Two batches of the same cream pass the incoming check with the same viscosity reading. One stays perfectly stable through a summer warehouse; the other separates within eight weeks. Both suppliers are telling the truth about their numbers — the problem is that the number does not describe the property the product actually needs. Most purchase specifications reduce a thickened system to a single viscosity point, and a single point on a non-Newtonian curve is close to meaningless.

The reading moves with the spindle, and that is the point

A Brookfield dial or cone reading is a viscosity at one shear rate, produced by one spindle at one speed. For a rotational instrument of the common RV class, that shear rate is typically somewhere between roughly 1 and 10 s-1 depending on spindle geometry and speed — and it is rarely quoted on the certificate. A shear-thinning fluid has no single viscosity: it has a curve. Read the same batch with a different spindle, a different speed, or after a different rest time, and you will get a different number that is every bit as honest and just as useless for comparison.

This is why "viscosity: 50,000 mPa·s at 25 °C" is not a specification. It is one coordinate on a curve whose shape is unknown, measured under conditions nobody records. Two fluids can agree at 10 s-1 and differ by a factor of ten at 0.1 s-1 — and it is the low-shear end that governs shelf behaviour.

Yield stress decides what stays suspended

The property that actually predicts whether particles stay distributed, whether emulsion droplets cream, and whether a gel holds a bead on a brush, is yield stress: the stress below which the material does not flow at all. The standard model that combines a yield stress with shear-thinning behaviour is Herschel–Bulkley:

τ = τ0 + k · γ̇n

where τ0 is the yield stress, k the consistency and n the flow index. Below τ0 the material behaves as a soft solid; above it, it flows and thins. This is the model drilling-fluid and suspension laboratories fit their data to, and it is the language worth demanding from a thickener supplier, because τ0 at rest is what a settling particle has to fight.

Xanthan gum (CAS 11138-66-2) is the classic yield-stress workhorse in water phases: strongly pseudoplastic, and able to hold particles in suspension at fractional dosages. Published supplier data make the contrast concrete — a 1% xanthan solution shows a viscosity around one hundred times that of a gelatin solution at the same concentration, and its viscosity climbs steeply as shear rate approaches zero. The same logic applies to the oil phase: a fat crystal or wax network (tribehenin, CAS 18641-57-1, is a neat example) gives the oil phase its own yield stress, which is why some creams carry actives in the oil droplets without any gum in the water.

Shear-thinning is the feature; slow recovery is the risk

A good suspension thickener is chosen for the ratio between its viscosity at rest and its viscosity during use, not for either number alone. Xanthan is prized because it is very thick at 0.01 s-1 and pours easily at 100 s-1. The risk hides in the recovery: after shearing stops, the structure rebuilds over some characteristic time. If it rebuilds in seconds, a pumped suspension re-forms its network before particles can move. If it takes hours, every filling line pass leaves the beads partially settled and the batch arrives separated despite a perfect viscosity certificate.

Recovery time is measurable in a thixotropic loop or a three-interval thixotropy test (rest – high shear – rest), and it is the single most useful number to add to a specification for anything that gets pumped, filled or shaken. Two thickeners with identical viscosity certificates can differ by an order of magnitude in recovery time.

Cup and KU numbers only rank one Newtonian family

Flow cups (Ford, Zahn, DIN) measure the time for a fixed volume to drain, which is only valid where viscosity is independent of shear rate — thin Newtonian fluids such as solvents and low-viscosity oils. Applied to a shear-thinning coating or gel, a cup reading ranks batches in an order that may not survive contact with the spray gun, because the cup shear rate and the application shear rate are different points on different curves. KU (Stormer) viscosity in coatings is more disciplined — a defined load and geometry — and is legitimate for QC within one product family, but it still cannot be compared across thickeners or across chemistries.

Writing a specification that survives contact

  • Fix the conditions, not just the number: temperature (±0.5 °C matters), spindle geometry, speed, and the rest time after loading the cup before the reading is taken.
  • Demand two shear points — one low (0.1–1 s-1, proxy for shelf behaviour) and one high (around 100 s-1, proxy for pouring or spreading). The ratio between them is the real product signature.
  • For suspensions, ask for yield stress (stress-ramp or vane method) rather than inferring it from a viscosity point.
  • For anything pumped or filled, add recovery time from a three-interval thixotropy test.
  • Note chemistry constraints: xanthan gum is anionic and is not usually compatible with cationic surfactants, polymers or preservatives (precipitation occurs); anionic and amphoteric surfactants above roughly 15% w/v can also precipitate it from solution.

Supply

For the water phase we supply xanthan gum (CAS 11138-66-2) and the inverse-emulsion acrylic thickener Acrylamide/Ammonium Acrylate Copolymer (and) Mineral Oil (and) Laureth-7 (CAS 26100-47-0; 8042-47-5; 9002-92-0), which thickens on neutralisation and hydration without fish-eye risk. For structuring the oil phase we offer tribehenin (CAS 18641-57-1), glyceryl stearate SE (CAS 11099-07-3) and the long-chain acrylate/methacrylate monomers behenyl methacrylate (CAS 16669-27-5) and stearyl acrylate (CAS 4813-57-4) used in associative and crystallising thickener systems.

For how to disperse xanthan gum without lumps and how to choose mesh grade, see our xanthan gum usage guide; for the three thickening mechanisms and their failure modes, see our emulsion thickening mechanisms guide. All grades are available for B2B supply with sample quantities shipped worldwide — requests for rheology data sheets and formulation support are welcome through the enquiry form on any product page.