Principles of Surface-Directed Liquid Flow in Microfluidic Channels

TLDR: To direct liquid flow inside microchannels, surface free energies were patterned by use of self-assembled monolayers (SAMs) in combination with either multistream laminar flow or photolithography, showing that aqueous liquids flow only along the hydrophilic pathways when the pressure is maintained below a critical value.

Abstract: To direct liquid flow inside microchannels, surface free energies were patterned by use of self-assembled monolayers (SAMs) in combination with either multistream laminar flow or photolithography. For the photolithographic method, two photocleavable SAMs were designed and synthesized. Carboxylic acid-terminated monolayers were obtained by photodeprotection, which was confirmed by contact angle and X-ray photoelectron spectroscopy. Using either of these patterning methods, we show that aqueous liquids flow only along the hydrophilic pathways when the pressure is maintained below a critical value; the liquids are referred to as being confined by virtual walls. Several principles of liquid flow in surface-patterned channels were derived analytically and verified experimentally. These principles include the maximum pressure that virtual walls can withstand, the critical width of the hydrophilic pathway that can support spontaneous flow, the smallest width of the liquid streams under an external pressure, the ...