(475h) Hydrodynamic Positioning of Buoyant Drops and Bubbles Inside Microfluidic Channels

We have investigated the positioning of buoyant drops and bubbles immersed in a liquid suspending phase that flows inside microfluidic channels. In Newtonian suspending fluids, in the absence of buoyancy, and for flow conditions characterized by small particle Reynolds numbers, ReP, (ReP < 0.1) drops and bubbles migrate across streamlines towards the center of the channel [1,2]. We observed that buoyant drops and bubbles flowing in a horizontal channel can travel without touching the walls of the channel and along a horizontal trajectory that is parallel with the centerline of the channel and displaced from it. Achieving such flow conditions can be critical for the operation of high-performance drop microfluidic applications such as ice nucleation measurements [3] or lasing with high-speed switching in trains of drops [4]. We quantified the position of drops by measuring the distance, d, between the trajectory of drops and bubbles and the centerline (see figure below), and by mapping how d varies with parameters such as the viscosity and the velocity of the suspending fluid, and the size of drops and bubbles. The distance varied in qualitative agreement with models that predict cross-streamline migration of deformable drops and bubbles, becoming smaller as the diameter of drops and bubbles became larger, and as the viscosity and velocity of the suspending phase increased. We have identified the range of experiment parameters (fluid properties, buoyant force, and channel geometry) for which drops and bubbles traveled without touching the walls. We will report for what range of parameters such travel was possible, and we will comment on the quantitative agreement between our measurements and predictions of d based on existing theoretical models [1,2]. 1. H. L. Goldsmith and S. G. Mason, J Coll. Sci. 17, 448-476 (1962). 2. P. C.-H. Chan and L. G. Leal, J . Fluid Mech. 92, 131-170 (1979). 3. C A. Stan, G. F. Schneider, S. S. Shevkoplyas, M. Hashimoto, M. Ibanescu, Benjamin J. Wiley and George M. Whitesides, Lab Chip 9, 2293-2305 (2009). 4. S. K. Y. Tang, Z. Li, A. R. Abate, J. J. Agresti, D. A. Weitz, D. Psaltis and G. M. Whitesides, Lab Chip 9, 2767-2771 (2009).