  Microfluidic techniques have been extensively developed to realize
micro-total analysis systems in a small chip. For microanalysis,
electro-magnetic forces have generally been utilized for the trapping of
objects, but hydrodynamics has been little explored despite its relevance to
pattern formation. Here, we report that water-in-oil (W/O) droplets can be
transported in the grid of an array of other large W/O droplets. As each
droplet approaches an interspace of the large droplet array, while exhibiting
persistent back-and-forth motion, it is conveyed at a velocity equal to the
droplet array. We confirm the appearance of closed streamlines in a numerical
simulation, suggesting that a vortex-like stream is involved in trapping the
droplet. Furthermore, more than one droplet is also conveyed as an ordered
cluster with dynamic reposition.
