Viscosity

Viscosity is the internal friction or resistance to flow in a fluid.

Viscosity arises because intermolecular interactions cause adjacent layers to resist sliding, creating a velocity gradient that scales from the stationary surface layer to the faster-moving center.

To maintain this gradient, an external force is applied that is proportional to the area and the velocity gradient described by Newton's law of viscosity, a relationship that connects applied force and flow rate to the fluid's physical properties.

The proportionality constant is the coefficient of viscosity

As temperature increases, the viscosity of a liquid generally decreases, whereas viscosity tends to increase with higher pressure.

The Reynolds number is a dimensionless ratio of inertial to viscous forces in a fluid. It determines flow type. High viscosity promotes laminar flow as viscous forces dominate, while low viscosity favors turbulent flow due to dominant inertial forces.

The volume of liquid in laminar flow is given by Poiseuille’s equation, where R is the pipe radius, p1 and p2 are the inlet and outlet pressures, t is the flow time, η is the viscosity coefficient, and l is the pipe length.