Wetting Behavior of Surface Nanodroplets Regulated by Periodic Nanostructured Surfaces.

Surfaces with nanostructure patterning are broadly encountered in nature, and they play a significant role in regulating various phenomena such as phase transition at the liquid/solid interface. Here, we designed two kinds of template substrates with periodic nanostructure patterns [i.e., nanotrench (NT) and nanopore (NP)]. Surface nanodroplets produced on these nanostructure surfaces were characterized to acquire their morphology and wetting properties. We show that nanostructure patterning could effectively regulate the shape, contact radius, and nucleate site of nanodroplets. While nanodroplets on the NT structure are constrained in one dimension, nanodroplets on the NP structure have enhanced the wetting property with constraints from two dimensions. Further numerical analysis indicates that the morphology and contact angles of nanodroplets on the NT structure depend on the substrate wettability and the droplet volume. These observations demonstrate how physical geometry and chemical heterogeneity of a substrate surface affect the growth and spreading of surface nanodroplets, which deepens our understanding on nanoscale phase separation.

Formation and Stability of Bulk Nanobubbles in Different Solutions.

The existence of bulk nanobubbles is still controversial in spite of their significance in a large range of applications. Here, we developed a new method of compression-decompression to produce controllably bulk nanobubbles. Then, we further investigated the generation of bulk nanobubbles in pure water, acid, alkaline, and salt solutions using nanoparticle tracking analysis. The results indicated that the concentration of bulk nanobubbles depends on the decompression time and would reach a maximum value when the decompression time is about 30 min for the pure water system. More importantly, we gave a relatively direct evidence of the existence of bulk nanobubbles by measuring the X-ray fluorescence intensity of Kr in acid, alkaline, and salt solutions. It is shown that the decrease tendency in intensity of Kr in alkaline solution is similar to that in the concentration of bulk nanobubbles with the deposited time, indicating that the bulk nanobubbles produced indeed have gas inside. Furthermore, the concentration and stability of bulk nanobubbles in an alkaline solution are greatest compared with other two solutions regardless of gas types. The concentration of bulk nanobubbles will decrease in the order alkaline > acid/pure water > salt solutions. We believe that our results should be very helpful in understanding the formation and stability of bulk nanobubbles in different solutions.

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.

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.

Formation and Stability of Bulk Nanobubbles Generated by Ethanol-Water Exchange.

Bulk nanobubbles have unique properties and find potential applications in many important processes. However, their stability or long lifetime still needs to be understood and has attracted much attention from researchers. Bulk nanobubbles are generated based on ethanol-water exchange, a method that is generally used in the study of surface nanobubbles. Their formation and stability is further studied by using a new type of dynamic light scattering known as NanoSight. The results show that the concentration of the bulk nanobubbles produced by this method is about five times greater than that in the degassed group, which indicates the existence of bulk gas nanobubbles. The effects of ethanol/water ratios and temperature on the stability of the bulk nanobubbles have also been studied and their numbers reach a maximum at a ratio of about 1:10 (v/v).

Size-Dependent Stiffness of Nanodroplets: A Quantitative Analysis of the Interaction between an AFM Probe and Nanodroplets.

The interfacial properties of nanodroplets are very significant for the exploration of the basic law governing the fluid behavior at the nanoscale and also the applications in some important processes in novel materials fabrication by forming a special and local reaction environment. However, many basic factors such as the interfacial tension or stiffness of nanodroplets are still lacking, partially because of the difficulty of making quantitative measurements of the interfacial interactions at the nanometer scale. Here, we used a novel atomic force microscopy (AFM) mode, PeakForce mode, to control the interaction between an AFM probe and nanodroplets, by which we could obtain the morphology and stiffness of nanodroplets simultaneously. The change in the stiffness with the size of the nanodroplets was observed where the smaller nanodroplets usually had a larger stiffness. To explain this phenomenon, we then established a theoretical model based on the Young-Laplace equation in which the deformation and size-dependent stiffness could be described quantitatively and the experimental observations could be explained with our numerical calculations very well. The general methodology presented here could also be extended to analyze the relevant behavior of nanobubbles and other wetting phenomena at the nanoscale.

The origin of the "snap-in" in the force curve between AFM probe and the water/gas interface of nanobubbles.

The long-range attractive force or "snap-in" is an important phenomenon usually occurring when a solid particle interacts with a water/gas interface. By using PeakForce quantitative nanomechanics the origin of snap-in in the force curve between the atomic force microscopy (AFM) probe and the water/gas interface of nanobubbles has been investigated. The snap-in frequently happened when the probe was preserved for a certain time or after being used for imaging solid surfaces under atmospheric conditions. In contrast, imaging in liquids rarely induced a snap-in. After a series of control experiments, it was found that the snap-in can be attributed to hydrophobic interactions between the water/gas interface and the AFM probe, which was either modified or contaminated with hydrophobic material. The hydrophobic contamination could be efficiently removed by a conventional plasma-cleaning treatment, which prevents the occurring of the snap-in. In addition, the adsorption of sodium dodecyl sulfate onto the nanobubble surface changed the water/gas interface into hydrophilic, which also eliminated the snap-in phenomenon.

Study on nanobubble generation: saline solution/water exchange method.

In a recently introduced method for nanobubble generation, water is replaced with NaCl solution. It has the same mechanism as alcohol/water exchange: a liquid of higher gas solubility is used to replace one of lower gas solubility. Herein, the opposite process is realized by replacement of saline solutions with water. Interestingly, nanobubbles are also observed by AFM when different concentrations and valences of saline liquids are employed.

Understanding the stability of surface nanobubbles.

Surface nanobubbles emerging at solid-liquid interfaces show extreme stability. In this paper, the stability of surface nanobubbles in degassed water is discussed and investigated by AFM. The result demonstrates that surface nanobubbles are kinetically stable and the liquid/gas interface is gas impermeable. The force modulation experiment further proves that there is a layer coating on nanobubbles. These critical properties suggest that surface nanobubbles may be stabilized by a layer which has a great diffusive resistance.

Imaging interfacial micro- and nano-bubbles by scanning transmission soft X-ray microscopy.

Synchrotron-based scanning transmission soft X-ray microscopy (STXM) with nanometer resolution was used to investigate the existence and behavior of interfacial gas nanobubbles confined between two silicon nitride windows. The observed nanobubbles of SF6 and Ne with diameters smaller than 2.5 µm were quite stable. However, larger bubbles became unstable and grew during the soft X-ray imaging, indicating that stable nanobubbles may have a length scale, which is consistent with a previous report using atomic force microscopy [Zhang et al. (2010), Soft Matter, 6, 4515-4519]. Here, it is shown that STXM is a promising technique for studying the aggregation of gases near the solid/water interfaces at the nanometer scale.

Volume transition and adhesion force of nanosized bifunctional spherical polyelectrolyte brushes observed by dynamic light scattering and atomic force microscopy.

Stimuli-responsive nanoparticles have attracted considerable interest due to their enormous potential applications. In this paper, nanosized bifunctional spherical polyelectrolyte brushes (BSPBs) were prepared through grafting random copolymer chains from N-isopropylacrylamide (NIPA) and acrylic acid (AA) onto polystyrene core by photoemulsion polymerization. The pH- and thermo-responses of the BSPB with different ratio of NIPA and AA were investigated by dynamic light scattering. Atomic force microscopy (AFM) was employed to in situ monitor the pH response of the morphology and surface adhesion of BSPB as well as to visualize BSPB individually or as monolayer in air and liquid. The brush-like shell was illegible in electron microscopy images but clearly observed in AFM images. The results demonstrated that AFM was a powerful tool for in situ observing the stimulus-response of brushes on the surface of nanoparticles.

Investigation on the temperature difference method for producing nanobubbles and their physical properties.

In recent years, the possibility of nanobubbles at the solid-liquid interface has drawn wide attention in the scientific community and industry. Thus the search for evidences for the existence of nanobubbles became a scientific hotspot. To produce interfacial nanobubbles, a systematic experiment, called the temperature difference method, is carried out by replacing low temperature water (LTW) with high temperature water (HTW) at the highly-oriented pyrolytic graphite (HOPG)-water interface. When LTW (4 °C) is mixed with HTW (25-40 °C), nanobubbles are observed by atomic force microscopy (AFM), and their size, density and total volume per square micrometer are measured. Furthermore, pancake-like gas layers and the coexistence of nanobubbles on top of the pancake layers are also observed.

Defouling and cleaning using nanobubbles on stainless steel.

The present work demonstrates that nanobubbles can be used as cleaning agents on stainless steel (SS) surfaces. Cleaning efficiency has been quantified. Using an Atomic Force Microscope (AFM), it was demonstrated that nanobubbles can be produced by electrochemical treatment on a SS surface either with or without adsorbed bovine serum albumin (BSA). After allowing adsorption on SS overnight, radio-labeled BSA was removed by electrochemically generated nanobubbles, and then the remaining BSA on the surface was quantified by radioactivity measurement. The results indicate that nanobubbles can remove >10% of the protein in each 3-min electrochemical treatment while in a control group, washing with water and electrolyte resulted in no more than 3% of the protein being removed each time. Cleaning of conducting surfaces by nanobubbles is promising in any system where fouling occurs in biomedia.

Cleaning using nanobubbles: defouling by electrochemical generation of bubbles.

Here we demonstrate that nanobubbles can be used as cleaning agents both for the prevention of surface fouling and for defouling surfaces. In particular nanobubbles can be used to remove proteins that are already adsorbed to a surface, as well as for the prevention of nonspecific adsorption of proteins. Nanobubbles were produced on highly oriented pyrolytic graphite (HOPG) surfaces electrochemically and observed by atomic force microscopy (AFM). Nanobubbles produced by electrochemical treatment for 20 s before exposure to bovine serum albumin (BSA) were found to decrease protein coverage by 26-34%. Further, pre-adsorbed protein on a HOPG surface was also removed by formation of electrochemically produced nanobubbles. In AFM images, the coverage of BSA was found to decrease from 100% to 82% after 50 s of electrochemical treatment. The defouling effect of nanobubbles was also investigated using radioactively labeled BSA. The amount of BSA remaining on a stainless steel surface decreased by approximately 20% following 3 min of electrochemical treatment and further cycles of treatment effectively removed more BSA from the surface. In situ observations indicate that the air-water interface of the nanobubble is responsible for the defouling action of nanobubbles.

Magnetic force microscopy analysis of apoptosis of HL-60 cells induced by complex of antisense oligonucleotides and magnetic nanoparticles.

Magnetic force microscopy (MFM) has been employed to observe antisense oligonucleotides (ASOs)-coupled silica-coated magnetic iron oxide nanoparticles (SMNPs) internalized into human leukemia (HL-60) cells. The experiment demonstrated that the ASOs-coupled SMNPs delivery into the cells really occurred. The nanoparticles were internalized into the cells and the apoptotic topography can be directly visualized simultaneously with MFM technology. These present observations offer direct morphology evidence on studying the apoptosis of tumor cells and provide useful information for better design of new diagnostic and therapeutic tools in tumor treatment.