Chih-Wen Yang

Molecular layer of gaslike domains at a hydrophobic-water interface observed by frequency-modulation atomic force microscopy.

It was numerically predicted that dissolved gas particles could enrich and adsorb at hydrophobic-liquid interfaces. Here we observe nucleation and growth of bright patches of ∼0.45 nm high on the graphite surface in pure water with frequency-modulation atomic force microscopy when the dissolved gas concentration is below the saturation level. The bright patches, suspected to be caused by adsorption of nitrogen molecules at the graphite-water interface, are composed of domains of a rowlike structure with the row separation of 4.2 ± 0.3 nm. The observation of this ordered adlayer might underline the gas segregation at various water interfaces.

Simultaneous detection of translational and angular displacements of micromachined elements

An astigmatic detection system is constructed with a modified digital-versatile-disk optical head. This system, with a detecting spot of ∼1μm, can simultaneously measure the vertical displacements and two-dimensional angular tilts of micromachined elements. It can detect thermal vibrations of microfabricated cantilevers with noise levels of 1.3pmHz−1∕2 for the linear displacement and of 3.2nradHz−1∕2 for angular displacements over a frequency range from 1to800kHz. The detecting frequency can even reach beyond 100MHz if high-speed electronic devices are adopted. Further optimization of the system will broaden its applications in diverse technological fields.

Imaging surface nanobubbles at graphite–water interfaces with different atomic force microscopy modes

We have imaged nanobubbles on highly ordered pyrolytic graphite (HOPG) surfaces in pure water with different atomic force microscopy (AFM) modes, including the frequency-modulation, the tapping, and the PeakForce techniques. We have compared the performance of these modes in obtaining the surface profiles of nanobubbles. The frequency-modulation mode yields a larger height value than the other two modes and can provide more accurate measurement of the surface profiles of nanobubbles. Imaging with PeakForce mode shows that a nanobubble appears smaller and shorter with increasing peak force and disappears above a certain peak force, but the size returns to the original value when the peak force is reduced. This indicates that imaging with high peak forces does not cause gas removal from the nanobubbles. Based on the presented findings and previous AFM observations, the existing models for nanobubbles are reviewed and discussed. The model of gas aggregate inside nanobubbles provides a better explanation for the puzzles of the high stability and the contact angle of surface nanobubbles.

Interface-Induced Ordering of Gas Molecules Confined in a Small Space

The thermodynamic properties of gases have been understood primarily through phase diagrams of bulk gases. However, observations of gases confined in a nanometer space have posed a challenge to the principles of classical thermodynamics. Here, we investigated interfacial structures comprising either O2 or N2 between water and a hydrophobic solid surface by using advanced atomic force microscopy techniques. Ordered epitaxial layers and cap-shaped nanostructures were observed. In addition, pancake-shaped disordered layers that had grown on top of the epitaxial base layers were observed in oxygen-supersaturated water. We propose that hydrophobic solid surfaces provide low-chemical-potential sites at which gas molecules dissolved in water can be adsorbed. The structures are further stabilized by interfacial water. Here we show that gas molecules can agglomerate into a condensed form when confined in a sufficiently small space under ambient conditions. The crystalline solid surface may even induce a solid-gas state when the gas-substrate interaction is significantly stronger than the gas-gas interaction. The ordering and thermodynamic properties of the confined gases are determined primarily according to interfacial interactions.

Soft-contact imaging in liquid with frequency-modulation torsion resonance mode atomic force microscopy

In this work, we demonstrate that high-resolution imaging in water with a soft contact between the tip and the sample can be achieved with frequency-modulation torsional resonance (FM-TR) mode atomic force microscopy (AFM). This mode is very sensitive to the contact of the tip with the sample surface. A sharp jump in the resonance frequency shift occurs when the tip is getting in touch with the sample. Individual atomic features on mica surfaces can be resolved with a relatively large tip. The tip applies very small normal and lateral forces on the surface. In addition, even a long and compliant AFM cantilever can achieve a high quality factor and a high resonant frequency for the torsional oscillation in water. Along with several other advantages, this mode is very suitable for future development of high-sensitivity, high-resolution, high-speed AFM for the study of dynamic biological processes in liquid.

Condensation of Dissolved Gas Molecules at a Hydrophobic/Water Interface

The non-wetting phenomenon of water on certain solid surfaces has been under intensive study for decades, but the nature of the hydrophobic/water interfaces remains controversial. Here a water/graphite interface is investigated with high-sensitivity atomic force microscopy. We show evidence of nucleation and growth of an epitaxial monolayer on the graphite surface, probably caused by the adsorption of nitrogen molecules dissolved in water. The subsequent adsorption process resembles the layer-plus-island, or Stranski-Krastanov, growth mode in heteroepitaxy. This finding underlines the importance of gas segregation at various water interfaces and may unravel many puzzles, especially the nature and the high stability of so-called nanobubbles at solid/water interfaces and in bulk water. Based on the hydrophobic effect, we propose that gas molecules dissolved in water may aggregate into clusters in bulk water as well as at solid/water interfaces. As a cluster grows above a critical size, it undergoes a transition into a gas bubble, which can explain the formation or nucleation of gas bubbles in water.

Nucleation processes of nanobubbles at a solid/water interface.

Experimental investigations of hydrophobic/water interfaces often return controversial results, possibly due to the unknown role of gas accumulation at the interfaces. Here, during advanced atomic force microscopy of the initial evolution of gas-containing structures at a highly ordered pyrolytic graphite/water interface, a fluid phase first appeared as a circular wetting layer ~0.3 nm in thickness and was later transformed into a cap-shaped nanostructure (an interfacial nanobubble). Two-dimensional ordered domains were nucleated and grew over time outside or at the perimeter of the fluid regions, eventually confining growth of the fluid regions to the vertical direction. We determined that interfacial nanobubbles and fluid layers have very similar mechanical properties, suggesting low interfacial tension with water and a liquid-like nature, explaining their high stability and their roles in boundary slip and bubble nucleation. These ordered domains may be the interfacial hydrophilic gas hydrates and/or the long-sought chemical surface heterogeneities responsible for contact line pinning and contact angle hysteresis. The gradual nucleation and growth of hydrophilic ordered domains renders the original homogeneous hydrophobic/water interface more heterogeneous over time, which would have great consequence for interfacial properties that affect diverse phenomena, including interactions in water, chemical reactions, and the self-assembly and function of biological molecules.

Imaging soft matters in water with torsional mode atomic force microscopy

We have developed a high-sensitivity atomic force microscopy (AFM) mode operated in aqueous environment based on the torsional resonance of the cantilever. It is found that the torsional mode can achieve a good spatial resolution even with a relatively large tip. We have used this mode to image different soft materials in water, including DNA molecules and purple membrane. High-resolution images of purple membrane can be obtained at a relatively low ion concentration under a long-range electrostatic force. Thus the torsional mode allows investigators to probe surface structures and their properties under a wide range of solution conditions.

Torsional resonance mode atomic force microscopy in liquid with Lorentz force actuation

In this work, we present a design based on Lorentz force induction to excite pure torsional resonances of different types of cantilevers in air as well as in water. To demonstrate the atomic force microscopy imaging capability, the phase-modulation torsional resonance mode is employed to resolve fine features of purple membranes in a buffer solution. Most importantly, force-versus-distance curves using a relatively stiff cantilever can clearly detect the characteristic oscillatory profiles of hydration layers at a water-mica interface, indicating the high force sensitivity of the torsional mode. The high resonance frequencies and high quality-factors for the torsional mode may be of great potential for high-speed and high-sensitivity imaging in aqueous environment.

High-sensitivity imaging with lateral resonance mode atomic force microscopy

In the operation of a dynamic mode atomic force microscope, a micro-fabricated rectangular cantilever is typically oscillated at or near its mechanical resonance frequency. Lateral bending resonances of cantilevers are rarely used because the resonances are not expected to be detected by the beam-deflection method. In this work, we found that micro-cantilevers with a large tip produced an out-of-plane displacement in lateral resonance (LR), which could be detected with the beam-deflection method. Finite-element analysis indicated that the presence of a large tip is the major source of the out-of-plane coupling for the LR. We also imaged a heterogeneous sample by operating a cantilever in LR, torsional resonance, and tapping modes. LR mode yielded a small deformation and noise level in the height maps as well as a high contrast and small noise level in the phase maps. LR mode also had a resonance frequency that was orders of magnitude higher than that of tapping mode. Operation with LR mode may have the benefits of high-speed scanning, high-sensitivity imaging, and mapping of in-plane mechanical properties of the sample surface. In general, LR mode may become a powerful new atomic force microscopy technique for characterizing sample materials.