Measures internal resistance to shear stress

Dynamic viscosity (also called absolute viscosity) quantifies how much force is needed to move one layer of fluid past another. It's the intrinsic property of the fluid itself, independent of density. Higher values mean more resistance.

Formula: τ = μ × (du/dy) where τ = shear stress, du/dy = velocity gradient

Units: Pa·s (SI), poise (P), centipoise (cP). Water @ 20°C = 1.002 cP

Dynamic viscosity divided by density

Kinematic viscosity measures how fast a fluid flows under gravity. It accounts for both internal resistance (dynamic viscosity) and mass per volume (density). Used when gravity-driven flow is important, like oil draining or liquid pouring.

Formula: ν = μ / ρ where μ = dynamic viscosity, ρ = density

Units: m²/s (SI), stokes (St), centistokes (cSt). Water @ 20°C = 1.004 cSt

• μ (dynamic) = 1.002 cP = 0.001002 Pa·s
• ρ (density) = 998.2 kg/m³
• ν (kinematic) = μ/ρ = 1.004 cSt = 1.004 mm²/s
• Ratio: ν/μ ≈ 1.0 (water is the reference)

• μ (dynamic) = 62 cP = 0.062 Pa·s
• ρ (density) = 850 kg/m³
• ν (kinematic) = μ/ρ = 73 cSt = 73 mm²/s
• Note: Kinematic is 18% higher than dynamic (due to lower density)

• μ (dynamic) = 1,412 cP = 1.412 Pa·s
• ρ (density) = 1,261 kg/m³
• ν (kinematic) = μ/ρ = 1,120 cSt = 1,120 mm²/s
• Note: Very viscous—1,400× thicker than water

• μ (dynamic) = 0.0181 cP = 1.81×10⁻⁵ Pa·s
• ρ (density) = 1.204 kg/m³
• ν (kinematic) = μ/ρ = 15.1 cSt = 15.1 mm²/s
• Note: Low dynamic, high kinematic (gases have low density)

ASTM D88 standard, widely used in North America for petroleum products

ν(cSt) = 0.226 × SUS - 195/SUS (valid for SUS > 32)

• Measured at specific temperatures: 100°F (37.8°C) or 210°F (98.9°C)
• Common range: 31-1000+ SUS
• Example: SAE 30 oil ≈ 300 SUS @ 100°F
• Saybolt Furol (SFS) variant for very viscous fluids: ×10 larger orifice

British IP 70 standard, common in UK and former Commonwealth

ν(cSt) = 0.26 × RW1 - 179/RW1 (valid for RW1 > 34)

• Measured at 70°F (21.1°C), 100°F, or 140°F
• Redwood No. 2 variant for thicker fluids
• Conversion: RW1 ≈ SUS × 1.15 (approximate)
• Largely replaced by ISO standards but still referenced in older specs

DIN 51560 German standard, used in Europe and petroleum industry

ν(cSt) = 7.6 × °E - 6.0/°E (valid for °E > 1.2)

• Measured at 20°C, 50°C, or 100°C
• °E = 1.0 for water @ 20°C (by definition)
• Common range: 1.0-20°E
• Example: Diesel fuel ≈ 3-5°E @ 20°C

The world's longest-running lab experiment (since 1927) at the University of Queensland shows pitch (tar) flowing through a funnel. It looks solid but is actually a very high-viscosity liquid—100 billion times more viscous than water! Only 9 drops have fallen in 94 years.

Basaltic lava (low viscosity, 10-100 Pa·s) creates gentle Hawaiian-style eruptions with flowing rivers. Rhyolitic lava (high viscosity, 100,000+ Pa·s) creates explosive Mount St. Helens-style eruptions because gases can't escape. Viscosity literally shapes volcanic mountains.

Blood is 3-4× more viscous than water (3-4 cP @ 37°C) due to red blood cells. High blood viscosity increases stroke/heart attack risk. Low-dose aspirin reduces viscosity by preventing platelet aggregation. Blood viscosity testing can predict cardiovascular disease.

Contrary to popular myth, old windows aren't thicker at the bottom due to flow. Glass viscosity at room temperature is 10²⁰ Pa·s (1 trillion trillion times water). To flow 1mm would take longer than the age of the universe. It's a true solid, not a slow liquid.

SAE 10W-30 means: 10W = winter viscosity @ 0°F (low-temp flow), 30 = viscosity @ 212°F (operating temp protection). The 'W' is winter, not weight. Multi-grade oils use polymers that coil when cold (low viscosity) and expand when hot (maintain viscosity).

Water striders exploit surface tension, but also leverage water's viscosity. Their leg movements create vortices that push against viscous resistance, propelling them forward. In zero-viscosity fluid (theoretical), they couldn't move—they'd slip without traction.

Isaac Newton describes viscosity in Principia Mathematica. Introduces concept of 'internal friction' in fluids.

Jean Poiseuille studies blood flow in capillaries. Derives Poiseuille's Law relating flow rate to viscosity.

George Stokes derives equations for viscous flow. Proves relationship between dynamic and kinematic viscosity.

Osborne Reynolds introduces Reynolds number. Relates viscosity to flow regime (laminar vs turbulent).

Saybolt viscometer standardized in USA. Efflux cup method becomes petroleum industry standard.

Poise and stokes named as CGS units. 1 P = 0.1 Pa·s, 1 St = 1 cm²/s become standard.

Pitch drop experiment begins at University of Queensland. Still running—longest lab experiment ever.

SI adopts Pa·s and m²/s as standard units. Centipoise (cP) and centistokes (cSt) remain common.

ASTM D445 standardizes kinematic viscosity measurement. Capillary viscometer becomes industry standard.

Rotational viscometers enable non-Newtonian fluid measurement. Important for paints, polymers, food.

Digital viscometers automate measurement. Temperature-controlled baths ensure precision to ±0.01 cSt.

Motor oil, hydraulic fluid, and bearing lubrication selection:

• SAE grades: 10W-30 means 10W @ 0°F, 30 @ 212°F (kinematic viscosity ranges)
• ISO VG grades: VG 32, VG 46, VG 68 (kinematic viscosity @ 40°C in cSt)
• Bearing selection: Too thin = wear, too thick = friction/heat
• Viscosity index (VI): Measures temperature sensitivity (higher = better)
• Multi-grade oils: Additives maintain viscosity across temperatures
• Hydraulic systems: Typically 32-68 cSt @ 40°C for optimal performance

Fuel, crude oil, and refining viscosity specifications:

• Heavy fuel oil: Measured in cSt @ 50°C (must be heated to pump)
• Diesel: 2-4.5 cSt @ 40°C (EN 590 spec)
• Crude oil classification: Light (<10 cSt), medium, heavy (>50 cSt)
• Pipeline flow: Viscosity determines pumping power requirements
• Bunker fuel grades: IFO 180, IFO 380 (cSt @ 50°C)
• Refining process: Viscosity breaking reduces heavy fractions

Quality control and process optimization:

• Honey grading: 2,000-10,000 cP @ 20°C (depending on moisture)
• Syrup consistency: Maple syrup 150-200 cP, corn syrup 2,000+ cP
• Dairy: Cream viscosity affects texture and mouthfeel
• Chocolate: 10,000-20,000 cP @ 40°C (tempering process)
• Beverage carbonation: Viscosity affects bubble formation
• Cooking oil: 50-100 cP @ 20°C (smoke point correlates with viscosity)

Paint, adhesives, polymers, and process control:

• Paint viscosity: 70-100 KU (Krebs units) for application consistency
• Spray coating: Typically 20-50 cP (too thick clogs, too thin runs)
• Adhesives: 500-50,000 cP depending on application method
• Polymer melts: 100-100,000 Pa·s (extrusion/molding)
• Printing inks: 50-150 cP for flexographic, 1-5 P for offset
• Quality control: Viscosity indicates batch consistency and shelf life

Dynamic viscosity (Pa·s, poise) measures the fluid's internal resistance to shear stress—its absolute 'thickness.' Kinematic viscosity (m²/s, stokes) is dynamic viscosity divided by density—how fast it flows under gravity. You need density to convert between them: ν = μ/ρ. Think of it this way: honey has high dynamic viscosity (thick), but mercury also has high kinematic viscosity despite being 'thin' (because it's very dense).

Not without knowing the fluid's density at the measurement temperature. For water near 20°C, 1 cP ≈ 1 cSt (because water's density ≈ 1 g/cm³). But for motor oil (density ≈ 0.9), 90 cP = 100 cSt. Our converter blocks cross-type conversions to prevent errors. Use this formula: cSt = cP / (density in g/cm³).

SAE viscosity grades specify kinematic viscosity ranges. '10W' means it meets low-temperature flow requirements (W = winter, tested at 0°F). '30' means it meets high-temperature viscosity requirements (tested at 212°F). Multi-grade oils (like 10W-30) use additives to maintain viscosity across temperatures, unlike single-grade oils (SAE 30) which thin dramatically when hot.

Saybolt Universal Seconds (SUS) measure how long 60mL of fluid takes to drain through a calibrated orifice. The empirical formula is: cSt = 0.226×SUS - 195/SUS (for SUS > 32). For example, 100 SUS ≈ 21 cSt. SUS is still used in petroleum specs despite being an older method. Modern labs use kinematic viscometers that directly measure cSt per ASTM D445.

Higher temperature gives molecules more kinetic energy, letting them slide past each other more easily. For liquids, viscosity typically drops 2-10% per °C. Motor oil at 20°C might be 200 cP but only 15 cP at 100°C (13× decrease!). Viscosity Index (VI) measures this temperature sensitivity: high VI oils (100+) maintain viscosity better, low VI (<50) thin dramatically when heated.

Most hydraulic systems operate best at 25-50 cSt @ 40°C. Too low (<10 cSt) causes internal leakage and wear. Too high (>100 cSt) causes sluggish response, high power consumption, and heat buildup. Check your pump manufacturer's spec—vane pumps prefer 25-35 cSt, piston pumps tolerate 35-70 cSt. ISO VG 46 (46 cSt @ 40°C) is the most common general-purpose hydraulic oil.

No theoretical maximum, but practical measurements become difficult above 1 million cP (1000 Pa·s). Bitumen/pitch can reach 100 billion Pa·s. Some polymer melts exceed 1 million Pa·s. At extreme viscosities, the boundary between liquid and solid blurs—these materials exhibit both viscous flow (like liquids) and elastic recovery (like solids), called viscoelasticity.

Poise honors Jean Léonard Marie Poiseuille (1840s), who studied blood flow in capillaries. Stokes honors George Gabriel Stokes (1850s), who derived the equations for viscous flow and proved the relationship between dynamic and kinematic viscosity. A reyn (pound-force second per square inch) is named after Osbourne Reynolds (1880s), famous for the Reynolds number in fluid dynamics.