Shivprasad Patil

Enhanced compositional sensitivity in atomic force microscopy by the excitation of the first two flexural modes

The authors demonstrate that the compositional sensitivity of an atomic force microscope is enhanced by the simultaneous excitation of its first two flexural eigenmodes. The coupling of those modes by the nonlinear probe-surface interactions enables to map compositional changes in several conjugated molecular materials with a phase shift sensitivity that is about one order of magnitude higher than the one achieved in amplitude modulation atomic force microscopy.

A highly sensitive atomic force microscope for linear measurements of molecular forces in liquids

We describe a highly improved atomic force microscope for quantitative nanomechanical measurements in liquids. The main feature of this microscope is a modified fiber interferometer mounted on a five axis inertial slider which provides a deflection sensitivity that is significantly better than conventional laser deflection based systems. The measured low noise floor of 572.0fm∕Hz provides excellent cantilever amplitude resolution. This allows us to operate the instrument far below resonance at extremely small cantilever amplitudes of less than 1 A. Thus linear measurements of nanomechanical properties of liquid systems can be performed. In particular, we present measurements of solvation forces in confined octamethylcyclotetrasiloxane and water with amplitudes smaller than the size of the respective molecules. In general, the development of the instrument is important in the context of quantitative nanomechanical measurements in liquid environments.

Study of the electrostatic force between a conducting tip in proximity with a metallic surface: Theory and experiment

A formula has been derived for the electrostatic force between the conducting tip and a planer metallic surface by exploiting the fact that the tip–sample geometry can be described by confocal hyperboloids of revolution. The prolate spheroidal coordinate system was found to be most convenient for this purpose. The general behavior of force curves obtained in the attractive regime using a conducting cantilever and an optical beam deflection system is in reasonably good agreement with the theory over a wide range of distances. The results are important in the context of design, development, and understanding of scanning probe microscopes involving voltage bias between the probe and sample.

Simultaneous normal and shear measurements of nanoconfined liquids in a fiber-based atomic force microscope

We have developed an atomic force microscopy (AFM) technique that can perform simultaneous normal and shear stiffness measurements of nanoconfined liquids with angstrom-range amplitudes. The AFM technique is based on a fiber-interferometric, small-amplitude, off-resonance AFM. This AFM is capable of providing linear quasistatic measurements of the local mechanical properties of confined liquid layers while only minimally disturbing the layers themselves. A detailed analysis of the measurement geometry reveals that shear stiffness measurements are extremely challenging, as even small deviations from perfect orthogonality can lead to data that is very difficult to interpret. We will show ways out of this dilemma and present results that show simultaneous measurement of the shear and normal stiffness of confined liquid layers.

The effect of boundary slippage and nonlinear rheological response on flow of nanoconfined water

The flow of water confined to nanometer-sized pores is central to a wide range of subjects from biology to nanofluidic devices. Despite its importance, a clear picture about nanoscale fluid dynamics is yet to emerge. Here we measured dissipation in less than 25 nm thick water films and it was found to decrease for both wetting and non-wetting confining surfaces. The fitting of Carreau-Yasuda model of shear thinning to our measurements implies that flow is non-Newtonian and for wetting surfaces the no-slip boundary condition is largely valid. In contrast, for non-wetting surfaces boundary slippage occurs with slip lengths of the order of 10 nm. The findings suggest that both, the wettability of the confining surfaces and nonlinear rheological response of water molecules under nano-confinement play a dominant role in transport properties.

Linear measurements of nanomechanical phenomena using small-amplitude AFM

Dynamic Atomic Force Microscopy (AFM) is typically performed at amplitudes that are quite large compared to the measured interaction range. This complicates the data interpretation as measurements become highly non-linear. A new dynamic AFM technique in which ultra-small amplitudes are used (as low as 0.15 Angstrom) is able to linearize measurements of nanomechanical phenomena in ultra-high vacuum (UHV) and in liquids. Using this new technique we have measured single atom bonding, atomic-scale dissipation and molecular ordering in liquid layers, including water.