Fluid physics is the study of how fluids move and transport mass and heat energy. A fluid is simply any material that flows in response to an external force, including liquids and gases. 

On Earth, fluid motion is strongly influenced by gravity-driven phenomena like convection, buoyancy, sedimentation, and hydrostatic pressure. But the weightless environment onboard the space station results in a loss of gravity-driven fluid motion. This loss can reveal previously obscured fluid behaviors, creating unique opportunities for researchers and entrepreneurs. 

Some of the most useful fluid motions revealed in microgravity include capillarity, diffusion, and surface tension. 

Capillary action is the movement of fluids through a thin tube or porous surface due to cohesion, adhesion, and surface tension. Cohesion is the intermolecular attraction that binds like molecules together in a fluid. This phenomenon is also responsible for surface tension. 

Molecules within a fluid attract and bind to the other molecules around them. But molecules on the surface of a fluid are only bound to the molecules below. As a result, the top layer of molecules experiences a net downward force. This compresses the surface molecules, causing them to form a kind of ‘skin’ on the top of the fluid. This surface tension ‘skin’ allows fluid surfaces to resist some external forces, like a paperclip floating on water. In general, the cooler a fluid, the stronger the surface tension. 

Adhesion is the opposite of cohesion—it is the molecular attraction between dissimilar molecules. Strong adhesive forces can cause a fluid to stick to other surfaces, like water drops on metal or glass. This is referred to as ‘wetting.’ 

Capillary action occurs when adhesion forces are stronger than cohesive ones. This action is responsible for phenomena like fluid rising in a pipette or water traveling from the tree roots up to the top branches. On Earth, stronger, gravity-driven flows can mask capillary motion. But in microgravity, capillary action can be fully revealed.

Another key driver of fluid motion on orbit is diffusion. Diffusion is the random motion of molecules from a place of high pressure to a place of low pressure within a confined space. One of the primary drivers of diffusion in microgravity is Marangoni convection, also known as the Marangoni effect. This phenomenon describes how differences in surface tension (surface tension gradients) influence molecular motion at a liquid or gas interface. 

Marangoni convection can be seen in everyday life on Earth, like when oil heats and spreads in a pan. The oil closest to the heat source has the highest temperature, meaning it also has the lowest surface tension. Cooler oil further from the source has a higher surface tension, so it pulls the hotter oil towards it. This is how Marangoni-driven flows spread oil around the pan. 

In microgravity, the Marangoni effect is also at least partly responsible for altering evaporation. On Earth, gravity-driven buoyancy and convection cause hot, evaporating gases to rise off a liquid surface and float away, like steam off boiling water. But in microgravity, hot gases remain close to the liquid interface. As a result, the cooler fluid’s higher surface tension pulls on the lower surface tension of the gas. This can cause condensation, the process of a gas returning to liquid form. Studies onboard the space station have found that fluids do not evaporate, even at 160 K above their boiling point. More research is needed to understand the effects of Marangoni convection in space, but the result of that research could enable the development of safer, more efficient cooling systems for future spacecraft. 

Understanding fluid physics in microgravity can offer significant insights for research and entrepreneurial fluid applications on Earth. Capillary motion, for example, does not require an external energy source to drive fluid motion. Capillarity research on orbit could lead to the development of passive micro- or even nano-scale capillary systems for electronics cooling, fuel cells, and medical fluid transport. Meanwhile, investigations into surface tension and Marangoni convection could help advance medical diagnostic tools for biofluid droplets like blood or even DNA. Fluid physics in microgravity could also lead to advances in complex fluids including gels, colloids, magnetic fluids, and foams.