Soft tribology using rheometers: a practical guide and introduction

Soft tribology (i.e., the measurement of friction as a function of speed between two compliant surfaces) has found applications in food science and there is a growing wealth of theoretical and practical knowledge of fundamental mechanisms of lubrication as well as increasingly strong correlations between tribology and sensory data. Soft tribology is generally conducted using either commercial or in-house built tribometers; however, the recent decade has seen a rise in the use of rheometers with tribology attachments. As such, soft tribological measurements using rheometers with tribological attachments are fast becoming a tool in the food scientists? toolbox when it comes to measuring mouthfeel, but the lack of fundamental studies and standardized measuring protocols makes interpretation and comparison across studies challenging. This review aims to provide an introduction to the basics of soft tribology as well as summarize current methodologies and fundamental knowledge on using rheometers for tribological measurements. Based on current literature, knowledge gaps and potential avenues for future research have been identified. These include investigations on hydrophobicity of surfaces, surface wear (running-in), cleaning procedure of the attachment and tribopairs, speed (range and method of increase/decrease) and measuring system configuration. Leaving the aim and design of experiments at the discretion of the individual researcher, recommendations for food scientists aiming to conduct studies on soft tribology of food are given, with a focus on reporting for better comparison between studies