Breaking and Rebuilding...Maybe
Thixotropy investigations are very helpful to provide insight into the effects of product manufacturing processes, handling, and dispensing. In this assay, the sample is subjected to various shear rate steps to model situations that typically cause thinning. The extent and rate of rebuilding, or lack thereof, is then determined. Depending on the product purpose, the desired extent and rebuild time after shear thinning will range between immediate and never. This test can also be used to model manufacturing processes such as pumping materials through pipes or dispensing from tubes. In particular for topical products, understanding thixotropic properties are important during application onto and penetration through skin that may affect efficacy.
Figure 1 shows two creams after being subjected to a brief increased shear have different rebuild curves that may effect product properties and performance.
Figure 2 is a simple example of designed thixotropy - toothpaste. The product stays in the tube until the squeeze force (yield stress) is achieved and product flows, yet quickly rebuilds to remain on the toothbrush. Ketchup and paint are designed to have highly controlled thixotropic properties.