David W. Spitzer

Which of the following common fluids is most viscous?
A. Water
B. Honey
C. Mustard
D. Peanut Butter
E. Cannot Be Determined

Commentary
Viscosity is the ability of a fluid to flow over itself. Viscosity is dependent upon composition, temperature, and the amount of stress applied to the fluid.

The composition of water is well-defined, so its viscosity is also well-defined, likely the lowest among these liquids. The composition of honey, mustard, and peanut butter can vary such that (for example) “heavier” mustard could be more viscous than honey, while “lighter” mustard could be less viscous. Peanut butter is likely more viscous than either honey or mustard. The correct answer appears to be Answer D.

Additional Complicating Factors
Complicating the above analysis is that temperature also affects viscosity, whereby increasing temperature generally reduces the viscosity of the fluid. Therefore, increasing the temperature of peanut butter will reduce its viscosity. Conversely, decreasing the temperature of honey will increase its viscosity. Therefore, it is conceivable that the viscosity of hot peanut butter could be lower than the viscosity of cold honey.

Without more detailed investigation, Answer E may be the pragmatic choice.

David W. Spitzer is a regular contributor to Flow Control with more than 35 years of experience in specifying, building, installing, startup, troubleshooting, and teaching process control instrumentation. Mr. Spitzer has written over 10 books and 150 technical articles about instrumentation and process control, including the popular “Consumer Guide” series that compares flowmeters by supplier. Mr. Spitzer is a principal in Spitzer and Boyes LLC, offering engineering, expert witness, development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.