Microscopic propulsion by sound waves
One of the first films microtechnology played a part in, was Fantastic Voyage dated back from 1966. In this film a submarine, with a medical team on board, is reduced so much that it is able to travel through the veins of a scientist who escaped from Eastern Europe. After having survived an attended murder, he suffers from a blood clot in his brain. According to reports the scientist solved the problem by reducing ordinary objects indefinitely, of which the CIA wanted to know all about. The task is to remove the blood clot in order to have the man regain consciousness. The image of a reduced submarine has helped to establish a certain image on medical microtechnology. The idea shows very little sense of reality, but to create a kind of vessel at a microscale may be closer to reality than we think. Researchers at the University of Twente have invented a one millimetre long device that is able to move through a liquid without a power of its own. They call the device an acoustic scallop. The researchers write about it in the Journal of Micromechanics and Microengineering of 11 July 2006.
Blowing out a candle is very easy. When blowing you produce a direct air beam that is cooling down the air around the burning taper so much that the flame is smothered. It is not possible to blow out a candle by sucking the air around the flame. By sucking, the air is being pulled from all directions and the effect on the flame is too small to smother. The effect on yourself also varies: blowing causes a bigger force reaction than sucking. We do not perceive any effect owing to the mass of our body, but a very light object will noticeably sense more power backwards by blowing than it does forwards by sucking. If you blow and suck alternately, then an asymmetric flow of air is being generated; repetition of this flow of air eventually causes a clear movement backwards. By blowing and drawing alternately, you are able to move. This may be a nice experiment for an astronaut of the International Space station, though.
Construing a microscopic device
In the past this principle has already been described and on the basis of a tube that was closed at one end, even a toy was made (see http://www.sciencetoymaker.org/boat/asembCartonl.html). In aviation its application is being investigated in order to control flow in the boundary layer. Andrea Prosperetti - a part-time professor of the Physics of Fluids Department at the University of Twente - lifted upon the idea to use asymmetric flow in order to set a microscopic device in motion. The device consists of a very small tube that is closed at one end and contains a bubble of air, immersed in a liquid. By using an acoustic sound field a pressure varying in time can be generated in the liquid. At a high pressure the bubble of air is being pushed and then extra liquid is able to flow into the tube. At a low pressure the bubble of air expands and presses liquid out of the tube. In the latter case (like blowing the candle) the power on the tube is larger than in the first case (like sucking air around the candle). During an acoustic cycle the tube moves two times forward and one time backward, just like the famous St. Vitus’ Procession at Echternach.
Doctoral student Rory Dijkink got to work with this idea. He construed a device consisting of a tube, three millimetre long and 0.25 millimetre inner diameter, and he closed the tube at one end with glue. Technical student Johan van der Dennen went deeply into the mechanism of the movement and, eventually, he was able to have the tube ‘swim’ in a tiny liquid channel (see film in figure 2). The way the tube moves on is very similar to the way a scallop (viz. the shell of the SHELL logo) does. Therefore, the researchers have called it an acoustic scallop. They also combined six devices that they attached to a bearing. In a sound field this operates like an underwater windmill (see film in figure 4).
Possible applications
Although the research started out of curiosity, nevertheless, ideas of application are raised, especially medical ones. The energy of the sound waves is considerably under the level that might damage biomaterial. A device that is attached to one point can be used as a micropump. Such a device does not have mechanical parts and sources of energy other than sound waves, which makes it work on every material in which sound waves are able to propagate. Claus-Dieter Ohl, who led the experiments, anticipates a possible implementation in lab-on-a-chip systems, for example, in processing liquids like protein solutions that are very sensitive to external disruptions. Far, far away, but perhaps not science fiction any more, one application of a similar device could be the transportation of drugs to very specific parts of the human body, say, a blood clot.
For more information, please contact Dr. Claus-Dieter Ohl, University of Twente,
telephone +31 (0)53 489 56 04.
For more data, see: http://stilton.tnw.utwente.nl/people/ohl/roboscallop.html
For more information on the Physics of Fluids Department, see http://pof.tnw.utwente.nl/
The research was financially supported by FOM and NWO.
All pictures by Physics of Fluids Department, University of Twente.