Direct studies of liquid flows near solid surfaces by total internal reflection fluorescence cross-correlation...

Area of Science:

• Fluid Dynamics
• Surface Science
• Spectroscopy

Background:

• Studying liquid flow near solid surfaces is crucial for understanding various phenomena.
• Traditional methods often lack the resolution to probe nanoscale flow dynamics accurately.

Purpose of the Study:

• To introduce and validate a novel method, Total Internal Reflection Fluorescence Cross-Correlation Spectroscopy (TIR-FCCS), for nanoscale fluid flow analysis.
• To investigate the flow of aqueous electrolyte solutions near a hydrophilic surface.

Main Methods:

• Utilized evanescent light generated via epi-illumination and a high numerical aperture objective for excitation.
• Employed two laterally shifted confocal pinholes to create distinct observation volumes.
• Measured time-resolved fluorescence intensity signals and computed cross-correlation to determine tracer velocities.
• Varied evanescent wave penetration depth for flow profiling within 200 nm of the interface.

Main Results:

• Demonstrated the capability of TIR-FCCS to measure fluid velocities with high sensitivity, accommodating tracers from single dye molecules to larger species.
• Successfully applied TIR-FCCS to study aqueous electrolyte solutions near a hydrophilic surface.
• Explored the influence of tracer size, ionic strength, and observation volume separation on flow measurements.

Conclusions:

• TIR-FCCS is a sensitive and versatile technique for characterizing nanoscale fluid flow dynamics at interfaces.
• The method allows for detailed investigation of factors affecting near-surface liquid behavior, such as ionic strength and tracer properties.