Every woman wants to see results from her anti-wrinkle serum. But before she gets what she wants she has to want to use it—again and again. Few attributes impact her decision to try, apply and buy again more than a product’s rheology profile. How a lotion looks and feels on the skin is nearly as important as how it smells and works on the skin.

With that in mind, the New York Chapter of the Society of Cosmetic Chemists held a two-day rheology symposium in March. It attracted more than 235 attendees, including formulators from companies such as Energizer, Avon and L’Oréal.

“By all accounts, the two-day 2014 NYSCC Rheology Symposium was a tremendous success,” noted Conference chairperson Joe Albanese of 3V. “It surpassed the first one-day Rheology Symposium held two years ago at the Liberty Science Center in Jersey City. I was told it that this year’s symposium drew the largest attendance for a single scientific education program held by the New York Chapter. As the chairperson for both of these symposiums, I couldn’t be prouder of my team of volunteers and their accomplishment to put together a world-class symposium.”

It was a fast-moving symposium, too, as Day 1 technical session speakers were given just 10 minutes to make their points.
“No one in the audience was caught napping. One attendee, Steve Herman, called this rapid-fire approach ‘speed dating’ for rheology.  It was very popular,” observed Albanese.

An All-Star Lineup of Speakers
On Day 2, the program followed a more traditional format of 40-45 minute in-depth presentations by eight renowned scientists in the field.

“It was an all-star lineup and they were equally fantastic. The more we learned the more we realize how much more there is to learn,” commented Albanese.

Albanese opened Day 1 with a presentation on rheology modifiers, specifically, associative thickeners such as acrylates/vinyl isodecanoate crosspolymers, which he noted are much better than traditional thickeners such as xanthan gum, as the crosspolymer is an excellent emulsifier with good thickening efficacy, high suspending properties, improved electrolyte tolerance and enhanced water-resistance for sunscreen actives. The material has applications in clear styling gels, surfactant systems, O/W emulsions and clear hydroalcoholic hand sanitizers. It may be dispersed in the oil or water phase and is functional over a broad pH range.

“Consumer perception depends on lubricity,” explained Rhyta Rounds of Fluid Dynamics, who provided an overview of lubricity measurements. But she also reminded the audience that the consumer’s skin covers a broad range of moduli that depends, among other things, on age, health and diet. Rounds explained how lubricity analyzers are used in lubricity tests and provided data demonstrating how two moisturizing lotions behave very differently on skin.

Daphne Benderly, Presperse, provided attendees with a look at how other industries view rheology. For example, viscosity plays a critical role in engine oil development, with a high viscosity index meaning a relatively small change of viscosity with temperature. In the coatings industry, shear rate is critical, since a thin paint is easier to apply, but drips more. Therefore, chemists study shear-thinning behavior to design paints that are thick enough not to drip and thin enough to spread easily. Benderly concluded her presentation by noting that concepts and measurement techniques from other industries are transferable to the beauty industry.
Christina Teng of Princeton University explained how foams could be employed for nanoparticle delivery. Inspired by whipped cream, her team developed temperature-responsive foams that contained cetyl alcohol, stearyl alcohol, polysorbate 60, propylene glycol, an active ingredient, potassium hydroxide and water.

According to Teng, foam volume is greatly affected by ethanol concentration and just a small window of ethanol concentrations produce acceptable foams. She explained how Tween 20 produced higher foam volumes compared to Tween 60 and 80, as well as PEG and PVA.

Yield stress; i.e., the stress above which a material flows, plays a key role in product stability, processing and sensory characteristics, noted Jeffrey Martin of Johnson & Johnson. He reviewed various test methods including flow curve, steady shear and amplitude sweep, before recommending researchers use amplitude sweep (0.1-1 rad/s) as it is fast, easy and robust without requiring model fitting.

How Does It Feel?
How it flows is one thing; more important is the relationship between rheological properties and skin feel, which was the topic of a presentation by Geng Li of Energizer, who defined the skin feel index and spectrum descriptive analysis. Li also gave attendees new things to think about when developing products.

He explained that psycho-rheology, a term coined by M.R. Wegener of Bristol University in 1997, is the study of the relationship between rheological and sensory properties of a material. Via predictive modeling, psychorheological studies predict what sensory characteristics and attributes appeal to the consumer.

“This saves time and money and makes product screening faster,” insisted Li.

The speaker also described how Brummer’s window could be a useful tool to not only predict a product’s skin feeling but also to fulfill the consumer’s expectations.

Gail Vance Civille, Sensory Spectrum, reminded the audience that when making rheological measurements, they must start with the consumer—and was quick to assert that sensory measurements using panelists is a real science. Sensory properties are the characteristics perceived through the senses, she explained, and not the liking or preference of products.

“We sometimes think that consumers aren’t very bright,” observed Civille. “I just think that we don’t ask the right questions.”

She gave examples of how expert panels and consumers can provide technical guidance when formulating products such as shampoo, hair gel and mousse.

“They can provide rich data for product development and marketing,” Civille noted.

Bharath Rajaram, TA Instruments, reviewed the rheological characterization of personal care products by detailing three different test methods: Tribology; interfacial rheology and orthogonal superposition. Tribology, the study of interacting surfaces in relative motion, can be used to study solid and liquid lubrication; lubricating oils and greases; friction, wear and surface damage; and surface modifications and coatings, explained Rajaram. As a result, tribology has a wide range of applications in the personal care industry. He explained how researchers can use interfacial rheology to compare such diverse products as makeup removers and hair sprays. Finally, orthogonal superposition enables researchers to monitor structural changes in materials, according to Rajaram.

The Power of Polymers
But not all numbers are equal, warned Nava Dayan of Dr. Nava Dayan LLC.

“Interpretation of data can be far from the truth,” she warned the audience. “We all arrive with different backgrounds.”

Dayan noted that study design, protocol and cells used can greatly modify results. She warned that infinite dosing may not reflect in use conditions and that thicker formulations may sometimes enhance the penetration of topical agents when applied “in use.” Moreover, these tests have their limits, Dayan cautioned.

“Diffusion cell studies should only be used to differentiate between formulations and, in the current set-up, correlation to in vivo is limited,” she explained. “This is my point; diffusion cell studies do not replace in vivo studies.”

Anna W. Tai, Merck & Co., explained how rheology and viscosity testing were used to evaluate emulsion stability and aesthetic properties. The effects of the surfactant level and the process parameters on the formulation scale up were characterized using tools such as Brookfield viscometer and TA Instruments’ AR2000 rheometer. Researchers found that the data show high correlation to the process conditions and surfactant levels, and that the data could be used for formulation and process optimization.

Kishore Shah of Polytherapeutics provided details on a hydrogel graft copolymer (INCI: dimethylacrylamide/acrylic acid/polystyrene ethyl methacrylate copolymer), which is said to provide excellent bioadhesion to skin and diffusion-controlled release of actives. As a result, the product can provide activity near the application site or systemically.

This “virtual patch” has applications in cosmetics (foundations, makeup and skin care), dermatological formulations and transdermal drug delivery.

Polymers of a different sort were the topic of a presentation by Brian Figura of Lubrizol Advanced Materials. Surfactant-Activated Microgels, or SAM polymers, are nonionic materials that don’t need to be neutralized, which enables formulators to go straight from batch to continuous process. SAMs create a yield stress fluid with smooth flow to provide uniform rheology and high optical clarity over a broad pH range. Figura also noted that SAMs may be tailored to enable silicone deposition and hair conditioning benefits in 2-in-1 shampoo formulations.

The always-entertaining Mark Chandler, ACT Solutions, reminded the audience that rheology plays a critical role in convincing consumers to repurchase products.

“We are building a long-term relationship (with consumers) and rheology plays a big role in developing products with staying power,” he explained.

Rheology, then, can impact a product’s value, price, appearance, and performance—but in order to create an effective system, it requires the cooperative efforts of trained and untrained panelists, Chandler concluded.

But thanks to an insightful, lively event from the New York Chapter of the SCC, attendees had the perfect platform to get their own questions answered by a range of industry experts.