Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

(EDB) to measure the thermophoretic force on single microsphere particles. In an EDB a vertical dc field cancels the gravitational
force on a particle and the thermophoretic force is measured in terms of this dc voltage. Another superimposed ac field keeps the
levitated particle stable. However, there are two challenges in using an EDB to measure thermophoretic forces. One is to keep
trapped particles stable at the very low ac voltage that is necessary to avoid gas glow discharge at reduced pressure. The other is
to determine the size and mass of the single particles, especially non-spherical ones, accurately in order to interpret the data. Particle
stability was improved by adding two independent dc fields in the radial direction with a new octuple double-ring design for
the EDB electrodes, as shown. A conventional double-ring EDB was modified by splitting each ring into four equal electrically
independent sections. Three dc sources were combined such that eight potentials were applied to the eight sections of the electrodes.
An additional ac voltage was superimposed on each ring section as in the conventional double-ring EDB. The resulting
electric field has dc components in the x, y and z-directions, which can be controlled independently by the three dc supplies. The
z component is equivalent to the dc field in the conventional EDB and can be used to balance and measure any vertical force such
as gravity. The x and y fields can be used to suppress radial oscillations of the trapped particles that arise due to gas convection
or distortion of the electric field by the view ports in the chamber walls. The aerodynamic size of the trapped particles can be determined
by a stable oscillation technique that we have developed previously. This technique involves partially balancing the particle
against gravity and allowing the particle to oscillate in the ac field. The oscillation trajectories are recorded with a fast linescan
camera and are fit to the solutions of the particle equation of motion. With the aid of a video microscopy system the shape and
orientation of the particle with respect to the electric field is determined. The geometric dimensions and mass of the particles can
be calculated from the aerodynamic size, the shape and the orientation with the knowledge of particle density. Using this method
we were able to determine the size and mass of PSL particles of two- and three-sphere aggregates. We have measured the thermophoretic
force on these PSL aggregates in nitrogen gas in the Knudsen regime. The thermophoretic force data, as well as particle
shapes and equivalent volume diameters, are shown. At a Knudsen number of one, the normalized thermophoretic force on the
two- and three-sphere aggregate is, respectively, 23% and 10% greater than that on a single sphere. This implies that two- and
three-sphere aggregates drift faster due to thermophoresis than single spheres. Once aggregate particles form there may be a positive
feedback mechanism for more aggregation due to the higher mobility of the aggregate spheres.
Author (revised)
Thermophoresis; Particle Motion; Spheres; Transport Properties; Trapped Particles; Convection; Direct Current; Electric
Fields; Microparticles
20010024966 Michigan Univ., Biomedical Engineering Dept., Ann Arbor, MI USA
Capillary Instabilities in the Microgravity Environment
Grotberg, James B., Michigan Univ., USA; Benintendi, Steven W., Michigan Univ., USA; Halpern, David, Alabama Univ., USA;
Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference; December 2000, pp. 1286-1288; In
English; See also 20010024890; No Copyright; Abstract Only; Available from CASI only as part of the entire parent document
The lung is comprised of a network of bifurcating airway tubes which are coated with a thin viscous film. Often times, especially
in the case of disease, the liquid film can form a meniscus which fills the whole tube, thus obstructing airflow. The formation
of the liquid meniscus is due to capillary driven instabilities which can arise in the lining, causing the lining to close up. In addition,
airflow can also be obstructed if the airway tube collapses in on itself. This occurs when the elastic forces of the tube are not large
enough to sustain the negative fluid pressures inside the tube caused by surface-tension, and the tube collapses. In both cases, the
instability is dependent on the surface tension of the liquid lining. In normal gravity, airway closure occurs at the end of a forced
expiration as a normal event even in health lungs. The amount of air remaining in the lungs when this happens is called the closing
volume. Fortunately, because of gravity the closure process tends to occur in the lower regions of the lung, due to the weight of
the tissue above. In microgravity, it is likely that closure occurs more homogeneously and thus exposes the lung to potential risks.
Once the airway is closed, it then must be reopened, usually this is done by taking a deep breath which forces the liquid plug to
move and deposit its volume onto a trailing film. There are other important issues regarding airway closure which relate to pulmonary
disease. For example, premature babies, whose lungs have not developed sufficient surfactant to maintain the surface tension
of the lung at a sufficiently low level for healthy functioning, are especially predisposed to problems caused by airway closure.
In such cases, the patients are sometimes put on high frequency ventilation machines to improve oxygenation. The frequency of
the breathing cycle, as well as the tidal volume, are two control parameters at the disposal of the clinician. Little is known, however,
of the effect that these two parameters can have on the occurrence of airway closure. Indeed, if airways can be kept open by using
some given imposed frequency, then this would important in the treatment of such patients. In addition to medical applications,
the general topic of multiphase flows over flexible boundaries is important for a number of technical fields including flows in
poroelastic media. We have investigated the closure phenomenon in flexible tubes in the past. Recently, we developed a theoretical
model of perturbed airway closure to study the effects of forced oscillatory air flow on the closure phenomenon. It consists of a
single rigid tube coated with a thin viscous layer. to examine the stability of the layer, we derived an evolution equation for the
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