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ISS Microgravity Marangoni Convection Simulator

In the microgravity environment of the International Space Station (ISS), buoyancy convection disappears and Marangoni convection — driven by surface-tension gradients — becomes the dominant transport mechanism. This free tool lets you adjust temperature difference, fluid properties, characteristic length and gravity level to see Ma and the dynamic Bond number update in real time.

Parameters
Fluid
Sets density, thermal expansion and dσ/dT automatically
Temperature difference ΔT
K
Surface tension σ
mN/m
For reference (Ma is driven by dσ/dT, not σ itself)
Dynamic viscosity μ
mPa·s
Characteristic length L
mm
Liquid bridge length or droplet diameter
Gravity level
Compare microgravity, Moon, Mars and Earth
Results
—
Marangoni number Ma
—
Rayleigh number Ra
—
Ratio Ma/Ra
—
Dominant convection
—
Surface velocity (mm/s)
—
Dynamic Bond number
—
Microgravity droplet — surface-tension flow visualisation

The temperature gradient (blue → red) creates a surface-tension imbalance that drives flow along the droplet surface and roll-cell circulation inside. In microgravity the droplet remains spherical and freely floating.

Marangoni number Ma vs temperature difference ΔT
Ma per fluid (at the current ΔT, L and μ)
Theory & Key Formulas

$$\mathrm{Ma} = \frac{|\partial\sigma/\partial T|\,\Delta T\,L}{\mu\,\alpha}, \qquad \mathrm{Ra} = \frac{g\,\beta\,\Delta T\,L^{3}}{\alpha\,\nu}$$

Marangoni number Ma and Rayleigh number Ra. ∂σ/∂T: temperature gradient of surface tension [N/m/K], α: thermal diffusivity [m²/s], β: volumetric expansion coefficient [1/K], ν = μ/ρ: kinematic viscosity [m²/s].

$$\mathrm{Bo}_d = \frac{\mathrm{Ra}}{\mathrm{Ma}} = \frac{\rho\,g\,\beta\,L^{2}}{|\partial\sigma/\partial T|}$$

Dynamic Bond number. Bo_d ≪ 1 → Marangoni-dominated; Bo_d ≫ 1 → buoyancy-dominated. On the ISS g drops by 10⁻⁶, so Bo_d collapses by six orders of magnitude and Marangoni flow takes over.

$$u_{\max} \sim \frac{|\partial\sigma/\partial T|\,\Delta T\,L}{\mu}$$

Scaling of the maximum surface velocity: proportional to ΔT and L, inversely proportional to viscosity μ.

ISS Microgravity Marangoni Convection — Surface-Tension-Driven Flow

🙋
Professor, I've seen ISS videos where astronauts float a perfectly spherical drop of water in mid-air. Is anything actually moving inside that drop, or is it completely still?
🎓
It looks calm at first glance, but in most cases there is surprisingly vigorous flow inside. In microgravity the everyday "hot fluid rises, cold fluid sinks" buoyancy convection essentially disappears. But the moment there's even a tiny temperature gradient on the droplet's surface, the surface tension difference starts pushing the interface around. That's Marangoni convection. On Earth, buoyancy crushes this signal — but in space, it takes over.
🙋
Why does a temperature change in surface tension cause flow? I always thought of surface tension as the thing that makes water bead up.
🎓
Right, surface tension is "the membrane that tries to shrink the surface". For water it gets weaker as temperature rises (dσ/dT is negative). So if one side of the drop is hot and the other is cold, the cold side pulls harder. Surface molecules get dragged from the hot side to the cold side, and viscosity drags the interior along — producing the roll cells you see in the animation. That's Marangoni flow in a nutshell.
🙋
When I flip the gravity selector from microgravity to Earth, the Ma/Ra ratio drops massively and the verdict switches to "buoyancy-dominated". Does that really mean we can ignore Marangoni flow on the ground?
🎓
Great catch. The dynamic Bond number Bo_d = Ra/Ma is what decides it, and it's proportional to L²·g. So either small g (space) or small L (thin films, tiny droplets) and Marangoni wins. Even on Earth, solder balls, evaporating thin films, weld pools and crystal growth melt zones all have small L and Marangoni matters. So "ignore on Earth" is wrong — it's "ignore only at large scales".
🙋
When I pick "molten metal" the Ma shoots up and the surface velocity gets very fast. What kind of research does that connect to?
🎓
Molten metals have a dσ/dT more than twice that of water. In the "floating zone" technique for growing semiconductor crystals (Si, GaAs, Ge) in space, you melt a column of material with a ring heater. You can't even keep a long liquid column on the ground — it sags under its own weight — but in microgravity you can. The catch is that if Marangoni convection turns oscillatory, the crystal ends up with periodic stripes (striations) that count as defects. Designing the thermal profile to keep Ma below the oscillation threshold is one of the main goals of ISS materials experiments.
🙋
So strong Marangoni flow basically means a noisy crystal. Can the lessons learned in space actually feed back into Earth-side processes?
🎓
Yes — the critical transition data from ISS Marangoni experiments (JAXA's MEIS / FPEF in the Kibo module, ESA's Geoflow, and many others) feed directly into Earth-side miniature systems: microfluidics, inkjet, laser welding pools, brazing. The "Ma threshold for steady → oscillatory → turbulent" measured cleanly in space gives CFD codes the validation data they need for predicting and controlling those terrestrial processes.

Frequently Asked Questions

Marangoni convection is the flow that arises when surface tension varies along a liquid surface due to temperature or composition gradients. For most liquids, surface tension drops as temperature rises (∂σ/∂T is negative), so the surface is pulled from hot regions toward cold ones, dragging the interior into a roll-like circulation. On Earth, buoyancy-driven convection is far stronger and masks Marangoni flow, but on the ISS — where gravity is effectively absent — Marangoni convection becomes dominant. This tool computes Ma from the temperature difference and fluid properties, and uses the Ma/Ra ratio to identify the controlling mechanism.
Ma = (∂σ/∂T)·ΔT·L / (μ·α), where ∂σ/∂T is the temperature gradient of surface tension (about 0.15 mN/m/K for water; the absolute value is used here), ΔT is the temperature difference, L is the characteristic length (liquid bridge length or droplet diameter), μ is dynamic viscosity, and α is thermal diffusivity. A larger Ma means surface-tension flow is faster than thermal diffusion; convection typically sets in above Ma ≈ 80, and turns oscillatory or turbulent above Ma ≈ 500-1000.
With the tool's default values (water, ΔT = 20 K, L = 20 mm, microgravity), Ma ≈ 400,000. That is four orders of magnitude larger than the conditions in which Earth-bound buoyancy convection completely dominates (Ra ≈ 10-100), so Marangoni convection fully takes over. Even on Earth, Ma can exceed Ra in thin films and small droplets (small L), which is why surface-tension flow is critical in soldering and crystal growth.
The dynamic Bond number Bo_d = Ra/Ma is a dimensionless ratio comparing buoyancy-driven flow to surface-tension-driven flow. Bo_d ≫ 1 means buoyancy convection dominates (normal Earth-bound flow); Bo_d ≪ 1 means Marangoni convection dominates (microgravity or very small scale). On the ISS, gravity is about 10⁻⁶ Earth gravity, so Bo_d is six orders of magnitude smaller than on the ground — making Marangoni flow take centre stage even in systems where it would be negligible on Earth.

Real-World Applications

Semiconductor crystal growth in space: When growing single crystals of Si, GaAs or Ge by the floating-zone method in microgravity, the Marangoni convection inside the liquid bridge directly controls crystal quality. A high Ma drives the flow oscillatory, leaving periodic striations as defects in the final crystal. This tool's Ma/Ra ratio lets you cleanly separate the surface-tension contribution from buoyancy — useful when planning the thermal profile of an actual ISS experiment.

JAXA Kibo fluid experiments (MEIS / FPEF): The Fluid Physics Experiment Facility on Japan's Kibo module heats silicone-oil liquid bridges to measure the critical Ma at which steady Marangoni flow transitions to oscillations. Select "silicone oil" in this tool and sweep ΔT from 10 to 50 K to see Ma reach the 10⁴ range — a regime impossible to reach on Earth without surface tension being swamped by buoyancy.

Earth-side soldering and laser welding: In molten solder balls and weld pools the characteristic length L is only 1-3 mm, and Ma can exceed Ra even on Earth. Choose "molten metal" with L = 2 mm and you'll see Marangoni convection dominate even at full Earth gravity. This affects bead shape and keyhole depth, so it feeds directly into real process design.

Microfluidics and inkjet: Microlitre droplets have L = 0.1-1 mm, so surface tension dominates from the start. Inkjet ejection, oil-isolated PCR droplets and lab-on-chip mixing enhancement all use deliberate temperature gradients to drive Marangoni flow for transport and mixing.

Common Misconceptions and Pitfalls

The first big trap is the idea that microgravity means perfectly still fluid. Even on the ISS, residual accelerations (g-jitter from crew motion, thruster firings) run 10⁻⁶ to 10⁻³ g and inject low-frequency disturbances. On top of that, as this simulator shows, just 1 K of temperature difference is enough to push Ma into the thousands and create flow that is as fast as ordinary natural convection on Earth. "No gravity" does not mean "nothing moves" — it means "surface tension and concentration gradients are now the protagonists". Experiments routinely require temperature uniformity to the 0.1 K level to suppress unwanted Marangoni currents.

The second trap is to assume that surface tension σ itself drives Ma. This tool exposes σ (in mN/m) as a reference input, but the driving force in the Ma formula is the temperature gradient ∂σ/∂T, not σ. Water has σ ≈ 72 mN/m yet ∂σ/∂T is only 0.15 mN/m/K. Molten metals have σ ≈ 500 mN/m but ∂σ/∂T ≈ 0.36 mN/m/K — large enough to produce vigorous flow once a temperature gradient is present. When you read a textbook expression for Ma, always check that it uses dσ/dT, not σ. Internally this tool uses fluidProps.sigma_T (= |dσ/dT|).

The third is the belief that "high Ma is good". In crystal growth and coating, once Ma exceeds a critical value (roughly 500-2000 depending on geometry and Prandtl number), the flow transitions from steady convection to oscillatory Marangoni convection. Surface waves form and imprint periodic defects on the crystal or thin film. Measuring this transition cleanly — without buoyancy interference — is one of the main reasons for running these experiments on the ISS. In this tool the verdict switches between warn and ok around Ma ≈ 500 to flag that boundary.

How to Use

  1. Enter the temperature difference across the liquid interface in Kelvin (typical range 1–50 K for ISS experiments)
  2. Input surface tension gradient in mN/m per Kelvin; silicone oil shows −0.08 mN/m·K, water −0.15 mN/m·K
  3. Specify kinematic viscosity in mm²/s (water 0.89 mm²/s at 25°C, silicone oils 10–100 mm²/s depending on grade)
  4. Set characteristic length scale in mm (pool width or droplet diameter: 5–50 mm for typical ISS fluid physics experiments)
  5. Click Calculate to obtain Marangoni number Ma, Rayleigh number Ra, surface velocity, and dominant convection regime

Worked Example

A 20 mm silicone oil layer aboard ISS with ΔT = 8 K, surface tension gradient −0.06 mN/m·K, viscosity 50 mm²/s: Ma = (0.06 × 8 × 400)/(50 × 0.00089) ≈ 2,150. With Ra ≈ 0.001 (no gravity term), Ma/Ra ≈ 2.15 million, indicating pure Marangoni-driven flow at surface velocity ~1.2 mm/s. Dynamic Bond number Bo ≈ 0.002, confirming surface tension dominates over residual acceleration effects.

Practical Notes

  1. In true microgravity (g < 10⁻⁶g₀), buoyancy Ra becomes negligible; Marangoni convection alone drives mixing—critical for crystal growth and protein crystallization experiments
  2. Surface contamination (surfactants, dust) alters surface tension gradient by 20–50%; contamination suppresses Marangoni flow, mimicking low-Ma regimes
  3. Oscillatory thermocapillary instability emerges when Ma exceeds 800–1200 depending on boundary conditions; ISS experiments use stable Ma < 500 to isolate laminar flow
  4. Residual g-jitter (0.01–1 g micro-accelerations) introduces secondary Rayleigh convection; Bond number < 0.1 ensures Marangoni dominates