Rodolfo Fernandez, Xiaohua Wang, Peter Rogen, and A. La Rosa Near-field Microscopy Group Department of Physics, Portland State University, P.O. Box 751; Portland, Oregon INTRODUCTION EXPERIMENTAL SETUP 2 GND Ref TF Signal generator Acoustic feedback control signal Acoustic Signal from mesoscopic film Piezo- Electric tube Lock-in #1 Lock-in #2 WORKING PRINCIPLE -1 WORKING PRINCIPLE -2 Left: The lateral oscillations of the probe (few nm amplitude), while immersed into the mesoscopic film, engenders acoustic waves, which are sensitively detected by a acoustic sensor located underneath the sample. Right: The intensity of the acoustic wave depends sensitively on the probe-substrate separation distance (z). The results shown above constitute an unprecedented detection of shear-forces via acoustic means. Understanding the dynamic behavior of confined mesoscopic fluids The physical properties of fluids confined to nanometer- sized regions and between sliding solid boundaries, differ greatly from those displayed by fluids in bulk: effective shear viscosity is enhanced, viscoelastic relaxation times are prolonged, layer molecular-order occurs adjacent to each surface. Gaining further insights into the properties of mesoscopic films are central to understanding: adhesion, wetting, fluid transport in biological membranes, flow behavior in granular material, friction phenomena. In spite of long standing interest, however, the dynamics, thermodynamics, and the molecular structure of fluids confined to nanometer-sized regions, is not yet well understood at the fundamental level. The UCNM offers a new alternative for characterizing such mesoscopic films. Indeed, a unique feature of the UCNM is its ability to concurrently and independently monitor the effects that surface interactions have on both i) the microscope’s tapered probe (attached to an mechanical motion sensor) and ii) the mesoscopic fluid. This dual sensing capability makes the UCNM a more comprehensive technique. ~ nm Acoustic sensor #1 Solid substrate Probe’s lateral oscillations Confined mesoscopic fluid z Mechanical motion driver Listening to the nanowaves engendered at confined fluids under shear Exploiting the microscope frame as acoustic cavity to sense probe-sample surface interactions We have developed a compact and versatile Shear-force/Acoustic Near-field Microscope (SANM), fully operated by acoustic sensory devices (U.S. Patent No. 11/809,196). It provides : a)An alternative method (acoustic) for characterizing surface phenomena (nanotribology, adhesion, wetting). b) a suitable characterization platform towards the development of surface related technologies ( industrial and bio-engineering lubricants,) and a)c) potential capability for imaging sub-surface materials properties. Further, unique to its novel development, the SANM operates using its own acoustic-based feedback control, which allows topographic characterization of the sample. S H E A R- F OR C E / A CO U S T I C N E A R – F I E L D M I C R O S C O P E (SANM) S A N M L I S T E N S T O S U R F A C E I N T E R A C T I O N S at the N A N O - S C A L E Substrate Acoustic sensor #1 Substrate PSU Near-field Microscopy Group Pictorially, SANM uses a simple “audiphone”, (placed around the microscope’s frame) for listening to the probe- sample interactions, allowing the probe to “walk” safely across the sample’s surface. Photo- detec tor LASER beam Sample AFM PROBE Tic, tac “artificial eye” Acoustic-based feedback control for scanning probe microscopy SANM acoustic feedback for controlling the probe’s vertical position. SANM acoustic feedback for controlling the probe’s vertical position. The inset shows the acoustic signal decreasing monotonically as the probe approaches the sample in the last 35 nm proximity. This behavior is exploited by the SANM to implement its own probe-sample distance feedback control and, hence, image the sample’s topography. SANM is the first scanning probe microscope operated via acoustic transducer. Principle: The probe- sample interactions affect the lateral motion of the probe. The latter, in mechanical contact Acoustic cavity Acoustic sensor #2 XYZ scanner Acoustic waves Probe Fluid film Probe-film Interaction region Tic, tac, tic, tac Tic, tac, Probe Acoustic sensor 2 Acoustic resonant cavity Sample Tic, tac, Sample with the cavity, sets waves that travel upwards towards the cavity where they interfere (see interference pattern). The acoustic is placed judiciously at the location of o constructive node for maximum sensitivity.