Principles of surface-directed liquid flow in microfluidic channels.

Area of Science:

• Microfluidics
• Surface Chemistry
• Materials Science

Background:

• Directing liquid flow within microchannels is crucial for various applications.
• Surface free energy plays a key role in controlling liquid behavior at the microscale.
• Existing methods for microchannel liquid control often have limitations.

Purpose of the Study:

• To develop and demonstrate a method for precise liquid flow control in microchannels.
• To utilize patterned surface free energies via self-assembled monolayers (SAMs) for flow direction.
• To establish analytical principles governing liquid confinement by 'virtual walls'.

Main Methods:

• Patterning surface free energies using self-assembled monolayers (SAMs).
• Employing multistream laminar flow and photolithography for SAMs patterning.
• Synthesizing and characterizing photocleavable SAMs for photodeprotection.
• Confirming monolayer formation using contact angle and X-ray photoelectron spectroscopy.

Main Results:

• Aqueous liquids were confined to hydrophilic pathways below a critical pressure, forming 'virtual walls'.
• Analytical principles for virtual wall behavior (pressure limits, pathway width, stream separation) were derived.
• Experimental results demonstrated strong agreement with the derived analytical predictions.

Conclusions:

• Surface patterning with SAMs offers effective control over liquid flow in microchannels.
• The concept of 'virtual walls' provides a robust framework for microfluidic design.
• The study establishes key parameters for designing stable and predictable microfluidic flows.