PIV measurements of flow within plugs in a microchannel

Abstract

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. The particle image velocimetry (PIV) technique offers a powerful nonintrusive 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 762 lm 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 1 to 13 mm with a bulk mean flow velocity ranging from 0.3 to 50 mm/s. The results demonstrate for the first time that circulation within the plugs is not always present and requires fluidic design considerations to ensure their generation.Keywords Two-phase flow Á Plug flow Á Internal circulation Á lPIV List of symbols m magnification NA numerical aperture R radius of tubing (mm) V velocity (mm/s) V avg average velocity of plug (mm/s) V max maximum velocity (mm/s) V mean bulk bulk mean velocity (mm/s) X distance (mm) Y distance (mm) d e effective diameter of particle on CCD (lm) d p diameter of particle (lm) d s point spread function (lm) n refractive index r radial position (mm)Greek symbols dx measurement uncertainty (lm) dz m measurement depth (lm) h light collection angle (rad) k 0 wavelength of light in a vacuum (nm)

On the flow topology inside droplets moving in rectangular microchannels

“…At low viscosity ratio (Fig. 3a) the droplet velocity is 15 higher than that of the continuous phase for the Ca range and droplet shapes studied. The v d /v c ratio is higher for spherical droplets, as expected due to the absence of wall effects and the intrinsic parabola-like flow profile in microchannels.…”

“…SB would like to thank Dantec Dynamics for the loan of the laser used in this study. Finally the authors wish to thank Dr Peter Vennemann for providing the free PIV analysis software JPIV 15 (http://www.jpiv.vennemann-online.de/). …”

“…Each image pair was first processed to ensure that it contained a full droplet. The images were binarised, and repeatedly dilated so that the droplet could be analysed as a single 15 particle. The image was accepted if the borders of the droplet were not in contact with the image edges.…”

“…10, 11 A number of such studies on the droplet flow topology of liquid-liquid systems can be found in the literature. [12][13][14][15][16][17] For example, Oshima et al 13 applied confocal 40 µPIV to measure the flow field inside a moving droplet; by acquiring the flow field at various planes they were able to reconstruct the 3D flow topology using continuity. The results highlighted the role of the flow surrounding the droplet and the liquid-liquid interface on the flow topology.…”

“…Very few studies have concentrated on the effects of 60 such parameters on flow topology or reported on distinct flow topology changes. King et al 15 observed, through a µPIV study, a change in the flow topology of aqueous droplets moving in Teflon tubing. By altering the length of the plugs and the bulk flow velocity, the strength of the internal circulation varied 65 proportionally.…”