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VISCOSITY FORUM: Ask the experts

How do you test construction materials for

Measuring Viscosity? Kinematic or Dynamic Method. How to Choose?

11/21/13 at 11am EST

An Interview With Pat Maggi

Capillary and rotational viscometers are used around the world to measure viscosity. Why are there two different methods? Does each have certain advantages that recommend its use for certain categories of materials? This interview will review various test methods that encompass one or the other technique and provide guidance on how best to proceed when in doubt on the choice. read more

How do you test construction materials for

How do you test construction materials for "flowability"?

11/6/13 at 11am EST

Dr. Guy Rosenthal Interview

How do you test construction materials for "flowability"? For example, when you open a tub of joint compound, does it have the right consistency? Do you know right away when you stick your trowel in? Or is the proof of the pudding in how it spreads? read more

Life Below the Yield

Viscometer vs. Texture Analyzer

10/23/12

An Interview with Ross Clark

When does a viscometer make sense and when does a texture analyzer make sense? read more

 

BROOKFIELD RESPONDS: Q+A

11/2014

CUSTOMER CHALLENGE: We're developing a thick shampoo that has particles in it. We are using a T-bar spindle to measure viscosity, but we don't feel our readings have been very accurate. For example, we can test one formulation that doesn't seem to flow when tipping the beaker and it will be 20,000 cP but another formulation seems to flow as soon as you move the container slightly will read 60,000 cP. Do you think that the viscosity measurements are correct? Is there a different test that could help us understand the viscosity behavior? The critical marketing issue is how fast the product will flow out of the bottle when opening the cap. Do you have any suggestions on how to test this type of flow behavior?

BROOKFIELD ANSWERS:

The important issue that you want to understand is how quickly the shampoo transitions from "non-flowing" to "pourable". You want the customer to admire the thick appearance in the bottle, but when used, you want the shampoo to come out easily so that it can be used.

There is a test method called "Thix Index" that measures viscosity at 2 different speeds that are an order of magnitude apart. In this case, you might try 1 rpm and 10 rpm or maybe 5 rpm and 50 rpm. Divide the viscosity reading at the low speed by the viscosity reading at the high speed. This ratio is called "Thix Index". If the value of the ratio is close to 1, then there is little change in the viscosity and it will not pour easily. If the value of the ratio is close to 10, then the shampoo thins out very easily and may flow out of the bottle too quickly. Values around 3 or 4 may be closer to what you are looking for.

Brookfield's Rheocalc T software has the Thix Index test as one of the standard methods that can be selected. You can also program the test directly in the DV3T Rheometer or DV2T Viscometer.

The choice of spindle is another issue to address. The T-bar is not the ideal spindle for measuring a shampoo with particles. Depending on particle size (average diameter of the particle), consider the vane spindle, which traps the particles in the space between the vanes and shears the mixture against similar material outside the circumference around the vane. If the particles are small, you might be able to use the Small Sample Adapter as an alternative.

10/2014

CUSTOMER CHALLENGE: What is the proper way to test yogurt for viscosity? When we use an RV disc spindle, the viscosity value seems to vary quite a bit. Sometimes the reading will be stable, but at other times it will drop as the spindle continues to rotate. Does the spindle position in the yogurt container make a difference? Please help; this issue is affecting our QC Department because we are rejecting several batches of yogurt each week.

BROOKFIELD ANSWERS:

The "best" approach for measuring viscosity may relate to the "thickness" of the product. Yogurts that are smooth, creamy, and flow easily off of a spoon can be measured in the way that you described; use an RV disc spindle (maybe RV3 or RV4), run at 10rpm and take the measurement after 30 seconds.

Yogurts that are thicker or that contain fruit particles/chunks may require a different spindle. The T-bar spindle with Helipath Stand has been used to measure viscosity of "thick" yogurts that do not flow easily. The disc spindle will create a hole as it rotates and the viscosity reading will drop. The T-bar cuts through fresh yogurt with each revolution of the spindle and gives a steady viscosity reading.

If the yogurt contains fruit, the T-bar may show viscosity readings that jump up and down when the spindle makes contact with the particles. In this case it may be helpful to take an average of the viscosity reading. The new DV3T Rheometer and DV2T Viscometer offer this feature.

The vane spindle may offer a better alternative for yogurts that contain fruit. When the vane rotates, the fruit trapped between the vanes will rotate with the spindle. The yogurt outside the circumference of the vane spindle shears against the yogurt within the vane. The viscosity signal may be more stable compared to the T-bar reading.

One added advantage of the vane spindle is that you can measure "yield stress", which is a measure of the "stiffness" of the yogurt. The DV3T Rheometer has this measurement method using the built-in "EZ-Yield" program.

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VISCOSITY OF WATER TABLE

View the viscosity of water at temperatures between 0 - 100° C.

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VISCOSITY OF MATERIALS TABLE

Instant view of some dynamic viscosities of Newtonian fluids.

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VISCOSITY CALCULATOR

The cup to viscosity conversion engine assumes fluid is Newtonian.

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YIELD STRESS VALUES

Examples of values for typical food and personal care products

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