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Nobuo Maeda

Publications and source records attributed to Nobuo Maeda.

12 recordsLinked to original sources
PubMed2006

Phase transitions of capillary-held liquids in a slit-like pore.

Dynamics of capillary held liquids plays important roles in a wide range of systems including adhesion, printing of paints and inks, the behavior of wet granular materials, and the mass transfer through porous media. Recent study suggested the presence of two distinct modes for the disappearance of capillary-held liquids in a slit-like pore of adjustable slit width that depended on the slit-opening rates. In contrast to the first mode that is well-documented in terms of the Young-Laplace equation, a novel and unexpected mode was observed when the liquid bridge was held in the vicinity of the thermodynamic phase boundary (equilibrium Kelvin length). Here we extended the study to three new compounds that have a wide range of vapor pressures. An evaporating liquid bridge developed large refractive index gradients that extended over a few micrometers from the edge of the meniscus once the slit width was increased beyond the equilibrium Kelvin length. This interfacial region with depleted refractive index retreated inward as the meniscus shrank with time, and the refractive index of the entire bridge subsequently fell from that of the liquid once the interfacial regions from the opposite sides of the shrinking bridge met at the center. The refractive index recovered to that of the liquid when the slit width was closed to below the Kelvin length and the vapor was allowed to recondense. The time scale of the evaporation and condensation depended on the size of the surface gap, and, when the surfaces were placed at a separation very close to the Kelvin length, it was possible to detect a stage in which the system was in an apparent kinetic equilibrium between two physical states--with and without the liquid connecting the two surfaces.

Journal Article↗
PubMed2006

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.

Gases↗
PubMed2006

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.

Journal Article↗
PubMed2006

[Study of influences of social support and social networks on participation in periodic health examination of the elderly at home comparison of three regions with different social backgrounds].

PURPOSE: Periodic health examination are presumed to be important with respect to the well-being of aged individuals. The purpose of this study was to analyze the relationships between elderly at home accepting periodic health examination and their social support and social networks. METHOD: The survey was performed targeting so-called young elderly persons living at home in 3 regions in Hokkaido Prefecture, a large city (Sapporo), an ex-coalmining town (Yuubari) and a small farming town (Takasu). The study populations were asked about social support and social networks, and the Mantel-Haenszel method was applied to analyze the data. RESULTS: 1) Both males and females who received periodic health examination within the last one year (participants) were members of various groups, such as neighborhood societies, clubs for the elderly and so on, and were more interested in public magazines and political issues than those who had never received periodic health examination (non-participants). Elderly males who accepted periodic health examination were those who were interested in elections, who had a hobby, and who thought that life was worthwhile. 2) Both elderly males and females who accepted periodic health examination were those who had friends, and elderly males who accepted periodic health examination were those who had close relatives and who lived in a friendly neighborhood. 3) Elderly males who accepted periodic health examination were more likely to both receive and supply instrumental and emotional support in interaction with others.

Aged↗
PubMed2005

Crystallization in thin liquid films induced by shear.

It is known that the thin-film structure of confined fluids and solids can be changed when the confining surfaces are sheared. Positional and orientational short- or long-range reordering can occur that often have no bulk counterparts. These multilayer, monolayer, or even sub-monolayer effects are important for understanding adhesion and friction processes, but they have proved difficult to measure, partly due to a lack of experimental techniques and partly to their apparent subtle dependence on many experimental parameters. Here we report the use of shear measurements and "optical absorption spectroscopy" in the surface forces apparatus to measure a shear-induced phase transition of an anisotropic (dye) molecule confined between two shearing mica surfaces in aqueous solution. Our studies on the shear-induced ordering and friction forces of highly anisotropic cyanine dye molecules in thin water films show only a weak effect of molecular anisotropy on shear-induced ordering, friction forces, and the onset of shear-induced crystallization, although dramatic changes do occur when the confined molecules ultimately crystallize.

Journal Article↗
PubMed2004

Preparing contamination-free mica substrates for surface characterization, force measurements, and imaging.

Due to its perfect cleavage that provides large areas of molecularly smooth, chemically inert surfaces, mica is the most commonly used natural substrate in measurements with the surface forces apparatus (SFA), in atomic force microscopy (AFM), and in many adsorption studies. However, preparing mica surfaces that are truly clean is not easy since mica is a high-energy surface that readily adsorbs water, organic contaminants, and gases from the atmosphere. Mica can also become charged on cleaving, which makes it prone to picking up oppositely charged particles or mica flakes from the surroundings. High refractive index particles, such as metals, will adhere to mica through van der Waals forces. Recent articles have demonstrated that particle contamination is obtained when inappropriate cutting and handling procedures for the mica are used. In this paper, we show that both particle and other critical contamination is easy to detect and provide proper steps to take during the sample preparation process.

Journal Article↗
PubMed2004

Further studies on the effect of degassing on the dispersion and stability of surfactant-free emulsions.

Recently reported results indicate that the formation of surfactant-free, oil-in-water emulsions can be significantly enhanced by the almost complete removal of dissolved gases and that the reintroduction of dissolved gases does not immediately destabilize the already-formed emulsions. These initial experiments have been repeated and extended to include a wider range of organic liquids and the application of light scattering to determine droplet size and distribution. The earlier observations have been confirmed. In addition, a systematic trend was found between the solubility of the oil in water and the stability (lifetime) of the degassed oil droplets in water. The lower the solubility, the more stable the emulsion, and for oils that are sparingly soluble in water such as squalane, the small droplets remain stable for several weeks, with buoyancy separation being the main cause of instability of the large droplets with time. The addition of electrolytes, up to molar concentrations, substantially reduces the enhancement of the dispersions on degassing but appears to have little effect on the stability of the already-formed emulsions. The reduction of pH to about 2 significantly reduces both the enhancement of the dispersions on degassing and the stability of the already-formed emulsions. In contrast, the increase of pH to about 11 hardly affects the enhancement of the dispersions on degassing or the stability of the already-formed emulsions. We have confirmed the importance of dissolved gas and its association with the electrostatic effects, but we still cannot provide a complete explanation for the effect of degassing on the hydrophobic dispersions.

Journal Article↗
PubMed2003

Evaporation and instabilities of microscopic capillary bridges.

The formation and disappearance of liquid bridges between two surfaces can occur either through equilibrium or nonequilibrium processes. In the first instance, the bridge molecules are in thermodynamic equilibrium with the surrounding vapor medium. In the second, chemical potential gradients result in material transfer; mechanical instabilities, because of van der Waals force jumps on approach or a Rayleigh instability on rapid separation, may trigger irreversible film coalescence or bridge snapping. We have studied the growth and disappearance mechanisms of laterally microscopic liquid bridges of three hydrocarbon liquids in slit-like pores. At rapid slit-opening rates, the bridges rupture by means of a mechanical instability described by the Young-Laplace equation. Noncontinuum but apparently reversible behavior is observed when a bridge is held at nanoscopic surface separations H close to the thermodynamic equilibrium Kelvin length, 2r(K)costheta, where r(K) is the Kelvin radius and theta is the contact angle. During the course of slow evaporation (at H > 2r(K)costheta) and subsequent regrowth by capillary condensation (at H < 2r(K)costheta), the refractive index of the bridge may vary continuously and reversibly between that of the bulk liquid and vapor. The evaporation process becomes irreversible only at the very final stage of evaporation, when the refractive index of the fluid attains virtually that of the vapor. Measured refractive index profiles and the time-dependence of evaporating neck diameters also seem to differ from predictions based on a continuum picture of bridge evaporation far from the critical point. We discuss these findings in terms of the probable density profiles in evolving liquid bridges.

Chemistry↗
PubMed2003

[Factors associated with participation in medical checkups of the elderly at home comparison of 3 regions with different social backgrounds].

PURPOSE: Annual medical checkups are presumed to be important with the well-being of aged individuals. The purpose of this study was to investigate factors related to participation in medical checkups of elderly persons at home. METHODS: A survey was performed targeting so-called young elderly persons living at home in 3 regions in Hokkaido Prefecture, a large city (Sapporo), an ex-coalmining town (Yubari) and a small farming town (Takasu). The study populations were asked about medical checkups, economic status, self-rated health status, activities of daily living (ADL) and basic characteristics such as age, sex and occupation. We used the Mantel-Haenszel method for statistical analysis. RESULTS: 1) Elderly males who received medical checkups within the last 1 year (participants) had a shorter period of education, were less likely to have or to have had the smoking habit, undertook more physical activity, had better eating habit, more often rated their own health as good or normal, and had a better physical ADL & instrumental ADL, than those who had never received medical checkups (non-participants). Regionally, the items that showed significant differences included: "live with the spouse", "do not smoke" and "do not have physical pain or disabilities" in Sapporo; "enjoy physical activity", "have good eating habit" and "have good instrumental ADL" in Yubari; and "engaged in agriculture/fishery work" and "had outpatient visits in the last three months" in Takasu. 2) Elderly female participants who had outpatient visits in the last three months had anxiety about their own health, significantly different from non-participants. Regionally, the items that showed significant differences included: "often drink alcohol" in Sapporo; "still working right now", "was engaged in agriculture/fishery work in the past" and "had outpatient visits in the last three months" in Yubari; and "have own house", "do not smoke" and "pay attention to eating habit" in Takasu. 3) The item which showed a significantly high rate both for male and female participants was "still working right now". CONCLUSIONS: This study indicates that medical checkups should be recommended to the elderly, taking into account not only their basic characteristics such as sex and occupation, but also lifestyle, self-rated health status and ADL, which are associated with medical checkup participation.

Activities of Daily Living↗
PubMed2002

Micromanipulation of phospholipid bilayers by atomic force microscopy.

The molecular details of adhesion mechanics in phospholipid bilayers have been studied using atomic force microscopy (AFM). Under tension fused bilayers of dipalmitoylphosphatidylcholine (DPPC) yield to give non-distance dependent and discrete force plateaux of 45.4, 81.6 and 113+/-3.5 pN. This behaviour may persist over distances as great as 400 nm and suggests the stable formation of a cylindrical tube which bridges the bilayers on the two surfaces. The stability of this connective structure may have implications for the formation of pili and hence for the initial stage of bacterial conjugation. Dimyristoylphosphatidylcholine (DMPC) bilayers also exhibit force plateaux but with a much less pronounced quantization. Bilayers composed of egg PC, sterylamine and cholesterol stressed in a similar way show complex behaviour which can in part be explained using the models demonstrated in the pure lipids.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2002

Adhesion and friction mechanisms of polymer-on-polymer surfaces.

The adhesion and friction of smooth polymer surfaces were studied below the glass transition temperature by use of a surface forces apparatus. The friction force of a crosslinked polymer was orders of magnitude less than that of an uncrosslinked polymer. In contrast, after chain scission of the outermost layers, the adhesion hysteresis and friction forces increase substantially. These results show that polymer-polymer adhesion hysteresis and friction depend on the dynamic rearrangement of the outermost polymer segments at shearing interfaces, and that both increase as a transition is made from crosslinked surfaces to surfaces with long chains to surfaces with quasi-free ends. The results suggest new ways for manipulating the adhesion and friction of polymer surfaces by adjusting the state of the surface chains.

Journal Article↗