Fingerprinting formulation texture
AUTHOR: Seher Ozkan, Research Scientist, Ashland Specialty Chemicals
Personal care formulations come in a wide variety of product formats, and their rheology/texture is one of the key factors influencing product functionality as well as consumer acceptance. An enjoyable sensorial experience is a key factor in repeat purchase patterns of a given personal care product by consumers. With the rise of online influencers, bloggers and social media, manufacturers need to develop products that delight all consumers and impart a positive emotional connection with little or no exception. Sensory panels are still the gold standard to describe and assess the sensorial customer experience; however, panels are difficult to set up and costly to run and maintain. There is certainly a place in the development process for instrumental techniques with good correlation to sensory assessments.
For instance, based on the current market trends, a hair conditioner, which is a complex fluid, should have a thick texture and a mounding look on the palm when first dispensed, a rich and pleasant feel during spreading on wet hair and slippery and soft feel during rinsing. All these desirable traits are either directly influenced by the rheology of the product or can be traced indirectly by monitoring rheology. Therefore, in the product design and development process, formulators rely on rheological techniques, regardless of the blind spots of conventional rheological methods [1]. Conventional rheological techniques are generally aimed at collecting linear viscoelastic data of a cosmetic product, which provides valuable insights on the architecture or spatial configuration of the microstructural components of the formula in its ‘at-rest’ state. However, linear rheological data fail to describe the textural experience associated with large and fast deformations during daily consumer application of personal care products. If we go back to desirable hair conditioner traits, linear viscoelastic data may be appropriate to trace the thick texture and mounding look on the palm, since the product is pretty much at rest. But, the feel during spreading on wet hair is experienced under large and fast deformations of the product, where the microstructure goes through a non-reversible evolution. Conventional rheological techniques are inadequate to capture the textural transitions experienced by the consumer. Recently, a non-linear rheological technique termed LAOS, was utilized to capture the textural expression perceived by consumers under large and fast deformations [2]. The LAOS test is also a sinusoidal oscillatory flow experiment in which the amplitude of the strain input is selected to be large enough to deform the material beyond the linear viscoelastic limit. The large strain amplitude deforms and changes the formulation architecture of the complex fluid. Therefore, the stress response becomes a function of strain. In practice, the sample is loaded between two parallel discs that are separated at a known gap. The rheometer oscillates one of the discs to a defined angle, deforming the sample under applied strain (distance) in alternating directions, whereas the torque (stress) data are collected simultaneously to capture the sample’s response to deformation. In this method, the non-sinusoidal output (stress) signal is assumed to be formed by superposition of a series of sinusoidal waves and can be expressed using Fourier expansion. In the Fourier expansion, higher harmonic contributions emerge and grow because of the increasing non-linearity in the stress response. Intensities of these higher harmonics can be used as a comparative tool to study the shear-induced microstructural changes of materials [2].
Aside from the potential applications of linking microstructure with higher harmonics, another practical tool used to plot the data from the LAOS technique is the Lissajous–Bowditch curve. The LAOS experiments can be set up in a way that it starts with small deformations and progressively moves up to larger deformations with each successive twist. For practical purposes, when the parallel plates of the rheometer twist to the left, the shear rate is negative; when twisting to the right, the shear rate is positive [1, 2]. The collected stress data as a function of strain input can be plotted into a composite graph. This special format was first proposed by the American mathematician Nathaniel Bowditch and later used by the French physicist Jules Antoine Lissajous. For the remainder of this article, we simply refer to these curves as Lissajous plots [1,2]. Figure 1 contains idealized diagrams of Lissajous plots for either purely elastic, purely viscous or a typical viscoelastic material. The Lissajous curve of a linear viscoelastic material for a given strain amplitude takes on the form of an ellipse, while it is a diagonal straight line for an ideal elastic material, and it is a circle for a Newtonian material in the stress vs. strain plot. If we plot the same data in the form of stress vs. shear rate,the elastic and viscous curves become inverted (Fig. 2). The evolution of the shapes and character of stress vs. shear rate curves in a composite graph of gradually increasing deformation rates provides a visual tool to understand how the texture of the product changes during application.
Figure 1 Lissajous plots for ideal materials.
Figure 2. Projection of curves from strain axis to strain rate axis.
These composite Lissajous plots provide a rheological fingerprint that can be used to differentiate the rheological behavior of formulations. These fingerprints of full formulas along with their selected ingredients are utilized to both analyze the thickening mechanism and build formulas with desired rheological attributes for skin, hair and home care formulations. Lissajous plots help to delineate the effects of key formulation variables, such as the intrinsic texture of base thickener systems and the influence of auxiliary ingredients of formulation on this texture. The plots provide formulators with insight into the sensorial behavior of the tested formulation and how certain ingredients may affect the rheology more than others. The technique is very useful to formulators who want to make changes to a certain formulation without changing the overall characteristic of the product texture. Take a look at sample fingerprints given below (Figure 3).
Figure 3. Selected fingerprints for various chemistries.
Inasmuch, this practical technique enables formulators to design formulas with desired textural attributes for cosmetic products. The Lissajous plots contain quantitative information, but more importantly they provide a visual depiction of the rheological breakdown of the products as a function of shear, temperature and colloidal interactions. The LAOS technique was specifically chosen to gain insight regarding the textural evolution under large and fast deformation rates that better represent the consumer experience in vivo during application. For example, rinse off hair conditioners are complex liquids which structure is designed to give the consumer both an enjoyable sensorial experience and good combability. A hair conditioner is instantly recognizable by its texture and spreading ability as well as appearance, all governed by rheological properties. The palette of ingredients is quite limited to create the lamellar structure necessary for technical performance and texture but as consumers demand for light conditioning is increasing globally, new formulations approaches are required. Conditioners contain cationic surfactants that self associate to form a lamellar phase that deposits onto negatively charged hair. A minimum amount of cationic surfactant is necessary to reach an acceptable viscosity and yield stress. Addition of hydrophobically modified hydroxy ethyl cellulose in the conditioner leads to unique fingerprints at low shear rates due to the hydrophobic network yielding (Figure 4, 5).
Figure 4. Diagram depicting the lamellar gel structure and their fingerprints.
Figure 5. DSC curves and cross-polarized light microscopy images for three hair conditioners: low gel phase without polymer, low gel phase with HMHEC and normal gel phase without polymer.
Probing the textures of composite skin care formulations using large amplitude oscillatory shear. T Gillece, RL Mcmullen, H Fares, L Senak, S Ozkan, L Foltis. J. Cosmet. Sci 67 (2), 121-159
Ozkan, S., Alonso, C. and McMullen, R.L. (2020), Rheological fingerprinting as an effective tool to guide development of personal care formulations. Int J Cosmet Sci, 42: 536-547. https://doi.org/10.1111/ics.12628
Seher Ozkan
Research Scientist
Ashland Specialty Chemicals
Currently, she works as a Research Scientist for Ashland Specialty Chemicals (formerly ISP), where her major research areas are correlation of rheological, mechanical and colloidal properties of personal and home care formulations to in vitro performance test results (sensory correlation, bioadhesion, tribology, etc.). Previously, she worked on areas such as process design for continuous twin screw extrusion of complex fluids and solubility enhancement via pharmaceutical hot melt extrusion. Her work is published in Biomaterials, Journal of Biomedical Materials Research, International Journal of Cosmetic Science, Journal of Applied Polymer Science and in meeting proceedings such as AAPS, MRS, SOR, ACS and NYSCC. Dr. Seher Ozkan has B.S. and Ms. Sci. degrees in Mechanical Engineering from Istanbul Technical University (ITU) and Ms. Eng. And Ph.D. degrees in Chemical Engineering from Stevens Institute of Technology (SIT), Hoboken, NJ. She was awarded a Merck Research Laboratories Fellowship, in Chemistry, Pharmaceutical Science, Material Science, and Engineering during her graduate studies. She also won the Exxon Mobile award for outstanding achievement in pursuing the PhD degree in Chemical Engineering.
