Materials: Viscosity

Viscosity: Basic Understanding

Viscosity is a measure of how a fluid resists flow; in essence, it quantifies internal friction within a fluid.
More viscous fluids will require a greater force to set them in motion compared to lesser viscous fluids.
For example, honey is more viscous than water and requires more force to flow.
Viscosity is typically expressed in the SI unit of Pascal-seconds (Pa.s), though poise (P) is also used in some contexts.

Viscosity depends significantly on temperature. For liquids, viscosity generally decreases as temperature increases, while for gases, viscosity increases with higher temperature.
Viscosity also depends on intermolecular forces. Stronger intermolecular forces, such as hydrogen bonding, can lead to higher viscosity.
The shape and size of molecules also influence viscosity. Larger or elongated molecules tend to get entangled more readily and as such, the substance exhibits a higher viscosity.

Stokes’ Law is a vital concept in understanding viscosity, which describes the force required to move a sphere through a viscous fluid at a certain speed.
The law states that the force (F) required is directly proportional to the speed (v), radius (r) of the sphere and the viscosity (η) of the fluid: F = 6πηrv.
Stokes’ Law assumes that the fluid is uniform and the flow is laminar.

Understanding viscosity is crucial in various fields such as food industry, petroleum engineering, pharmaceuticals, and more.
For instance, lubricants used in machinery need to have specific viscosity to ensure they adequately reduce friction without causing overheating or energy loss.
In medicine, the viscosity of substances like blood serum is essential to understand in relation to certain health conditions.