Abstract

Force measurements with atomic force microscopy (AFM) in liquid are subjected to the hydrodynamic drag force artifact (Fd) due to viscous friction of the cantilever with the liquid. This artifact may be especially relevant in microrheological studies of soft samples. Common approaches estimate Fd at a certain distance above the sample and subtract its value from the contact force measured on the sample. However, this procedure can underestimate Fd at contact. The aim of this work was to assess the effect of the hydrodynamic drag in microrheological AFM measurements of soft samples in liquid at low Reynolds numbers (Re < 1). Drag forces of water on rectangular and V-shaped cantilevers were measured in noncontact when subjecting the substrate to low-amplitude (35 nm) sinusoidal oscillations at different frequencies (1−200 Hz) and tip−substrate distances (h) (0.2−3 μm). Fd increased proportionally with the relative velocity (v). Moreover, the drag factor b(h) defined as Fd/v rose when the cantilever approach...

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