Experimental and numerical hydrodynamic studies of ionic liquid-aqueous plug flow in small channels

- Detailed plug flow characteristics in liquid-liquid microchannel flows.
- Velocity profiles and circulation patterns in plugs were obtained with bright field PIV.
- CFD simulations predicted well experimental plug lengths.
- Variation of pressure along the microchannel during plug flow.

The hydrodynamic characteristics of liquid-liquid plug flow were studied in microchannels with 0.2 and 0.5 mm ID both experimentally and numerically. For the experiments high speed imaging and bright field micro-Particle Image Velocimetry were used, while the numerical simulations were based on the volume-of-fluid (VOF) method. The two immiscible liquids were a 1 M HNO3 aqueous solution which formed the dispersed plugs and a mixture of 0.2 M n-octyl(phenyl)-N,N-diisobutylcarbamoylmethyphosphine oxide (CMPO) and 1.2 M Tributylphosphate (TBP) in the ionic liquid 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide ([C4min][NTf2]). The thickness of the film surrounding the plugs, and the plug velocity and length were measured and compared against literature correlations. For the cases studied (0.0224 < Ca < 0.299) it was observed that the liquid film was largely affected by the changes in the shape of the front cap of the plug. The plug length was affected by both the Capillary number and the ratio of the aqueous to ionic liquid phase flow rates while the plug volume depended on the channel diameter and the mixture velocity. The numerical simulations showed that, in agreement with the measurements, a parabolic velocity profile develops in the middle of the plugs while the circulation patterns in the plug are affected by the channel size. The pressure profile along the channel with a series of plugs and slugs was predicted numerically while the pressure drop agreed well with a correlation which included the dimensionless slug length and the ratio Ca/Re.