Hydrodynamics of torsional probes for atomic force microscopy in liquids

Abstract

Improving the force resolution of atomic force microscopy for soft samples in liquid requires soft cantilevers with reduced hydrodynamic cross section. Single and dual axis torsion levers ͓Beyder and Sachs, 2006͔ are an attractive technology. They have reduced area and reduced drift due to the symmetric support ͓Beyder et al., 2006͔ can add a second dimension using two independent axes. Here we investigate the hydrodynamics of these probes using three-dimensional transient fluid-structure interaction models with comparison to the experimental data. The computed Q factors and wet/dry resonance frequencies of different modes compare well with experimental measurements indicating that continuum viscous hydrodynamics can be used effectively to predict probe performance. The modeling further explores cross-axis hydrodynamic coupling and the influence of a nearby sample plane to provide guidance on approach algorithms and the possibilities of parametric detection.

Unmasking Imaging Forces on Soft Biological Samples in Liquids When Using Dynamic Atomic Force Microscopy: A Case Study on Viral Capsids

“…In summary, based on the scaling laws and experimental results, we suggest that the major reasons the SL applies reduced imaging forces are a) its stiffness is ;40% (nominal) smaller, and b) its Q-factor close to the sample is much larger than that of the CL, which actually becomes overdamped when brought into the vicinity of the sample. The dependence of cantilever damping on its vicinity to the sample is well known (19,28,29). The fact that the SL has a much longer tip length than the CL assures us that its distance from the sample is larger, thus minimizing the hydrodynamic damping of the viscous squeeze film between the cantilever and the sample.…”