Objective

To learn and practice how to measure viscosity  

Background

Viscosity is an important parameter that relates to texture and appearance of food and hence sensory experience while eating. Viscosity is also important in the rate of fluid flow and the amount of energy that may be required to move liquids in pipes. As it relates to fluid flow, there are generally two types. There is Newtonian flow and Non-Newtonian flow. When you think of Newtonian flow you can think of water and similar non-viscous fluids like beverages and wines. Regardless of how much shear stress you add to these liquids, their viscosity remains the same. Viscosity refers to the resistance of flow of a liquid. Shear stress is the force applied to the liquid, for example when we shake, mix or pump it. When we apply a shear stress we cause the liquid to move at a certain shear rate i.e. the velocity at which they flow. If you increase the speed of your blender for example, you are increasing shear force on the liquid which will cause it to move at a faster rate. In Newtonian fluids the thickness or viscosity will not change no matter how fast you mix the fluid.

Non-Newtonian fluids do not function the same way. They may change their viscosity depending on the amount of shear stress applied. Take for example ketchup. This fluid will behave like a solid until a force is applied to move it. That is why your ketchup will remain stuck in the bottle even when you turn it upside down. But as soon as you shake it, it starts to flow. Starch/water mixture is also Non-Newtonian. It becomes extremely viscous behaving like a solid when a very high shear force is applied but behave like a liquid when the force is removed.

Types of Non-Newtonian Flow

Non-Newtonian flow can be broken down into various types as follows:

Pseudoplastic fluids – Fluids that display a shear-thinning behavior. That means the more stress you apply the faster they flow. Pseudoplastic fluids that are depended on time are called thixotropic. For these fluids if you maintain a constant stress, the fluid will still get less viscous over time. Example of fluids showing pseudoplastic behavior include concentrated fruit juices, egg white, mayonnaise, yogurt and sour cream

Dilatant fluids – Fluids that display a shear thickening behavior. That means the more you apply a stress, the more the fluid thickens. Dilatant fluids that are depended on time are called rheopectic. For these fluids, if you 

Newtonion and Non-Newtonion Fluidsmaintain the same shear stress, it will still get thicker over time. Example of fluids showing dilatant behaviors include wheat flour dough and corn starch in water.

Bingham Plastic – Fluids that will not move until a minimum amount of force is applied. However, once they begin to flow they behave like a Newtonian fluid e.g. ketchup

Determination of Viscosity

Viscosity of fluids can be determined using a viscometer. The viscometer we will use in this lab will measure dynamic viscosity which is the internal resistance of a fluid to flow when an external force is acting upon the fluid. The external force in this case would be the mixer in the viscometer. It will simulate an external force that would be exerted by say, an industrial mixer or other external force such as friction when trying to pump the liquid through a pipe. Measurement is given in mPa.s. (also called centipoise). Note the following conversions that you will find useful:

  1. 1 Pa.s = 1000 mPa.s
  2. 1 Pa.s. = 1 kg/m.s.

Viscosity can also be reported as kinematic viscosity. This is viscosity of a fluid due to its internal resistance to flow when nothing is acting upon it except gravity. In this lab you will be using a Bostwick consistometer to measure the kinematic viscosity. This instrument is a standard tool in the ketchup industry for measuring viscosity by determining how fast it moves down an inclined slope within 30 seconds. The viscosity is reported in any one of the following units. 

  1. m2/s
  2. Stoke (St): 1 cm2/s 
  3. Centistoke (cSt): 1 mm2/s   

Note the following conversions that you will find useful:

  1. 1 St = 1 cm2/s = 10−4 m2/s
  2. 1 cSt = 1 mm2/s = 10−6 m2/s  

Procedure – Using Bostwick Consistometer

  1. Ensure that the Bostwick consistometer is clean and dry
  2. Adjust the angle of the instrument by locating the two twist screws in the rear of the instrument, and adjust until the leveling bubble on the front of the instrument is centered 
  3. Close the gate by pulling it down and while holding it in this depressed position, pull the lever arm up as far as it will go. This puts the product gate in a cocked position
  4. Pour ketchup sample (refrigerated, room temperature and 50oC sample) into the sample reservoir and fill the reservoir up to the top of the product gate
  5. Release the product by pressing down on the lever arm. Allow the product to run along the slope for 30 seconds
  6. Look at how far the ketchup ran down the slope during 30 seconds (use graduation marks on slope) and record this value as the consistency of the product.
  7. Clean the Bostwick consistometer with warm soapy water. Dry the instrument completely before using it again.

Procedure – Using Brookfield DV2T Viscometer 

  1. Turn on the viscometer (Brookfield DV2T)
  2. Level the viscometer by adjusting legs until leveling bulb is centered 
  3. Allow the instrument to complete “auto zero” action
  4. Fill 100 beaker with ketchup sample (slightly angle to prevent air bubbles from being trapped while you are pouring)
  5. Attach spindle
    1. To prevent air bubbles from being trapped under the spindle, tip the spindle at a 45o angle as you dip it into the sample
    2. With the motor off, attach the spindle by gently lifting the coupling nut on the viscometer with one hand and screwing the spindle with the other (lifting the coupling nut protects the suspension system in the viscometer)
  6. Turn the gear knob up and down until the fluid is level with the spindle’s immersion grove 
  7.  Use the touch screen of the viscometer to configure your test
    1. Spindle type = RV03
    2. Speed = 10 rpm
    3. Test type = Single point
    4. End condition = 1 minute
  8. Press “run” to measure viscosity

Lab Questions

  1. Plot a graph showing temperature versus average consistency.
  2. Plot a second graph showing temperature versus average dynamic viscosity in mPa.s
  3. Convert mPa.s values obtained to Pa.s and kg/s.m.
  4. What is the difference between dynamic and kinematic viscosity?
  5. Calculate the kinematic viscosity of the ketchup at room temperature if its density is 1.45 g/ml
  6. What was the effect of temperature on the consistency of the ketchup? Explain your results in relation to effect of temperature on shear stress. 
  7. Why was it necessary to completely dry the consistometer before measuring the consistency of the ketchup?
  8.  How would temperature of the ketchup affect the energy requirement for fluid transportation through pipes in the processing plant?

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Courtney Simons
Courtney Simons
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Dr. Simons is a food science educator. He earned his bachelor’s degree in food science, and Ph.D. in cereal science at North Dakota State University.