On Earth, gravity usually overpowers surface tension
On Earth, a puddle of water spreads out because gravity pulls the liquid down and the floor pushes back, flattening it against a surface. Surface tension still tries to pull the water into a compact shape, but for anything larger than a small raindrop, gravity wins. In space, the situation reverses: the spacecraft and everything inside it are in free fall together, so there is no effective downward pull to flatten a floating drop. The paper on spherical capillary water waves describes the motion of an almost spherical water droplet under zero gravity governed by water-air interface tension [1], which is exactly this gravity-free regime.
The space station experiments give a concrete sense of how clean that regime is. The DropCoal device operated aboard the ISS under residual gravity of about 10⁻⁵ to 10⁻⁶ g, which is roughly a millionth of Earth's gravity, providing a platform free from buoyancy and convection [2]. That matters because on Earth, buoyancy deforms droplets and drives convective flows that interfere with measurements [2]. In orbit, those effects are essentially switched off, leaving surface tension as the dominant force shaping the liquid.
Surface tension pulls water into the smallest possible skin
Surface tension is best pictured as a thin elastic skin that wants to shrink. Water molecules at the surface are pulled inward by their neighbors more than outward, so the surface behaves as if it is under tension. A free-floating blob of water will therefore contract until it cannot shrink any further without changing its volume. The shape that encloses a given volume with the least surface area is a sphere—the same reason soap bubbles are round.
The mathematical paper treats this directly: it models an almost spherical water droplet under zero gravity, with the motion governed by water-air interface tension [1]. The word 'almost' is important. A real droplet can wobble and oscillate around the spherical shape, and the paper studies those small departures mathematically. The sphere is the resting shape, not a rigid cage.
In orbit, droplets behave as surface-tension objects
The ISS experiments make the surface-tension picture visible. Water droplets 2 to 5 millimeters across were generated at coaxial needles and brought into contact, and high-speed video captured a liquid bridge forming and growing rapidly under surface tension [2]. Capillary wave packets launched at the moment of contact were clearly resolved propagating along the droplet interface [2]. These are the kinds of motions you would expect when surface tension, not gravity, is calling the shots.
The measurements also show how predictable this regime is. The bridge neck radius followed a power law with an exponent of 0.49 ± 0.03 across eight configurations, in excellent agreement with the theoretical value of 0.5 [2]. That level of agreement is evidence that the gravity-free, surface-tension-dominated description is capturing the real physics, not just approximating it. The experiments were done with droplets a few millimeters wide, so the results describe that size range under microgravity rather than every possible droplet.
Why a space droplet is not always a perfect sphere
A sphere is the preferred shape, but a floating water drop can be pushed away from it. If the drop is set oscillating—by a touch, a vibration, or a coalescence event—it will wobble around the spherical shape, and the mathematical paper on spherical capillary water waves is precisely about those small departures from a sphere [1]. The paper's focus on 'almost spherical' droplets reflects that the sphere is a resting state, not a guarantee.
Size and the surrounding environment also matter. The ISS experiments used droplets of 2 to 5 millimeters and found that even asymmetric pairs collapsed onto the same universal behavior when normalized by an effective radius [2]. That suggests the surface-tension description is robust across that range, but it does not by itself tell us what happens to much larger water masses in space, where other effects could come into play. The evidence supports the sphere-forming role of surface tension in the absence of gravity; it does not claim that every water blob in every space setting is a perfect ball.
Sources used in this answer
On the Cauchy Problem of Spherical Capillary Water Waves
The paper models an almost spherical water droplet under zero gravity governed by water-air interface tension, explaining the role of surface tension and the absence of gravity in shaping droplets.
BINARY MILLIMETRIC DROPLET COALESCENCE UNDER MICROGRAVITY CONDITIONS ON ISS
Under microgravity on the ISS, water droplets coalesce with surface-tension-driven dynamics that match theoretical scaling, free from buoyancy and convection effects that interfere on Earth.