A. Prosperetti - prosperetti@jhu.edu
H. Yuan
Dept. of Mechanical Engineering
The Johns Hopkins University
Baltimore MD 21218
Contact information during ASA meeting:
The lead author (A. Prosperetti) can be reached at his office at The Johns Hopkins University by phone: (410) 516-8534 or e-mail prosperetti@jhu.edu. He will be at the conference only on Monday and Tuesday.
Popular version of paper 2pPA1
Presented Tuesday afternoon, October 13, 1998
136th ASA Meeting, Norfolk, VA
New manufacturing techniques render possible the production of
"intelligent," very low-cost silicon chips in which mechanical and
electronic functions are integrated. Due to their combined ability to
process information and to act on the external environment, this
technology -- known by the acronym MEMS for Micro-Electro-Mechanical
Systems -- promises to foster a technological revolution equal and possibly
even greater than that of the electronic age. For the time being the only
widespread practical application has been as impact sensors for the
deployment of air bags in automobiles, but many companies are busy at work
trying to develop a new generation of micromachines for all sorts of
applications. The dimensions of these devices span a very broad range of
scales, from a few millimeters down to microns (thousandths of millimeters).
In many applications currently under study there is the need to move fluids
around suitable passages in the silicon chips. For example, one may envisage
small disposable devices for the analysis of blood or other biological fluid
samples, tiny surgical implants for the precise and precisely timed delivery
of minute amounts of expensive new medicines, and many other such
applications. The pumping of fluids in such small devices by miniaturized
pumps of more-
or-less conventional type is a highly non-trivial task due to complexity of
fabrication, presence of limiting factors (such as surface effects)
unimportant at larger scales, need to power the device, etc.
It would seem that gas and vapor bubbles can be used to great advantage in such applications due to their intrinsic short time scales and high energy densities. The only widely successful bubble-powered device so far is the ink-jet printer, but many others can be envisaged. The one described in this paper is an example.
In this study we explore the possibility of using vapor bubbles generated thermally at suitably spaced time intervals for the purpose of pumping a liquid through a small channel. In the current version of the model the bubbles are one-dimensional ``slices'' of vapor in which the law of variation of the internal pressure with time is prescribed. This represents a simplified model of the switching on and off of microheaters embedded in the channel walls. When the thickness of the bubble falls below a certain value, the bubble is removed and that portion of the channel available for flow. It is shown that, with a suitable timing of the application of heat, a net pumping flow can be generated. The effects of liquid viscosity and surface tension are also included in the model and shown to be important in determining the pumping action.
In another paper presented at the same session (2pPA3, The Oscillations of a
Gas Bubble in a Tube, by X. Geng, H. Yuan, H. Oguz, and A. Prosperetti), we
carry out a fundamental study of bubbles containing a permanent gas, rather
than a vapor as before. The potential importance of such systems is that
they can be driven remotely by exposure to ultrasound, which forces them to
pulsate. This oscillatory motion can be converted to uni-directional motion
and used for a variety of purposes. For example, one may imagine tiny
micro-machines immersed in the bloodstream and powered by an ultrasonic
field applied through the skin and interposed body tissue. An important
quantity characterizing any oscillator is its natural frequency and damping.
The theoretical predictions of these two quantities are confirmed by
experimental data described in that paper.
Work supported by the Air Force Office of Scientific Research.