Yagodnitsyna A.   Kovalev A.   Bilsky A.  

Visualization and velocity field measurements in immiscible liquid-liquid flow in microchannels

Reporter: Yagodnitsyna A.

In last decade microreactors have become alternative to traditional reactors in chemical industry and biotechnology [1]. Channels with submillimeter hydraulic diameter have extremely large surface to volume ratio which leads to intensification of heat and mass transfer. Besides that microreactors have such advantages as small volume of react-ing fluids and better reaction control. Flows of immiscible liquids in microchannels are important for different kinds of technical applications such as micromixers in chemistry, bioMEMS devices, extraction reactions, emulsions production etc. In the present paper experimental investigation of immiscible liquids flow in a T-shaped microchannel was carried out.
A flow of two pairs of immiscible liquids in a T-shaped microchannel made of SU-8 material with high wettabilitywas studied: kerosene and distilled water and paraffin oil and distilled water. The size of inlet channels was 200x200 um, the size of outlet channel was 200x400 um. The length of outlet channel was 22.5 mm, the length of inlet channels was 11.5 mm. We used high speed camera (pco.1200 hs) and inverted microscope (Zeiss Axio Oberver.Z1) with 5x magnification to visualize flow regimes and dynamics of slug and plug formation.  Liquid flow was organized by using KDS Gemini 88 double syringe pump. The flow rate of water was varied from 0.3 ul/min to 5 ml/min, flow rate of kerosene was varied from 2 ul/min to 5 ml/min, Therefore, Weber number of water was in the range from 2.3*10-8 to 6.4 and Weber number of kerosene was in the range from 8*10-7 to 5. Flow rate of paraffin oil was limited to 0.24 ml/min because of high viscosity.
For water-kerosene flow we found out that there were the following flow regimes (Fig.1): plug flow (water in kerosene), slug flow (for both liquids), droplet flow (water in kerosene), rivulet flow (for both liquids), parallel flow and serpentine flow (stationary and non-stationary). It is important to notice that we didn’t observe annular flow because of low contact angle of both liquids. High surface wettability was the reason for occurrence of slug flow for both liquids and rivulet flow instead of annular flow. As flow visualization was done we constructed flow regime map using Weber number of both liquids.  The flow map was different from that obtained by [2]. So we can conclude that Weber number is not the only parameter which defines flow regime. Contact angle of liquids and channel surface should be also taken into account. Flow visualization of paraffin oil and water showed that there were the following flow patterns: plug flow (water in oil), droplet flow (water in oil), parallel flow and slug flow (oil in water). Comparison of two flow maps for both pairs of liquids showed that those were similar to each other but pattern transition was on different Weber numbers of kerosene and oil. In order to take into account fluid viscosity we constructed new flow maps by multiplying Weber number of kerosene and oil by ratio of viscosities of liquids to water viscosity. After that flow maps coincided with each other (Fig.2).
For typical flow regimes we made micro-PIV measurements for water and kerosene in order to evaluate convective flows inside both phases which is important for microreactors development and optimization.

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