration. Experimentally measured currents, i^ss, corresponding to the electrogeneration of H₂, at or near the three-phase interface, needed to sustain the nanobubble are between 0.2 and 2.4 nA for Pt nanodisk electrodes with radii between 2.5 and 40 nm. However, simple theoretical analysis shows that the diffusion-limited currents required to sustain such a single nanobubble at an inlaid Pt nanodisk are 1-2 orders larger than the observed values. Finite element simulation of the dynamic steady state of a nanobubble at an inlaid disk also demonstrates that the expected steady-state currents are much larger than the experimental currents. Better agreement between the simulated and experimental values of i^ss is obtained by considering recession of the Pt disk nanoelectrode below the plane of the insulating surface, which reduces the outward flux of H₂ from the nanobubble and results in smaller values of i^ss.

Noncontact Viscoelastic Measurement of Polymer Thin Films in a Liquid Medium Using Long-Needle Atomic Force Microscopy

We report the noncontact measurement of the viscoelastic property of polymer thin films in a liquid medium using frequency-modulation atomic force microscopy with a newly developed long-needle probe. The probe contains a long vertical glass fiber with one end adhered to a cantilever beam and the other end with a sharp tip placed near the liquid-film interface. The nanoscale flow generated by the resonant oscillation of the needle tip provides a precise hydrodynamic force acting on the soft surface of the thin film. By accurately measuring the mechanical response of the thin film, we obtain the elastic and loss moduli of the thin film using the linear response theory of elastohydrodynamics. The experiment verifies the theory and demonstrates its applications. The technique can be used to accurately measure the viscoelastic property of soft surfaces, such as those made of polymers, nanobubbles, live cells, and tissues.

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.

Interfacial gas nanobubbles or oil nanodroplets?

The force curves on nanobubbles and PDMS nanodroplets are quite different. The peculiar plateaus on nanobubbles can be used to distinguish these two easily confusing objects.

Probing Interactions between Air Bubble and Hydrophobic Polymer Surface: Impact of Solution Salinity and Interfacial Nanobubbles

The interactions between air bubbles and hydrophobic polymer surfaces in aqueous media play important roles in many industrial and engineering processes. In this work, the interaction forces between air bubble and a model hydrophobic polymer-polystyrene (PS) in NaCl solutions (1 mM to 1000 mM) were directly measured using a bubble probe atomic force microscope (AFM) technique, and the measured forces were analyzed by a theoretical model based on Reynolds lubrication theory and augmented Young-Laplace equation including the influence of disjoining pressure. It was found that the theoretical analysis, by assuming that the PS surface was a pristine and bare polymer surface in aqueous solutions, could not fully agree with the experimental force measurements at intermedium salinity condition (i.e., 100 mM NaCl), and the discrepancy could not be described by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory even including the effects of non-DLVO interactions such as hydrophobic interaction. Atomic force microscope (AFM) imaging demonstrated that the above discrepancy was caused by the presence of interfacial nanobubbles (INBs) on the hydrophobic PS surface. The solution salinity was found to significantly affect the size and surface coverage of INBs on the PS surface, thereby influencing the surface forces. At high NaCl concentration (e.g., 500 and 1000 mM), the INB formation (and its impact on the surface interactions) and electric double layer repulsion were highly suppressed, and the bubble-PS attachment was observed attributing to their hydrophobic attraction with a decay length of ∼0.75 ± 0.05 nm. The results agree with our previous surface force measurements between two PS surfaces using a surface forces apparatus. This work provides useful insights into the interaction mechanism between air bubbles and hydrophobic polymer surfaces, as well as the influence of solution salinity and interfacial nanobubbles on the bubble-polymer interaction.

Pinning Down the Reasons for the Size, Shape, and Stability of Nanobubbles

A thermodynamic analysis of the size, shape, and stability of nanobubbles is carried out by modifying classical nucleation theory to include the supersaturation dependence of the surface tension. It is shown that the free energy of a nanobubble is a minimum at the critical radius when the contact line is pinned, that the size of the nanobubble is determined by the decrease in the surface tension caused by the degree of supersaturation, and that together these explain the increased exterior contact angle of a nanobubble compared with the corresponding angle of a macroscopic bubble or droplet on the same surface.

Nanobubbles at Hydrophilic Particle-Water Interfaces

The puzzling persistence of nanobubbles breaks Laplace's law for bubbles, which is of great interest for promising applications in surface processing, H₂ and CO₂ storage, water treatment, and drug delivery. So far, nanobubbles have mostly been reported on hydrophobic planar substrates with atomic flatness. It remains a challenge to quantify nanobubbles on rough and irregular surfaces because of the lack of a characterization technique that can detect both the nanobubble morphology and chemical composition inside individual nanobubble-like objects. Here, by using synchrotron-based scanning transmission soft X-ray microscopy (STXM) with nanometer resolution, we discern nanoscopic gas bubbles of >25 nm with direct in situ proof of O₂ inside the nanobubbles at a hydrophilic particle-water interface under ambient conditions. We find a stable cloud of O₂ nanobubbles at the diatomite particle-water interface hours after oxygen aeration and temperature variation. The in situ technique may be useful for many surface nanobubble-related studies such as material preparation and property manipulation, phase equilibrium, nucleation kinetics, and relationships with chemical composition within the confined nanoscale space. The oxygen nanobubble clouds may be important in modifying particle-water interfaces and offering breakthrough technologies for oxygen delivery in sediment and/or deep water environments.

A History of Nanobubbles

We follow the history of nanobubbles from the earliest experiments pointing to their existence to recent years. We cover the effect of Laplace pressure on the thermodynamic stability of nanobubbles and why this implies that nanobubbles are thermodynamically never stable. Therefore, understanding bubble stability becomes a consideration of the rate of bubble dissolution, so the dominant approach to understanding this is discussed. Bulk nanobubbles (or fine bubbles) are treated separately from surface nanobubbles as this reflects their separate histories. For each class of nanobubbles, we look at the early evidence for their existence, methods for the production and characterization of nanobubbles, evidence that they are indeed gaseous, or otherwise, and theories for their stability. We also look at applications of both surface and bulk nanobubbles.

Hidden Nanobubbles in Undersaturated Liquids

Here, we propose theoretically the existence of a new type of nanobubble in undersaturated liquids. These nanobubbles have a concave vapor-liquid interface featured with a negative curvature rather than a positive curvature for nanobubbles in supersaturated liquids, so that they often hide inside of the substrate textures and it might not be easy to characterize them through atomic force microscopy (AFM) measurements. However, these hidden nanobubbles are still stabilized by the contact line pinning effect and stay at the thermodynamically metastable state. We further demonstrate that similar to the nanobubbles in supersaturated liquids the contact angle of the hidden nanobubbles is more sensitive to the nanobubble size rather than the substrate chemistry, and their curvature radius is dependent on the chemical potential but independent of the base radius. Finally, we show several potential situations for the appearance of the hidden nanobubbles.

Optimized Model Surfaces for Advanced Atomic Force Microscopy Studies of Surface Nanobubbles

The formation of self-assembled monolayers (SAMs) of binary mixtures of 16-mercaptohexadecanoic acid (MHDA) and 1-octadecanethiol (ODT) on ultraflat template-stripped gold (TSG) surfaces was systematically investigated to clarify the assembly behavior, composition, and degree of possible phase segregation in light of atomic force microscopy (AFM) studies of surface nanobubbles on these substrates. The data for SAMs on TSG were compared to those obtained by adsorption on rough evaporated gold, as reported in a previous study. Quartz crystal microbalance and surface plasmon resonance data acquired in situ on TSG indicate that similar to SAM formation on conventional evaporated gold substrates ODT and MHDA form monolayers and bilayers, respectively. The second layer on MHDA, whose formation is attributed to hydrogen bonding, can be easily removed by adequate rinsing with water. The favorable agreement of the grazing incidence reflection Fourier transform infrared (GIR FTIR) spectroscopy and contact angle data analyzed with the Israelachvili-Gee model suggests that the binary SAMs do not segregate laterally. This conclusion is fully validated by high-resolution friction force AFM observations down to a length scale of 8-10 nm, which is much smaller than the typical observed surface nanobubble radii. Finally, correspondingly functionalized TSG substrates are shown to be valuable supports for studying surface nanobubbles by AFM in water and for addressing the relation between surface functionality and nanobubble formation and properties.

Surface Nanobubbles Studied by Time-Resolved Fluorescence Microscopy Methods Combined with AFM: The Impact of Surface Treatment on Nanobubble Nucleation

The impact of surface treatment and modification on surface nanobubble nucleation in water has been addressed by a new combination of fluorescence lifetime imaging microscopy (FLIM) and atomic force microscopy (AFM). In this study, rhodamine 6G (Rh6G)-labeled surface nanobubbles nucleated by the ethanol-water exchange were studied on differently cleaned borosilicate glass, silanized glass as well as self-assembled monolayers on transparent gold by combined AFM-FLIM. While the AFM data confirmed earlier reports on surface nanobubble nucleation, size, and apparent contact angles in dependence of the underlying substrate, the colocalization of these elevated features with highly fluorescent features observed in confocal intensity images added new information. By analyzing the characteristic contributions to the excited state lifetime of Rh6G in decay curves obtained from time-correlated single photon counting (TCSPC) experiments, the characteristic short-lived (<600 ps) component of could be associated with an emission at the gas-water interface. Its colocalization with nanobubble-like features in the AFM height images provides evidence for the observation of gas-filled surface nanobubbles. While piranha-cleaned glass supported nanobubbles, milder UV-ozone or oxygen plasma treatment afforded glass-water interfaces, where no nanobubbles were observed by combined AFM-FLIM. Finally, the number density of nanobubbles scaled inversely with increasing surface hydrophobicity.

AFM Study of Surface Nanobubbles on Binary Self-Assembled Monolayers on Ultraflat Gold with Identical Macroscopic Static Water Contact Angles and Different Terminal Functional Groups

All experimental findings related to surface nanobubbles, such as their pronounced stability and the striking differences of macroscopic and apparent nanoscopic contact angles, need to be addressed in any theory or model of surface nanobubbles. In this work we critically test a recent explanation of surface nanobubble stability and their consequences and contrast this with previously proposed models. In particular, we elucidated the effect of surface chemical composition of well-controlled solid-aqueous interfaces of identical roughness and defect density on the apparent nanoscopic contact angles. Expanding on a previous atomic force microscopy (AFM) study on the systematic variation of the macroscopic wettability using binary self-assembled monolayers (SAMs) on ultraflat template stripped gold (TSG), we assessed here the effect of different surface chemical composition for macroscopically identical static water contact angles. SAMs on TSG with a constant macroscopic water contact angle of 81 ± 2° were obtained by coadsorption of a methyl-terminated thiol and a second thiol with different terminal functional groups, including hydroxy, amino, and carboxylic acid groups. In addition, surface nanobubbles formed by entrainment of air on SAMs of a bromoisobutyrate-terminated thiol were analyzed by AFM. Despite the widely differing surface potentials and different functionality, such as hydrogen bond acceptor or donor, and different dipole moments and polarizability, the nanoscopic contact angles (measured through the condensed phase and corrected for AFM tip broadening effects) were found to be 145 ± 10° for all surfaces. Hence, different chemical functionalities at identical macroscopic static water contact angle do not noticeably influence the apparent nanoscopic contact angle of surface nanobubbles. This universal contact angle is in agreement with recent models that rely on contact line pinning and the equilibrium of gas outflux due to the Laplace pressure and gas influx due to gas oversaturation in the aqueous medium.

In situ measurement of contact angles and surface tensions of interfacial nanobubbles in ethanol aqueous solutions

The astonishing long lifetime and large contact angles of interfacial nanobubbles are still in hot debate despite numerous experimental and theoretical studies. One hypothesis to reconcile the two abnormalities of interfacial nanobubbles is that they have low surface tensions. However, few studies have been reported to measure the surface tensions of nanobubbles due to the lack of effective measurements. Herein, we investigate the in situ contact angles and surface tensions of individual interfacial nanobubbles immersed in different ethanol aqueous solutions using quantitative nanomechanical atomic force microscopy (AFM). The results showed that the contact angles of nanobubbles in the studied ethanol solutions were also much larger than the corresponding macroscopic counterparts on the same substrate, and they decreased with increasing ethanol concentrations. More significantly, the surface tensions calculated were much lower than those of the gas-liquid interfaces of the solutions at the macroscopic scale but have similar tendencies with increasing ethanol concentrations. Those results are expected to be helpful in further understanding the stability of interfacial nanobubbles in complex solutions.

Modeling the Interaction between AFM Tips and Pinned Surface Nanobubbles

Although the morphology of surface nanobubbles has been studied widely with different AFM modes, AFM images may not reflect the real shapes of the nanobubbles due to AFM tip-nanobubble interactions. In addition, the interplay between surface nanobubble deformation and induced capillary force has not been well understood in this context. In our work we used constraint lattice density functional theory to investigate the interaction between AFM tips and pinned surface nanobubbles systematically, especially concentrating on the effects of tip hydrophilicity and shape. For a hydrophilic tip contacting a nanobubble, its hydrophilic nature facilitates its departure from the bubble surface, displaying a weak and intermediate-range attraction. However, when the tip squeezes the nanobubble during the approach process, the nanobubble shows an elastic effect that prevents the tip from penetrating the bubble, leading to a strong nanobubble deformation and repulsive interactions. On the contrary, a hydrophobic tip can easily pierce the vapor-liquid interface of the nanobubble during the approach process, leading to the disappearance of the repulsive force. In the retraction process, however, the adhesion between the tip and the nanobubble leads to a much stronger lengthening effect on nanobubble deformation and a strong long-range attractive force. The trends of force evolution from our simulations agree qualitatively well with recent experimental AFM observations. This favorable agreement demonstrates that our model catches the main intergradient of tip-nanobubble interactions for pinned surface nanobubbles and may therefore provide important insight into how to design minimally invasive AFM experiments.

Surface Nanobubbles in Nonaqueous Media: Looking for Nanobubbles in DMSO, Formamide, Propylene Carbonate, Ethylammonium Nitrate, and Propylammonium Nitrate

Surface nanobubbles produced by supersaturation during the exchange of ethanol for water are routinely observed on hydrophobic surfaces, are stable for days, and have contact angles that are very much greater than observed macroscopically. Here, we test the hypothesis that nanobubbles can also be observed in nonaqueous solvents in order to ascertain if their anomalous lifetimes and contact angles are related to properties of the solvent. Nanobubbles were seen in the protic solvents formamide, ethylammonium nitrate, and propylammonium nitrate, but not in propylene carbonate or dimethyl sulfoxide. Solvents in which nanobubbles were observed exhibit a three-dimensional hydrogen-bonding network. Like in aqueous systems, the nanobubbles were stable for days and exhibited high contact angles (∼165°).

Humidity-dependent surface tension measurements of individual inorganic and organic submicrometre liquid particles

Atomic force microscopy has been utilized to measure the surface tension of atmospherically relevant droplets smaller than one micron.

Surface tension, an important property of liquids, is easily measured for bulk samples. However, for droplets smaller than one micron in size, there are currently no reported measurements. In this study, atomic force microscopy (AFM) and force spectroscopy have been utilized to measure surface tension of individual submicron sized droplets at ambient pressure and controlled relative humidity (RH). Since the surface tension of atmospheric aerosols is a key factor in understanding aerosol climate effects, three atmospherically relevant systems (NaCl, malonic and glutaric acids) were studied. Single particle AFM measurements were successfully implemented in measuring the surface tension of deliquesced particles on the order of 200 to 500 nm in diameter. Deliquesced particles continuously uptake water at high RH, which changes the concentration and surface tension of the droplets. Therefore, surface tension as a function of RH was measured. AFM based surface tension measurements are close to predicted values based on bulk measurements and activities of these three chemical systems. Non-ideal behaviour in concentrated organic acid droplets is thought to be important and the reason for differences observed between bulk solution predictions and AFM data. Consequently, these measurements are crucial in order to improve atmospheric climate models as direct measurements hitherto have been previously inaccessible due to instrument limitations.

Measuring forces and spatiotemporal evolution of thin water films between an air bubble and solid surfaces of different hydrophobicity

A combination of atomic force microscopy (AFM) and reflection interference contrast microscopy (RICM) was used to measure simultaneously the interaction force and the spatiotemporal evolution of the thin water film between a bubble in water and mica surfaces with varying degrees of hydrophobicity. Stable films, supported by the repulsive van der Waals-Casimir-Lifshitz force were always observed between air bubble and hydrophilic mica surfaces (water contact angle, θ(w) < 5°) whereas bubble attachment occurred on hydrophobized mica surfaces. A theoretical model, based on the Reynolds lubrication theory and the augmented Young-Laplace equation including the effects of disjoining pressure, provided excellent agreement with experiment results, indicating the essential physics involved in the interaction between air bubble and solid surfaces can be elucidated. A hydrophobic interaction free energy per unit area of the form: WH(h) = -γ(1 - cos θ(w))exp(-h/D(H)) can be used to quantify the attraction between bubble and hydrophobized solid substrate at separation, h, with γ being the surface tension of water. For surfaces with water contact angle in the range 45° < θ(w) < 90°, the decay length DH varied between 0.8 and 1.0 nm. This study quantified the hydrophobic interaction in asymmetric system between air bubble and hydrophobic surfaces, and provided a feasible method for synchronous measurements of the interaction forces with sub-nN resolution and the drainage dynamics of thin films down to nm thickness.

Forces and thin water film drainage in deformable asymmetric nanoscale contacts

Gas-liquid interfaces are omnipresent in daily life, and processes involving these interfaces are the basis for a broad range of applications that span from established industrial processes to modern microengineering, technology, and medical applications for diagnosis and treatment. Despite the rapid progress in understanding intermolecular forces at such interfaces from a theoretical point of view and, in particular, from an experimental point of view down to sub-nanometer length scales, the quantitative description of all relevant forces, particularly the hydrophobic interaction and the dynamic behavior of nanometer-scale confined water films, was until now unsatisfactory. This situation is particularly the case for the elusive description and understanding of the origins of the so-called hydrophobic interaction. For soft, deformable interfaces, such as those found in asymmetric contacts between gas bubbles and a solid, a complete picture has begun to emerge that has direct consequences for interfacial water at (bio)interfaces, functionalized gas microbubbles, surface nanobubbles, and beyond.

Photoresistance switching of plasmonic nanopores

Fast and reversible modulation of ion flow through nanosized apertures is important for many nanofluidic applications, including sensing and separation systems. Here, we present the first demonstration of a reversible plasmon-controlled nanofluidic valve. We show that plasmonic nanopores (solid-state nanopores integrated with metal nanocavities) can be used as a fluidic switch upon optical excitation. We systematically investigate the effects of laser illumination of single plasmonic nanopores and experimentally demonstrate photoresistance switching where fluidic transport and ion flow are switched on or off. This is manifested as a large (∼ 1-2 orders of magnitude) increase in the ionic nanopore resistance and an accompanying current rectification upon illumination at high laser powers (tens of milliwatts). At lower laser powers, the resistance decreases monotonically with increasing power, followed by an abrupt transition to high resistances at a certain threshold power. A similar rapid transition, although at a lower threshold power, is observed when the power is instead swept from high to low power. This hysteretic behavior is found to be dependent on the rate of the power sweep. The photoresistance switching effect is attributed to plasmon-induced formation and growth of nanobubbles that reversibly block the ionic current through the nanopore from one side of the membrane. This explanation is corroborated by finite-element simulations of a nanobubble in the nanopore that show the switching and the rectification.

Dimensions and the profile of surface nanobubbles: tip-nanobubble interactions and nanobubble deformation in atomic force microscopy

The interactions between argon surface nanobubbles and AFM tips on HOPG (highly oriented pyrolitic graphite) in water and the concomitant nanobubble deformation were analyzed as a function of position on the nanobubbles in a combined tapping mode and force-volume mode AFM study with hydrophilic and hydrophobic AFM tips. On the basis of the detailed analysis of force-distance curves acquired on the bubbles, we found that for hydrophobic tips the bubble interface may jump toward the tip and that the tip-bubble interaction strength and the magnitude of the bubble deformation were functions of vertical and horizontal position of the tip on the bubble and depended on the bubble size and tip size and functionality. The spatial variation is attributed to long-range attractive forces originating from the substrate under the bubbles, which dominate the interaction at the bubble rim. The nonuniform bubble deformation leads to a nonuniform underestimation of the bubble height, width, and contact angle in conventional AFM height data. In particular, scanning with a hydrophobic tip resulted in severe bubble deformation and distorted information in the AFM height image. For a typical nanobubble, the upward deformation may extend up to tens of nanometers above the unperturbed bubble height, and the lateral deformation may constitute 20% of the bubble width. Therefore, only scanning with a hydrophilic tip and no direct contact between the tip and the bubble may reduce nanobubble deformation and provide reliable AFM images that can be used to estimate adequately the unperturbed nanobubble dimensions. The deformation of the bubble shape and underestimation of the bubble size lead to the conclusion that the profile of surface nanobubbles is much closer than previously thought to a nearly flat bubble profile and hence that the Laplace pressure is much closer to the atmospheric pressure. Together with line pinning, this may explain the long nanobubble lifetimes observed previously. The findings presented in this report hold independently of the material that constitutes the interrogated nanoscale surface features.

Hydrodynamic effects of the tip movement on surface nanobubbles: a combined tapping mode, lift mode and force volume mode AFM study

We report on an Atomic Force Microscopy (AFM) study of AFM tip-nanobubble interactions in experiments conducted on argon surface nanobubbles on HOPG (highly oriented pyrolytic graphite) in water in tapping mode, lift mode and Force Volume (FV) mode AFM. By subsequent data acquisition on the same nanobubbles in these three different AFM modes, we could directly compare the effect of different tip-sample interactions. The tip-bubble interaction strength was found to depend on the vertical and horizontal position of the tip on the bubble with respect to the bubble center. The interaction forces measured experimentally were in good agreement with the forces calculated using the dynamic interaction model. The strength of the hydrodynamic effect was also found to depend on the direction of the tip movement. It was more pronounced in the FV mode, in which the tip approaches the bubble from the top, than in the lift mode, in which the tip approaches the bubble from the side. This result suggests that the direction of tip movement influences the bubble deformation. The effect should be taken into account when nanobubbles are analysed by AFM in various scanning modes.