Feel of skin cream

Dr. Neil Canter, Contributing Editor | TLT Tech Beat March 2010

Three phases of friction are major factors where the skin cream is applied to the skin.

KEY CONCEPTS
• Selection of a skin cream is based in part on its effect on the tribological properties of the skin surface.
• Skin cream application moves through three phases: a hydrodynamic or mixed lubrication regime, boundary lubrication and reduction in the coefficient of friction.
• A number of parameters, including contact angle, change in skin roughness and friction, are evaluated to determine the tribological properties of the skin cream on the nanoscale.

As an old surfactant chemist, I found out early in my career that there is a direct relationship between the components used in personal-care products and in lubricants. Formulators in both markets share components such as emulsifiers and lubricity additives. The latter are known to the personal-care industry as emollients.

One personal-care product that we use everyday is skin cream. While we may not pay much attention to the product’s content, skin creams are complex emulsions that contain such additive types as humectants (attract and hold water in the skin) and occlusives (form a layer on the skin surface and moisturize by retarding water evaporation). In a sense, a skin cream has comparable components to an emulsified oil used in metalworking applications.

STLE fellow Bharat Bhushan, professor and director of the Nanoprobe Laboratory for Bio- and Nanotechnology and Biomimetics at The Ohio State University in Columbus, Ohio, says, “Skin cream is used for a number of reasons. One of the main uses is to moisturize dry skin. Skin cream also helps to soften skin, as a rough feel is no good.”

Bhushan maintains that there are a number of factors to consider in selecting a skin cream from a scientific viewpoint. He says, “Skin cream performs by altering the tribological properties of the skin surface. These include the roughness, friction and adhesion of the skin surface.”

Friction is a major factor when the skin cream is applied. Bhushan says, “An individual places skin cream on his or her skin leading to the generation of friction between the cream and the finger, as shown in Figure 1. Sensations in the finger are sent to the brain indicating how the cream is perceived. Initially, skin vibrations will be produced that are minimized once the cream becomes smooth.”


Figure 1. Application of skin cream onto skin is a complicated process in which the friction generated between the skin and the individual’s finger is a major factor in determining how the cream is perceived. (Courtesy of The Ohio State University)

Bhushan indicates that there are three phases in this process. Initially, the skin cream film is quite thick so that lubrication is in the hydrodynamic or mixed lubrication regime. This means that the skin feels slippery, and the coefficient of friction is low. As the cream is thinned out on the skin, a boundary lubrication regime is created in the second phase. At this point, the skin becomes more moist and smooth, leading to high adhesive forces and friction. Finally, either through water evaporation or some other physical or chemical change in the skin surface, a reduction in the coefficient of friction and adhesive force is achieved in the third phase.

Bhushan summarizes, “The progression through the three phases of friction is dependent upon the thickness of the cream, time and finally the quality of the cream.”

A better understanding of how skin cream interacts with skin at the nanoscale would provide greater insights into the three phases of friction and, as a consequence, determine which components in a skin cream provide better performance characteristics. Such a study has not been conducted until now.

PARAMETERS EVALUATED
Bhushan and his research group evaluated the tribological properties of skin cream on the nanoscale at ambient temperature through the use of an Atomic Force Microscope (AFM). The skin cream examined is a well-known commercial product, Vaseline Intensive Care Lotion. Studies were conducted on the raw skin itself and on skin treated with the cream.

The first parameters evaluated are how the contact angle and the roughness of the skin change when the cream is applied on the skin. Bhushan says, “Contact angle is a very important factor because it measures how readily the skin repels water. The key angle is 90 degrees because contact angles above this figure indicate skin is sufficiently hydrophobic to repel water, while a lower angle indicates that the skin attracts water.”

The contact angle for skin treated with skin cream is lower, which means that the cream imparts hydrophilic properties to the skin. Surface roughness is also lower, indicating that the cream improves the condition of the skin surface.

Friction increases upon application of the skin cream because of its impact on the AFM probe. Bhushan says, “As the probe encounters the skin cream film, the hydrodynamic drag rises as the tip tries to move through the cream, leading to an increase in friction. This force is not seen with the skin itself.

The thickness of the cream on the skin surface also is a factor in influencing the magnitude of friction seen. The researchers applied skin cream with a thickness between 80 and 450 nanometers. These values are approximate because the cream is continuously being absorbed by the skin.

There is a direct correlation between cream thickness and friction. Bhushan says, “Cream thickness is directly related to viscosity, which means that a greater value leads to higher friction.”

Two important external factors, humidity and temperature, also affect the tribological properties of skin cream. Intuitively, the goal is to moisturize skin, which would suggest that higher levels of humidity are helpful. But in actuality, humidity is a negative factor.

Bhushan explains, “In a high-humidity environment, a meniscus force is formed between the condensed water and the lipids constituting the surface of the skin. This increases the thickness of the fi lm present on the skin, resulting in an increase in friction.”

Friction is higher with cream-treated skin because some of the components used to prepare the cosmetic are humectants that attract water. Result: A higher concentration of water is hydrogen-bonded to the humectants, leading to a thicker film.

When asked about the properties of a good skin cream, Bhushan indicated that viscosity, thickening and rate of applications all must be considered. He adds, “The key element is how long the skin remains smooth or moist. A superior skin cream will maintain this effect for more than a day, while an inferior product may last for less than an hour.”

Bhushan intends to do further evaluation of other skin creams and their components. Additional details can be found in two recent articles (1, 2) or by contacting Bhushan at bhushan.2@osu.edu. 

REFERENCES
1. Tang, W. and Bhushan, B. (2010), “Adhesion, Friction and Wear Characterization of Skin and Skin Cream using Atomic Force Microscope,” Colloids and Surfaces B: Biointerfaces, 76 (1) , pp. 1–15.
2. Tang, W. and Bhushan, B. (2009), “Friction, Adhesion and Durability and Influence of Humidity on Adhesion and Surface Charging of Skin and Various Skin Creams using Atomic Force Microscopy,” Journal of Microscopy, in press, doi: 10.1111/j.1365-2818.2009.03362.x.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.