Boy, if that isn't food for thought!
Imagine getting your hands on a piezo-electric transducer from say, an old ultrasonic parts cleaner and integrating it into a water block. With a fairly slow lateral flow rate through the block, the water as it passes between the transducer and the cups/pins/whatever shape lends well, would be excited both towards and away from them. I think you get the drift. Lots of parameters to work out and hoping the transducer wouldn't dump too much heat into the water, but just imagine...
Hoot