William A. Ducker

LATERAL, NORMAL, AND LONGITUDINAL SPRING CONSTANTS OF ATOMIC FORCE MICROSCOPY CANTILEVERS

For a V‐shaped atomic force microscopy cantilever beam, the spring constants in the three principal directions are given in terms of the beam geometry and material properties. For the lateral stiffness, a closed‐formed expression is presented. Also, the normal and the longitudinal stiffness are obtained from a few simple equations. The results are compared with a finite element study and found to be very accurate. All spring constants depend strongly on the cantilever thickness, which is difficult to measure. In addition, the lateral and longitudinal stiffness are sensitive to the location and the height of the attached pyramid.

Nanobubbles at the Interface between Water and a Hydrophobic Solid

A very thin layer (5-80 nm) of gas phase, consisting of discrete bubbles with only about 40 000 molecules, is quite stable at the interface between a hydrophobic solid and water. We prepare this gas phase from either ambient air or from CO(2)(g) through a solvent exchange method reported previously. In this work, we examine the interface using attenuated total internal reflection infrared spectroscopy. The presence of rotational fine structure in the spectrum of CO(2) and D(2)O proves that molecules are present in the gas phase at the interface. The air bubbles are stable for more than 4 days, whereas the CO(2) bubbles are only stable for 1-2 h. We determine the average gas pressure inside the CO(2) bubbles from the IR spectrum in two ways: from the width of the rotational fine structure (P(gas) < 2 atm) and from the intensity in the IR spectrum (P(gas) = 1.1 +/- 0.4 atm). The small difference in gas pressure between the bubbles and the ambient (1 atm) is consistent with the long lifetime. The dimensions and curvature of a set of individual bubbles was determined by atomic force microscopy. The pressures of individual bubbles calculated from the measured curvature using the Laplace equation fall into the range P(gas) = 1.0-1.7 atm, which is concordant with the average pressure measured from the IR spectrum. We believe that the difference in stability of the CO(2) bubbles and the air bubbles is due to a combination of the much lower pressure of CO(2) in the atmosphere and the greater solubility of CO(2) in water, compared to N(2) and O(2). As expected, smaller bubbles have a shorter average lifetime than larger bubbles, and the average pressure and the curvature of individual bubbles decreases with time. Surface plasmon resonance measurements provide supporting evidence that the film is in the gas state: the thin film has a lower refractive index than water, and there are few common contaminants that satisfy this condition. Interfacial gas bubbles are not ubiquitous on hydrophobic solids: bubble-free and bubble-decorated hydrophobic interfaces can be routinely prepared.

Measuring surface forces in aqueous electrolyte solution with the atomic force microscope

Abstract Surface forces determine the behaviour and properties of colloids, including biological molecules, micelles and membranes. Recently it has been realized that the atomic force microscope, which is normally used to image the topography of surfaces with high resolution, can also be used to measure surface forces. The advantages of the atomic force microscope are that virtually any surface of interest can be investigated and that measurements are relatively fast and easy to perform. Furthermore, since the interacting areas are small (typically 10 2 –100 2 nm 2 ) samples only need to be smooth and homogeneous on a small scale. Local surface properties, like the surface charge density or micromechemical properties, can be determined.

Contact Angle and Stability of Interfacial Nanobubbles

Small bubbles of gas are known to exist at the interface between hydrophobic solids and water. Two features of these bubbles are unexplained: the very low contact angle and the stability. A self-consistent explanation of both of these effects is that there is a film of contaminant at the air-water interface that decreases the surface tension and thus the contact angle, and also hinders diffusion of gases from the bubble, thereby increasing the lifetime. If, during the lifetime of the bubble, the surface tension increases faster than the area of the air-water decreases, the interfacial energy can lead to a stabilization of the bubbles.

Celery (Apium graveolens L.) parenchyma cell walls examined by atomic force microscopy: effect of dehydration on cellulose microfibrils.

Abstract. Atomic force microscopy (AFM) was used to image celery (Apium graveolens L.) parenchyma cell walls in situ. Cellulose microfibrils could clearly be distinguished in topographic images of the cell wall. The microfibrils of the hydrated walls appeared smaller, more uniformly distributed, and less enmeshed than those of dried peels. In material that was kept hydrated at all times and imaged under water, the microfibril diameter was mainly in the range 6–25 nm. The cellulose microfibril diameters were highly dependent on the water content of the specimen. As the water content was decreased, by mixing ethanol with the bathing solution, the microfibril diameters increased. Upon complete dehydration of the specimen we observed a significant increase in microfibril diameter. The procedure used to dehydrate the parenchyma cells also influenced the size of cellulose microfibrils with freeze-dried material having larger diameters than air-dried material.

A deliberation on nanobubbles at surfaces and in bulk

Surface and bulk nanobubbles are two types of nanoscopic gaseous domain that have recently been discovered in interfacial physics. Both are expected to be unstable to dissolution because of the high internal pressure driving diffusion and the surface tension which squeezes the gas out, but there is a rapidly growing body of experimental evidence that demonstrates both bubble types to be stable. However, the two types of bubbles also differ in many respects: surface nanobubble stability is most probably assisted by the nearby wall, which can repel the water (in the case of hydrophobicity), accept physisorbed gas molecules, and reduce the surface area through which outfluxing can occur; bulk nanobubbles, on the other hand, must stabilise themselves. This is perhaps through ionic shielding, perhaps through diffusive shielding, or perhaps through both. Herein, the features of both bubble types are described individually, their common and disparate features are discussed, and emerging applications are examined.

The mechanism for hydrothermal growth of zinc oxide

The mechanism for hydrothermal growth of ZnO was studied in ammonium hydroxide solution at pH near 11 (0.3 M NH4OH). The products formed at 20–90 °C and ambient pressure were characterised using Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photon Spectroscopy (XPS), X-ray Diffraction (XRD) and Secondary Ion Mass Spectroscopy (SIMS). Under these conditions, the growth of ZnO occurs via the initial precipitation of e-Zn(OH)2 (Wulfingite), which subsequently dehydrates, to form Wurtzite ZnO. Isotope tracking experiments show that most of the oxygen atoms do not mix with water during the conversion from Wulfingite to ZnO, and thus that the reaction proceeds primarily in the solid phase rather than through dissolution then reprecipitation. XPS results show that the surface of hydrothermal ZnO consists primarily of zinc hydroxide at lower temperatures, and there is a significant fraction of zinc hydroxide persisting at higher temperatures. Together these results show that much of the solid-solution interface is mainly comprised of zinc hydroxide not ZnO. These findings may have implications for understanding how small organic molecules can be used to control the morphology of zinc oxide crystals grown under hydrothermal conditions.

Shear flow promotes amyloid-β fibrilization

The rate of formation of amyloid fibrils in an aqueous solution of amyloid-beta (Abeta) is greatly increased when the solution is sheared. When Abeta solution is stirred with a magnetic stirrer bar at 37 degrees C, a rapid increase in thioflavin T fluorescence is observed. Atomic Force Microscopy (AFM) images show the formation of aggregates, the growth of fibrils and the intertwining of the fibrils with time. Circular dichroism (CD) spectroscopy of samples taken after stirring shows a transition from random coil to alpha-helix to beta-sheet secondary structure over 20 h at 37 degrees C. The fluorescence, AFM and CD measurements are all consistent with the formation of amyloid fibrils. Quiescent, non-stirred solutions incubated at 37 degrees C showed no evidence of amyloid formation over a period of 3 days. Couette flow was found to accelerate the formation of amyloid fibrils demonstrating that the primary effect of stirring is not mixing but shearing. Only very small shear forces are applied to individual molecules in our experiments. Simple calculation suggests that the force is too small to support a hypothesis that shearing promotes partial unfolding of the protein as is observed.

Effects of Surfactants on the Formation and the Stability of Interfacial Nanobubbles

Contamination has previously been invoked to explain the flat shape and the long lifetimes of interfacial nanobubbles (INBs). In this study, the effects of surfactants on the formation and the stability of INBs were investigated when surfactants were added to the system before, during, and after the standard solvent exchange procedure (SSEP) for the formation of INBs. The solutions of sodium dodecyl sulfate (SDS) above critical micelle concentration were found to have little effect on the bubble stability. Likewise, cleaning of the substrate with a surfactant solution had little effect. In contrast, addition of a water-insoluble surfactant during the formation dramatically reduced the INBs. Finally, repeated application of SSEP to surfactant-coated substrates progressively rinsed the surfactant off the system. Thus, we found no evidence to support the hypothesis that (1) INBs are stabilized by a layer of insoluble organic contaminant or that (2) SSEP introduces surface-active materials to the system that could stabilize INBs.

Do stable nanobubbles exist in mixtures of organic solvents and water

Several recent papers have described the existence of stable nanobubbles in bulk, which is surprising given that the high curvature of these bubbles is expected to place such bubbles under a high pressure and therefore lead to rapid dissolution. Here, we investigate the possible existence of nanobubbles in mixtures of water plus an organic solvent using both static and dynamic light scattering and infrared spectroscopy. The mixing of solvents was designed to introduce nanobubbles into bulk solution via supersaturation of the solution. The solutions scatter light for a long period (days) after mixing, which is consistent with the formation of nanoscale objects, but we show that these scattering objects originate from water-insoluble impurities in the organic solvents. Our results are inconsistent with the presence of gas nanobubbles in bulk solution: Degassing the solutions, either before or after mixing, has a minimal effect on the scattering, and purification of the organic solvent before mixing reduces the scattering after mixing. Therefore, previous reports of nanobubbles based on scattering experiments should be reconsidered with the hypothesis that the scattering objects are not actually gaseous.