Viscosity is a fluid’s internal resistance to flow, caused by friction between adjacent layers of the fluid as they move past one another. Viscosity measurement is the process of quantifying this resistance, usually as dynamic (absolute) viscosity in pascal-seconds (Pa·s) or poise, or as kinematic viscosity in square metres per second (m²/s) or stokes. In simple terms, a high-viscosity fluid like honey flows slowly, while a low-viscosity fluid like water flows freely.
Viscosity is one of the most important transport properties in fluid mechanics, chemical engineering, and pharmaceutics. It governs pipe flow, lubrication, pump sizing, drug formulation, and heat transfer. This guide explains what viscosity is, its types, the governing formula and units, and how it is measured and demonstrated in a teaching laboratory.
What causes viscosity in a fluid?
Viscosity arises from intermolecular cohesion and momentum exchange between fluid layers. When a fluid flows, layers near a solid boundary move slower than layers farther away, creating a velocity gradient. The friction between these layers, the fluid’s shear stress, is what we experience as viscosity.
Newton’s law of viscosity
For many common fluids, shear stress is directly proportional to the velocity gradient (shear rate). This relationship, known as Newton’s law of viscosity, is written as:
τ = μ × (du/dy)
- τ (tau) = shear stress between fluid layers, in N/m² (Pa)
- μ (mu) = dynamic viscosity, the constant of proportionality, in Pa·s
- du/dy = velocity gradient or shear rate, in s⁻¹
Fluids that obey this linear relationship are called Newtonian fluids (water, air, light oils). Fluids that do not, where viscosity changes with shear rate, are called non-Newtonian fluids (blood, paints, ketchup, many polymer and pharmaceutical suspensions).
Dynamic vs kinematic viscosity
Two viscosity quantities are used in engineering. Dynamic (absolute) viscosity (μ) measures internal friction directly. Kinematic viscosity (ν) relates that friction to the fluid’s density and is widely used in flow and Reynolds-number calculations:
ν = μ / ρ
where ρ (rho) is the fluid density in kg/m³, giving kinematic viscosity in m²/s.
What are the units of viscosity?
Both SI and CGS units are commonly encountered. Students should be comfortable converting between them.
| Quantity | SI unit | CGS unit | Conversion |
|---|---|---|---|
| Dynamic viscosity (μ) | pascal-second (Pa·s) | poise (P) | 1 Pa·s = 10 poise; 1 cP = 1 mPa·s |
| Kinematic viscosity (ν) | m²/s | stokes (St) | 1 m²/s = 10,000 St; 1 cSt = 1 mm²/s |
For reference, water at 20°C has a dynamic viscosity of about 1.0 mPa·s (1 centipoise) and a kinematic viscosity of about 1.0 mm²/s (1 centistokes). Honey may be thousands of times more viscous.
What are the main types of viscosity behaviour?
Beyond the Newtonian/non-Newtonian split, fluids are classified by how their apparent viscosity responds to shear rate and time. This classification matters for pharmacy formulation, food processing, and polymer engineering.
| Type | Behaviour | Common examples |
|---|---|---|
| Newtonian | Constant viscosity at all shear rates | Water, glycerine, light mineral oil |
| Pseudoplastic (shear-thinning) | Viscosity decreases as shear rate rises | Paint, blood, polymer melts |
| Dilatant (shear-thickening) | Viscosity increases as shear rate rises | Cornstarch suspension, some slurries |
| Bingham plastic | Needs a yield stress before it flows | Toothpaste, drilling mud |
| Thixotropic | Viscosity drops over time under shear | Gels, certain creams and lotions |
How does temperature affect viscosity?
Temperature has a strong effect, and the direction depends on whether the fluid is a liquid or a gas:
- Liquids: viscosity decreases as temperature rises, because thermal energy weakens intermolecular cohesion. Hot oil flows more easily than cold oil.
- Gases: viscosity increases with temperature, because faster molecular motion increases momentum exchange between layers.
Because of this sensitivity, viscosity must always be reported at a stated temperature, and lab viscometers are often used with a temperature-controlled water bath.
How is viscosity measured in a teaching lab?
Several standard instruments are used to measure or demonstrate viscosity. The choice depends on the fluid type, the required accuracy, and whether the goal is a teaching demonstration or a precise measurement.
- Capillary (Ostwald / U-tube) viscometer: measures the time a fixed volume of liquid takes to flow through a fine glass capillary under gravity. The flow time is proportional to kinematic viscosity, calculated using the relation ν = C × t, where C is the capillary constant and t is the efflux time. This is the classic method for Newtonian liquids.
- Falling-sphere (Stokes’ law) viscometer: a sphere of known diameter and density falls through the liquid at a steady terminal velocity. Dynamic viscosity is found from Stokes’ law: μ = [2 r² (ρs − ρf) g] / (9 v), where r is the sphere radius, ρs and ρf are sphere and fluid densities, g is gravity, and v is terminal velocity. It is excellent for demonstrating the physics of viscous drag.
- Rotational (Redwood / Saybolt / rotational) viscometers: measure the torque needed to rotate a spindle in the fluid, suitable for oils and non-Newtonian fluids.
- Redwood viscometer: widely used in Indian engineering and oil-testing curricula to measure the kinematic viscosity of lubricating oils by efflux time in Redwood seconds.
A typical capillary viscometer experiment
In a fluid mechanics or applied chemistry lab, students typically:
- Bring the liquid and viscometer to a controlled temperature in a water bath.
- Draw the liquid above the upper timing mark, then release it.
- Record the efflux time between the upper and lower marks with a stopwatch.
- Repeat for a reference liquid (often distilled water) of known viscosity.
- Calculate the unknown viscosity by comparison, then compare to standard tables.
This hands-on procedure connects Newton’s law of viscosity, units, and temperature dependence into a single repeatable measurement, the core learning objective for the topic.
Why does viscosity matter in engineering and pharmacy?
Viscosity is not just a textbook property; it drives real design decisions:
- Pipe and pump design: viscosity sets the pressure drop and the Reynolds number that decides laminar versus turbulent flow.
- Lubrication: engine and machine oils are graded by viscosity to protect moving parts.
- Pharmacy and formulation: syrups, suspensions, creams, and injectables are formulated to target viscosities for pourability, dosing accuracy, and patient comfort.
- Food and chemical processing: mixing, coating, and heat-transfer rates all depend on viscosity.
Sourcing viscosity lab apparatus for colleges and labs
Reliable, well-calibrated apparatus is essential for accurate viscosity experiments. Fluid Mechanics Lab Equipment such as capillary viscometers, falling-sphere setups, Redwood viscometers, and temperature-controlled baths is used across engineering, polytechnic, and pharmacy teaching labs in India and abroad.
Scientico is an ISO 9001:2015 and CE certified manufacturer and exporter of fluid mechanics and viscosity lab apparatus, based in Ambala, Haryana, India, and exporting to 60+ countries since 1993. As a GeM-registered supplier, Scientico provides conformity and calibration documentation with its instruments and can issue a CIF proforma invoice within 24 hours for institutional and export buyers. For technical specifications or a quotation, you can reach the team on WhatsApp at +91-7015865225.
Key takeaways
- Viscosity is a fluid’s resistance to flow, defined by Newton’s law τ = μ(du/dy).
- Dynamic viscosity uses Pa·s or poise; kinematic viscosity uses m²/s or stokes, with ν = μ/ρ.
- Fluids are Newtonian or non-Newtonian (shear-thinning, shear-thickening, Bingham, thixotropic).
- Liquid viscosity falls with temperature; gas viscosity rises with it.
- Common lab instruments include capillary, falling-sphere, and Redwood viscometers, each with a clear governing formula.
Frequently Asked Questions
What is viscosity in simple terms?
Viscosity is a fluid’s resistance to flow. It is the internal friction between fluid layers as they slide past one another. A high-viscosity fluid like honey flows slowly, while a low-viscosity fluid like water flows easily.
What is the formula for viscosity?
Newton’s law of viscosity is τ = μ × (du/dy), where τ is shear stress, μ is dynamic viscosity, and du/dy is the velocity gradient (shear rate). Kinematic viscosity is ν = μ/ρ, where ρ is the fluid density.
What are the SI units of viscosity?
Dynamic viscosity is measured in pascal-seconds (Pa·s) in SI or poise in CGS, where 1 Pa·s = 10 poise. Kinematic viscosity is measured in m²/s in SI or stokes in CGS, where 1 cSt = 1 mm²/s.
What is the difference between dynamic and kinematic viscosity?
Dynamic (absolute) viscosity measures internal friction directly in Pa·s. Kinematic viscosity divides dynamic viscosity by fluid density (ν = μ/ρ) and is given in m²/s; it is used in flow and Reynolds-number calculations.
How is viscosity measured in a laboratory?
Common instruments include the capillary (Ostwald) viscometer, which times flow through a fine tube; the falling-sphere viscometer, which uses Stokes’ law on a sphere falling at terminal velocity; and Redwood or rotational viscometers for oils and non-Newtonian fluids. Measurements are taken at a controlled temperature, often in a water bath.
Does temperature affect viscosity?
Yes. For liquids, viscosity decreases as temperature rises because heat weakens intermolecular cohesion. For gases, viscosity increases with temperature because faster molecules exchange more momentum. Viscosity must always be reported at a stated temperature.
Lab Equipment Featured in This Guide
Manufactured in-house by Scientico India — ISO 9001:2015 & CE certified, exported to 60+ countries. Request a CIF quote within 24 hours.
Multi-Purpose Teaching Flume (Length 1m) | FluidoSurge-X 392View details & get quote →
Fluid Properties & Hydrostatics Bench | FluidoSurge-X 212View details & get quote →
Pelton Turbine Apparatus | FluidoSurge-X 156View details & get quote →
Multi-Purpose Teaching Flume (Length 10 m) | FluidoSurge-X 248View details & get quote →
Groundwater Flow Apparatus | FluidoSurge-X 310View details & get quote →
Multi Turbine Test Rig | FluidoSurge-X 314View details & get quote →