Before evaluating an inkjet ink, we need to understand the following important parameters:

Surface tension plays an important role in inkjet printers because it affects droplet formation, as well as interaction with the substrate.

Viscosity is a key parameter for regulating the flow of the printhead. Due to the high flow rate and small nozzle diameter, the fluid operates in a high rate regime. Viscosity also affects the filling of the ink into the ink chamber of printhead.

In the nozzle, the ink is simultaneously sheared and stretched, and extensional viscosity is important because it affects the formation and breakage of ink filaments and subsequent droplet formation.

Viscoelasticity is used to describe materials that have both viscous (liquid) and elastic (solid) properties. The degree of viscoelasticity depends on the concentration of the additives. Viscoelasticity can be characterized using rotational techniques to some extent, but oscillatory measurement techniques are usually used.

The particle content in the inkjet ink significantly affects the rheology of the ink, with the key parameters of particle size, particle size distribution and particle shape. These parameters depend primarily on the grinding of the particles used as colorants or, in some cases, specifically as rheology modifiers.

Instant viscosity depends on the time scale over which the shear stress or shear rate is applied. Instant viscosity is different because it measures a non-bulk property.

Equilibrium viscosity is obtained when the measured viscosity equals the bulk viscosity over a period of time. The equilibrium flow curve is the reference curve for all viscosity data and is the result of measuring a sample as a function of shear rate or shear stress.

The yield stress is the minimum shear stress required to break the internal structure of the pigment ink and start flowing. The yield stress is determined as a linear slope of shear stress and is defined as the maximum in the measured viscosity curve. All viscosity data are instantaneous viscosity data as defined above.

Many high viscosity samples have compositions where the components form a three-dimensional structure. This structure is time dependent and therefore thixotropic, breaking down under shear. The rate of breakdown varies at different shear rates and between different samples.