Hydrophilic and Hydrophobic Modifications of Microchannel Inner Walls for Liquid-Liquid Laminar Layered Flows

Abstract

Microchannels, which have a wide surface area per unit volume, have the potential for use in efficient chemical synthesis, separation and analysis. When microchannels are applied to mass transfer operations between immiscible liquids, a layered laminar flow is preferable because the two liquids can be spontaneously separated at the outlets. In this study, the flow of a water-octane system in a horizontal microchannel 200 ?m in width and 200 ?m in depth was simulated using CFD software. The simulation suggested that a stable layered flow was realized when the contact angle of octane on the hydrophobic inner walls was smaller than ca. 60 degrees and when that on the hydrophilic inner walls was larger than ca. 120 degrees irrespective of the inlet planes into which water and octane were introduced. To fulfill these requirements, PMMA plates were modified using five procedures, and the contact angles of one of the liquids were determined in the presence of the counterpart liquid. Measurement of the contact angles was carried out by lateral observation (in the case of spherical cap drops) and the fringe pattern of laser interference (in the case of thin films). When the two liquids coexisted, a combination of modification with 1-octadecanethiol for hydrophobicity and coating with TiO2 for hydrophilicity was the most workable of the treatments tested. Microchannels modified by this combination were then fabricated. When either water or octane were introduced into the hydrophilic and hydrophobic half-sections, respectively, a stable two-phase layered flow was obtained.