Abstract
The functionality of atomic force microscopy (AFM) and nanomechanical sensing can be enhanced using higher-mode microcantilever vibrations. Both methods require a resonating microcantilever to be placed close to a surface, either a sample or the boundary of a microfluidic channel. Below a certain cantilever-surface separation, the confined fluid induces squeeze-film damping. Since damping changes the dynamic properties of the cantilever and decreases its sensitivity, it should be considered and minimized. Although squeeze-film damping in gases is comprehensively described, little experimental data is available in liquids, especially for higher-mode vibrations. We have measured the flexural higher-mode response of photothermally driven microcantilevers vibrating in water, close to a parallel surface with gaps ranging from ~200 μm to ~1 μm. A modified model based on harmonic oscillator theory was used to determine the modal eigenfrequencies and quality factors, which can be converted into co-moving fluid mass and dissipation coefficients. The range of squeeze-film damping between the cantilever and surface decreased for eigenfrequencies (inertial forces) and increased for quality factors (dissipative forces) with higher mode number. The results can be employed to improve the quantitative analysis of AFM measurements, design miniaturized sensor fluid cells, or benchmark theoretical models. PACS: 07.10.Cm (Micromechanical devices and systems), 46.40.Ff (Resonance and damping of mechanical waves), 07.79.-v (Scanning probe microscopes and components), 07.07.Df (Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing).
Highlights
Damping is an important design criterion for micro- and nanometer sized resonators, because surface forces dominate body forces at small dimensions [1]
The influence of the cantileversurface gap became substantial for H = g/b < 1, causing the resonance peaks to shift towards lower frequencies and broaden significantly
We have measured the squeeze-film damping on higher flexural mode vibrations of microcantilevers placed in proximity to a parallel surface in liquid
Summary
Damping is an important design criterion for micro- and nanometer sized resonators, because surface forces dominate body forces at small dimensions [1]. E.g., a microcantilever, in fluid drastically changes its dynamic properties. Squeeze-film damping starts to dominate other dissipative effects and, needs to be considered and characterized [4]. The functionality of atomic force microscopy (AFM) and nanomechanical sensing can be enhanced using higher-mode microcantilever vibrations. Both methods require a resonating microcantilever to be placed close to a surface, either a sample or the boundary of a microfluidic channel. A certain cantilever-surface separation, the confined fluid induces squeeze-film damping. Since damping changes the dynamic properties of the cantilever and decreases its sensitivity, it should be considered and minimized. Squeeze-film damping in gases is comprehensively described, little experimental data is available in liquids, especially for higher-mode vibrations
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