Surface Modification Method of Microchannels for Gas−Liquid Two-Phase Flow in Microchips

Abstract

A capillarity restricted modification method for microchannel surfaces was developed for gas--liquid microchemical operations in microchips. In this method, a microstructure combining shallow and deep microchannels and the principle of capillarity were utilized for chemical modification of a restricted area of a microchannel. A hydrophobic--hydrophilic patterning in microchannels was prepared as an example for guiding gas and liquid flows along the respective microchannels. Validity of the patterning was confirmed by measuring aqueous flow leak pressure from the hydrophilic microchannel to the hydrophobic one. The leak pressure of 7.7-1.1 kPa agreed well with that predicted theoretically from the Young-Laplace equation for the microchannel depth of 8.6-39 microm. In an experiment to demonstrate usefulness and effectiveness of the method, an air bubble was first introduced into the hydrophilic microchannel and purged from the hydrophobic-hydrophilic patterned microchannels. Next, the patterning structure was applied to remove dissolved oxygen by contacting the aqueous flow with a nitrogen flow. The concentration of dissolved oxygen decreased with contact time, and its time course agreed well with numerical simulation. These demonstrations showed that the proposed patterning method can be used in general microfluidic gas-liquid operations.

Mixing with bubbles: a practical technology for use with portable microfluidic devices

“…In order to avoid both potential problems in processing the mixed analytes and in detection associated with the presence of bubbles in the stream of mixed liquids, it is possible to separate the gaseous slugs from the continuous fluid with the use of capillary pressure. Such separation has been demonstrated independently by Gunther et al, 17,46 and by Hibara et al 47 The residence times of the analytes-a parameter important in many biological assays-can be tuned easily (either in the fabrication process, or directly in the field by adjusting an imbedded TWIST valve). The device functions efficiently both in the presence or absence of surface-active agents, should be applicable for diagnostic assays involving proteins, and is resistant to many variations in the properties of biological fluids.…”

Surface patterning of bonded microfluidic channels

“…The result is a self-assembled monolayer ͑SAM͒ at the interface, with an alkyl chain ͑with the desired functional group͒ exposed at the microchannel walls. 23,42,43 Depending on their functionality, SAMs and other thin organic layers provide resistance to biofouling, [44][45][46] a wettability contrast, 26,32 or intermediates for surface patterning. 25,26 The silanol groups present at glass and oxidized PDMS surfaces lead to a native negative surface potential in aqueous phases, except at very low pH ͑the point of zero charge for silica is pH ϳ 2-4͒.…”

“…24,32 Hibara et al used silane chemistry and capillarity to selectively hydrophobize one channel of a multichannel gas-liquid extraction chip. 32 The authors employed a step in the channel geometry to guide liquid containing a hydrophobizing agent ͑OTS͒ along only one of the channels during capillary filling. Hydrophobicity of the channel ͑for the gas phase͒ ensured stability of the liquid-gas interface due to the Laplace pressure that opposes entry of the aqueous phase ͑leak pressures were several kilopascals͒.…”

“…Most of these methods draw on knowledge from conventional techniques of surface modification, with some degree of adaptation to microscale confinement, e.g., photolithography [25][26][27] or microplasmas, [28][29][30][31] while other methods are emerging which take advantage of specific physical behavior in microchannels, e.g., laminar and capillary flow. 26,27,32,33 This Review discusses prominent examples of postbonding channel patterning that are, or have the potential to become, routine tools for the microfluidics community. The discussion is organized as follows.…”

Active liquid degassing in microfluidic systems

“…There are passive and active methods, and the main techniques used are bubble traps, bubble formation prevention methods, and bubble removal methods using pressurisation and/or extraction of bubbles, see Table 1. Bubble traps and multiphase flow systems function for certain applications, 6,7 but suffer from increased gas pressures at elevated temperatures, which may cause significant bubble expansions. Prevention of gas transport from the surroundings into microchannels using claddings 1 or barriers 4,8 have been shown to partly solve the problem of bubble formation in porous materials, though problems with bubble formation via trapping of large bubbles during liquid pumping and outgassing remain unsolved.…”