You knew the first response would be from me, didn't you :-)?
No, this cannot be scaled up to the scale of automotive or pump devices. There are several reasons for this. One, the energy released with the 'stroke' of the small bubble dissipating into the big one cannot be effectively harvested as it is used almost exclusively to increase the surface area of the big droplet. If you hinder that growth, the small bubble won't disappear fully into the big one and your engine will experience 'knocking'.
One-and-a-half, if you put some sort of thin sheet across the bubbles to have that stretch and contract thus producing a voltage difference, the surface tension will immediately see to it that your precious bubbles become a thin smear. Or in other words, you create a capillary effect. The device works because it relies on surface tension producing bubbles. Anything interfering with that process ruins the engine.
Two, the researchers only gave numbers about the energy released from the disappearance of the small bubble. You don't get that energy for free. The big bubble decreases in size, requiring energy. Some energy---but not all---is returned as the small bubble grows. Only when the small bubble touches the big one is the energy balance completed and is the energy difference equalised with a big jolt. It is therefore not really an engine, but more a little device capable of producing pulses.
Three, simply making larger bubbles doesn't work, as the forces scale with the reciprocal of bubble size. Or, in layman's terms: the bigger the bubble, the less important the surface tension.
Four, this device would require materials of utmost purity (any contiminant would influence the surface tension and thus the properties of the pulse) and mechanical stability (these bubbles have almost nothing to hold on to). The shaking you'd get in a car would render the engine useless: all the droplets would be torn from their anchoring points.
Nice piece of nano-engineering, but for all intents and purposes, utterly useless.
