Wettability-driven separation of two-phase liquid–liquid emulsions in laser-engraved PMMA microchannels

Abstract

Separating immiscible liquid phases at the microscale is a persistent challenge in process intensification, particularly where sample volumes are small and conventional equipment is impractical. In this study, a passive radial multi-channel microfluidic device was fabricated from poly (methyl methacrylate) (PMMA) by CO₂ laser micromachining and sealed by solvent-free thermal bonding a route that requires no cleanroom facilities and is achievable with standard laboratory equipment. The device was used to investigate wettability-driven separation of water and paraffin oil emulsions under both hydrophilic and hydrophobic surface conditions, across four volumetric flow rates (0.25, 0.38, 0.50-, and 1.60-mL min⁻¹) and four water-to-oil feed ratios (30:70, 40:60, 50:50, and 60:40). The underlying transport mechanism was interpreted through capillary number analysis. In the hydrophilic configuration, water was preferentially drawn into the narrower branches by surface tension while oil became retained in the wider sections; in the hydrophobic configuration, the preferential pathways were reversed. The highest separation efficiency (0.92) was achieved under both surface conditions at 0.25 mL min⁻¹, and declined progressively with increasing flow rate as viscous forces grew dominant over capillary forces. Three-dimensional response surfaces confirmed that flow rate exerted a substantially greater influence on separation efficiency than feed composition across all tested conditions. Because separation relies on wettability contrast rather than density difference, the device performs effectively where gravity-driven settling is insufficient. Its passive operation, low fabrication cost, and continuous-flow design make it a promising candidate for integration into laboratory-scale essential-oil recovery systems as an inline, solvent-free phase-splitting step.