Showing papers by "Arvind Raman published in 2006"

Ultrasensitive mass sensing using mode localization in coupled microcantilevers

Abstract: We use Anderson or vibration localization in coupled microcantilevers as an extremely sensitive method to detect the added mass of a target analyte. We focus on the resonance frequencies and eigenstates of two nearly identical coupled gold-foil microcantilevers. Theoretical and experimental results indicate that the relative changes in the eigenstates due to the added mass can be orders of magnitude greater than the relative changes in resonance frequencies. Moreover this sensing paradigm possesses intrinsic common mode rejection characteristics thus providing an alternate way to achieve ultrasensitive mass detection under ambient conditions.

Hydrodynamic loading of microcantilevers vibrating in viscous fluids

Abstract: The hydrodynamic loading of elastic microcantilevers vibrating in viscous fluids is analyzed computationally using a three-dimensional, finite element fluid-structure interaction model. The quality factors and added mass coefficients of several modes are computed accurately from the transient oscillations of the microcantilever in the fluid. The effects of microcantilever geometry, operation in higher bending modes, and orientation and proximity to a surface are analyzed in detail. The results indicate that in an infinite medium, microcantilever damping arises from localized fluid shear near the edges of the microcantilever. Closer to the surface, however, the damping arises due to a combination of squeeze film effects and viscous shear near the edges. The dependence of these mechanisms on microcantilever geometry and orientation in the proximity of a surface are discussed. The results provide a comprehensive understanding of the hydrodynamic loading of microcantilevers in viscous fluids and are expected to be of immediate interest in atomic force microscopy and microcantilever biosensors.

Chaos in atomic force microscopy.

Abstract: Chaotic oscillations of microcantilever tips in dynamic atomic force microscopy (AFM) are reported and characterized. Systematic experiments performed using a variety of microcantilevers under a wide range of operating conditions indicate that softer AFM microcantilevers bifurcate from periodic to chaotic oscillations near the transition from the noncontact to the tapping regimes. Careful Lyapunov exponent and noise titration calculations of the tip oscillation data confirm their chaotic nature. AFM images taken by scanning the chaotically oscillating tips over the sample show small, but significant metrology errors at the nanoscale due to this ``deterministic'' uncertainty.

Parametric resonance based scanning probe microscopy

Abstract: We propose a mode of dynamic scanning probe microscopy based on parametric resonance for highly sensitive nanoscale imaging and force spectroscopy. In this mode the microcantilever probe is excited by means of a closed-loop electronic circuit that modulates the microcantilever stiffness at a frequency close to twice its natural resonance frequency. Under ambient conditions this parametric pumping leads to self-sustained oscillations in a narrow frequency bandwidth thereby resulting in exquisitely sharp, controllable, and non-Lorentzian resonance peaks. We discuss and demonstrate the potential of imaging and force spectroscopy using this mode.

An Experimental Study of Fluidic Coupling Between Multiple Piezoelectric Fans

Abstract: Piezoelectric fans have been shown to provide large enhancements in heat transfer over natural convection while consuming very little power. These fans consist of a piezoelectric material attached to a flexible cantilever. When driven at resonance, large oscillations at the cantilever tip cause fluid motion, which in turn, results in improved heat transfer rates. In this work, the performance of two fans operating simultaneously is analyzed. A coupling phenomenon is observed which, for a given input, causes an increase in vibration amplitude of as large as 40 percent compared to an isolated single fan. Understanding this coupling is essential in order to create design tools for implementing piezoelectric fans in practical cooling systems. Mylar fans are analyzed, and multiple experiments performed in air and within a vacuum chamber to isolate the source of coupling and determine its magnitude. The results suggest that coupling is almost entirely due to fluid-structure interaction, and the impact on the characteristic vibration parameters is explored. The collective motion of the fans decreases the fluidic damping, and the coupling magnitude is determined for a range of fan pitches

Nonlinear micromechanical filters based on internal resonance phenomenon

Abstract: A microresonator concept based on 1:2 internal resonance between the modes of the resonator is explored in this study. The response of the structure under electrostatic actuation is computed and the simulated currents at the input and output ports are presented. Results show that the output current for the T-beam is non-zero for a very small band of frequencies. Unlike linear filters, the proposed non-linear resonator provides filtering and mixing since the output signal is at half the input signal frequency. Furthermore, the proposed device has significantly higher out-of-band rejection as compared to an equivalent linear micromechanical filter. Because of these unique characteristics these microresonators hold a great potential for use in RF filtering and mixing applications.

Nonlinear internal resonance based micromechanical resonators

Abstract: A micromechanical resonator having a structure defining a first mode and a second mode and permitting non-linear internal resonance between the first and second modes. The resonator may further include a first component embodying a first mode and a second component embodying a second mode such that the first and second modes are substantially completely non-linearly coupled with each other while the second component vibrates at a frequency approximately twice the at least one first natural resonance frequency.

Experimental Mapping of Local Heat Transfer Coefficients Under Multiple Piezoelectric Fans

Abstract: Piezoelectric fans have been shown to provide large enhancements in heat transfer over natural convection while consuming very little power. These fans consist of a piezoelectric material attached to a flexible cantilever. When driven at resonance, large oscillations at the cantilever tip cause fluid motion, which in turn, results in improved heat transfer rates. In this study, the local heat transfer coefficients are determined experimentally for piezoelectric fans vibrating close to an electrically heated stainless steel foil, and the entire temperature field is observed by means of an infrared camera. Various vibration amplitudes, distances from heater to fan tip (or gap), and fan pitches are considered for both single-fan and two-fan configurations in impinging orientations. Of particular interest is the increase in heat transfer performance with an additional fan present and the dependence of this increase on the variable parameters. Results show nearly uniform cooling within the envelope of vibration for single-fan experiments with small gaps, and the existence of an optimal gap distance which is dependent on vibration amplitude. The benefits of an additional fan include greater coverage area, but the resulting increase in peak convection coefficient is highly dependent on the fan pitch. Conditions exist where constructive interference is observed which causes a roughly 10% increase in peak convection coefficient while significantly increasing the coverage area. Understanding the local performance of piezoelectric fans provides an important tool to help implement these devices in practical cooling systems.Copyright © 2006 by ASME

Dynamics of Microcantilevers Tapping on Nanostructures in Liquid Environments in the Atomic Force Microscope

Abstract: The Atomic Force Microscope (AFM) has become an indispensable tool in biology because it permits the probing of nanomechanical properties under physiological (liquid environments) conditions. AFM has been used in liquid environments to image, manipulate and probe atoms, living cells, bacteria, viruses, subcellular structures such as microtubules, individual proteins and DNA. Probably the most popular method used for AFM in liquids is the tapping mode wherein a resonant microcantilever is scanned over a sample. Yet very little is known about the dynamics of microcantilevers in liquid environments while interacting with nanostructures. This problem is especially challenging because viscous hydrodynamics couples strongly with cantilever motions, and the contribution from the electric double layer forces, which is not significant in air, must be taken into account. Previous attempts in the analysis and simulation of tapping mode in liquid modeled the tip-sample interaction forces using either a Lennard-Jones potential [1, 2], an exponentially growing force of small duration of the cantilever oscillation cycle [3] without any contact mechanics, or an unrealistic discontinuous interaction force [4]. Moreover, in all these papers the microcantilever was modeled by a point (lumped) mass, and the hydrodynamic effects were not derived rationally from basic hydrodynamic theory. Instead, a low quality factor (Q factor) and an added fluid mass were simply assumed [1–4]. A direct method to systematically deal with the AFM microcantilever using continuous beam theory in liquids governed by the unsteady Stokes equations and experiencing intermittent contact with the sample is not available in the literature.Copyright © 2006 by ASME