direct application to the science of liquid phase sintering. It should be possible to test models
for liquid drop coalescence used to understand liquid phase sintering. Insight into the materials
deposition processes is also important to the industrial process of rapid prototyping.
SPACE BENEFITS
Understanding the viscosity of molten materials and coalescence of liquid drops is important
for everything from glass formation laboratory experiments to industrial materials processes
such as sintering (a method of fusing together particles of material at lower temperatures and
without melting). Viscosity is one of the key parameters that materials scientists must measure
to create accurate models predicting the best methods for materials production. Understanding
and controlling viscosity can even enable researchers to make new materials or improve
existing ones. Microgravity advantageously eliminates the gravitational distortion of liquid
drops, permitting liquids to be suspended in a free-floating condition. Scientists can measure
the viscosity of low-viscosity liquids such as molten metal in low-gravity levitators, by
measuring vibrations of liquid drops. However, the oscillation method cannot be used on more
viscous liquids like molten glass, since they will not oscillate. The FMVM experiment can
ultimately lead to a greater understanding of glass formation from melted lunar soil. It will also
lead to a better understanding of liquid phase sintering processes for in-space fabrication
methods that can be used for constructing surface habitat structures from sintered lunar soil
and vehicle components for longer-term space missions.
RESULTS
FMVM tests a new method for measuring the viscosity of high-viscosity materials by measuring
the time it takes 2 nearly free-floating drops of a liquid to merge. Preliminary results from data
analysis indicate agreement with the predicted coalescence time (Ethridge 2006).
The experiments demonstrate that when the surface tension of a liquid is known, the
coefficient of viscosity for that liquid can be determined by the contact radius speed. This data
can be fit to numerical results to calculate the viscosity, thus validating the model for this new
viscosity measurement method (Antar 2007).
The original data tapes for the FMVM investigation were returned to Earth on space shuttle
flight STS-114/LF-1 in August 2005; later experiments were carried out aboard the International
Space Station (ISS) in July of 2004 and May of 2005. The behavior of two coalescing drops in
microgravity was examined; calculations were run on 7 different conditions to determine the
theoretical coalescence half time (time at which the neck diameter of the converging drops was
half the original drop diameter) for each condition. These results were then compared with the
observed times from experiments conducted aboard ISS and were found to be in close
agreement, with the exception of the high-viscosity silicone oil condition (which may have been
due to invalid initial conditions input to the analytical model). The results of this experiment
validate this numeric model used to calculate the viscosity of a liquid from measured half time
coalescence and known surface tension (Ethridge 2009).