Centrifuge Test for Cosmetics: Fast Screening of Emulsion Stability

Waiting three months to find out whether a new emulsion separates is frustrating, especially when you have five formula variants to compare. The centrifuge test for cosmetics gives a first answer in minutes. In short: a centrifuge spins a sample so that it experiences many times the force of gravity, which speeds up creaming, oil separation and sedimentation; if an emulsion separates in the centrifuge, it is likely to be weak on the shelf, but passing the test does not prove stability or give you a shelf life. This guide explains what the test shows, how to run and report it, and where it fits in a full stability study.

The short answer

A centrifuge test is a quick physical screening tool for emulsions and suspensions. It is useful during development, for comparing variants and for checking batches, and it is often included in stability programmes. It takes little sample and little time, so it suits early decisions. It does not replace storage at different temperatures, freeze–thaw cycling or real-time data, and it says nothing about chemical, microbiological or packaging stability. ISO/TR 18811, the ISO technical report on cosmetic stability testing, includes centrifugation among the methods it reviews, but no standard sets one universal speed or time for cosmetics.

What happens in a centrifuge?

In an emulsion, oil droplets are dispersed in water (or water droplets in oil). Because the two phases have different densities, droplets slowly rise or sink under gravity. Lighter oil droplets move upwards and form a cream layer; heavier particles such as pigments or exfoliants settle at the bottom. Over time, droplets that touch may merge, and the emulsion can break.

The speed of this movement depends mainly on three things, described by Stokes’ law:

• Droplet size: larger droplets move much faster; doubling the radius roughly quadruples the speed.
• Density difference between the droplet and the surrounding phase.
• Viscosity of the continuous phase: a thicker outer phase slows everything down.

A centrifuge multiplies the gravitational force. If a sample is spun at around 1,000 times gravity, gravity-driven separation happens much faster than on a shelf. That is why a few minutes of centrifugation can reveal a weakness that would take weeks or months to appear.

Read also Why Emulsions Separate and How Stability Tests Catch It

Typical test conditions

There is no single official setting. Laboratories and formulators commonly use a few thousand revolutions per minute for around 15 to 30 minutes, sometimes after warming the sample to add stress. What matters is that the conditions are fixed, recorded and repeated the same way every time.

A key detail is often overlooked: revolutions per minute (rpm) alone do not describe the force. The relative centrifugal force (RCF, expressed as × g) depends on both the speed and the radius of the rotor. For example, 3,000 rpm at a radius of 10 cm gives roughly 1,000 × g, while the same speed in a smaller rotor gives less. When you compare results between labs or machines, compare the force, not the rpm.

What results look like

• No change: the sample remains uniform from top to bottom. This is a good sign, not a guarantee.
• Creaming: a denser, whiter layer at the top of an oil-in-water emulsion. It may still redisperse on shaking, but it shows a tendency to separate.
• Free oil: a clear oily layer or ring at the top. This usually points to coalescence and a weak emulsion.
• Free water: a watery layer at the bottom, typical of a weak water-in-oil system or a failing gel.
• Sediment: pigments, mineral filters, glitter or exfoliating particles settled at the bottom of a suspension.

Record the result with a photo and, where possible, measure the height of any separated layer, so that variants and time points can be compared objectively.

When to use a centrifuge test

• Early development: to rule out weak prototypes before investing in long storage studies.
• Comparing variants: different emulsifier levels, thickeners or oil phases under identical conditions.
• Scale-up: to check that the production batch behaves like the lab batch.
• During a stability study: at time zero and at each time point, to see whether the emulsion becomes more sensitive with age or after freeze–thaw cycles.
• Batch checks: as a quick quality control test for each production batch.

Read also Natural Cosmetics Stability: Special Challenges and Solutions

What a centrifuge test cannot tell you

• Chemical changes: oxidation, rancidity, loss of active ingredients, colour or odour drift.
• pH and viscosity drift during storage.
• Microbiological stability and preservation; those need a microbiological test and a challenge test.
• Packaging interactions: leaks, deformation, migration or pump failure.
• Slow processes such as gradual droplet growth or temperature-driven changes in structure.
• A shelf life: there is no reliable conversion from minutes in a centrifuge to months on a shelf.

Some products also behave unusually. Thick creams and gels with a yield stress may resist the centrifuge simply because their structure holds droplets in place, and very high forces can damage structures that would be fine in normal use. Bi-phase products are designed to separate, so the test is not meaningful for them.

How the centrifuge fits into a full stability study

Think of the centrifuge as a filter, not a verdict. A typical approach is to screen variants by centrifugation, then put the best candidates into a stability and compatibility test with storage at several temperatures, freeze–thaw cycles and the final packaging. The results of that study, not the centrifuge alone, support the shelf life and PAO in the CPSR.

A worked example

A brand compares three versions of a body lotion with different emulsifier levels. After the same time at the same force, version A shows a clear cream layer of several millimetres, version B a thin oily ring and version C no visible change. Version A is dropped. B and C go into accelerated storage in the final bottle. After three months at 40 °C, C is still uniform while B shows slight creaming. The centrifuge saved weeks by eliminating A early, but the storage study made the final decision.

Read also What Is Cosmetic Stability Testing? A Plain-English Guide

How to report a centrifuge test

• Speed in rpm and the rotor radius, or the RCF in × g.
• Duration and sample temperature.
• Tube type, sample volume and how the sample was filled (avoid trapped air).
• Result for each sample, with photos and measured layer heights.
• Batch number, formula version and storage history of the sample.

Common mistakes

• Comparing rpm values from different centrifuges.
• Testing a sample full of air bubbles, which rise and look like creaming.
• Testing a hot sample straight from the production line without recording its temperature.
• Treating a pass as proof of a 30-month shelf life.
• Changing speed or time between variants, so results can no longer be compared.

Frequently asked questions

Is a centrifuge test required by law?

No. EU law requires stability data for the safety assessment but does not prescribe specific methods. Centrifugation is a common, useful part of many programmes.

Can a product that fails the centrifuge still be stable?

Sometimes, particularly if the conditions were very harsh. Treat a failure as a warning and confirm with storage tests.

Does the centrifuge test work for powders or anhydrous balms?

It is mainly meaningful for liquid and semi-solid dispersions such as emulsions and suspensions. Powders and solid balms need other checks.

Key takeaways

• A centrifuge test speeds up gravity-driven separation and screens emulsions in minutes.
• Report the force (× g), time and temperature, not just rpm.
• Passing the test is a good sign but does not prove stability or set a shelf life.
• Use it to screen variants, then confirm with a full stability study in the final packaging.