Characterization of nanobubbles on hydrophobic surfaces in water

The aim of this paper is to quantitatively characterize the appearance, stability, density, and shape of surface nanobubbles on hydrophobic surfaces under varying conditions such as temperature and temperature variation, gas type and concentration, surfactants, and surface treatment. The method we adopt is atomic force microscopy (AFM) operated in the tapping mode. In particular, we show (i) that nanobubbles can slide along grooves under the influence of the AFM tip, (ii) that nanobubbles can spontaneously form by substrate heating, allowing for a comparison of the surface topology with and without the nanobubble, (iii) that a water temperature increase leads to a drastic increase in the nanobubble density, (iv) that pressurizing the water with CO2 also leads to a larger nanobubble density, but typically to smaller nanobubbles, (v) that alcohol-cleaning of the surface is crucial for the formation of surface nanobubbles, (vi) that adding 2-butanol as surfactant leads to considerably smaller surface nanobubbles, and (vii) that flushing water over alcohol-covered surfaces strongly enhances the formation of surface nanobubbles.     

固液界面纳米气泡的研究进展

根据经典热力学理论,在水中纳米级的气泡难以长期稳定存在.近年来却有大量的实验结果表明固液界面存在纳米气泡,原子力显微镜也直接观察到了纳米气泡.有关纳米气泡的研究具有巨大的理论和实际意义,它对表面科学、流体动力学、生物科学以及一些应用领域都有深远的影响.纳米气泡会引起流体在界面的滑移,减少流动阻力,并与表面粘附、胶体分散、矿石浮选、废渣处理等方面密切相关.目前关于纳米气泡的研究才刚刚开始,对于它的基本物化性质的了解还不多,但其重要性已经引起相关领域的极大关注.本文综述了从提出纳米气泡存在一直到实验证明的过程、纳米气泡的形成机制和形貌、分布特征等基本性质以及纳米气泡的存在对疏水长程作用和流体滑移的影响,并阐述了生物学中一些与纳米气泡存在有关的问题.     

Scanning of silicon wafers in contact with aqueous CTAB solutions below the CMC

Hydrophilic silicon wafers are studied against aqueous solutions of hexadecyl trimethyl ammonium bromide (CTAB) at concentrations between 0.05 mM up to 1 mM (CMC). AFM studies show that nanobubbles are formed at concentrations up to 0.4 mM. From 0.5 mM upward, no bubbles could be detected. This is interpreted as the formation of hydrophobic domains of surfactant aggregates, becoming hydrophilic at about 0.5 mM. The high contact angle of the nanobubbles (140-150° through water) indicates that the nanobubbles are located on the surfactant domains. A combined imaging and colloidal probe AFM study serves to highlight the surfactant patches adsorbed at the surface via nanobubbles. The nanobubbles have a diameter between 30 and 60 nm (after tip deconvolution), depending on the surfactant concentration. This corresponds to a Laplace pressure of about 30 atm. The presence of the nanobubbles is correlated with force measurements between a silica probe and a silicon wafer surface. The study is a contribution to the better understanding of the short-range attraction between hydrophilic surfaces exposed to a surfactant solution.     

固液界面纳米气泡研究

固液界面纳米气泡是近十年来表面科学的重要发现之一。从利用原子力显微镜(AFM)在固液界面上观察到纳米气泡以来,科学工作者们已经证实了纳米气泡在固液界面上存在。由于其在微机电系统(MEMS)、微生化系统、表面科学、流体动力学等领域潜在的应用价值,各国学者们对纳米气泡的自身性质及影响因素已经开展了多方面的研究。但纳米气泡稳定性(反常的长寿)的原因仍然是未解决的问题之一。本文综述了纳米气泡的形成及影响因素,重点评述了纳米气泡稳定性理论,包括线张力理论、动态平衡理论、杂质理论和克努森气体理论等。同时,介绍了固液界面纳米气泡的应用,并展望了未来研究的重点和方向。     

Hydrophilic Domains Enhance Nanobubble Stability

Highly stable nanoscale gas states at solid/liquid interfaces, referred to as nanobubbles, have been widely studied for over a decade. In this study, nanobubbles generated on a hydrophobic Teflon amorphous fluoroplastic thin film in the presence and absence of hydrophilic carbon domains are investigated by peak force quantitative nanomechanics. On the hydrophobic surface without hydrophilic domains, a small number of nanobubbles are generated and then rapidly decrease in size. On the hydrophobic surface with hydrophilic domains, the hydrophilic domains have a significant effect on the generation and stability of nanobubbles, with bubbles remaining on the surface for up to three days.     

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.     

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.     

Physical properties of nanobubbles on hydrophobic surfaces in water and aqueous solutions

In recent years there has been an accumulation of evidence for the existence of nanobubbles on hydrophobic surfaces in water, despite predictions that such small bubbles should rapidly dissolve because of the high internal pressure associated with the interfacial curvature and the resulting increase in gas solubility. Nanobubbles are of interest among surface scientists because of their potential importance in the long-range hydrophobic attraction, microfluidics, and adsorption at hydrophobic surfaces. Here we employ recently developed techniques designed to induce nanobubbles, coupled with high-resolution tapping-mode atomic force microscopy (TM-AFM) to measure some of the physical properties of nanobubbles in a reliable and repeatable manner. We have reproduced the earlier findings reported by Hu and co-workers. We have also studied the effect of a wide range of solutes on the stability and morphology of these deliberately formed nanobubbles, including monovalent and multivalent salts, cationic, anionic, and nonionic surfactants, as well as solution pH. The measured physical properties of these nanobubbles are in broad agreement with those of macroscopic bubbles, with one notable exception: the contact angle. The nanobubble contact angle (measured through the denser aqueous phase) was found to be much larger than the macroscopic contact angle on the same substrate. The larger contact angle results in a larger radius of curvature and a commensurate decrease in the Laplace pressure. These findings provide further evidence that nanobubbles can be formed in water under some conditions. Once formed, these nanobubbles remain on hydrophobic surfaces for hours, and this apparent stability still remains a well-recognized mystery. The implications for sample preparation in surface science and in surface chemistry are discussed.     

Influence of surface topography on the interactions between nanostructured hydrophobic surfaces

Nanostructured particle coated surfaces, with hydrophobized particles arranged in close to hexagonal order and of specific diameters ranging from 30 nm up to 800 nm, were prepared by Langmuir-Blodgett deposition followed by silanization. These surfaces have been used to study interactions between hydrophobic surfaces and a hydrophobic probe using the AFM colloidal probe technique. The different particle coated surfaces exhibit similar water contact angles, independent of particle size, which facilitates studies of how the roughness length scale affects capillary forces (previously often referred to as "hydrophobic interactions") in aqueous solutions. For surfaces with smaller particles (diameter < 200 nm), an increase in roughness length scale is accompanied by a decrease in adhesion force and bubble rupture distance. It is suggested that this is caused by energy barriers that prevent the motion of the three-phase (vapor/liquid/solid) line over the surface features, which counteracts capillary growth. Some of the measured force curves display extremely long-range interaction behavior with rupture distances of several micrometers and capillary growth with an increase in volume during retraction. This is thought to be a consequence of nanobubbles resting on top of the surface features and an influx of air from the crevices between the particles on the surface.     

Removal of nanoparticles from plain and patterned surfaces using nanobubbles

It is the aim of this paper to quantitatively characterize the capability of surface nanobubbles for surface cleaning, i.e., removal of nanodimensioned polystyrene particles from the surface. We adopt two types of substrates: plain and nanopatterned (trench/ridge) silicon wafer. The method used to generate nanobubbles on the surfaces is the so-called alcohol-water exchange process (use water to flush a surface that is already covered by alcohol). It is revealed that nanobubbles are generated on both surfaces, and have a remarkably high coverage on the nanopatterns. In particular, we show that nanoparticles are-in the event of nanobubble occurrence-removed efficiently from both surfaces. The result is compared with other bubble-free wet cleaning techniques, i.e., water rinsing, alcohol rinsing, and water-alcohol exchange process (use alcohol to flush a water-covered surface, generating no nanobubbles) which all cause no or very limited removal of nanoparticles. Scanning electron microscopy (SEM) and helium ion microscopy (HIM) are employed for surface inspection. Nanobubble formation and the following nanoparticle removal are monitored with atomic force microscopy (AFM) operated in liquid, allowing for visualization of the two events.     

Nanobubbles give evidence of incomplete wetting at a hydrophobic interface

The appearance of a hydrophobic surface, namely a crystalline (111) Si wafer coated with a thick soft polystyrene film, and the morphological changes along this interface depending on the polarity of an adjoining liquid phase were studied with magnetic tapping mode atomic force microscopy. Interfacially associated nanobubbles of decreasing size and number are observed as the hydrophobicity of the subphase increases. The disturbance of the water structure in the contact region induces the formation of nanobubbles. The topology of the interface is visualized, starting with the dry polymer under normal atmosphere conditions and observing the changes as air is replaced by a series of liquids. With water, the surface coverage of the substrate with bubbles is almost a close-packed configuration. The bubble shape is well approximated by spherical caps of a rather low aspect ratio. The Gaussian size distributions of bubble shape parameters are discussed. The contact angle of the nanobubbles is substantially smaller than the corresponding number measured for a macroscopic droplet. This apparent discrepancy might be resolved if the nanobubbles were assumed to exist along the interface as a connecting sublayer between a depleted water film at the hydrophobic polymer surface and an adsorbed macrodroplet.     

Stability of self-assembled hydrophobic surfactant layers in water

Using contact angle measurements, surface force balance experiments, and AFM imaging, we have investigated the process of self-assembly of surfactants onto mica and the subsequent stability of those layers in pure water. In the case of cetyltrimethylammonium bromide (CTAB), the stability of a monolayer when immersed in pure water is found to be dependent on initial immersion time in surfactant, which is likely to be caused by an increase in the proportion of ion-exchange to ion-pair adsorption when incubated in surfactant for longer periods of time. Infinite dilution of the surfactant solution before withdrawal of the sample is found to have little effect on the stability of the resulting layer in pure water. The nature of the counterion is found to affect dramatically the stability of a self-assembled surfactant monolayer: cetyltrimethylammonium fluoride (CTAF) forms a layer that is much more stable in water than CTAB, which is likely to be due to faster and more complete ion-exchange with the mica surface for CTAF. Surface force balance experiments show that when the hydrophobic monolayer is immersed in pure water it does not simply dissolve into the water; instead it rearranges, possibly to patches of bilayer or hemimicelles. The time scale of this rearrangement agrees well with the time scale of the change from a hydrophobic to more hydrophilic surface observed using contact angle measurements. AFM imaging has also in some cases shown an evolution from an even monolayer to patches of bilayer.     

Surface morphology of nanostructured polymer-based activated carbons

Complementary techniques, including nitrogen adsorption, small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM), have been utilized to characterize the surface features of highly microporous carbon materials prepared from highly aromatic polymers. Nitrogen adsorption measurement interpreted by BET, DR, HK, and NLDFT methods reveals these nanostructured activated carbons exhibit a high surface area of up to 4000 m2/g, a micropore volume up to approximately 1.75 mL/g, and an average pore size of approximately 10-20 angstroms. A modified equation, based on Porod's law, the Debye-Bueche equation, and fractal dimension theories, has been proposed and successfully applied to analyze the SAXS spectra and to extract the porous texture of these unique activated carbons. AFM 3D imaging combined with the Fourier transform technique has been applied to statistically quantify pore sizes on the carbon surface.     

Transient behavior of the hydrophobic surface/water interface: from nanobubbles to organic layer

We report the formation and subsequent change of the water-depleted layer at a hydrophobic surface/water interface. With water as the solvent, surface plasmon resonance measurements indicate time dependent evolution of two separate states. The first state is the water-depleted layer, and it is characterized by a layer of nanobubbles on the surface and is short-lived in time (order of 10 min). The second state is a final equilibrium state, which occurs in approximately 30 h, where a layer is formed with organic characteristics. If, instead of water, an aqueous solution is exposed to the hydrophobic surface, the evolution from nanobubbles to an organic like layer shows dependency on the surface energy of the liquid media.     

Nanohybrids from liquid crystalline extended amphiphilic dendrimers

A novel extended amphiphilic dendrimer with linear poly(ethylene oxide) (PEO) attached to a PEO-like dendritic core as hydrophilic fraction and eight docosyl chain branches as hydrophobic fraction has been prepared for the use as structure-directing agent for silica-type materials. The extended dendrimer exhibits a hexagonal columnar liquid crystalline phase in the melt. Organically modified inorganic precursors and the extended dendrimer co-assemble into nanostructured hybrids. Hybrids with 0.44 weight fraction (fw) of aluminosilicate show a lamellar morphology, while hybrids with 0.21 fw exhibit a cylindrical structure. Nanostructures were characterized by a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The results suggest that dendrimer-based amphiphiles may provide an exciting platform for the formation of multifunctional organic-inorganic nanostructured hybrid materials with unique structural characteristics.     

电化学控制产生纳米氧气气泡及其对电化学聚合吡咯形貌的影响

利用原位电化学原子力显微镜(in-situ EC-AFM)研究了纳米氧气气泡在高定向热解石墨(HOPG)表面的电化学控制产生与生长. AFM原位图像表明电化学产生的氧气在HOPG表面形成了纳米气泡, 并且纳米气泡的产生与生长可以通过改变外加电压和反应时间来进行控制. 随后, 研究了电化学产生的纳米氧气气泡对于在HOPG表面电化学聚合吡咯反应的影响, 结果表明伴随聚合反应过程产生的纳米氧气气泡使得生成的聚吡咯膜表面形成了气泡状的缺陷.     

Contact angles of surface nanobubbles on mixed self-assembled monolayers with systematically varied macroscopic wettability by atomic force microscopy

The dependence of the properties of so-called "surface nanobubbles" at the interface of binary self-assembled monolayers (SAMs) of octadecanethiol (ODT) and 16-mercaptohexadecanoic acid (MHDA) on ultraflat template-stripped gold and water on the surface composition was studied systematically by in situ atomic force microscopy (AFM). The macroscopic water contact angle (θ(macro)) of the SAMs spanned the range between 107° ± 1° and 15° ± 3°. Surface nanobubbles were observed on all SAMs by intermittent contact-mode AFM; their size and contact angle were found to depend on the composition of the SAM. In particular, nanoscopic contact angles θ(nano) < 86° were observed for the first time for hydrophilic surfaces. From fits of the top of the bubble profile to a spherical cap in three dimensions, quantitative estimates of nanobubble height, width, and radius of curvature were obtained. Values of θ(nano) calculated from these data were found to change from 167° ± 3° to 33° ± 58°, when θ(macro) decreased from 107° ± 1° to 37° ± 3°. While the values for θ(nano) significantly exceeded those of θ(macro) for hydrophobic SAMs, which is fully in line with previous reports, this discrepancy became less pronounced and finally vanished for more hydrophilic surfaces.     

Removal of induced nanobubbles from water/graphite interfaces by partial degassing

Nanobubbles at an interface between a hydrophobic solid and water have a wide range of implications, but the evidence for their existence is still being debated. Here we artificially induced nanobubbles on freshly cleaved HOPG substrates in water using the protocol developed previously and subjected the system to moderate levels of degassing (approximately 0.1 atm for 0.5 to 3 h). The AFM images after the partial degassing revealed that some nanobubbles had coalesced and detached from the substrate because of buoyancy, whereas others apparently remained unaffected. The size and spatial distributions of the nanobubbles after the partial degassing suggest that there is a critical size for a nanobubble above which it may grow. The contact angle of water next to nanobubbles (approximately 160 degrees) is much larger than the advancing contact angle of a macroscopic water droplet on the same substrate (approximately 80 degrees) both before and after the partial degassing and concomitant growth and shrinkage of the nanobubbles. The contact angle of a nanobubble also remained unchanged as the nanobubble was moved along the substrate by the AFM tip. The apparent lack of contact angle hysteresis in the nanobubble systems may suggest that the very large contact angle may correspond to a local minimum of the free-energy landscape.     

Preparation and characterization of core-shell nanoparticles hardened by gamma-ray

Core-shell nanoparticles have been prepared by irradiation of gamma-ray on block copolymer micelles consisting of hydrophilic polyacrylic acid and hydrophobic polyisoprene with each 40 monomer units. The structure was determined by means of dynamic light scattering (DLS), small angle X-ray scattering (SAXS) and atomic force microscopy (AFM). The size distribution of the core-shell nanoparticles determined by DLS and AFM was very narrow. The average diameter of the particles decreased from 48 nm for the original micelles to 26 nm by the irradiation of 30 kGy. The core size determined by SAXS combined with DLS was roughly constant of 10 nm, irrespective of irradiation dose, whereas the shell thickness of the micelles was twice as large as the core size, and decreased with increasing irradiation dose.     

Non-DLVO silica interaction forces in NMP-water mixtures. II. An asymmetric system

The interaction between energetically asymmetric hydrophilic and hydrophobic surfaces has fundamental and practical importance in both industrial and natural colloidal systems. The interaction forces between a hydrophilic silica sphere and a silanated, hydrophobic glass plate in N-methyl-2-pyrrolidone (NMP)-water binary mixtures were measured using atomic force microscopy (AFM). A strong and long-range attractive force was observed in pure water and was attributed to the formation of capillary bridges associated with nanoscale bubbles initially present on the hydrophobic surface. When NMP was added, the capillary force and corresponding pull-off force became less attractive, which was explained readily in terms of the surface wettability by the binary solvent mixture. Similar to the case of symmetric (two hydrophilic) surfaces, the range of attraction between the asymmetric surfaces was maximized at around 30 vol % NMP, which is consistent with the formation of a thick adsorbed macrocluster layer on the hydrophilic silica surface.