Simple. Dropping a water droplet in bulk water you get some bouncing due to the surface tension and kinetic energy and then coalescence into the bulk. But if you keep on bouncing the bulk water using acoustic waves the originally droplet will be suspended on top. It is not explained how big the drop are or how big the waves are to suspend the drop. There is likely a maximum and a minimum that this could work. Also different surface tensions of the droplet and the bulk water could also be explored.