 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 vacuums, 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 station's 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 a 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 describe 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-24. In buoyancy-induced convection, otherwise known as Rayleigh convection, gravity is the dominant force. In stratified fluids, stability exists when heavier fluids lie 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 an 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 two 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 droplet's free surface, the temperature there decreases, known here by the broadening yellow rim and its 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, which 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 droplet's free surface. Let's compare our computational results with what we observe experimentally, using laser shadow graphing. 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. Four 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-scale 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 the counterclockwise surface tension flow in Region 4 displaces Region 3 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 1 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 thermocapillary-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 counterclockwise 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 temperature 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 thermocapillary convection. The fundamental differences 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 homogeneously mixed. Thermocapillary 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. Thermocapillary 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 thermocapillary 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 will 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.