Amplitude and phase spectra of a microcantilever vibrating at different gaps g to a surface. The amplitude (upper plot) and phase (lower plot) response of a microcantilever (250 μm × 35 μm × 2 μm) vibrating in water at different distances from a surface are shown as a function of frequency and the corresponding Reynolds number (Re). The flexural mode numbers are written above the resonance peaks. Absolute (g) and normalized (H) cantilever-surface separations are indicated. The color scale is not linear; far from the surface the increment in g was set larger because the effect diminishes (superimposed purple curves). The spectra are overlayed; the red curves (small g) are at the back.