Aggregates were observed to form very suddenly in a lab-contained dust cloud,
transforming (within seconds) an opaque monodispersed cloud into a clear volume
containing rapidly-settling, long hair-like aggregates. The implications of such
a “phase change” led to a series of experiments progressing from the lab, to
KC-135, followed by micro-g flights on USML-1 and USML-2, and now EGM slated for
Space Station. We attribute the sudden “collapse” of a cloud to the effect of
dipoles. This has significant ramifications for all types of cloud systems, and
additionally implicates dipoles in the processes of cohesion and adhesion of
granular matter. Notably, there is the inference that like-charged grains need
not necessarily repel if they are close enough together: attraction or repulsion
depends on intergranular distance (the dipole being more powerful at short
range), and the D/M ratio for each grain, where D is the dipole moment and M is
the net charge. We discovered that these ideas about dipoles, the likely
pervasiveness of them in granular material, the significance of the D/M ratio,
and the idea of mixed charges on individual grains resulting from tribological
processes --are not universally
recognized in electrostatics, granular material studies, and aerosol science,
despite some early seminal work in the literature, and despite commercial
applications of dipoles in such modern uses as “Krazy Glue”, housecleaning dust
cloths, and photocopying.
The overarching goal of EGM is to empirically prove that (triboelectrically)
charged dielectric grains of material have dipole moments that provide an
“always attractive” intergranular force as a result of both positive and
negative charges residing on the surfaces of individual grains. Microgravity is
required for this experiment because sand grains can be suspended as a cloud for
protracted periods, the grains are free to rotate to express their electrostatic
character, and Coulombic forces are unmasked. Suspended grains will be
“interrogated” by applied electrical fields. In one module, grains will be
immersed in an inhomogeneous electric field and allowed to be attracted towards
or repelled from the central electrode of the module: part of the grain’s speed
will be a function of its net charge (monopole), part will be a function of the
dipole. Observed grain position vs. time will provide a curve that can be
deconvolved into the dipole and monopole forces responsible, since both have
distinctive radial dependencies. In a second approach, the inhomogeneous field
will be alternated at low
frequency (e.g., every 5-10 seconds) so that the grains are alternately
attracted and repelled from the center of the field. The resulting “zigzag”
grain motion will gradually drift inwards, then suddenly change to a
unidirectional inward path when a critical radial distance is encountered (a
sort of “Coulombic event horizon”) at which the dipole strength supersedes the
monopole strength --thus proving the presence of a dipole, while also
quantifying the D/M ratio. In a second module, an homogeneous electric field
eliminates dipole effects (both Coulombic and
induced) to provide calibration of the monopole and to more readily evaluate net
charge statistical variance. In both modules, the e-fields will be exponentially
step-ramped in voltage during the experiment, so that the field “nominalizes”
grain speed while spreading the response time --effectively forcing each grain
to “wait its turn” to be measured.
In addition to rigorously quantifying M, D, and the D/M ratio for many hundreds
of grains, the experiment will also observe gross electrometric and RF discharge
phenomena associated with grain activity. The parameter space will encompass
grain charging levels (via intentional triboelectrification), grain size, cloud
density, and material type.
Results will prove or disprove the dipole hypothesis. In either case, light will
be shed on the role of electrostatic forces in governing granular systems.
Knowledge so gained can be applied to natural clouds such as protostellar and
protoplanetary dust and debris systems, planetary rings, planetary dust palls
and aerosols created by volcanic, impact, aeolian, firestorm, or nuclear winter
processes. The data are also directly applicable to adhesion, cohesion,
transport, dispersion, and collection of granular materials in industrial,
agricultural,
pharmaceutical applications, and in fields as diverse as dust contamination of
space suits on Mars and crop spraying on Earth.
Marshall, J., Sauke, T., Farrell, W., Electrostatics of Granular Material (EGM): Space Station Experiment, Fifth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 670-688, August 9, 2000.