Eden ReidJune 29, 20213 min read

# What is Rheology?

Rheology is a branch of physics dealing with the deformation and flow of materials. Wherever a material flows, rheology is involved, meaning it affects us nearly everywhere we go. While the term “rheology” was not coined until 1929, Sir Isaac Newton and Robert Hooke are both attributed “in setting the boundaries of the modern science of Rheology” hundreds of years earlier. (History of Rheology – Kenneth Walters). Hooke introduced his famous linear law relating stress and strain, while Newton described the flow behavior of fluids with a simple linear relation between shear stress [mPa] and shear rate [1/s] - now known as Newton's Law of Viscosity. Rheology has applications in materials science, industrial processes, engineering, geophysics, physiology, biology and pharmaceutics, yet there are less than 5,000 rheologists in the world. In this post we’ll further define rheology, describe the parameters measured, and show examples of common applications.

Rheology is defined as the study of the flow of matter. Generally accounting for the behavior of non-Newtonian fluids (their viscosity is dependent on shear rate), rheology characterizes the “minimum number of functions that are needed to relate stresses with rate of change of strain or strain rates”. In simpler terms, rheology is used to establish the relationship between strains and stresses of substances with complex microstructures and establish predictions for the mechanical behavior of these substances. Rheology measurements are focused on the rheological properties of materials:

• Shear stress τ (mPa-s)
• Deformation γ [%]
• Shear rate γ (1/s)

When measuring the rheological properties of substances, it is crucial to consider the 3 factors that most significantly impact flow or deformation: internal structure, external forces, and environmental conditions (e.g temperature). Measuring rheological properties is critical in production of industrial substances (paints, inks, cement), determining the mechanical performance of polymer-based substances (rubber, plastic, textile), and in formulation of many dosage forms of biophamaceuticals (subcutaneous injections, ointments, creams). With accurate data, you can gain a deep understanding of the composition of various substances, predict the impact of that composition on your process, and optimize your process based on the characteristics revealed by the data. A few specific applications of rheological measurements are:

1. Cosmetics – The rheological properties (mentioned above) of cosmetic products varies across a wide range from serums, foundations, face wash and mascara, to list a few. Viscosity testing is one important rheological parameter for both production and quality control of cosmetics process. Measuring the viscosity of cosmetics and cosmeceuticals allows manufacturers to predict how the products will behave once distributed to consumers and helps to ensure the product quality and user experience are excellent.

2. Food Rheology - Food and beverage production is a complex rheological field to work with because of the wide range of sample types and behaviors encountered. Understanding the rheology of food and beverage products helps in both development (texture, shelf stability, spreadability, squeezability, etc) and processability (can the products be processed on your existing lines).

3. Pharmaceuticals – “Study of flow properties of liquids is important for pharmacists working in the manufacture of several dosage forms, such as simple liquids, ointments, creams, pastes etc.” Rheology as a tool can contribute to formulation characterization as well as formulation analysis and application development. Injection force estimates are one such rheological tool. One of the most common uses of viscosity is to assess the ability to deliver pharmaceutical formulations to the patient by injection. Incorporating injectability analysis earlier in therapeutic development can guide development and mitigate the risk of developing a formulation that is impossible to deliver.

Rheology and its effects on materials affect us everywhere we go. Every substance has rheological properties, and we can measure and utilize knowledge of those properties in our formulation, production and other processes. The term "rheology" is relatively new, however the study of rheological concepts has been at the forefront for scientists for hundreds of years. While there are less than 5,000 rheologists in the world, understanding the rheological properties of substances is crucial for most industries, from food and beverage, to cosmetics, pharmaceuticals, engineering, and material sciences.  Working with someone who specializes in measuring and understanding these rheological properties can improve formulation, production processes and determine the performance of your products at various conditions - which directly impacts end user/ customer experience. If you would like to learn more about the rheological properties you should be measuring for your applications, contact us to speak with one of our rheology experts today!

Written by: Eden Reid - RheoSense Senior Marketing and Sales Operations

#### Eden Reid

Eden Reid is the RheoSense Senior Marketing Associate. She has a Bachelor's of Science degree in Biology from the University of California, San Diego and has over 5 years of marketing experience.