Numerical Study of Hydrodynamic Forces for AFM Operations in Liquid

Area of Science:

• Materials Science
• Surface Science
• Fluid Dynamics

Background:

• Atomic Force Microscopy (AFM) requires operation in liquid for soft samples and surface chemistry.
• Liquid environments introduce hydrodynamic drag, affecting AFM cantilever mechanics and obscuring molecular forces.
• Understanding hydrodynamic drag is critical for accurate AFM measurements in fluid media.

Purpose of the Study:

• To develop and validate a computational fluid dynamics (CFD) model for hydrodynamic drag forces on AFM cantilevers in liquids.
• To investigate the impact of cantilever geometry and fluid conditions on drag forces during Peakforce Tapping (PFT).
• To provide insights for optimizing AFM experiments in various liquid environments.

Main Methods:

• Development of a numerical model using Computational Fluid Dynamics (CFD).
• Simulation of hydrodynamic drag forces for various cantilever geometries (triangular vs. rectangular, short vs. long).
• Analysis of drag forces under different fluid conditions (ultrapure water, ethanol-water mixtures) and temperatures.

Main Results:

• The CFD model accurately predicted drag forces, showing a linear dependence on tip speed.
• Triangular AFM cantilevers exhibited significantly lower drag forces compared to rectangular ones.
• Shorter cantilevers demonstrated reduced flow resistance, and drag forces increased with ethanol concentration and temperature.

Conclusions:

• CFD modeling is effective for analyzing hydrodynamic drag in AFM liquid environments.
• Cantilever geometry (triangular, short) and fluid properties significantly influence drag forces.
• Optimizing cantilever design and fluid selection is essential for high-resolution AFM imaging in liquids.