Hemispherical nanobubbles reduce interfacial slippage in simple liquids

Using an electrochemical quartz crystal microbalance (EQCM), we have produced bubbles of nanoscopic size at the front electrode of an acoustic shear wave resonator. Nanobubbles are usually expected to increase the resonance frequency because they have a low density and, also, because a liquid slides easily at a liquid-air interface. However, the bubble-induced frequency shift in many cases was negative, which implies positive hydrodynamic thickness and reduced slippage. The explanation is based on Laplace pressure. Due to the bubbles' inherent stiffness, the space in-between neighboring bubbles may turn into an assembly of pockets which move with the underlying substrate in the same way as a solid film. If, first, the bubbles are so small that the Laplace pressure can overcome the viscous drag, and, second, the contact angle is in the range of 90°, the latter effect dominates. This interpretation was corroborated by a calculation using the finite element method (FEM). The argument as such is not limited to acoustic shear waves: hemispherical nanobubbles increase the surface drag in stationary flows in the same way.     

Surface reduction of freely floating smectic bubbles

Elongated freely floating smectic bubbles are observed during their relaxation to equilibrium sphere shape. Unlike soap bubbles that perform weakly damped oscillations into equilibrium, this relaxation is overdamped in smectics by internal structure reorganisation processes. The bubble area reduction of centimetre-sized freely floating bubbles with few nanometres film thickness is recorded with high-speed optical imaging in microgravity and analysed quantitatively. We find a nearly linear reduction of the film area with time, driven by capillary forces and inhibited by smectic layer reorganisations. Characteristic times are in the milliseconds range, with little correlation to the film thickness and bubble size. Instead, the homogeneity of the films and the number and sizes of islands of excess layers that spontaneously form on the films appear to have crucial influence on the dynamics. The efficiency of this process sets the time scale of the film area shrinkage. We discuss the limitations of a minimalistic model that captures smectic layer reorganisation processes.     

Measurement of thickness and composition of a solvent film on a bubble

Solvent-coated air bubbles in the air-assisted solvent extraction (AASX) process achieve the dual role of high solvent specific surface area and ease of phase separation. The properties and thickness of the solvent film control the process. As an approach to the study, the layer interferometry (in the UV-vis region) and FT-IR spectroscopy were used to measure the time dependent thickness and chemical composition, respectively, of a film formed by blowing an air bubble in kerosene-based solvents. The film was stabilized by the presence of 1.5 ppm silicone oil, as employed in AASX. The film appears to comprise two layers; an outer layer of almost constant thickness and an inner layer which decreased in thickness with time. The latter is considered relevant to AASX. Generally, the initial thickness was approximately 3 microm which decreased over several minutes to a final rupture thickness of 500 nm. The initial thickness is of the order determined indirectly. The chemical composition of the layer did not change with time.     

纳米多孔阳极氧化铁膜的形成及其形貌演变

阳极氧化法制备具有纳米多孔结构的阳极氧化铁膜因其潜在的应用价值而倍受关注。然而,在阳极氧化过程中多孔结构的形成机制至今尚不清楚。本文结合电流密度-电位响应（I-V曲线）及法拉第定律的推导,分析了形成纳米多孔阳极氧化铁膜的过程中阳极电流的组成。结果表明,离子电流（导致离子迁移形成氧化物）和电子电流（导致析出氧气）共同组成阳极电流,并且纳米多孔阳极氧化铁膜的形成与两种电流的占比相关。分段式氧化物之间的空腔以及在阳极氧化初期纳米孔道上覆盖的致密膜,表明氧气泡可能是从氧化膜内部析出。此时,阳离子和阴离子绕过作为模具的氧气泡实现传质,最终导致纳米多孔结构的形成。此外,在阳极氧化铁膜形貌演变过程中,氧气泡不断向外溢出会使表面氧化物被冲破,导致表面孔径不断增大。     

Effect of Surfactants on the Film Drainage

Drainage of a partially mobile thin liquid film between two deformed and nondeformed gas bubbles with different radii is studied. The lubrication approximation is used to obtain the influence of soluble and insoluble surfactants on the velocity of film thinning in the case of quasi-steady state approach. The material properties of the interfaces (surface viscosity, Gibbs elasticity, surface diffusivity, and/or bulk diffusivity) are taken into account. In the case of deformed bubbles the influence of the meniscus is illustrated assuming simple approximated shape for the local film thickness. Simple analytical solutions for large and small values of the interfacial viscosity, and for deformed and nondeformed bubbles, are derived. The correctness of the boundary conditions used in the literature is discussed. The numerical analysis of the governing equation shows the region of transition from partially mobile to immobile interfaces. Quantitative explanation of the following effects is proposed: (i) increase of the mobility due to increasing bulk and surface diffusivities; (ii) role of the surface viscosity, comparable to that of the Gibbs elasticity; and (iii) significant influence of the meniscus on the film drainage due to the increased hydrodynamic resistance. Copyright 1999 Academic Press.     

Isoelectric state and stability of foam films, bubbles and foams from PEO–PPO–PEO triblock copolymer (P85)

Influence of pH of P85 copolymer solutions on stability of microscopic foam films (static conditions), lifetime of single bubbles at solution surface (quasi-static conditions), volume of the foam formed (dynamic conditions) and time of rupture of the foam column was investigated. Variations of the film equilibrium thickness as a function of pH were determined for different ionic strengths of the solutions using microinferometric method, while the combined pneumatic–mechanical method was applied in experiments on foams. It was found that lowering the pH reduced stability of the foam films, and at lower ionic strength the films ruptured at pH 2.9 (isoelectric point). Simultaneously, the lifetime of single bubbles was much shorter at lower pH of the P85 solutions. The average life, t _av, was 11.1 s at pH 5.8, while at pH 3.0, only 3.1 s. Under dynamic conditions the pH lowering did not significantly influence the solution foamability.     

Coalescence of bubbles covered by particles

The interaction between two bubbles coated with glass particles in the presence of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) was studied experimentally. The time taken for two bubbles to coalesce was determined as a function of the fractional coverage of the surface by particles. The results suggested that the coalescence time increases with the bubble surface coverage. Interestingly, it was found that although the particles did not have any physical role in film rupture at low surface coverage, they still added resistance to film drainage. For particle-loaded bubbles, the initial resistance was due to the lateral capillary interactions between particles on the interface, which hold the particles firmly together. The coalescence dynamics of bubbles was also observed to be affected by the presence of attached particles.     

Surface forces and the stability of colloids and disperse systems

There are discussed two factors controlling the thickness h of a free film of liquid between two bubbles of radius R pressed against one another, the mechanical properties of the film and the temperature. The role of the viscosity, η, is analyzed on the basis of the formula h = 2.64 R(ηu/σ)^2/3 (σ surface tension, u thereceding velocity of the film perimeter), which is the simple consequence of the formula for the thickness of the film left behind the receding wetting perimeter (Derjaguin, 1943). A similar formula is also used for the analysis of the role of yield value of the film. The temperature does not change the thickness of the black free films (100A.) in contrast to the thicker films governed by electric repulsion. The jumpwise thinning of free films is studied by microfilming.

The method of crossed polarized metal wires is described, which permits measurement of the potential barrier preventing metallic contact in liquid media. The measurements of this barrier in water solutions of electrolytes as a function of potential of the wires are interpreted on the basis of our theory of colloid stability. The existence of repulsive forces (disjoining pressure) at high electrolyte concentrations independent of the potential proves their nonelectrostatic origin. These measurements were also used to calculate Hamaker's constant of molecular attraction and to determine the potential of zero charge of some metals.     

Detachment force of particles from air-liquid interfaces of films and bubbles

The detachment force required to pull a microparticle from an air-liquid interface is measured using atomic force microscopy (AFM) and the colloidal probe technique. Water, solutions of sodium dodecyl sulfate (SDS), and silicone oils are tested in order to study the effects of surface tension and viscosity. Two different liquid geometries are considered: the air-liquid interface of a bubble and a liquid film on a solid substrate. It was shown that detaching particles from liquid films is fundamentally different than from bubbles or drops due to the restricted flow of the liquid phase. Additional force is required to detach a particle from a film, and the maximum force during detachment is not necessarily at the position where the particle breaks away from the interface (as seen in bubble or drop systems). This is due to the dynamics of meniscus formation and viscous effects, which must be considered if the liquid is constrained in a film. The magnitude of these effects is related to the liquid viscosity, film thickness, and detachment speed.     

Recent advances in studies of bubble-solid interactions and wetting film stability

Research in the area of bubble-solid interactions is reviewed and highlighted, with a focus on studies of wetting film drainage using theoretical approaches and experimental (interferometric) approaches, and also studies probing the stability of wetting films, where the stability has been affected by physical and chemical modification/factors. Significant advances have been made in recent years in the area of interferometry and force measurement of bubble-surface encounters, with multiple light wavelengths used to improve accuracy and certainty with regard to thickness of wetting films, as well as high speed interferometry. These advances have been accompanied with improvements to models to describe nonequilibrium aspects of opposing interfaces. Experimental studies of the influence of air bubbles and surface roughness have highlighted the importance of dissolved gas and surface condition in determining whether wetting films are stable. Finally, many new studies on the influence of polymer layers on wetting film stability and rupture have been published, and these are described in relation to the increase in our understanding of the role of adsorbed polymers in altering surface chemistry and physics of the underlying substrate.     

Time-resolved light scattering studies of Spin-coated titanium dioxide

Spin coating of titanium alkoxides is a fast and straightforward method of forming compact and durable titanium dioxide thin films. In this report, an optical interference technique is used to understand the dynamics of titanium dioxide thin film formation during the spin coating process of titanium (IV) ethoxide sols, and a 4-stage thinning model is proposed. Optical monitoring of interference fringes as a function of time enables determination of the time-dependent film thickness through measurement of the interference fringe separation while the sol to gel transition can be monitored by analyzing the fringe visibility. For films that are spun at 2,000 rpm or faster, a spin duration of 5 s is sufficient for the film to reach its final thickness. At a given spin speed, spin times longer than 5 s do not affect the final film thickness. However, thinner final film thicknesses can be achieved by increasing the spin speed. All films were found to densify on annealing with an annealed thickness approximately 0.4 times that of the pre-anneal thickness regardless of spin speed. Spin-coated titanium dioxide films are expected to play an important role in low-cost and scalable next generation solar cells employing dyes and quantum dots as the light harvesting agents.     

金属铝诱导法低温制备多晶硅薄膜

以氢气稀释的硅烷(SiH₄)和硼烷(B₂H₆)为气源,利用等离子体增强化学气相沉积法(PECVD)制备出p型a-Si薄膜.采用铝诱导晶化技术对不同厚度的铝膜对a-Si薄膜晶化的影响进行了研究.实验中发现,铝膜溅射为10 s的非晶硅薄膜样品在450℃下退火10 min后,p型a-Si结构仍为非晶态,铝膜溅射为20 s的非晶硅薄膜在450℃下退火20 min后,p型a-Si薄膜开始晶化为poly-Si薄膜,并且铝膜厚度越厚,则a-Si薄膜晶化程度越强.     

Electrochemically produced responsive hydrogel films: influence of added salt on thickness and morphology

We report on electrochemically prepared hydrogel layers of poly-N-isopropylacrylamide (pNIPAm) and on the influence that the supporting electrolyte has on their thickness and morphology. Ions that are destabilizing in the Hofmeister sense increase the thickness. The effect correlates well with the ion's tendency to lower the lower critical solution temperature (LCST) of pNIPAm films. AFM micrographs show small-scale globules. When the films were produced in the presence of a destabilizing salt (such as ammonium sulfate) one also observes larger features, resembling wrinkles. We attribute the globules to nucleated growth of surface-attached microgels, whereas the wrinkles presumably are produced by the collapse of hydrogen bubbles underneath a well-crosslinked film. Adding a chain transfer agent to the reactant solution reduces the lateral heterogeneities.     

Decay of standing foams: drainage,coalescence and collapse

A summary of recent theoretical work on the decay of foams is presented. In a series of papers, we have proposed models for the drainage, coalescence and collapse of foams with time. Each of our papers dealt with a different aspect of foam decay and involved several assumptions. The fundamental equations, the assumptions involved and the results obtained are discussed in detail and presented within a unified framework.Film drainage is modeled using the Reynolds equation for flow between parallel circular disks and film rupture is assumed to occur when the film thickness falls below a certain critical thickness which corresponds to the maximum disjoining pressure. Fluid flow in the Plateau border channels is modeled using a Hagen-Poiseuille type flow in ducts with triangular cross-section.The foam is assumed to be composed of pentagonal dodecahedral bubbles and global conservation equations for the liquid, the gas and the surfactant are solved to obtain information about the state of the decaying foam as a function of time. Homogeneous foams produced by mixing and foams produced by bubbling (pneumatic foams) are considered. It is shown that a draining foam eventually arrives at a mechanical equilibrium when the opposing forces due to gravity and the Plateau-border suction gradient balance each other. The properties of the foam in this equilibrium state can be predicted from the surfactant and salt concentration in the foaming solution, the density of the liquid and the bubble radius.For homogeneous foams, it is possible to have conditions under which there is no drainage of liquid from the foam. There are three possible scenarios at equilibrium: separation of a single phase (separation of the continuous phase liquid by drainage or separation of the dispersed phase gas via collapse), separation of both phases (drainage and collapse occurs) or no phase separation (neither drainage nor collapse occurs). It is shown that the phase behavior depends on a single dimensionless group which is a measure of the relative magnitudes of the gravitational and capillary forces. A generalized phase diagram is presented which can be used to determine the phase behavior.For pneumatic foams, the effects of various system parameters such as the superficial gas velocity, the bubble size and the surfactant and salt concentrations on the rate of foam collapse and the evolution of liquid fraction profile are discussed. The steady state height attained by pneumatic foams when collapse occurs during generation is also evaluated.Bubble coalescence is assumed to occur due to the non-uniformity in the sizes of the films which constitute the faces of the polyhedral bubbles. This leads to a non-uniformity of film-drainage rates and hence of film thicknesses within any volume element in the foam. Smaller films drain faster and rupture earlier, causing the bubbles containing them to coalesce. This leads to a bubble size distribution in the foam, with the bubbles being larger in regions where greater coalescence has occurred.The formation of very stable Newton black films at high salt and surfactant concentrations is also explained.     

Thermal hysteresis and thickness dependence of the molecular orientation of poly(di-n-hexylsilane) in the film state

Molecular orientation of poly(di-n-hexylsilane) adsorbed on poly(vinyl alcohol) film has been studied by making use of the stretching technique. Dichroic ratio, Rd, strongly depended on the thickness of poly(di-n-hexylsilane) thin film and the highest value ca. 19 was observed at the film thickness of 110 +/- 30 nm. The thermal hysteresis of the molecular orientation was observed in the heating-cooling cycles. By studying the fluorescence spectrum it was confirmed that a portion of the poly(di-n-hexylsilane) molecules were in transoid conformation even at 320 K, although most of poly(di-n-hexylsilane) molecules were in disordered conformation (conformation D). This poly(di-n-hexylsilane) in transoid conformation is formed in the stretching process and may play a role of crystallization nucleus to induce the whole orientation of the poly(di-n-hexylsilane) in the film state.     

A method to measure the mean thickness and non-uniformity of non-uniform thin film by alpha-ray thickness gauge (author's transl)

The alpha-ray thickness gauge is used to measure non-destructively the thicknesses of thin films, and up to the present day, a thin film with uniform thickness is only taken up as the object of alpha-ray thickness gauge. When the thickness is determined from the displacement between the absorption curves in the presence and absence of thin film, the absorption curve must be displaced in parallel. When many uniform particles were dispersed as sample, the shape of the absorption curve was calculated as the sum of many absorption curves corresponding to the thin films with different thicknesses. By the comparison of the calculated and measured absorption curves, the number of particles, or the mean superficial density can be determined. This means the extension of thickness measurement from uniform to non-uniform films. Furthermore, these particle models being applied to non-uniform thin film, the possibility of measuring the mean thickness and non-uniformity was discussed. As the result, if the maximum difference of the thickness was more than 0.2 mg/cm2, the non-uniformity was considered to distinguish by the usual equipment. In this paper, an alpha-ray thickness gauge using the absorption curve method was treated, but one can apply this easily to an alpha-ray thickness gauge using alpha-ray energy spectra before and after the penetration of thin film.     

Effect of film thickness on the antifouling performance of poly(hydroxy-functional methacrylates) grafted surfaces

The development of nonfouling biomaterials to prevent nonspecific protein adsorption and cell/bacterial adhesion is critical for many biomedical applications, such as antithrombogenic implants and biosensors. In this work, we polymerize two types of hydroxy-functional methacrylates monomers of 2-hydroxyethyl methacrylate (HEMA) and hydroxypropyl methacrylate (HPMA) into polymer brushes on the gold substrate via surface-initiated atom transfer radical polymerization (SI-ATRP). We systematically examine the effect of the film thickness of polyHEMA and polyHPMA brushes on their antifouling performance in a wide range of biological media including single-protein solution, both diluted and undiluted human blood serum and plasma, and bacteria culture. Surface plasmon resonance (SPR) results show a strong correlation between antifouling property and film thickness. Too thin or too thick polymer brushes lead to large protein adsorption. Surfaces with the appropriate film thickness of ～25-45 nm for polyHPMA and ～20-45 nm for polyHEMA can achieve almost zero protein adsorption (<0.3 ng/cm(2)) from single-protein solution and diluted human blood plasma and serum. For undiluted human blood serum and plasma, polyHEMA brushes at a film thickness of ～20-30 nm adsorb only ～3.0 and ～3.5 ng/cm(2) proteins, respectively, while polyHPMA brushes at a film thickness of ～30 nm adsorb more proteins of ～13.5 and ～50.0 ng/cm(2), respectively. Moreover, both polyHEMA and polyHPMA brushes with optimal film thickness exhibit very low bacteria adhesion. The excellent antifouling ability and long-term stability of polyHEMA and polyHPMA brushes make them, especially for polyHEMA, effective and stable antifouling materials for usage in blood-contacting devices.     

Drying characteristics and evolution of the pore space in alginate scaffold with embedded sub-millimeter voids

Alginate scaffold has potential use in the controlled release of drugs and as a three dimensional structure for the formation of tissue matrix. This article describes the changes in the alginate scaffold when the moisture was removed from the scaffold under vacuum. Here, some scaffolds have self-aligned gas bubbles with average diameter of 500 μm, introduced through fluidic arrangement, prior to the crosslinking of the aqueous alginate film. The crosslinked gel film was dried in a vacuum oven at a constant temperature. The image of the alginate film prior to crosslinking was acquired under digital microscope, and was compared with the images of the dried scaffolds from the scanning electron microscope. The voids retained their identity at the time of drying, while the diameter was reduced to half of the initial value. The thickness of the scaffold was reduced ten folds. The presence of voids enhanced the drying rate when the drying was conducted at higher temperature. The drying primarily occurred in the falling rate period. The constant rate period was approached at lower moisture content for thin scaffolds without voids indicating the presence of surface moisture for substantial period. This feature was not observed for the scaffolds with voids. For these scaffolds, the shrinkage was insignificant except for the initial phase of drying. Based on this information, the conclusions were drawn on how the de-saturation of the various parts of the scaffold was phased.     

Charge separation from the bursting of bubbles on water

Film droplets formed from the bursting of 2.4 mm diameter bubbles on the surface of pure water are predominantly negatively charged. The charge generated per bubble varies chaotically; a few bubbles generate more than -3 × 10(6) elementary charges (e) but the vast majority generate much less. The average is -5 × 10(4)e/bubble, and it is not significantly affected by bubbling rate or temperature. The charge diminishes with increasing salt concentration and vanishes for concentrations above 10(-3) M. We propose a mechanism consistent with the observed charge separation. The model relies on the assumption that the surface of pure water has a slight excess of hydroxide ions. The charge separation results when water with entrained counterions (H(3)O(+)) flows out of the thinning film of the bubble cap, leaving behind the excess OH(-) on the surface. Addition of salt reduces the Debye length, and the charge separation mechanism becomes less effective as the Debye length becomes small compared with the film thickness. The excess charge near the surface of pure water is very small, around -4 nC/m(2).