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Static surface tension

Pronunciation: /ˈstætɪk ˈsɜːfɪs ˈtɛnʃən/

Static surface tension is the surface tension of a liquid once its surface has reached equilibrium and no longer changes with time. It is the value quoted in data tables, and for solutions it is reached only after dissolved surfactant has finished adsorbing at the surface.

Also called

Equilibrium surface tension Steady state surface tension

Related terms

Surface tension Dynamic surface tension Surfactant

Topics

Surface tension Equilibrium Surfactants
Book a 15 minute demo Read the surface tension measurement guide

At a glance

What static surface tension means, how it differs from the dynamic value, and what numbers to expect.

What it is

A fresh liquid surface does not start at equilibrium. Molecules rearrange and, in solutions, surfactant diffuses up and adsorbs. Once that process has finished, the surface tension stops changing. That final, time independent value is the static surface tension.

  • Units: mN/m, equal to mJ/m² and dyn/cm
  • Pure liquids reach it within a fraction of a second
  • Dilute surfactant solutions can take seconds to hours

Why it matters

Static surface tension is the right number for slow processes: a drop resting on a surface, a liquid filling a pore, or a formulation sitting in storage. It is also the value that surfactant curves and CMC are read from. Our article on the science behind water's high surface tension shows why water sits so high.

Typical values

  • Water: 72.8 mN/m at 20 °C, about 72 mN/m at 25 °C
  • Formamide: about 58 mN/m at 20 °C
  • Diiodomethane: 50.8 mN/m at 20 °C
  • Ethylene glycol: about 48 mN/m at 20 °C
  • Hexadecane: about 27.5 mN/m at 20 °C

More reference liquids are in our surface tension of common liquids dataset.

Expert view

“Surface tension versus concentration is one of the few measurements that lets you watch a surfactant reorganise an interface in real time: read the curve, not just the endpoint.”

Dr. Alidad Amirfazli, Co-founder and Scientific Advisor, Droplet Lab

What the terms mean

How surface tension changes with surface age until it becomes static.

Fresh surface (dynamic)

Just after the surface forms, little surfactant has reached it. The tension is close to that of the pure solvent.

Ageing surface (dynamic, falling)

Surfactant diffuses up and adsorbs. The tension falls over time, quickly at first and then more slowly.

Equilibrium surface (static)

Adsorption and desorption balance. The tension stops changing, and this plateau is the static surface tension.

For a pure liquid the three stages collapse into one, because the surface reaches equilibrium almost at once.

γ Surface tension, mN/m
γeq Static (equilibrium) surface tension, mN/m
γ(t) Dynamic surface tension at surface age t, mN/m
t Surface age: time since the surface was created, s
T Temperature, °C

Key equations

How static surface tension is defined and how it shifts with temperature.

Equation: γeq = lim (t → ∞) γ(t)

Variable legend:

  • γeq: static surface tension (mN/m)
  • γ(t): surface tension at surface age t (mN/m)
  • t: surface age (s)

Interpretation: Static surface tension is where the dynamic curve ends. In practice you report it once the change over a set time is smaller than your measurement scatter.

When to use: Use it to decide whether a reading is static or still part of the dynamic curve.

Assumptions:

  • Constant temperature and composition
  • No evaporation or contamination while the surface ages

Common mistakes:

  • Reading too early on a surfactant solution and calling it static
  • Waiting so long that evaporation changes the concentration

Equation: γ(T) ≈ γ(T0) − k (T − T0)

Variable legend:

  • γ(T0): surface tension at a reference temperature (mN/m)
  • k: temperature coefficient (mN/m per °C); about 0.15 to 0.17 for water near room temperature

Worked example, water: 72.8 − 0.16 × (25 − 20) = 72.8 − 0.8 = 72.0 mN/m at 25 °C.

Interpretation: Static surface tension falls almost linearly as temperature rises, because thermal motion weakens the pull between surface molecules.

When to use: Use it to correct a reference value to your lab temperature, over a range of a few tens of degrees.

Assumptions:

  • Pure liquid, well below its boiling point

Common mistakes:

  • Comparing values measured at different temperatures without correcting
  • Extrapolating the straight line far outside the range it was fitted on

How to read it

What a static surface tension reading tells you about a liquid.

Before you trust a static value, check that it has stopped drifting and that the temperature is stated. If your work also involves wetting, our free contact angle tool pairs well with a known static surface tension.

From reading to meaning

If you see It usually means Check next
Pure water well below 72 mN/m at room temperature The water or the equipment is contaminated Fresh water, cleaned needle or plate
Reading still falling after several minutes Surfactant is still adsorbing, so the value is not yet static Wait for a plateau, or report it as dynamic with its age
Static value stops falling as concentration rises The solution has reached its critical micelle concentration A full surface tension versus concentration curve
Batch to batch differences of a few mN/m Formulation or impurity level is changing Raw material lots and storage conditions

Common misconceptions

Where static surface tension is most often misread.

Static surface tension is the only value that matters

Fast processes such as spraying, inkjet printing and foaming create new surface in milliseconds. Surfactant cannot reach it in time, so the tension that matters there is the dynamic value, which can be much higher than the static one.

A single quick reading is static

For pure liquids it usually is. For surfactant solutions, especially dilute ones, a reading taken in the first seconds can sit well above equilibrium. Watch the trend and only call it static once it stops moving.

Static means measured with a static method

Static describes the state of the surface, not the instrument. A pendant drop, plate or ring can all give a static value if the surface is allowed to reach equilibrium first.

Temperature makes little difference

Water loses about 0.16 mN/m for every degree Celsius, so a 5 °C change shifts it by almost 1 mN/m. Always state the temperature. Teaching labs choosing equipment can compare options in our guide to the best tensiometer for teaching labs.

Related terms

Where static surface tension sits among other surface tension concepts.

Broader term

  • Surface tension

Related terms

  • Surfactant
  • Critical micelle concentration (CMC)
  • Pendant drop method
  • Tensiometer

Often confused with

  • Dynamic surface tension (time-resolved pendant)

FAQ

Short answers to the questions people ask most about static surface tension.

What is the difference between static and dynamic surface tension?

Static surface tension is the equilibrium value, reached once the surface has stopped changing. Dynamic surface tension is the value at a given moment while the surface is still ageing, usually because surfactant is still arriving. For pure liquids the two are practically the same. For surfactant solutions the dynamic value starts higher and falls towards the static one.

What is the static surface tension of water?

Pure water has a static surface tension of 72.8 mN/m at 20 °C and about 72 mN/m at 25 °C. The value falls by roughly 0.16 mN/m for each degree of warming. Readings well below this at room temperature usually point to contamination of the water or the equipment.

How long does it take to reach static surface tension?

Pure liquids reach equilibrium within a fraction of a second. Surfactant solutions take longer, from seconds for concentrated solutions to many minutes or more for dilute ones, because molecules must diffuse to the surface. The practical rule is to wait until the reading no longer changes beyond your measurement scatter.

References

Primary sources and further reading.

1. Vargaftik, N. B., Volkov, B. N., and Voljak, L. D. (1983). International tables of the surface tension of water. Journal of Physical and Chemical Reference Data, 12(3), 817 to 820. https://doi.org/10.1063/1.555688
2. IAPWS R1-76(2014). Revised Release on the Surface Tension of Ordinary Water Substance. International Association for the Properties of Water and Steam. https://iapws.org/relguide/Surf-H2O.html
3. Ward, A. F. H., and Tordai, L. (1946). Time-dependence of boundary tensions of solutions I. The role of diffusion in time-effects. Journal of Chemical Physics, 14, 453 to 461. https://doi.org/10.1063/1.1724167
4. Rosen, M. J., and Kunjappu, J. T. (2012). Surfactants and Interfacial Phenomena, 4th edition. Wiley, Hoboken. https://doi.org/10.1002/9781118228920
5. Surface tension measurement guide
6. Surface tension
7. Dynamic surface tension