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Flow profile near a wall measured by double-focus fluorescence cross-correlation
1Max-Planck-Institut für Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany.
Summary
We developed a new method for precise flow profiling in tiny volumes using tracer particles and microscopy. This technique reveals colloidal lift forces crucial for transport in microfluidic devices.
Area of Science:
- Fluid dynamics
- Microfluidics
- Nanotechnology
Background:
- Accurate flow profiling in microfluidic devices is essential for understanding transport phenomena.
- Traditional methods struggle with femtoliter volumes and solid interfaces.
- Colloidal forces significantly impact fluid behavior at the microscale.
Purpose of the Study:
- To present an experimental approach for high-precision flow profiling in femtoliter sample volumes.
- To enable accurate measurements at the solid-fluid interface.
- To investigate the role of colloidal forces in microfluidic flow.
Main Methods:
- Spatial cross-correlation of fluorescence from labeled tracer particles (nanospheres or dye molecules).
- Utilizing two laser foci under confocal microscopy to create separated excitation volumes.
- Measuring tracer particle velocity with 0.1% accuracy in microchannels and estimating wall positions independently.
Main Results:
- Demonstrated precise flow profiling at the solid interface in femtoliter volumes.
- Quantified apparent fluid velocity at the wall, influenced by colloidal (electrostatic) forces.
- Illustrated the method with Poiseuille flow of a Newtonian liquid exhibiting slip.
Conclusions:
- The developed optical method allows accurate flow profiling and wall position estimation in microfluidic channels.
- Colloidal lift forces significantly affect apparent fluid velocity near the wall.
- Understanding colloidal lift is critical for optimizing transport processes in microfluidic devices.