The use of two phase flow in lab-on-chip devices, where chemical and biological reagents are enclosed within plugs separated from each other by an immiscible fluid, offers significant advantages for the development of devices with high throughput of individual heterogeneous samples. Lab-on-chip devices designed to perform the polymerase chain reaction (PCR) are a prime example of such developments. The internal circulation within the plugs used to transport the reagents affects the efficiency of the chemical reaction within the plug, due to the degree of mixing induced on the reagents by the flow regime. It has been hypothesised in the literature that all plug flows produce internal circulation. This work demonstrates experimentally that this is false, and seeks to elucidate the parameters influencing the internal circulation of plugs. The particle image velocimetry (PIV) technique offers a powerful non-intrusive tool to study such flow fields. This paper presents micro-PIV experiments carried out to study the internal circulation of aqueous plugs in two phase flow within 762μm internal diameter FEP Teflon tubing with FC-40 as the segmenting fluid. Experiments have been performed and the results are presented for plugs ranging in length from 1mm to 13mm with an average fluid velocity ranging from 0.3mm/s to 50mm/s. The results demonstrate that circulation within the plugs is not always present and requires design considerations to benefit from this phenomenon.

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