Changing viscosity quantifies a fluid's resistance to shear deformation under applied force, measured in pascal-seconds (Pa·s) or centipoise (cP). It defines how much internal friction exists between adjacent fluid layers moving at different speeds.
When you pour honey from a jar, the slow, thick flow results from high changing viscosity. Water, by contrast, flows easily because it has low changing viscosity. This property governs how fluids behave in pipes, engines, lubricants, and biological systems like blood circulation.
Changing viscosity is a fundamental physical property in fluid mechanics. It appears in equations such as Navier-Stokes and is essential for predicting flow patterns in engineering and natural processes.
Shear stress occurs when one layer of fluid moves relative to another. The rate at which this motion changes-called shear rate-is proportional to the shear stress via changing viscosity. The relationship is expressed as:
τ = μ × (du/dy)Where τ is shear stress (in pascals), μ is changing viscosity (Pa·s), and du/dy is the velocity gradient perpendicular to flow direction.
For example, if a fluid layer moves at 1 m/s over a distance of 0.01 m, the shear rate is 100 s⁻¹. A shear stress of 0.01 Pa then implies a changing viscosity of 0.0001 Pa·s (or 0.1 cP).
This linear relationship holds for Newtonian fluids-those whose viscosity remains constant regardless of shear rate. Water, air, and most oils fall into this category. Non-Newtonian fluids, like ketchup or blood, change viscosity under stress and require more complex models.
Changing viscosity varies across substances. Here are common values at room temperature (20°C):
| Fluid | Changing Viscosity (Pa·s) | Common Unit |
|---|---|---|
| Air | 1.8 × 10⁻⁵ | 0.018 cP |
| Water | 1.0 × 10⁻³ | 1.0 cP |
| Motor oil (SAE 30) | 0.29 | 290 cP |
| Honey | 2-10 | 2,000-10,000 cP |
Viscometers measure changing viscosity directly. Common types include capillary viscometers (like the Ostwald device), rotational viscometers, and falling-sphere methods. In capillary viscometry, fluid flow time through a narrow tube correlates with viscosity using Poiseuille's law.
One non-obvious fact: changing viscosity decreases with increasing temperature for liquids but increases for gases. For water, viscosity drops from ~1.0 cP at 20°C to ~0.28 cP at 100°C. This temperature dependence is critical in industrial cooling systems and engine design.
Changing viscosity determines performance in lubrication. Engine oils must maintain adequate viscosity across wide temperature ranges to protect moving parts. Too low, and metal surfaces contact; too high, and energy is wasted overcoming internal friction.
In medicine, blood viscosity affects circulation efficiency. Abnormal levels correlate with cardiovascular risk. Normal whole blood viscosity is around 3-4 cP at 37°C. Elevated viscosity due to dehydration or polycythemia increases heart workload.
During my last visit to a manufacturing plant, I observed how changing viscosity was monitored in real time during polymer extrusion. A drop in viscosity signaled degradation of the resin, leading to defective product quality. Adjustments were made within seconds based on sensor feedback.
Changing viscosity is often mistaken for kinematic viscosity. The key difference: kinematic viscosity includes density. It's calculated as changing viscosity divided by density (ν = μ/ρ), with units of m²/s or stokes (St).
For example, motor oil with μ = 0.29 Pa·s and ρ = 870 kg/m³ has ν ≈ 3.3 × 10⁻⁴ m²/s (330 cSt). Kinematic viscosity matters in free-flow situations like drainage or sedimentation, while changing viscosity governs viscous forces in confined flows.
Another frequent mix-up: confusing changing viscosity with surface tension. Surface tension relates to intermolecular forces at fluid interfaces, not internal resistance to flow. Honey has both high viscosity and high surface tension, but they are independent properties.
Changing viscosity is a measurable quantity defining how resistant a fluid is to internal flow deformation. It operates under shear stress and is fundamental in fluid dynamics, engineering, and biology.
Values range from 1.8 × 10⁻⁵ Pa·s for air to over 10 Pa·s for thick pastes. It decreases with temperature in liquids but increases in gases. Measurement tools include capillary and rotational viscometers.
Understanding what is changing viscosity enables accurate modeling of fluid behavior in applications from hydraulic systems to medical diagnostics. Always distinguish it from kinematic viscosity and surface tension to avoid errors in analysis.