Abstract: In one aspect of the disclosed subject matter, a method for isolating target microbubbles having a predetermined size range from polydisperse microbubbles is disclosed. The method includes applying a first centrifugal field having a first field strength to a suspension comprising the polydisperse microbubbles for a first duration of time, thereby forming a first infranatant comprising at least a portion of target microbubbles and a first supernatant cake comprising microbubbles having a greater size than the target microbubbles; removing the first supernatant cake; applying a second centrifugal field having a second field strength to the first infranatant for a second duration of time, the second field strength being greater than the first field strength, thereby forming a second supernatant cake comprising at least a portion of the target microbubbles and second infranatant comprising microbubbles having a smaller size than the target microbubbles; and isolating the second supernatant cake.