Scientists have been monitoring and investigating the properties of water and other liquids since the dawn of time.

    Archimedes, a mathematician in Ancient Greece is credited to have invented the concepts of volume and density (both important measurements for liquids) in the 3rd century B.C.E.

    We've come a long way since then. In this educational Q & A, we'll be focusing on dynamic viscosity. For help with conversions, use our free calculator to go from dyne·s/cm2 to N·s/m2 and more.

    FREQUENTLY ASKED QUESTIONS


    What is dynamic viscosity?

    To answer this question, it is probably easiest to first define what viscosity is.

    Viscosity is a trait or measure of a liquid that has resistance to "deformation" or movement when put under stress, whether that's gravity or some other force.

    Example

    Water has a very low viscosity because it flows easily and freely. It is relatively frictionless when compared to a liquid like honey or molasses, which are both extremely viscous.

    So if you were to flip a cup of water upside down, the water would spill everywhere immediately.

    If you were to flip a cup of honey upside down, you might be there a while before it fully leaves the cup.

    So, What Is Dynamic Viscocity?

    Now that we know what viscosity is, let's move on to dynamic viscosity.

    Definition

    It is defined as, "the force needed by a fluid to overcome its own internal molecular friction so that the fluid will flow."

    Specifically, it's a measure of liquid's resistance to move across a horizontal plane.

    Example

    To illustrate this, imagine pouring a cup of water over a metal baking sheet.

    Water, since it has a low viscosity, it will likely spread out all across the baking sheet. Its internal friction is just too low to keep its own shape (unless it's frozen).

    Now imagine dripping honey on the sheet. It will mostly retain the shape it was in when it came in contact with the shape.

    Now, if you came back to that baking sheet later, the honey might have spread out across the baking sheet.

    But that difference between water and honey, and the time it took to move across that plane is dynamic viscosity.

    Dynamic viscocity is not to be confused with kinematic viscocity.

    Summary

    Dynamic viscocity is a measure of liquid's internal molecular resistance to move across a horizontal plane.

    What Is the standard unit of dynamic viscosity?

    Dynamic viscosity (DV), when expressed in SI units, can look pretty complex.

    It is most commonly expressed as N·s/m (Newton seconds per square meter).

    However, you can also refer to it is Pa·s or kg/(m·s).

    How do you convert dynamic viscocity?

    Because DV can be measured in a variety of ways, the method to convert units will differ.

    The easiest way to get accurate conversions is using our dynamic viscocity calculator, but we'll also review some common conversion factors.

    SI Units

    Previously, we mentioned the three typical SI units used for dynamic viscosity.

    These measurements can be converted with a 1:1 ratio.

      1 N·s/m2 = 1 Pa·s = 1 kg/(m·s)

    Dynamic viscosity can also be measured with the metric CGS (centimeter/gram/second) system, which we'll detail next.

    CGS Units

    Here's how these measurements are converted:

      1 Poise = 100 cP = 1 dyne·s/cm2 = 1 g/(cm·s) = 1/10 Pa·s = 1/10 N·s/m2

    Here, P, or Poise, is the largest unit of measurement.

    It's often actually too large to be used for dynamic viscosity, so cP is the predominant unit of measurement, but g/(cm·s) or dyne s/cm2 are also used.

    Why measure dynamic viscosity?

    Understanding how various liquids will react when coming into contact with solid materials is important for a variety of different reasons in engineering and mechanical fields.

    Understanding Fluid Dynamics

    Car engines use oil to lubricate its internal moving parts.

    The metal pistons of a car engine move up and down in their metal cylinder housing and require lubrication to minimize wear and tear and prevent overheating from friction.

    But the viscosity of the oil used is crucial. You can't just use any liquid to lubricate a car engine.

    You can't use a liquid like water to use as lubrication because it isn't viscous enough to cling to the engine parts, and you can't use a liquid like honey because it's too viscous and would likely prevent the pistons from moving effectively.

    By measuring the dynamic viscosity of different liquids, engineers were able to determine that motor oil had the perfect viscosity for that specific task.

    What's the dynamic viscosity of water?

    An important thing to take note of is that the dynamic viscosity of a liquid is largely reliant on the temperature of the liquid itself.

    You've probably encountered this phenomenon yourself when pouring syrup on pancakes.

    If you use the syrup that was chilling in your fridge, it flow out of the bottle much slower than the syrup sitting in your room-temperature pantry.

    The same applies to water, of course, though there is much less variance.

    The short answer to this question, however, is that at room temperature, water has a dynamic viscosity of .001002 kg/(m·s), or 1 centipoise (cP).

    Compared To Other Fluids

    To help you understand that value, here's the DV of other common fluids:

    • milk has a dynamic viscosity of 3 cP
    • honey has a dynamic viscosity of 10,000 cP
    • peanut butter sits at 250,000 cP.

    Summary

    At room temperature, water has a dynamic viscosity of 0.001002 kg/(m·s), or 1 centipoise (cP).

    Who uses dynamic viscosity?

    As we've already mentioned, understanding dynamic viscosity is crucial for a variety of engineering fields.

    Car engine designers, motor oil manufacturers, and mechanics have to be knowledgeable about this concept to varying degrees of abstraction.

    More broadly, people that work in hydrodynamics or fluid dynamics need to be well-acquainted with this concept.

    Hopefully, this quick Q&A has helped you understand what dynamic viscosity is and how it's measured, and why it's important.

    Whether you're going to be creating motor oil blends or going into hydraulics, it's an important concept to understand.

    If you ever need help with the conversions, be sure to check out our dynamic viscocity calculator. It'll make your physics homework ten times easier.

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