Insights to the tribology of toothpaste

Texture is a commonly used term that encompasses the multi-dimensional sensory characteristics influenced by a material’s structure at both macro and micro-levels. Also, the rheology and surface properties of products that come into contact with human skin, such as food and oral care items considers the texture preferences. Oral contact is particularly significant in shaping the texture sensation of these products. One specific sensory aspect related to texture is mouthfeel, described as the tactile property perceived from the moment a food (solid, semi-solid, or fluid) or a beverage is placed in the mouth until it is swallowed. Various studies have explored the use of rheology and tribology as imitative techniques to capture tactile mechano-sensations, but many of these approaches fall short in recognizing that texture is a multi-modal sensory feature unlikely to be fully captured by a single property.

Additionally, once a food or oral care product enters the oral cavity, saliva becomes involved. Consequently, the perceived sensory property shifts from that of the product itself to its mixture with saliva. Importantly, the controlling mechanism of texture sensation is likely to shift from rheology dominance to tribology (surface properties) dominance in the presence of saliva. This complexity may explain why the traditional approach of instrumentally measuring the sensory properties of pure material (such as food) is often not directly applicable to oral sensory perception. Soft surface tribology, especially in the context of biotribology like oral lubrication deals with these aspects. While the origin of lubricating effects in both cases lies in the lifting pressure between two surfaces in relative motion, the friction on a soft surface can significantly differ from that on a hard surface, particularly when moving against a ball, owing to Hertzian deformation. Adding complexity to the scenario is the presence of suspended particles in the lubricating fluid, a situation highly pertinent to oral processing.

In oral processing, suspended particles such as oil/fats, starch granules, polysaccharide particles, and protein aggregates are common in many food formulations. Additionally, suspended filler particles are essential in toothpaste formulations. The effects of suspended particles hinge on their size, surface properties, and morphology. These particles can engage in three distinct friction mechanisms: sliding, rolling, or exfoliation. In cases of oral processing, the first two mechanisms, sliding and rolling, are highly likely, while exfoliation is less probable for filler particles in toothpaste applications. This intricate interplay of factors underscores the complexity of understanding the frictional dynamics involved in oral sensations and product formulations.

Reference:

[1] Cai, H., Li, Y. and Chen, J., 2017. Rheology and tribology study of the sensory perception of oral care products. Biotribology, 10, pp.17-25.