Rheology of Semi-Solid Materials

In the world of semi-solid material, rheological properties such as yield stress, thixotropy, and viscoelasticity have a direct impact on pharmaceuticals, foods, cosmetics, and consumer products.

Semi-solid materials exhibit liquid and solid like properties depending on temperature, as well as on being subjected to stress and strain over time. Rheological instrumentation is used for various reasons, for example, to determine yield stress (stress that must be exceeded for flow to occur) to help predict shelf-life and strain sweeps to determine the critical strain (minimum energy needed to disrupt structure, where the higher the critical strain the better the systems is dispersed).

Since the materials act like elastic solids under some conditions and viscous liquids under others, they are viscoelastic materials. If one is interested in influencing consumer perception by having a product appear solid under “at rest” conditions, rheological testing should be carried out at low stress and long time periods (low frequencies) to simulate this. If predicting the pumping of a material for proper sizing of the pump is of interest, rheological tests should be carried out under similar shear rate and stress conditions.

Some of the commonly used tests for characterizing rheology of semi-solid materials are:

• flow behavior of non-Newtonian materials (to determine yield stress, shear thinning and/or thixotropy)
• strain sweep (to determine viscoelasticity) and critical strain (to determine how stable a system is)
• creep/recovery (to determine relaxation times, zero shear viscosity and viscoelastic properties to determine strength of bonds)
  

Available rheology products:

	Bohlin Gemini HR nano and Bohlin Gemini II An advanced range of modular and compact rheometers with 'fluids to solids' capability. They are optimized for both strain controlled and stress controlled rheology measurements, can accept an extensive range of temperature control options, and have wide application in research and product development.
	Bohlin CVO A flexible rheometer system suitable for research, product development, and quality control rheology measurements.
	Bohlin V88 An easy to use viscometer which provides accurate viscosity measurements at single or multiple shear rates.
	Rosand RH7/10-D Advanced, powerful floor standing capillary rheometry units suitable for research and product development rheology.
	Rosand RH2000 Bench-top, capillary rheometers suitable for rheology measurements for research, product development, and quality control.