Non-Contact Mechanics of Soft and Liquid Interfaces by Hydrodynamic Confinement Using a Frequency-Modulated AFM

Area of Science:

• Physics
• Materials Science
• Surface Science

Background:

• Measuring the mechanical response of liquid interfaces without direct contact is challenging, especially for liquid-liquid systems lacking solid references.
• Existing methods often require direct contact or lack sensitivity for soft/liquid interfaces.

Purpose of the Study:

• To develop and validate a noncontact frequency-modulation atomic force microscopy (FM-AFM) method for probing liquid interfaces.
• To simultaneously measure in-phase and dissipative mechanical responses under hydrodynamic confinement.
• To apply the method to both liquid-solid and liquid-liquid interfaces.

Main Methods:

• Utilized frequency-modulation atomic force microscopy (FM-AFM) with hydrodynamic confinement of a viscous liquid film.
• Validated the technique on a liquid-solid interface against elastohydrodynamic theory.
• Applied the method to analyze the mechanical response of a liquid-liquid interface.

Main Results:

• The FM-AFM method accurately measures confinement thickness and mechanical impedance for liquid-solid interfaces.
• The liquid-liquid interface exhibited a predominantly viscous response with a measurable in-phase component.
• Hydrodynamic confinement provided a sensitive, noncontact measurement of interfacial mechanics.

Conclusions:

• Hydrodynamic confinement via FM-AFM offers a sensitive, noncontact approach to characterize soft and liquid interfaces.
• This method is promising for studying complex, deformable systems like biological membranes and nanoparticle assemblies.
• Opens new avenues for interfacial mechanics research in diverse soft matter systems.