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Fluid viscosity is the property that describes fluid friction between liquid layers and between liquid and surrounding material. It determines how easily a fluid flows—water has low viscosity and flows quickly, while honey has high viscosity and flows slowly. Viscosity influences whether flow is laminar or turbulent, making it essential for understanding fluid behavior.

Higher viscosity promotes laminar flow, where fluid layers flow in parallel sheets with minimal mixing. Lower viscosity allows turbulent flow, characterized by eddies and swirls. Smoke from an incense stick demonstrates this transition: it rises smoothly in laminar flow initially, then swirls as speed increases and drag causes turbulent flow.

The dragging force is directly proportional to the laminar flow velocity, plate area, and coefficient of viscosity, but inversely proportional to the distance between plates. Greater viscosity requires greater force to move the plate at constant velocity. This relationship quantifies how viscosity resists fluid motion.

Viscosity is temperature-dependent and varies significantly across fluids. Water viscosity decreases with increasing temperature, from 1.002 mPa·s at 20°C to 0.282 mPa·s at 100°C. In contrast, honey maintains much higher viscosity values between 2000–10000 mPa·s at 20°C, demonstrating how viscosity varies by orders of magnitude.

Viscosity is measured in newton·second per square meter (N·s/m²) or pascal·second (Pa·s) in the SI system. These units reflect the relationship between force, area, and velocity gradient in fluid flow. Common viscosity values include air at 0.0181 mPa·s and water at 1.002 mPa·s at 20°C.

Honey flows slowly and sticks to surfaces because it has much higher viscosity than water. This greater fluid friction between honey's layers and with surrounding material creates more resistance to flow. Water's lower viscosity allows it to fall freely and quickly, demonstrating how viscosity directly controls flow behavior.

In laminar flow, viscosity causes drag between fluid layers and with fixed surfaces, creating a velocity gradient. The top layer moving with the plate travels faster than bottom layers near the stationary surface. This gradient decreases progressively from the moving surface toward the fixed surface due to viscous friction.