Partner Perspective: Rethinking Viscosity with Neil Cunningham

In collaboration with Neil Cunningham, Founder of The Centre for Industrial Rheology, we’re excited to share his expert perspective on the evolving role of rheology in formulation science. Neil draws from years of hands-on lab experience to highlight the limitations of traditional viscometry and the transformative potential of high-performance rheology. Through relatable examples and clear explanations, he shows how understanding material structure, beyond just flow, can unlock better product performance and process control. This piece is a must-read for anyone working with complex formulations in today’s fast-paced market.

Abraham Maslow once said: “If all you have is a hammer, everything starts to look like a nail.” After years of teaching companies how to perform rheology and viscosity measurements, and measuring thousands of samples here at The Centre for Industrial Rheology, I can tell you the same thing happens when all you have is a viscometer. Every material starts to look like a liquid. And that’s where problems begin.

I’ve lost count of the times I’ve walked into a lab and seen someone with a Brookfield RV viscometer with a spindle number seven poking holes in a thick suspension or paste. Spindle number seven is just a straight rod and is often resorted to because every other spindle leaves a gaping hole in the sample. The LV spindle number four has the same reputation. It’s a sure sign that what you’re working with isn’t a simple liquid at all, but a structured material. Structured materials such as suspensions, emulsions, gels, waxes and pastes don’t flow back neatly around a spindle. They’ve got internal networks that give them yield stress and a built-in resistance to flow. Yield stress is why your ketchup won’t budge until you give the bottle a good whack, and why meringue forms stiff peaks after the correct amount of whisking.

This is where the limits of viscometry start to show.

Viscometers typically have a tiny operating torque range. Users will be all too familiar with flashing displays associated with hitting the 100% torque ceiling or the 10% torque floor. This 10:1 maximum-to-minimum torque ratio is tiny and highly restrictive and navigating these constraints requires experimenting with multiple spindle and speed selections and even purchasing additional viscometers of higher or lower operating torques.

Furthermore, the bewildering selection of potential spindle and speed combinations is hard to navigate. For example, at a single, defined speed, different spindles produce different shear conditions, making cross-site method alignment, or generating numbers for pump and plant specification, or CFD modelling, a guessing game.

High-performance rheology takes a lot of that pain away. Even an entry-level rheometer, like TA Instruments’ Discovery Core Rheometer, has a torque range more than 25,000 times that of a viscometer. That means one instrument covers the delicate “nudge and wobble” of oscillatory rheology to characterize colloidal networks, right through to the high shear rates needed to simulate pumping, spreading, and coating. Furthermore, because rheometers use well-defined geometries, you get data you can rely on – and repeat – across multiple locations or with a wide variety of samples.

Here at the lab, we often start with simple “Rheology Snapshots” that capture, for example, flow behaviour (i.e. viscosity), along with structural properties of colloidal dispersions and gels, including their presence – storage modulus, their strength – yield stress, and timescale-dependent behaviour of viscoelastic liquids.

Here we sweep through a wide range of deformation rates to generate a flow curve. This curve reveals viscosity across a range of shear conditions that may be relevant to a variety of processes, from low shear storage and pouring to high shear spreading, coating and atomization. The plot reveals whether a material is Newtonian, where viscosity remains independent of applied shear, or otherwise. The most common form of non-Newtonian behaviour is shear-thinning, where viscosity decreases with increasing shear rate.

The plot below shows shear rate sweeps for two haircare products: one shampoo and one conditioner. The shear rate range spans over three decades and viscosity drops significantly over that range. The viscosity profiles demonstrate significant shear thinning for both products but a pronounced zero-shear viscosity plateau for the shampoo.

This test gently increases oscillatory stress on a sample, gently nudging and probing, looking for elastic responses that indicate the presence of structure. Storage and loss modulus, or complex modulus and phase angle, are plotted as a function of stress. At small stresses, the material holds firm like a soft solid (storage modulus dominates), but at a critical stress the network collapses as the yield is reached. This test identifies two aspects of the yielding process: the upper limit of the linear viscoelastic region (the LVR), where the first perceptible structural disruption ensues, and the flow point, where sufficient yielding results in a loss of elastic dominance, signified by a storage and loss modulus crossover. Yield stress gives insights into texture, handling, application and suspending ability.

The oscillatory stress sweeps performed on our two haircare products demonstrates an interesting relationship. The conditioner demonstrates obvious, well-formed gel structure compared to the shampoo: in the former, complex modulus is (relatively) high, phase angle is low and there is a cliff-edge yielding, signified by phase angle heading skyward and modulus dropping, around an applied stress of 50Pa.

Oscillatory frequency sweeps maintain a small amplitude oscillatory deformation whilst sweeping the frequency of oscillation. This mechanical spectroscopic method provides an insight into the nature of the structure present. By plotting storage and loss modulus vs frequency it is possible to identify timescale-dependency of storage modulus and a tendency towards viscous dominance at lower frequencies, both indicators of a “relaxable” weak gel network, rather than a more established colloidal gel.

The oscillatory frequency sweeps performed on our two haircare samples demonstrate the significantly differing nature of their rheologies. The conditioner shows a classic well-developed colloidal gel; storage modulus dominates throughout the frequency/timescale window. In contrast, the shampoo shows a strong timescale-dependent viscoelastic response: It flows like honey (viscous dominant at low frequencies) but wobbles like jello (crosses over into elastic-dominance)!

For many of our customers here at The Centre, the richness of insights delivered by high performance rheology can present quite a contrast to the single-point viscosity measurements obtained from a viscometer. That’s when the conversation shifts: viscosity alone isn’t enough, and rheology isn’t just a luxury – it’s the practical way to understand and control complex formulations for a demanding marketplace.

So, if you ever find yourself pushing a spindle number seven into a reluctant sample, take it from someone who’s seen it all: it may not be viscosity you need to measure at all. That’s when rheology stops being an abstract science and becomes an essential tool.