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Monitoring structure rebuild (thixotropy) following extrusion from a bottle, tube or spray head

Many consumer products are packaged in tubes or bottles where product application involves pumping the product through a nozzle. Such products tend to be shear thinning products where the viscosity drops during the extrusion process due to the increasing shear rate, and then recovers on exiting the orifice as the shear rate is reduced. The shear rate encountered during this process is related to the radius, r of the orifice and the volumetric flow rate Q.

The parameter n is the power law index, which is 1 for a Newtonian liquid and between 0-1 for a non-Newtonian fluid. This value can be readily attained from a variable shear rate test by fitting a power law model to the resultant data.

By measuring the volumetric flow rate (volume dispensed in given time) and the internal radius of the orifice, it is possible to estimate the shear rate encountered during extrusion. 

Because many consumer products including foodstuffs, coatings and toiletries/ cosmetics can have very delicate microstructures, these structures are easily broken down during the extrusion process and may not recover their original structure until a finite time has passed. Materials which have a time-dependent structural recovery or rebuild are termed thixotropic. By calculating the shear rate encountered during extrusion and using this in the intermediate stage of a step strain-shear rate test it is possible to mimic structural breakdown resulting from extrusion.

 By immediately following structural recovery through the elastic modulus G’ as a function of time it is possible to determine structural recovery of the system and ultimately the structural integrity of the product at time of use. This can be important in terms of physical appearance (slump resistance), in use texture or product functionality, for example the ability to cling on to a vertical surface.