What Is Viscosity and How Is It Measured?

Updated

Viscosity is a fluid's resistance to flow: the internal friction that makes honey pour slowly and water pour quickly. You measure it by applying a known shear (flow) or stress and recording the response, then reporting the value with its conditions—temperature, shear rate, and time—because viscosity usually changes with all three. If your material is simple (water, thin oils, some solvents), a single number from a rotational viscometer is often enough. If it is structured, filled, or applied by hand (creams, adhesives, paints, foods, slurries), you need a flow curve or oscillatory data, not one point.

Dynamic vs. kinematic viscosity: two numbers, two uses

Dynamic (shear) viscosity Kinematic viscosity
Symbol η (eta) ν (nu)
Definition Shear stress ÷ shear rate Dynamic viscosity ÷ density
Common units Pa·s, mPa·s, cP (1 cP = 1 mPa·s) cSt, mm²/s
Depends on density? No Yes
Typical use Rheology, processing, formulation Flow through pipes, orifices, gravity-driven systems

They are not interchangeable. A dense fluid can have the same dynamic viscosity as a light one but a very different kinematic viscosity. When a supplier quotes "viscosity" without specifying which, ask—the number may not apply to your calculation.

Newtonian vs. non-Newtonian behavior

A Newtonian fluid has one viscosity regardless of shear rate. Water and many mineral oils behave this way over practical ranges.

A non-Newtonian fluid's viscosity depends on shear rate, time, or both:

  • Shear-thinning (pseudoplastic): viscosity drops as shear rate rises. Common in polymer solutions, cosmetics, paints, and foods. This is why shaking or pumping can make a thick product flow.
  • Shear-thickening (dilatant): viscosity rises with shear rate. Seen in some concentrated suspensions and pastes.
  • Thixotropy: viscosity decreases under sustained shear and recovers when shear stops. A gel that "breaks down" when stirred and re-thickens on standing is thixotropic.
  • Yield stress: a stress threshold below which the material behaves more like a solid. Toothpaste and ketchup are everyday examples—they hold shape until enough force is applied.

The distinction matters for testing: a single-point viscometer reading on a shear-thinning fluid tells you almost nothing about how it behaves at other shear rates.

Matching the method to your question

Method What it applies Best for Limitations
Rotational viscometer Controlled shear rate or stress Quick single-point or simple flow curves; QC checks Limited shear-rate range; geometry choice affects results
Capillary Pressure-driven flow through a tube High-shear behavior, process-relevant flow, some melts Indirect shear rate; corrections needed for non-Newtonian fluids
Oscillatory rheometry Small-amplitude sinusoidal strain or stress Structure, gelation, viscoelasticity, yield stress, frequency and strain sweeps Requires more interpretation; not a direct "viscosity number"

Fluid Dynamics Testing, for example, offers steady shear viscosity, oscillatory rheometry, yield stress, texture analysis, lubricity/coefficient of friction, and powder-related measurements—so the choice depends on whether you need a flow number, a structural picture, or a sensory/application property.

Choosing by question

  • "Will it pump?" → steady shear viscosity across the shear rates your pump produces.
  • "Will it sag or hold shape?" → yield stress and low-shear viscosity.
  • "How does it feel on skin or in the mouth?" → oscillatory rheometry plus texture analysis.
  • "Does it stay stable on the shelf?" → time-dependent (thixotropy) and frequency sweep data.

Why conditions must be reported

Three variables change the number you get:

  1. Temperature. Most fluids thin as they warm. A viscosity quoted at 25 °C may be far from the value at 5 °C or 60 °C. Always record temperature.
  2. Shear rate. For non-Newtonian fluids, viscosity at 1 s⁻¹ can differ by orders of magnitude from viscosity at 1000 s⁻¹. State the shear rate or show the full curve.
  3. Time / shear history. Thixotropic materials give different readings depending on how long and how hard they were sheared before measurement. Note the protocol.

A viscosity value without these conditions is not reproducible and cannot support a specification.

Reading a flow curve

A viscosity flow curve plots viscosity (often on a log scale) against shear rate (also often log). What to look for:

  • Plateau at low shear: zero-shear viscosity, relevant to sagging, settling, and storage.
  • Slope in the mid-range: indicates how strongly shear-thinning the material is.
  • High-shear plateau: relates to spraying, coating, or high-speed processing.
  • Hysteresis between up and down curves: a sign of thixotropy.

Use the curve to pick the shear rate that matches your process. For pipe flow, estimate the shear rate from flow rate and geometry; for brushing or spraying, use application-relevant ranges. Then compare materials at the same shear rate and temperature—never at different ones.

Practical checklist before you test

  • Define the decision the data must support (specification, scale-up, sensory claim, QC limit).
  • Identify the relevant shear rate range and temperature.
  • Decide whether you need a single point, a flow curve, or oscillatory data.
  • Confirm the lab reports conditions and units (Pa·s vs. cP; cSt vs. mm²/s).
  • For structured or unfamiliar materials, ask for a method recommendation rather than specifying the instrument yourself.

If you are unsure which measurement fits your material, describe the product, the process it goes through, and the decision you need to make—that information determines the method more reliably than the material name alone.