Numerical Study of Hydrodynamic Forces for AFM Operationsin Liquid

For advanced atomic force microscopy (AFM) investigation of chemical surface modifications or very soft organic sample surfaces,the AFM probe tip needs to be operated in a liquid environment because any attractive or repulsive forces influenced by themeasurement environment could obscure molecular forces. Due to fluid properties, the mechanical behavior of the AFM cantileveris influenced by the hydrodynamic drag force due to viscous friction with the liquid. This study provides a numerical model basedon computational fluid dynamics (CFD) and investigates the hydrodynamic drag forces for different cantilever geometries andvarying fluid conditions for Peakforce Tapping (PFT) in liquids. The developed model was verified by comparing the predictedvalues with published results of other researchers and the findingsconfirmed that drag force dependence on tip speed is essentiallylinear in nature. We observed that triangular cantilever geometry provides significant lower drag forces than rectangular geometryand that short cantilever offers reduced flow resistance. The influence of different liquids such as ultrapure water or an ethanol-water mixture as well as a temperature induced variation of the drag force could be demonstrated. The acting forces are lowest inultrapure water, whereas with increasing ethanol concentrations the drag forces increase.