# Basic Concepts

• Werner-Michael Kulicke
• Christian Clasen
Part of the Springer Laboratory book series (SPLABORATORY)

## Abstract

The viscosity η is defined as the ratio of the shear stress σ21 and the shear rate γ̇
$$\eta = \frac{{{\sigma _{21}}}}{{\dot \gamma }}$$
(2.1)
The shear stress σ21, as can be seen in Fig. 2.1, is the force F [N] that is exerted on the surface element A [m2] parallel to the direction of the flow:
$${\sigma _{21}} = \frac{F}{A}$$
(2.2)
The shear stress has the unit [Pa], which is the same as for a pressure. In former times, the unit [dyn/cm2] was often used:
$$1Pa = 1N/{m^2} = 10dyn/c{m^2} = 1kg/\left( {m/{s^2}} \right)$$
(2.3)
The shear rate γ̇ describes how fast the fluid layers move in respect to one another in a laminar flow profile. It is described as the ratio of the velocity difference Δv [m/s] of two fluid layers and the distance Δh between them.

## Keywords

Shear Stress Shear Rate Dynamic Viscosity Molar Mass Kinematic Viscosity
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.