Micro-object manipulation by AC-EWOD-driven twin bubbles: (a) Initial state; (b) When twin bubbles actuated by AC-EWOD at a certain frequency (100 Hz) generate streaming, a micro-object is pushed by the streaming and transported to the next EWOD electrodes; (c) The object is carried by the sequential AC-EWOD operations with the twin bubbles to the right end of EWOD electrodes; (d) When the twin bubbles actuated by AC-EWOD at 1 Hz are transported to the original position without streaming, the object is released from the bubbles and remains where it is. Note that the bubble-induced streaming is generated only at a certain frequency.