Kiss and make up: Quantifying the lubrication of lip balms and lipstick

Here at the Centre for Industrial Rheology kissing couplewe pride ourselves on how we quantify life’s pleasures to help our customers, the researchers and developers of the world, make their products a little more pleasurable. So, following on from our work on the sensory aspects of indulgent dairy products, moisturizing lotions and intimate lubricants, we thought we’d turn our attention to kissing – more specifically, to products that may or may not improve on the kiss, and the feeling of the lips in general: lip balms and lipsticks.

The sensory qualities of balms and lipsticks arelip-balm crucial to their acceptance by consumers. This is evident in the range of terms used by sensory panels to describe their qualities, such as smoothness, greasiness and spreadability. Lips are sensitive tactile areas with a high density of nerve endings, so the mechanical shearing forces from lip movements can be felt acutely. The friction-reducing abilities of any product on the lips will therefore impact greatly on how that product is perceived.

Tribology of lip balms and lipsticks

Tribology, from the Greek word “tribo” meaning “to rub” is the study of friction, lubrication and wear of closely-interacting surfaces. We compared the lubricating abilities of four lip products: two balms and two lipsticks. Tribology testing typically entails measuring the friction between closely interacting surfaces, lubricated or otherwise, often across a range of sliding speeds under defined “squeeze” loads. In this case the surfaces were a steel-on-soft-elastomer contact, lubricated with a layer of the product tested. “Stribeck Curves” of coefficient of friction as a function of sliding speed were generated:

lip balm and lipstick tribology

The increase in friction with sliding speed seen in the Stribeck curves seen here suggest that hydrodynamic lubrication is in effect, where a defined fluid film separates the surfaces. The striking thing about our results here is that as sliding speed is increased the lubricating qualities of the four products diverge so significantly, the two lip balms showing the greatest differentiation, with lip balm B showing over six times higher friction than lip balm A.

The climax of our study is…

From this we conclude that all the products tested will perform similarly for slow sensual kissing but as the passion rises and the pace picks up Lip Balm A will out-lubricate all others. The researchers are now looking for volunteers for extensive trials.

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