with the advent of space technology new
opportunities are emerging for
scientific research with exciting
applications in many fields to take
advantage of this new era in space
exploration basic research into the
fundamental characteristics of the
environment of space such as long-term
microgravity and ultra-high vacuum
should first be undertaken the purpose
of the research presented in this video
is to understand more clearly the
effects of surface tension on fluid
convection the predominance of surface
tension effects over buoyancy effects is
a unique characteristic of microgravity
environments and is critically important
to many microgravity applications such
as material processing and fluid
management the fluid system chosen for
this study the liquid sessile droplet
has been selected because among many
fluid systems most affected by surface
tension liquid droplets are particularly
important to many industrial and
biological processes evaporation induced
surface tension effects on liquid
droplets and their associated transport
process can significantly enhance heat
and mass transfer
they are also critical to a wide range
of technologies such as single crystal
growth the spray drying and cooling of
metal and the advanced droplet radiators
of the space stations power systems
while quantitative research on surface
tension induced flow is pivotal to our
understanding of the microgravity
environment it is difficult to achieve
on earth not only do surface tension
driven flow phenomena take place solely
in small-scale systems under normal
gravity surface tension properties
themselves are very easily disturbed by
experimental observation
thus computational simulations
offer an attractive way to investigate
internal convection within liquid
droplets during this video a
cross-sectional representation of the
hemispherical liquid droplet under ideal
conditions is used to show internal
fluid motion the droplet is placed on an
unheated plate with evaporation
occurring along the free surface the
black dots within the liquid droplet are
fluid tracers used to visualize fluid
flow because the processes that occur
are assumed symmetric with respect to
the centerline we will be describing
only the flow field in the right half of
the droplet two types of evaporation
induced convection are presented one
under normal gravity where buoyancy is
the most significant force the other
under microgravity where buoyancy force
subsides and surface tension effects
predominate the patterns of flow that
described them were obtained
computationally using an Adi scheme
while a runga cotta scheme tracks the
motion of the fluid tracers the
calculations required about one hour of
CPU time on the Cray XMP 24in buoyancy
induced convection otherwise known as
Rayleigh convection gravity is the
dominant force in stratified fluids
stability exists when heavier fluids
live beneath lighter fluids that is why
when a heavier fluid lies above a
lighter one the heavier fluid will tend
to descend when the criteria for
stability are exceeded the result will
be fluid motion induced by gravity we
begin with a liquid droplet having a
uniform temperature shown in orange
which is the same as the plate upon
which it rests as evaporative cooling
occurs along the free surface of the
drop the temperature of that surface
decreases which makes the fluid there
heavier as evaporation proceeds the
temperature continues to decrease and a
yellow rim representing the colder fluid
at the surface broadens eventually as
the fluid turns even colder the rim
becomes green and then blue as this
increasingly heavier fluid descends the
relatively warmer fluid of the interior
ascends setting up a clockwise fluid
convection because buoyancy force is a
body force it induces a global motion
and convection occurs almost everywhere
within the liquid drop after an
initially rapid flow the rate of
convection gradually slows down as the
temperature field becomes vertically
more stable the clockwise fluid motion
in the upper layer is reduced and it
almost still predominantly lateral
motion results eventually however as
evaporative cooling continues to reduce
the temperature at the surface a
coupling between internal convection and
that cooling induces to thermal
stratifications
the first stratification moves clockwise
at the lower edge of the droplet surface
the second moves counterclockwise and
begins at the top of the surface as
another vortex of cooler fluid moves
downward into the interior demonstrating
that evaporation induced buoyancy
convection is a dynamic unsteady process
unlike buoyancy dominant flow surface
tension dominant flow has not been well
studied however we do know that it is
partly a function of temperature and
concentration the present study focuses
on the effect of temperature on surface
tension the warmer the fluid the weaker
the surface tension and conversely the
colder the fluid the stronger the
surface tension again we begin with a
liquid droplet having a uniform
temperature which is the same as the
plate upon which it rests
as evaporative cooling occurs along the
droplets free surface the temperature
there decreases shown here by the
broadening yellow rim and it's gradual
transformation from yellow to green
because the temperature at the bottom of
the plate is still warm a temperature
gradient begins to form where the plate
meets the lower edge of the relatively
colder droplet the surface tension is
weakest near the warm plate but stronger
at the colder surface as a result
surface tension is able to pull the
fluid upwards that is away from the
plate and fluid viscosity transfers
momentum into the interior conservation
of mass dictates that a circular
counterclockwise motion or vortex wraps
cold fluid into the interior of the drop
to form a cold pocket the end result is
a self propelling process with a maximum
velocity and the largest temperature
gradient coupled in the cold pocket to
maintain the surface tension induced
fluid flow which pushes the vortex all
the way up the droplets free surface
let's compare our computational results
with what we observe experimentally
using laser shadowgraph
traces of aluminum particles allow us to
visualize evaporation induced convective
patterns inside a chloroform drop a few
seconds after it was placed on an
isothermal plate for regions of flow can
be identified region 1 located at the
top of the drop is stagnant region 2
consists of weak clockwise convection
currents driven by buoyancy force region
3 is composed of a few layers of
hexagonal flow cells finally region 4
has relatively stronger counterclockwise
convection currents which according to
our computational results are driven by
surface tension since the liquid drop is
a small
system surface tension is expected to
play a dominant role in the evaporation
process and this is also what we observe
experimentally as evaporation proceeds
the weak buoyancy induced flow region
shrinks while a counter clockwise
surface tension flow in Region four
displaces Region three upwards this is
exactly the result we saw earlier in the
graphical presentation of surface
tension dominant convection thus the
computational results qualitatively
match the experimental results well
however the observations also reveal a
drawback of the experimental method not
only is the stagnant nature of region
one exaggerated by the particles of
aluminum that accumulate there but the
accumulation eventually destroys the
surface tension properties altogether
let's review the graphical presentation
of buoyancy dominant and thermo
capillary dominant convection using
numerical methods the elapsed time
during this segment of the video will
more accurately reflect real time
processes in buoyancy dominant
convection gravity is the dominant force
as evaporation occurs the temperature at
the droplet surface decreases a
clockwise convection results as the
increasingly colder and heavier fluid
descends and the relatively warmer fluid
ascends eventually as the rate of
convection slows down the clockwise
fluid motion is reduced and lateral
motion predominates until a second
counter clockwise motion begins when we
observe the surface tension dominant
flow characteristics of microgravity
environments a different pattern of
convection occurs the resulting fluid
motion is generated by the surface
tension gradient along the droplet
surface which pulls the fluid upward in
a circular counterclockwise motion the
end result is a self propelling process
with velocity and ten
coupled to maintain the surface tension
induced flow in this video we have seen
a direct simulation of natural
convection inside liquid droplets caused
by evaporative cooling of the two
mechanisms that induce convection
the first was gravitational buoyancy
force and the resulting fluid transport
the well studied rayleigh natural
convection the other mechanism which is
less familiar to the thermal fluid
society but more relevant to future
space venture is surface tension induced
by a thermal gradient and the resulting
fluid motion the so called Marangoni or
thermo capillary convection the
fundamental difference is between these
two convection patterns can be briefly
summarized as follows gravitational
buoyancy force is a body force thus the
induced fluid dynamics is global and the
resulting temperature field in the fluid
is more homogeneous li mixed thermo
capillary force is a surface force and
here both the induced fluid motion and
the thermal stratification is more
intensified along the free surface under
conditions of decreasing gravity the
pattern of fluid flow will gradually
become less dominated by buoyancy
convection eventually when the force of
gravity has been reduced significantly
the flow pattern will be dominated by
surface tension this research video has
illustrated the clear differences
between two mechanisms of fluid
transport thermo capillary convection
and buoyancy dominant convection in the
future the investigator will expand his
studies of fluid flow to explore the
effects of surface tension on double
diffusion and two phase related problems
such as crystal growth he will also
explore the potential of thermo
capillary convection for the droplet
radiators involved in the Space
Station's power system such
computational studies are laying the
groundwork that will enable NASA
and its partners in academia and
industry to design the expensive
experimental ventures in outer space of
the coming decade together numerical and
experimental studies we'll be able to
meet the challenge of exploiting the
revolutionary capabilities the space
station offers thus helping to fulfill
the Space Station's promise as the
laboratory of the future