Let things flow naturally forward in whatever way they like.
—Lao Tzu
Rheology is the study of flow, and it plays a pivotal role in the performance of beauty and personal care products. We deal with it constantly in our everyday lives, from squeezing a toothpaste tube to banging a ketchup bottle. In beauty, it is a key to successful products, and it determines what happens when applying lipstick, the aesthetics of rubbing in a cream and how the suspension of beads in a gel are stabilized. Multi-day conferences even deal with the complexities and nuances of the subject.
Whenever a force is applied to a material causing it to deform or flow, rheology offers a quantitative description of what is happening. Squeeze a rubber ball and there will be a deformation—the relative displacement of points in a body when a force is applied to it. If you stretch a rubber band and then let it go, it has experienced a reversible deformation.
Viscosity is a measure of resistance to flow and is the ratio of the amount of force divided by the amount of movement. Viscosity has some strange-looking units. The most common is the centipoise (cP), which in fundamental units is millipascal sec. We use centipoise, one hundredth of a poise, rather than the poise itself because it is closer in value to most of the systems we measure. There is fortunately one simple thing about this unit: the viscosity of water is 1 cP. By comparison castor oil is 1000 cP and honey is 10,000 cP.
There seems to be no limit to the exotic terms used in rheology. For example, kinematic viscosity is the absolute viscosity of a liquid divided by its density at a specified temperature, thinness is the reciprocal of viscosity, and diffusion is inversely related to viscosity. In defense of these terms and more, they all describe phenomena that we encounter in real life or in evaluating beauty personal care products. Giving an exact meaning to “thin” actually makes the description of a material more precise.
Understanding Viscosity
The easiest way to visualize many different features of viscosity is to use graphs. Figure 1 shows graphs of the common viscosity responses to an applied force. The most straightforward type of flow is called Newtonian, which has a linear relationship between applied force and motion. The more force applied to a Newtonian fluid, the more it moves—and the viscosity of a Newtonian fluid is a constant. Water, glycerin and mineral oil are examples of Newtonian fluids.
The viscosity of non-Newtonian fluids varies with the material and conditions. With a pseudoplastic fluid, the viscosity decreases as more force is applied. This effect is called “shear thinning.” Such behavior allows a lotion to flow through a pump and spread on skin, and therefore, the majority of beauty products are pseudoplastic. The opposite property, dilatant or “shear thickening,” can be a disaster in many scenarios. If, in mixing an emulsion, more shear creates greater resistance, the machinery would break.
Thixotropic materials have different flow characteristics when the force is removed than when the force is applied. A recovery is needed because a structure has been disrupted, like a polymer network held together by polar bonds or where the molecular chains had been interlocked. Many beauty products are thixotropic, and it is usually desirable.
Viscoelasticity is a property combining both viscous and elastic behavior, and the analysis of such systems can be complex. Biopolymers are viscoelastic—think Silly Putty. Silly Putty’s unique rheology is due to the presence of 4% dimethicone in its formulation.
Delving Deeper
Yield value occurs when a minimum force is required to start movement. It is very important when suspending particles and is very different from viscosity. A formula can be very thick but not able to suspend while a relatively thin formula may have good suspending properties, often due the presence of a polymer gel network. Honey has high viscosity and no yield value, while mayonnaise has much lower viscosity but high yield value.
The graphs in Figure 1 do not tell the whole story of rheological behavior. For example, it is common to thicken the water phase of an emulsion or to make a gel with a carbomer as the thickener. In this case, another series of graphs may be needed. There can be a graph of viscosity vs. percent carbomer, another at a particular percent carbomer vs. pH, yet another vs. temperature. Plus, there are many variations of materials under the generic description “carbomer,” each with its own behavior. In addition, they each perform differently in the presence of salt or surfactants, so the data sheets and specifications must be considered carefully to choose the correct type.
Measurement of rheological properties has evolved. The standard has been the Brookfield viscometer for basic measurements. Different models handled different viscosity ranges, and a heliopath stand can be used to spiral into very thick materials. Now, very sophisticated rheometers are available to measure a range of shear stress and shear rate. The sophisticated models can be processed by computer software that can create multiple graphs using real data, as well as mathematical models for complex analysis.
Rheology has assumed a prominent position as a key component of beauty product development, evaluation and performance. While the subject is formidable and filled with special terminology and many graphs, everyone in our industry would do well to be familiar with at least the basic ideas to appreciate its importance.
Steve Herman is president of Diffusion LLC, a consulting company specializing in technology development in the beauty and cosmetic industry. He is a principal in PJS Partners, offering formulation, marketing and technology solutions for the personal care and fragrance industries. He served the New York Society of Cosmetic Chemists (NYSCC) as chapter chair in 1992 and 2013, and is an adjunct professor in the Fairleigh Dickinson University masters in cosmetic science program. He also is a fellow in the Society of Cosmetic Chemists (SCC).
