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.

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.

Towards Understanding the Role of Surface Gas Nanostructures: Effect of Temperature Difference Pretreatment on Wetting and Flotation of Sulfide Minerals and Pb-Zn Ore

Surface nanobubbles at hydrophobic interfaces now attract much attention in various fields but their role in wetting-related phenomena is still unclear. Herein, we report the effect of a preliminary contact of "hot" solids with cold water previously proposed for generation of surface nanobubbles, on wettability of compact materials and flotation of particulate galena (PbS), sphalerite (ZnS), and Pb-Zn sulfide ore. Atomic force microscopy was applied to visualize the nanobubbles at galena crystals heated in air and contacted with cold water; X-ray photoelectron spectroscopy was used to characterize the surface composition of minerals. Contact angles measured with the sessile drop of cold water were found to increase when enhancing the support temperature from 0 to 80 °C for sphalerite and silica, and to pass a maximum at 40-60 °C for galena and pyrite (FeS2) probably due to oxidation of sulfides. The temperature pretreatment depressed the recovery of sulfides in collectorless schemes and improved the potassium butyl xanthate-assisted flotation both for single minerals and Gorevskoye Pb-Zn ore. The results suggest that the surface nanobubbles prepared using the temperature difference promote flotation if minerals are rather hydrophobic and insignificantly oxidized, so the addition of collector and activator (for sphalerite) is necessary.

Oxygenation and synchronous control of nitrogen and phosphorus release at the sediment-water interface using oxygen nano-bubble modified material

Synchronously controlling the nitrogen (N) and phosphorus (P) release from sediments is an important basis for eutrophication management in lakes, but it is still a technical challenge at present. Loading nano-bubbles on the surface of natural minerals to increase dissolved oxygen(DO) level at the sediment-water interface (SWI)provides a possible solution to this problem. In this study, oxygen nano-bubble modified mineral (ONBMM) technology was developed, and its efficiency of oxygenation at the SWI and effect on the removal of internal nutrient input were evaluated under simulated conditions. The results showed that ONBMM effectively improved DO levels near the SWI; the highest concentration reached 6.55 mgL^-1. Meanwhile, adding ONBMM remarkably reduced the concentrations of total P(TP), total N(TN) and ammonia N(NH₃-N) in the overlying water. Compared with the control group, the fluxes of TP, NH₃-N, and TN loading from sediments in simulation cores treated with ONBMM reduced by 96.4%, 51.1%, and 24.9%, respectively. The high-resolution data obtained by DGT showed that ONBMM effectively inhibited the reduction and release of FeP through increasing the oxygen level at the SWI. The results of 16S rRNA high-throughput sequencing showed that adding ONBMM strengthened the role of nitrobacteria, denitrifying bacteria, and ammonia oxidation bacteria at the SWI. The ONBMM technology provides a new tool to achieve oxygenation at the SWI and in situ control of internal pollution in eutrophic lakes.

How Bulk Nanobubbles Might Survive

The existence of bulk nanobubbles has long been regarded with scepticism, due to the limitations of experimental techniques and the widespread assumption that spherical bubbles cannot achieve stable equilibrium. We develop a model for the stability of bulk nanobubbles based on the experimental observation that the zeta potential of spherical bubbles abruptly diverges from the planar value below 10  μm. Our calculations recover three persistently reported-but disputed-properties of bulk nanobubbles: that they stabilize at a typical radius of ∼100  nm, that this radius is bounded below 1  μm, and that it increases with ionic concentration.

Mechanical Properties of Sub-Microbubbles with a Nanoparticle-Decorated Polymer Shell

Nanoparticle-decorated polymer-coated sub-microbubbles (NP-P-coated SMBs), as proved, have shown promising application prospects in ultrasound imaging, magnetic resonance imaging, drug delivery, and so forth. However, the quantitative evaluation of the stability and mechanical properties of single NP-P-coated SMB is absent. Here, we first reported the stiffness and Young's modulus of single NP-P-coated SMB obtained by the PeakForce mode of atomic force microscopy. Such NP-P-coated SMBs could maintain perfect spherical shapes and have a thinner shell thickness (about 10 nm), as determined by characterization using a transmission electron microscope. Young's modulus of NP-P-coated SMBs is about 4.6 ± 1.2 GPa, and their stiffness is about 15.0 ± 3.1 N/m. Both modulus and stiffness are obtained from the linear region in the force-deformation curve and are nearly independent of their sizes. These results should be very useful to evaluate their stability, which plays a key role in maintaining the shell drug loading and acoustic capabilities.

Surface nanobubbles on the rare earth fluorcarbonate mineral synchysite

Surface nanobubbles have been identified to play an important role in a range of industries from mineral processing to food science. The formation of surface nanobubbles is of importance for mineral processing in the extraction of complex ores, such as those containing rare earth elements. This is due to the way minerals are extracted utilising froth flotation. In this study, surface nanobubbles were imaged using non-contact atomic force microscopy on a polished cross section containing rare earth minerals. Nanobubbles were found on synchysite under reagent conditions expected to induce hydrophobicity in rare earth minerals, which is required for efficient processing. Synchysite -(Ce) is a rare earth fluorcarbonate mineral containing over 30% rare earth elements. Relatively little research has been conducted on synchysite, with only a few papers on its surface behaviour and flotation. The resulting nanobubbles were analysed and showed an average contact angle of 24° ± 8. These are in line with contact angles found on dolomite and galena by previous studies.

Label-Free Optical Imaging of the Dynamic Stick-Slip and Migration of Single Sub-100-nm Surface Nanobubbles: A Superlocalization Approach

The past decade has witnessed theoretical and experimental debates on the extraordinary long lifetime and low contact angle of surface nanobubbles. While several kinds of imaging techniques have provided promising evidence on the lifetime and gaseous nature of single surface nanobubble, each of them suffered from its own limitations before a consensus can be reached. In the present work, we employ a recently developed surface plasmon resonance microscopy (SPRM) to nonintrusively visualize single sub-100-nm surface nanobubble without labeling for the first time. The quantitative dependence between optical signal and nanobubble volume allows for resolving the dissolution kinetics, which is a key for understanding the lifetime. A superlocalization method is further introduced to monitor the trajectory of its mass center during dissolution, which uncovers the stick-slip behavior in the early stage and the migration behavior in the late stage. The label-free, nonintrusive, quantitative and sensitive features of SPRM and the potential compatibility with atomic force microscopy shed new light on the long-standing puzzle behind surface nanobubbles.

Properties of Blisters Formed on Polymer Films and Differentiating them from Nanobubbles/Nanodrops

When studying surface nanobubbles on film-coated substrates, a class of bubble-like domains called blisters are probably forming at the solid-liquid interface together with nanobubbles. This may easily lead to a misunderstanding of the characteristics and applications of surface nanobubbles and thus continue to cause problems within the nanobubble community. Therefore, how to distinguish surface nanobubbles from blisters is a problem. Herein, the morphology and properties of blisters are investigated on both smooth and nanopitted polystyrene (PS) films in degassed water. The morphology and contact angle of blisters are similar to those of surface nanobubbles. However, blisters were observed to be punctured under the tip-blister interaction and be torn broken by an atomic force microscope tip during the process of scanning. At the same time, nanopits on the surface of blisters that formed on a pitted PS film can be seen clearly. These provide direct and visual evidence for distinguishing blisters from surface nanobubbles. In addition, surface nanobubbles and blisters on smooth and pitted PS films in air-equilibrated water are studied. No punctured surface nanobubble was observed, and the force curves obtained on surface nanobubbles and the change in height of blisters and surface nanobubbles under a large scanning force show that surface nanobubbles are much softer than blisters.

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.

CH4 Nanobubbles on the Hydrophobic Solid-Water Interface Serving as the Nucleation Sites of Methane Hydrate

The surface hydrophobicity of solid particles plays a critical role in the nucleation of gas hydrate formation, and it was found that the hydrophobic surface will promote this nucleation process, but the underlying mechanism is still unveiled. Herein, we proposed for the first time our new theory that the formation of methane nanoscale gas bubbles on the hydrophobic surface provides the nuclei sites for further formation of methane hydrate. First, we studied the effect of hydrophobicity of particles on the nucleation of hydrate. It was found that the hydrophobic graphite and silica particles would promote the nucleation of hydrate, but the hydrophilic silica particles did not promote the methane hydrate nucleation. Then, we designed the atomic force microscopy experiment to explain this mechanism from a nanometer scale. The results showed that the methane nanobubbles were formed on the hydrophobic highly ordered pyrolytic graphite surface, but they were hard to form on the hydrophilic mica surface. These results indicated that the methane nanobubbles on the hydrophobic surface could provide the gas hydrate nucleation sites and may induce a rapid nucleation of methane hydrate.

The Role of Nanobubbles in the Precipitation and Recovery of Organic-Phosphine-Containing Beneficiation Wastewater

Dissolved air flotation (DAF) is broadly applied in wastewater treatment, especially for the recovery of organic pollution with low concentration. However, the mechanism of interaction between nanoscale gas bubbles and nanoparticles in the process of DAF remains unclear. Here, we investigated the role of nanobubbles in the precipitation of styryl phosphoric acid (SPA)-Pb particles and recovering organic phosphine containined in beneficiation wastewater by UV-vis (ultraviolet-visible) spectra, microflotation tests, nanoparticle tracking analysis, dynamic light scattering, and atomic force microscopy measurements. As suggested from the results, nanobubbles can inhibit the crystallization of SPA-Pb precipitation, which makes the sediment flotation recovery below 20%. After the precipitation crystallization is completed, nanobubbles can flocculate precipitated particles, which can promote the flotation recovery of precipitated particles to 90%. On the basis of the results, we proposed a model to explain the different roles of nanobubbles in the process of precipitation and flotation of SPA-Pb particles. This study will be helpful to understand the interaction between nanobubbles and nanoparticles in the application of flotation.

Robust nanobubble and nanodroplet segmentation in atomic force microscope images using the spherical Hough transform

Interfacial nanobubbles (NBs) and nanodroplets (NDs) have been attracting increasing attention due to their potential for numerous applications. As a result, the automated segmentation and morphological characterization of NBs and NDs in atomic force microscope (AFM) images is highly awaited. The current segmentation methods suffer from the uneven background in AFM images due to thermal drift and hysteresis of AFM scanners. In this study, a two-step approach was proposed to segment NBs and NDs in AFM images in an automated manner. The spherical Hough transform (SHT) and a boundary optimization operation were combined to achieve robust segmentation. The SHT was first used to preliminarily detect NBs and NDs. After that, the so-called contour expansion operation was applied to achieve optimized boundaries. The principle and the detailed procedure of the proposed method were presented, followed by the demonstration of the automated segmentation and morphological characterization. The result shows that the proposed method gives an improved segmentation result compared with the thresholding and circle Hough transform method. Moreover, the proposed method shows strong robustness of segmentation in AFM images with an uneven background.

Formation of surface nanodroplets of viscous liquids by solvent exchange

Surface nanodroplets are essential units for many compartmentalised processes from catalysis, liquid-liquid reactions, crystallization, wetting and more. Current techniques for producing submicron droplets are mainly based on top-down approaches, which are increasingly limited as scale reduces. Herein, solvent exchange is demonstrated as a simple solution-based approach for the formation of surface nanodroplets with intermediate and extremely high viscosity (1 000 000 cSt). By solvent exchange, the viscous droplet liquid dissolves in a good solvent that is then displaced by a poor solvent to yield surface droplets for the oversaturaion pulse at the mixing front. Within controlled flow conditions, the geometry of droplets of low and intermediate viscosity liquids can be tailored on the nano and microscale by controlling the flow rate. Meanwhile for extremely viscous liquids, the droplet size is shown to be dependent on the liquid temperature. This work demonstrates that solvent exchange offers a versatile tool for the formation of droplets with a wide range of viscosity.