Numerical Study of Hydrodynamic Forces for AFM Operations in Liquid.

Tobias Berthold,Guenther Benstetter,Rosana Rodríguez,Montserrat Nafría,Werner Frammelsberger

doi:10.1155/2017/6286595

Tobias Berthold, Guenther Benstetter + Show 3 more

Open Access

https://doi.org/10.1155/2017/6286595

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Abstract

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 the measurement environment could obscure molecular forces. Due to fluid properties, the mechanical behavior of the AFM cantilever is influenced by the hydrodynamic drag force due to viscous friction with the liquid. This study provides a numerical model based on computational fluid dynamics (CFD) and investigates the hydrodynamic drag forces for different cantilever geometries and varying fluid conditions for Peakforce Tapping (PFT) in liquids. The developed model was verified by comparing the predicted values with published results of other researchers and the findings confirmed that drag force dependence on tip speed is essentially linear in nature. We observed that triangular cantilever geometry provides significant lower drag forces than rectangular geometry and 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 in ultrapure water, whereas with increasing ethanol concentrations the drag forces increase.

Highlights

Since the invention by Binnig et al in 1986 [1], atomic force microscopy (AFM) is finding increasing use in the characterization of chemical surface modifications [2, 3] to achieve a chemical mapping of the surface in the nanoscale
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 the measurement environment could obscure molecular forces
This study provides a numerical model based on computational fluid dynamics (CFD) and investigates the hydrodynamic drag forces for different cantilever geometries and varying fluid conditions for Peakforce Tapping (PFT) in liquids

Summary

Introduction

Since the invention by Binnig et al in 1986 [1], atomic force microscopy (AFM) is finding increasing use in the characterization of chemical surface modifications [2, 3] to achieve a chemical mapping of the surface in the nanoscale. Mendez-Mendez et al [11] introduced a numerical model to predict the hydrodynamic forces acting on a V-shaped tip for velocities up to 105 μm/s and varying cantilever-sample distances They show that the usage of different fluids significantly influences the drag force acting on the cantilever and confirm that the drag force dependence on tip speed is essentially linear in nature. PFT enables the mapping of nanomechanical properties [20, 22] This advanced characterization technique provides advantages for the investigation of chemical modification by CFM because this dynamic operation mode enables a complete chemical mapping of the surface [23, 24] and is not just limited to applying individual force distance curves at distinct measurement points. The present work addresses these needs and provides a numerical model to investigate the hydrodynamic drag forces for different cantilever geometries and varying fluid conditions for tip speeds associated with PFT measurements. Therewith, the measurement setup can be optimized by minimizing parasitic drag forces and by reducing experimental uncertainties for a more precise force control regarding studies of soft samples in liquids

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Results and Discussion

Conclusion