Bubbles dancing in a vortex: trapping air at a T-junction

We present an unusual phenomenon that occurs to low density material, and in particular air bubbles, entrained in a fluid when flowing through a T-junction. For a range of Reynolds numbers, the flow develops two symmetric vortices. Air bubbles are forced to the center of the vortex due to the centrifugal force and, for Reynolds number, $Re$, greater than $\approx 220$, are then ``trapped'', i.e. they accumulate inside the vortex. Bubbles eventually oscillate (i.e. ``dance'') in the vortex when the flow becomes unsteady for $Re>550$. Experiments were conducted by generating H$_2$, O$_2$ or simply air bubbles in the range $Re = 100$ to $\approx 6,000$ in a variety of T-junction devices. We have also observed a size dependence of the trapping phenomenon. In addition, our 3D numerical simulations have revealed a gradient of pressure, similar to vortex breakdown, that drives the flow towards the center of the T-junction creating two recirculating zones, which trap air bubbles. The presence of light material or air trapped in a flow could be relevant to industrial systems and biological flows, such as blood vessels, and may contribute to unexpected complications and/or failures in these systems.