How Rheology Helps Personal Care Products Meet Consumer Expectations

Morgan Ulrich | Sarah Cotts | Laura Lupton
February 10, 2025

From smooth moisturizers to voluminous shampoo, the products that support everyday self-care routines are the result of the scientific interplay between deformation and flow. The personal care industry (and your morning routine) is only made possible because of rheology – the study of how a material moves (flow) or changes under varying degrees of stress (deformation).

What is Rheology?

Rheology comes from the Greek root logy, meaning the “study of” and rheos, meaning “flow.” It provides an invaluable (and fundamental) understanding of how a material moves under varying stress in different environments. Specifically, personal care product designers and materials scientists use rheology to examine:

Deformation: How a material changes when force is applied; essential data if you are, for example, optimizing the amount of liquid soap dispensed on your hand after you press the bottle’s pump.

Flow: The measurement of a fluid’s movement from one point to another. It has an inverse relationship to viscosity. Toothpaste has a high viscosity because it needs to maintain structure on a toothbrush, while mouthwash has greater flow, allowing it to swish inside the mouth.

The Most Important Rheological Measurements in Personal Care

With any personal care product, four key measurements provide accurate and predictable information for developers to better understand outcomes in the real world. They include:

• Viscosity: The material’s resistance to movement, or “thickness.” Water has a low viscosity and high flow. Honey has a high viscosity and low flow.
• Yield stress: The transition between solid-like and fluid-like behaviors. Shaving cream at rest has a high viscosity but becomes more liquid-like when stress is applied (like when you rub it on skin or wash it with water).
• High shear viscosity: The material’s thickness under a force applied parallel to its surface. Soap that’s designed to foam as it exits a pump changes its viscosity under a high shear rate.
• Zero-shear viscosity: The material’s thickness at rest. If this is too low, different ingredients in a formula might begin to separate, causing sedimentation.

Why Rheology Matters in the Personal Care Industry

Personal care products aren’t designed to stay static. You need to spread lotion on skin or wash shampoo out of hair. Movement is an essential part of their everyday use. That’s why rheology provides critical information to improve nearly every stage of any product’s lifecycle, from its initial design to the mass manufacturing process to its final application. It’s not enough to understand the properties of a material at a single point in time under a single degree of stress (the capability of a viscometer). To get a complete picture, you need to measure a wide range of data points – from rest to high shear – on a rheometer.

When chemists and developers aim to launch a new product or refine the formula and manufacturing process of an existing one, they use rheometers to predict or improve performance in four key areas.

1. Sensory Perception

Virtually every personal care and cosmetic product is “non-Newtonian,” meaning its viscosity changes depending on the shear rate. More specifically, most of those products are “shear-thinning”; their viscosity decreases (thins) the more force is applied.

In most personal care products, shear thinning is a desirable behavior for consumer sensory perception. Sunscreen needs to resist flow at rest, otherwise its ingredients might separate. But it must be absorbed by the skin upon application; nobody wants streaks on their face before jumping into the pool.

Since personal care products are used in various environments and require unique shear rates for their application, rheology is essential for characterizing products under application-specific conditions in a lab setting. Rheology predicts consumer preferences quickly and effectively, reducing the need for costly consumer panels and bringing quality products to market faster. For example, rheometers help developers ensure:

• Chapstick still spreads evenly in freezing temperatures
• Lipstick doesn’t melt after being stored in the car during summer
• Face cream maintains a stable thickness over several months on the shelf before being smoothly applied

The feeling of a personal care product is intrinsic to both its performance and appeal. Only through rheology can this be quickly developed, refined, or changed efficiently and at scale.

2. Package Design

Packaging, of course, ensures a product remains shelf-stable over its many months of shipment and storage. But many personal care products require the packaging itself to “activate” specific behaviors during use. The best example? Soap that foams while being pumped.

A rheometer can apply the specific shear rate the product experiences during dispensing, whether it is pumped from a nozzle, squeezed from a tube or poured from a bottle. This gives scientists the exact data they need to formulate products to work with their packaging and dispense the right amount of product every time.

Package design can also accomplish the opposite, ensuring the shear rate stays under a maximum threshold. Shampoo in a steamy shower must keep moisture out and remain viscous under warm temperatures. Otherwise, it could turn slimy and thin. Rheology sets that acceptable range.

3. Manufacturing Process and Efficiency

How you make, package, ship, and store a non-Newtonian fluid matters. Predicting how materials change during mixing or pouring is essential for optimizing the speed, temperature, and method of packaging personal care products. Even fill-level matters for low viscous fluids. Extra space in a bottle could cause the product to slosh and change during shipment, ruining it before it makes it to the store.

Thorough rheological testing guides every stage of the manufacturing, shipment, and storage pipeline. Without it, manufacturers could waste both money and resources on spoiled product.

4. Formula Design

Ingredients in products change all the time. Whether from supply chain woes, cost of goods, or just the need to improve or match consumer trends. Each change, though, can create cascading and unknown consequences unless a rheometer is employed first. Some examples include:

• Thickeners, stabilizers, surfactants, film-forming agents, and silicones: These substances are used to intentionally alter the product’s viscosity. That may become necessary because of consumer preference. But when you alter the formula to improve one quality, others might be negatively impacted.
• Colorants, fragrances, preservatives, and other additives: Each provides variety across a product line. But when you need a citrus-smelling soap to perform the same as a vanilla-smelling soap, both require testing for consistency.

How is Rheological Data Captured?

Rheology is a notoriously complex field, and inexperienced users may view it as a massive challenge. Luckily, new technology simplifies rheology down to its core so you can reliably characterize personal care products without difficult training, measurements, or calculations.

The new Discovery Core Rheometer is designed for scalability and speed without compromising accuracy. While some personal care labs rely on viscometers, rheometers provide significantly more valuable information, including viscosity across varying shear rates – essential for characterizing personal care products in application.

Personal care R&D labs also benefit from the Discovery Core Rheometer’s new RheoGuide software, which guides users of every experience level to complete tests quickly and accurately.

Learn more about how the Discovery Core Rheometer accelerates and simplifies rheology testing in this brief product demo.

Personal Care is about Change

In an ever-changing personal care industry, products are constantly competing to improve formulations and streamline development. Rheology helps personal care developers quantify the impacts of formulation changes, optimize properties to align with consumer preferences, and manufacture at scale to bring quality products to market faster.