Have you ever wondered what keeps soap bubbles, well, as bubbles? What about why some types of insects are able to "walk" on water?

    Those things are possible because of surface tension. Surface tension is a property of liquids that is important in the study of natural sciences including physics, chemistry, and biology.

    For help converting surface tension units, use this handy calculator to go from N/m to dyn/cm and more.

    To learn what surface tension is, including examples of surface tension, and how you can calculate it, read this comprehensive guide.


    What is surface tension?

    Surface tension is what happens when liquids act like they're elastic, almost as though there is a stronger film on the surface of the liquid, making it harder for other things to penetrate/break that surface.

    This phenomenon is caused by the stronger cohesion of liquid molecules on the surface of the liquid.

    How It Works

    In the center of a liquid, let's say a glass of water, there are water molecules on all sides of each other, which distributes the forces between these molecules more evenly all around.

    The molecules on the surface of the liquid, however, don't have like molecules on all sides.

    This causes the forces between the surface molecules and the like molecules that do surround it to bond much more strongly to each other.

    Since the bonds between the surface molecules are stronger than normal, it creates a stronger sort of "film" at the surface of the liquid that has stronger bonds than the liquid below.

    This stronger "film" on the surface is harder to break through, which allows for things like bubbles to form and bugs to walk on water.

    This entire phenomenon is what we call "surface tension."

    What are some examples of surface tension?

    A few of the most well-known examples are ones we already went over.

    Walking on Water

    Bugs are able to "walk" one water because their weight is so low that the force they exert on the water is not enough to penetrate the water's surface.

    This is also why smaller leaves and needles are able to stay on top of the water as well: they should sink because they're denser than water, but their weight on the water isn't enough to break the surface tension.


    Bubbles are another example of surface tension.

    The surface tension of the water and soap combination pulls the liquid into a spherical shape.

    Medical Applications

    Some medical tests rely on surface tension to test for certain diseases. An example of this is when doctors are testing patients for jaundice.

    A normal patient's urine will have a surface tension of 66 dyn/cm. When a patient is jaundiced, bile infiltrates the urine and decreases the surface tension to around 55 dyn/cm.

    Doctors can use this difference in surface tension to test whether a patient has jaundice simply by using their urine.

    The Hay test tests urine for bile presence by putting sulfur powder on a urine sample.

    If the sulfur floats on the surface, this means the patient has normal urine surface tension.

    If the sulfur sinks, this means the urine's surface tension has been lowered by the presence of bile, meaning the patient does have jaundice.

    What is the standard unit of surface tension?

    The standard unit is in Newtons/meter (N/m).

    Surface tension is represented by a lowercase gamma and is more technically defined as the force per unit length (force/length).

    The higher the surface tension, the more difficult it is to penetrate the surface of the liquid.

    How do you convert surface tension?

    While the standard unit is in N/m, you can convert this to any other form of force/length.

    Some examples of other units would be:

  • dynes per centimeter (dyn/cm)
  • millinewton per meter (mN/m)
  • joules per square meter (J/m2)
  • Conversion Factors

    The conversion rates for these 4 common units of surface tension are:

    1 dyn/cm = 1 mN/m = 0.001 N/m = 0.001 J/m2

    You can either do this by hand, or try our surface tension conversion calculator.

    How is surface tension calculated?

    You can calculate surface tension by dividing the force (F) over the unit length (l) where it acts and the cosine of the angle (cosθ) of that length where the force is acting.

    So the equation looks something like this:

    Surface Tension = F / (l * cosθ)

    However, each liquid has a defined surface tension at each individual temperatures.

    As we went over, the surface tension of water at 20 degrees Celcius (room temperature) is 0.073 N/m. At 100 degrees, it's 0.059.

    In most problems you'll encounter dealing with surface tension, you'll be given a few of the variables and be asked to find the missing pieces.


    An example problem would be something like this: find the surface tension of a liquid that as a force of 50 Newtons acting over 10 mm at a 30-degree angle.

    You can then use the units of surface tension and your knowledge of force and unit conversion to calculate the surface tension.

    The solution would look something like this:


    Surface Tension = Force / (length * cosθ)

    = 50 N / (0.01 m * cos(30))

    = 50 N / (0.01 m * 0.0867)

    = 5767.01 N/m

    This one example shows how you can use given variables to find the surface tension of liquids.

    You might also be asked to use the known surface tension of liquids to find the weight of objects resting on it, the length at which a force is acting, and more.

    Once you know the formula and the units, this should be simple math. And you can use our conversion calculators if you need a little help.

    Surface tension can be a complex topic to understand as it involves concepts from chemistry, physics, and math.

    Hopefully, this guide helped make sense of this topic so you can successfully solve problems and get through your work.

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