How do you measure viscosity of cosmetics
There was a question on one of the cosmetic science forums I subscribe to about viscosity. This reader wanted to know whether they needed an “expensive” Brookfield viscometer or whether they could take viscosity readings another way. It turns out there are a variety of choices.
Why measure viscosity
Viscosity is one of those terms that wasn’t covered much in college but tends to be used a lot in the cosmetic industry. We’ve previously writte about fluid viscosity and I would encourage you to read that to get up to speed on what it is and why you would want to measure it. But basically you measure viscosity to ensure your product is of consisten quality and that it remains stable.
How to measure viscosity
In the cosmetic industry the overwhelming instrument of choice for measuring viscosity is the Brookfield Viscometer. Viscosity is measured by looking at the force required to spin a plate through the fluid. All the calculating is done by the instrument.
The Brookfield viscometer is rather simple to use and has a number of benefits over other options including
1. Speed - you can get a reading in less than 2 min
2. Easy clean up - you can get multiple readings fast
3. Reliability - readings are consistent
4. Versatile - you can test lots of different rheologies
Of course, there are some downsides. For shear thinning products your readings might not be accurate due to the drilling effect. And the device needs to be callibrated to give consistent readings. This adds to the cost of operating it, in addition to the >$1500 that it costs up front.
However, if you are a serious formulation chemist you need to have a viscometer and Brookfield makes the industry standard.
Here’s a viscometer in action.
Alternatives to Brookfield Viscometer
While most every company uses a Brookfield viscometer, there are other options.
For example, there is the Ubbelohde glass capillary viscometer. This is a glass tube made with some curves and bulbs. It has no moving parts and simply requires a stop watch and a vacuume bulb. The fluid is drawn up the tube and the time it takes to flow back down is related to its viscosity. Unlike the Brookfield, there is no issue with shear thinning products. Of course, the major drawback is that it takes 5 to 10 minutes to get a viscosity reading and it would be impractical for most cosmetic labs.
You can see one in action here.
The primary benefit of this option is that it is much less expensive than the Brookfield and it doesn’t require recallibration.
And then there is the Rheometer. This device uses a flat plate in which a small sample of the product is put between. The plate is rotated and the force required for rotating the plate is related to the product viscosity.
This device is easy to use but generally not as accurate as the Brookfield. But for a cosmetic lab, it might be good enough.
Finally, if you don’t want to pay much but want to have a way of measuring the relative viscosity of your product, you can do something like this device. It is a series of 4 glass tubes that contains liquids of varying viscosity. A metal ball is dropped through the liquid and the time required to flow to the bottom is tracked. This is then related to the product viscosity. It’s a pretty crude method but it works.
Viscosity and the cosmetic chemist
Of all the measurements that you will take as a formulating cosmetic chemist, viscosity will be most frequently done. The only readings you might take more often is pH. That’s why to be a great cosmetic chemist you need to know and appreciate the fundamentals of viscosity. And you also have to have a way of measuring it.
Were there any viscosity measuring methods we missed? Leave a comment below and let us know.