Influence of squeeze-film damping on higher-mode microcantilever vibrations in liquid

Background: 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. Methods: 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. Results: 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. Conclusions: 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). © 2014, Bircher et al.; licensee Springer on behalf of EPJ.