Fluid Testing Guide: The 3 Viscosity Behaviors

Mastering your fluid's behavior is the secret to consistent, high-quality production.

While the terminology around newtonian vs thixotropic fluids can seem complex, we believe in simplifying it. Clarifying whether your material is thixotropic vs non newtonian is the key to selecting the right cost-effective equipment and ensuring your process runs smoothly.

The Straightforward Fluid: What is a Newtonian?

Before exploring more complex materials, we have to start with the most basic fluid type: the Newtonian.

Scientific literature confirms that these fluids possess a constant viscosity that does not change with the applied shear rate, meaning their flow behavior is linear and predictable (Mustafa et al., 2020; Kapadia et al., 2025).

Water and simple oils are prime examples. Stir them slowly or agitate them vigorously, and their core consistency does not change. When comparing thixotropic vs newtonian fluids, this unwavering stability is the key difference.

Viscometers for these fluids simply measure this constant viscosity, often using methods like micropillar deflection or capillary flow. Therefore, testing a Newtonian fluid is a simple affair.

Often, a reliable analog dial unit like the ViscoQT DR-100 is all that's required to confirm quality assurance.

Where Things Get Interesting: Non-Newtonian Fluids

Most materials used in industrial applications simply don't follow these basic rules. This is where the discussion of thixotropic vs non newtonian fluids becomes critical for any QC department.

In contrast to the stable Newtonian types, non-Newtonian fluids exhibit viscosity that varies with shear rate or time, showing complex flow behaviors such as shear thinning or thickening (Srivastava & Burns, 2006; Kornaeva et al., 2022; Lee et al., 2020).

To visualize this, look at everyday items:

- Nail Polish: It must flow smoothly off the brush (high shear) but stay put on the nail without running into the cuticles (low shear).

- Printing Ink: It needs to be thin enough to jet through a nozzle but thick enough to remain crisp on the paper without bleeding.

This category generally splits into two types:

- Shear-thinning fluids: These become less viscous the more you work with them, like shampoo.

- Shear-thickening fluids: These become more viscous when agitated, like a cornstarch and water mixture.

Here’s a critical point many labs overlook. While all thixotropic fluids are non-Newtonian, not every non-Newtonian fluid possesses thixotropic properties. Assuming they are the same is a frequent source of inconsistent test results.

The Deciding Factor: What Makes Thixotropy Unique?

Thixotropy introduces the element of time into the equation.

As noted in rheological studies, thixotropic fluids are a subset of non-Newtonian fluids whose viscosity decreases over time under constant shear and recovers when the shear is removed (Chhabra, 1999).

To understand this, compare honey with yogurt. If you stir honey, it flows exactly the same way (Newtonian). If you stir yogurt vigorously, the structure breaks down and it becomes runny. Stop stirring, and the yogurt doesn't instantly solidify. It takes time to rebuild that structure. That delay is thixotropy.

This time-dependent behavior creates a massive trap for quality control. If you treat a thixotropic sample like a Newtonian one, your data will be unreliable. It's that simple.

For example, if you measure a cosmetic cream immediately after mixing, you will get one viscosity reading. If you measure that same sample an hour later, the reading will be completely different. This isn't a product failure. It's a measurement failure.

Rheological characterization of these fluids is challenging due to their nonlinear and time-dependent properties, often requiring advanced measurement strategies (Chhabra, 1999).

You need an instrument capable of tracking that recovery curve, such as the ViscoQT 1000-Pro or ViscoQT 2000 Series, which allow you to program speeds and capture data points over a set period.

A Quick Reference Guide: Fluid Behaviors

To help simplify your process, we've created this reference table. It’s a useful tool for discussing newtonian vs thixotropic testing needs with your team.

How This Affects Your Operations

Knowing the science is one thing, but applying it is another. Here is why the distinction between thixotropic vs non newtonian fluids is so important for your ROI across various industries:

Paints & Coatings

The thixotropic property is non-negotiable. Ceiling paint, for example, must be thick in the can to prevent separation, thin on the roller for application, and then thicken rapidly on the ceiling to prevent drips.

For this industry, we often recommend the ViscoQT KS-Series (KS-100 or KS-300), built specifically for the Krebs units (KU) used in these standards.

Cosmetics & Personal Care

Consumers are incredibly picky about texture. A face cream that is too runny feels "cheap," while toothpaste that won't squeeze out is frustrating.

Ensuring the right thixotropic breakdown means the product feels luxurious on the skin but stable on the shelf.

Adhesives & Sealants

Industrial caulking and epoxies need to be pumpable during application but must stay in place immediately after dispensing. If you misjudge the thixotropic vs newtonian behavior here, you end up with messy bonds and structural weaknesses.

Food & Beverage

Mouthfeel dictates sales. Salad dressing must pour easily but cling to the lettuce. We find that getting this wrong leads to a product that just "feels" off to the customer.

Pharmaceuticals

Topical gels must remain stable in the tube but spread easily upon application. Proper analysis ensures the correct dosage and efficacy.

Meeting Industry Standards (ASTM & ISO)

In a professional setting, you are testing to prove compliance and quality.

Whether your lab follows ASTM D2196 or ISO 2555, our viscometers are built to produce the reliable data you need. We consider these standards to be the common language of quality control.

Knowing if your material is thixotropic vs newtonian is the first step to selecting the correct testing protocol outlined by these bodies.

Qualitest Solution: Precision Without the Premium

At Qualitest, our position is that accurate testing shouldn't require an exorbitant budget.

Whether you need the rugged ViscoQT DR-100 for quick Newtonian checks or the advanced, touch-screen ViscoQT 1700/S Rheometers for complex thixotropic analysis, we provide the exact level of technology required—nothing more, nothing less.

We offer a full lineup of cost-effective, high-precision instruments to back up your product quality. Explore our complete range of viscometers today and find the perfect match for your lab!

References

- Chhabra, R. (1999). Chapter 2 – Rheometry for non-Newtonian fluids. Rheology of Complex Fluids, 37-72. 

- Kapadia, W., Giri, N., Qin, N., Zhao, P., Phan, C., Haines, L., Jones, L., & Ren, C. (2025). A novel microfluidic viscometer for measuring viscosity of ultrasmall volumes of Newtonian and non-Newtonian liquids. Journal of Micromechanics and Microengineering, 35. 

- Kornaeva, E., Stebakov, I., Kornaev, A., Dremin, V., Popov, S., & Vinokurov, A. (2022). A method to measure non-Newtonian fluids viscosity using inertial viscometer with a computer vision system. International Journal of Mechanical Sciences. 

- Lee, E., Kim, B., & Choi, S. (2020). Hand-held, automatic capillary viscometer for analysis of Newtonian and non-Newtonian fluids. Sensors and Actuators A-physical, 313, 112176. 

- Mustafa, A., Eser, A., Aksu, A., Kiraz, A., Tanyeri, M., Erten, A., & Yalcin, O. (2020). A micropillar-based microfluidic viscometer for Newtonian and non-Newtonian fluids. Analytica chimica acta, 1135, 107-115. 

- Srivastava, N., & Burns, M. (2006). Analysis of non-Newtonian liquids using a microfluidic capillary viscometer. Analytical chemistry, 78 5, 1690-6.