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.     

Polymeric Foaming with Nanoscale Nucleants: A Surface Nanobubble Mechanism

The dimensionally restricted, diffusion‐driven volumetric change of almost flat nucleated surface nanobubbles hosted on dispersed nanoscale surfaces is proposed as the probable mechanism of heterogeneous bubble generation during polymer–nanoscale‐nucleant suspension foaming. By conducting numerical simulations, this hypothesis is used to predict the final bubble sizes upon polymeric foaming with nanoscale nucleants and to compare them with reported experimentally determined values. The volumetric change in the bubble hosted on the miniscule surface is envisaged to occur due to two parallel diffusion processes: 1) through the contact line of the bubble cap with the surface, and 2) through the curved gas–polymer interface. The foaming conditions determine the direction and molar rate of both these diffusions. The mechanism explains the relative nucleating efficiency of nanoscale surfaces experimentally observed during reactive and nonreactive polymeric foaming by predicting the growth or dissolution of the bubble. In the case of nonreactive thermoplastic foaming, the size of the bubbles released to the bulk from the nanoscale surface varies in a near linear fashion with respect to the size of the nucleants, limited to a maximum nucleant size. Beyond this maximum, the size of bubble generated is independent of the nucleant size. However, increase in the initial nanoscopic contact angle does not significantly affect the bubble size upon detachment from the surface.     

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 relationship between nanobubbles and the hydrophobic force

The formation of nanobubbles on hydrophobic self-assembled monolayers has been examined in a binary ethanol/water titration using small angle X-ray scattering (SAXS) and atomic force microscopy (AFM). The AFM data demonstrates a localized force effect attributed to nanobubbles on an immersed hydrophobic surface. This evidence is arguably compromised by the possibility that the AFM tip actually nucleates nanobubbles. As a complementary noninvasive technique, SAXS has been used to investigate the interfacial region of the immersed hydrophobic surface. SAXS measurements reveal an electron density depletion layer at the hydrophobic interface, with changing air solubility in the immersing liquid, due to the formation of nanobubbles.     

Movement of fine particles on an air bubble surface studied using high-speed video microscopy

A CCD high-speed video microscopy system operating at 1000 frames per second was used to obtain direct quantitative measurements of the trajectories of fine glass spheres on the surface of air bubbles. The glass spheres were rendered hydrophobic by a methylation process. Rupture of the intervening water film between a hydrophobic particle and an air bubble with the consequent formation of a three-phase contact was observed. The bubble-particle sliding attachment interaction is not satisfactorily described by the available theories. Surface forces had little effect on the particle sliding with a water film, which ruptured probably due to the submicrometer-sized gas bubbles existing at the hydrophobic particle-water interface.     

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.     

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.     

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.     

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.     

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.     

Wetting properties at the surface of iota-carrageenan-based edible films

Surface properties of edible films composed of a polymeric matrix of carrageenan in association with hydrophobic material were studied by contact angle measurements. The use of this technique not only in a static mode but also in a dynamic way enables investigation of surface hydrophobicity as well as surface wettability. The absorption flux inside the material can be estimated from the wetting kinetic, which can be very useful to quickly compare water barrier efficiency of the tested films. Comparison of carrageenan films with films containing known amounts of additives enables understanding and correlation of changes of the surface properties with the nature of used additives (glycerol used as a plasticizer, glycerol monostearate used as a surfactant, and fat) and their influence on the orientation of polymer chains at the surface during film formation. Very different responses were observed from one surface of the film (film-casting-support interface) to the other (film-air interface), which could be also attributed to the influence of the support on the polymer and to macromolecular orientation during drying after casting.     

Surface modification of PET films by atmospheric pressure plasma exposure with three reactive gas sources

The surface modification of poly (ethylene terephthalate) (PET) film was carried out using an atmospheric pressure plasma (APP) jet device with three reactive gases: air, N₂, and Ar. The water contact angles on the PET film were found to decrease considerably after the APP exposure. The changes in the advancing and receding contact angles of water on the APP-exposed PET film with aging time were examined by the wetting force measurements employing the Wilhelmy method. The hydrophobic recovery due to the rinsing with water as well as the aging in air was observed only for the advancing angle, which was probably caused by the dissolution of low molecular weight oxidized materials into water, the loss of volatile oxidized species to the atmosphere and the reorientation and the migration of polymer chains. The wettability and the surface free energy of the APP-exposed PET film after diminishing hydrophobic recovery was sufficiently large compared with the untreated film. X-ray photoelectron spectroscopy confirmed that the PET film surface was oxidized due to the APP exposure. When N₂ gas was used for the APP exposure, the surface nitrogen concentration was found to increase with decreasing D. The surface oxygen concentration on the APP-exposed PET film was reduced by rinsing with water, in accordance with the hydrophobic recovery behavior. From atomic force microscopy, surface topographical change due to the APP exposure was observed. The changes in the PET surface properties due to the APP exposure as mentioned above were remarkable for using N₂ gas.     

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.     

Influence of aqueous electrolytes on the wetting behavior of hydrophobic solid polymers-low-rate dynamic liquid/fluid contact angle measurements using axisymmetric drop shape analysis

The interaction of inorganic ions with low-energy hydrophobic surfaces was examined using model systems of solid polymers without ionizable functional surface groups in aqueous electrolyte solutions. Low-rate dynamic contact angle measurements with captive bubbles in conjunction with axisymmetric drop shape analysis (ADSA) were performed to study the influence of electrolyte ions (in the aqueous test solutions) on the wettability of the polymers. When various types of ions were used, no significant change in advancing and receding contact angles was observed. The contact angle hysteresis was small. The zeta potential of the model polymers in aqueous electrolyte solutions was determined from streaming potential measurements. The variation of the zeta potential at different pH levels indicates preferential adsorption of hydroxyl ions at this interface. However, the presence of electrolytes at the interface between water and the different model polymers did not influence the macroscopic contact angle. The results may suggest the absence of any specific interaction between the ions and the solid polymer, as this should result in changes of hydrophobicity. Similar to the air/water interface, the composition and the potential of the polymer/water interface are obviously determined predominantly by the aqueous phase with only slight influence from the solid phase.     

Effects of pH and ionic strength on the stability of nanobubbles in aqueous solutions of alpha-cyclodextrin

Formation of stable nanobubbles in aqueous solutions of water-soluble organic molecules is a spontaneous process. Using a combination of laser light scattering (LLS) and zeta-potential measurements, we investigated the effects of salt concentration and pH on their stability in alpha-cyclodextrin (alpha-CD) aqueous solutions. Our results reveal that the nanobubbles are unstable in solution with a higher ionic strength, just like colloidal particles in an aqueous dispersion, but become more stable in alkaline solutions. The zeta-potential measurement shows that the nanobubbles are negatively charged with an electric double layer, presumably due to adsorption of negative OH- ions at the gas/water interface. It is this double layer that plays a critical dual role in the formation of stable nanobubbles in aqueous solutions of water-soluble organic molecules, namely, it not only provides a repulsive force to prevent interbubble aggregation and coalescence but also reduces the surface tension at the gas/water interface to decrease the internal pressure inside each bubble.     

Gas permeability in polymer- and surfactant-stabilized bubble films

The gas permeabilities of thin liquid films stabilized by poly(N-isopropylacrylamide) (PNIPAM) and PNIPAM-SDS (sodium dodecyl sulfate) mixtures are studied using the "diminishing bubble" method. The method consists of forming a microbubble on the surface of the polymer solution and measuring the shrinking rates of the bubble and the bubble film as the gas diffuses from the interior to the exterior of the bubble. PNIPAM-stabilized films exhibit variable thicknesses and homogeneities. Interestingly, despite these variable features, the gas permeability of the film is determined principally by the structure of the adsorbed polymer layer that provides an efficient gas barrier with a value of gas permeability coefficient that is comparable to that of an SDS Newton black film. In the presence of SDS, both the film homogeneity and the gas permeability coefficient increase. These changes are related to interactions of PNIPAM with SDS in the solution and at the interface, where coadsorption of the two species forms mixed layers that are stable but that are more porous to gas transfer. The mixed PNIPAM-SDS layers, studied previously for a single water-air interface by neutron reflectivity, are further characterized here in a vertical free-draining film using X-ray reflectivity.     

Adhesion forces in liquid media: effect of surface topography and wettability

This work was motivated by the unexpected values of adhesion forces measured between an atomic force microscopy tip and the hydrophobic surface of ultra-high-molecular-weight polyethylene. Two types of samples with different roughness but similar wettability were tested. Adhesion forces of similar magnitude were obtained in air and in polar liquids (water and Hank's Balanced Salt Solution, a saline solution) with the rougher sample. In contrast, the adhesion forces measured on the smoother sample in air were much higher than those measured in water or in the aqueous solution. Those experimental results suggested the presence of nanobubbles at the interface between the rough sample and the polar liquids. The existence of the nanobubbles was further confirmed by the images of the interface obtained in noncontact tapping mode. The adhesion forces measured in a nonpolar liquid (hexadecane) were small and of the same order of magnitude for both samples and their values were in good agreement with the predictions of the London-Hamaker approach for the van der Waals interactions. Finally, we correlate the appearance of nanobubbles with surface topography. The conclusion of this work is that adhesion forces measured in aqueous media may be strongly affected by the presence of nanobubbles if the surface presents topographical accidents.     

Water and ice in contact with octadecyl-trichlorosilane functionalized surfaces: a high resolution x-ray reflectivity study

We present a high energy x-ray reflectivity study of the density profiles of water and ice at hydrophobic and hydrophilic substrates. At the hydrophobic water/octadecyl-trichlorosilane (water-OTS) interface, we find clear evidence for a thin density depletion layer with an integrated density deficit corresponding to approximately 40% of a monolayer of water molecules. We discuss the experimental results in terms of a simple model of hydrophobic/hydrophilic solid-liquid interfaces. Our results also exclude the presence of nanobubbles. A detailed study of possible radiation damage induced by the intense x-ray beam at the dry OTS surface and at the ice-OTS, as well as at water-OTS interfaces, discloses that noticeable damage is only induced at the water-OTS interface, and thus points to the dominant role of highly mobile radicals formed in bulk water close to the interface.     

Synthesis and Characterization of β‐CD‐Coated Polystyrene Microspheres by γ‐Ray Radiation Emulsion Polymerization

Polystyrene (PS) microspheres coated with β‐cyclodextrin (β‐CD) were fabricated via γ‐ray‐induced emulsion polymerization in a ternary system of styrene/β‐CD/water (St/β‐CD/water). The solid inclusion complex of St and β‐CD particles formed at the St droplets–water interface can stabilize the emulsion as the surfactant. TEM and XPS results showed that β‐CD remains on the surface of PS particles. The average size of the PS particles increases from 186 to 294 nm as the weight ratio of β‐CD to St rises from 5% to 12.5%. The water contact angle (CA) of PS latex film is lower than 90°, and reduces with the β‐CD content even to 36°. Thus, this work provides a new and one‐pot strategy to surface hydrophilic modification on hydrophobic polymer particles with cyclodextrins through radiation emulsion polymerization.     

Contact angle hysteresis: study by dynamic cycling contact angle measurements and variable angle spectroscopic ellipsometry on polyimide

The phenomenon of contact angle hysteresis was studied on smooth films of polyimide, a polymer type used in the microelectronic industry, by dynamic cycling contact angle measurements based on axisymmetric drop shape analysis-profile in combination with variable angle spectroscopic ellipsometry (VASE). It was found that both advancing and receding contact angles became smaller with increasing the number of cycles and are, therefore, not a property of the dry solid alone. The changes of the wetting behavior during these dynamic cycling contact angle measurements are attributed mainly to swelling and/or liquid retention. To reveal the water-induced changes of the polymer film, the polyimide surface was studied before and after the contact with a water droplet by VASE. Both the experimental ellipsometric spectrum for Delta and that for Psi as well as the corresponding simulations show characteristic shifts due to the contact with water. The so-called effective medium approximation was applied to recover information about the thickness and effective optical constants of the polymer layer from the ellipsometrically measured values of Delta and Psi. On the basis of these results, the swelling and retention behavior of the polyimide films in contact with water droplets were discussed.