Understanding the effects of processing conditions on the formation of lamellar gel networks using a rheological approach

Lamellar gel networks are multiphase systems which form the basis of many cosmetic and pharmaceutical cream products, thanks to their superior stability compared to typical oil-in-water emulsions, and highly desirable rheological properties inferred by the interconnected structure. There has previously been considerable interest in the formulation of lamellar gel networks, but little interest has been given to the effects of processing conditions on the formation of the desired structure, or the possibility for process optimisation through understanding power consumption. The evolution of the microstructure of an incipient lamellar gel network during processing was investigated by varying the temperature, vane speed and time using a rheometer equipped with a four-bladed vane in cup geometry. Torque and vane speed measurements were recorded at 2 Hz for the duration of the experiment, from which apparent viscosity (taken at a reference shear rate of 200 s^-1) and power input were calculated. Samples were then characterised by yield stress and flow curve measurements to determine the impact of processing conditions on the final product microstructure. Increasing vane speed increased the maximum apparent viscosity achieved and yield stress of the sample, and reduced the time taken to reach the peak apparent viscosity. However, the increased power requirements from the higher vane speed were not counteracted by shorter processing times. Increasing the temperature reduced the rate of apparent viscosity increase but did not affect the yield stress of the final lamellar gel network, offering a reduction in power consumption due to a lower apparent viscosity for the majority of the process.