The tribology of personal care

Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge December 2009

Researchers are improving cosmetic products with the help of novel tribometer measurements.
 

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While many of us in the tribology community concern ourselves with the sliding contact of metallic components, including their lubrication with oil and all the associated sealing, condition monitoring and additive technologies, we should not forget that tribological issues go far beyond this into the most personal aspects of our everyday lives.

Tribological problems in the personal-care industry abound, whether they be in the design of hair-care products (which probably also involves some condition monitoring), the development of nail polish, where adhesion and wear phenomena are important issues, or the formulation of skin creams where the lubrication behavior of the cream during application can determine the degree of consumer acceptance.

In a recent paper published in Tribology Letters, Ken Nakano and his co-workers from Yokohama National University and the Kao Corp. in Japan have looked into the tribological phenomena involved in the application of cosmetic foundation. A mainstay of the personal-care industry has been the use of sensory panels that report on the tactile sensations involved during the application of various skin products.

While this information is useful, it tends to be subjective and vague and involves a large number of panel participants. Nakano’s work represents a step toward more objective prediction of sensory response. It involves the correlation of sensory-panel data with measured tribological quantities from a specially designed tribometer by means of multiple-regression analysis.

The sensory panel was asked to rub a number of formulations resembling cosmetic foundation into the inner surface of the wrist with a forefinger and to report the sensation and comfort level according to smoothness, silkiness, velvety feeling, softness and skin-adhesion ability. The formulations consisted of a mixture of mica particles (10 x 0.2 μm platelets), acrylic spheres (5 μm dia.) and silicone oils in different ratios.

The tribometer involved sliding a silicone rubber sphere against a silicone rubber plate (both with moduli of 0.5 Mpa), both sides being supported by double-cantilever springs, with lasers being used to monitor displacement in both x and z directions. A number of different experiments were carried out, including a dry-sliding control, a fully lubricated test, with the sample on the plate, a starved lubrication experiment, with the sample on the sphere, and another fully lubricated experiment, with grooves on the plate, to simulate a fingerprint structure. 

The quantities measured included static and kinetic friction and also dynamic-response parameters such as the time required to reach asymptotic values of friction and the amplitude/degree of damping of the vibrations due to natural system resonance and the oscillations caused by sliding over the simulated fingerprint structure. A minimum value of kinetic friction was found for a composition capable of forming a layered structure of spheres and platelets.

By means of multiple-regression analysis, the sensory-panel data were correlated with the tribometer measurements. A reasonable degree of correlation between the two datasets could be obtained despite a simple linear correlation being used to investigate the very non-linear behavior of our senses.

Meanwhile, some interesting conclusions could be drawn. For example, skin-adhesion ability was strongly correlated to the degree of damping of the natural system frequency. A surprise was that comfort was not simply related to static and kinetic friction. Greater comfort in certain cases was reported for higher static friction and also greater vibrational amplitudes. The next steps will involve the use of non-linear approaches such as neural networks to better model the sensory response.

It is clear to all of us that tribology is far from being a straightforward field. The tribology of sensory phenomena represents a significant added level of complexity, with many challenges and opportunities, in terms of greater understanding of our senses and improved products for the personal-care industry.

FOR FUTHER READING:
Horiuchi, K., Kashimoto, A., Tsuchiya, R., Yokoyama, M. and Nakano, K. (2009), “Relationship Between Tactile Sensation and Friction Signals in Cosmetic Foundation,” Tribology Letters, 36 (2), pp. 113-123.


Eddy Tysoe is a Distinguished Professor of Physical Chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.


Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland. Both serve as editors-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.