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Bubble Pumps

Professor Andrea Prosperetti has done the engineering equivalent of pulling a rabbit out of a hat—by designing a pump with no moving parts. These pumps are not likely to take over municipal water delivery, however, because they are about the same size as a human hair. Prof. Prosperetti’s pump consists of a channel about 100 to 200 microns in diameter and several hundred microns long, connecting two reservoirs of liquid. Within the channel a single vapor bubble expands and then collapses in response to a pulse of current. In the process, liquid is displaced, moving from one reservoir to the other. The entire cycle is completed in a few milliseconds and can be repeated hundreds of times a second. This simple device is surprisingly powerful; flow rates of hundreds of microliters per minute and pressure heads of several tenths of atmospheres are easily achieved. Another way to power these bubble pumps is by means of a sound wave, which causes areas of relative low and high pressure to form. A bubble will change in volume, or oscillate, in response to these pressure changes. When the bubble vibrates like this, the fluid in the channel moves. Prof. Prosperetti and his team are modeling these kinds of bubble behaviors, as well as experimenting with bubble pumps in the laboratory.

Practical applications have started to materialize, and many more appear possible. For example: In the interest of eventually putting humans in space, NASA might have a good use for bubble pumps—growing food. If people are going to spend much time in space, says Prof. Prosperetti, they will need to eat their spinach, and it will have to be grown in space. Every pound that exits the Earth’s atmosphere costs around $10,000, and every drop of water in space will have to be recycled. If the plants are grown hydroponically with a legion of bubble pumps circulating the water through their root zone, nutrients could be delivered in an ultra-efficient manner with minimal water inventory. Or, imagine a drug-delivery system that could be implanted under the skin and deliver medication (such as insulin) on demand via a pulse-driven bubble pump. Or perhaps a bubble pump could be activated automatically with a sensor that would monitor the insulin level in the blood. The use of an ultrasonic field—which propagates harmlessly through living tissue—might make it possible to power such devices remotely with no need to undergo periodic operations to replace implanted batteries, as for example is currently the case with pacemakers.