In order to make honey mustard, I mix honey and mustard. But the resulting mixture is less viscous then both honey and mustard. How does mixing two viscous substances give a less viscous one?

  • $\begingroup$ Would you say that the mix of mustard and honey is less viscous than the less viscous component (the mustard)? Also, what treatment do you use (do you whip the preparation)? $\endgroup$
    – Buck Thorn
    Commented Oct 15, 2019 at 16:52
  • $\begingroup$ Mustard takes a long time to flow, but isn't dense or sticky like honey. Honey mustard flows easier than both. To mix them I whip them together with a fork. $\endgroup$
    – SuperWild1
    Commented Oct 15, 2019 at 17:20
  • 2
    $\begingroup$ Physical chemistry of soft matter and complex fluids are perfectly on topic for chem.SE. $\endgroup$
    – Karl
    Commented Oct 15, 2019 at 20:29

1 Answer 1


Honey is a homogeneous substance, a solution of around 98% sugar and water.

Mustard is a heterogeneous mixture, containing about 1:1:1 oils, proteins and water, all in very small particles. The proteins act as a stabiliser to the small oil droplets. They cant slide past each other easily, and act more like a dense network of small solid particles. Like a heap of wet sand.

Now if the aqueous phase gets much larger due to the honey, the oil+protein droplets loose direct contact, and you no longer have the solid-like behaviour of wet sand.

Your aqueous honey-phase however now contains perhaps 25 or 30 percent of water instead of 15 or so in pure honey, and that lowers its viscosity a lot.

You have a major phase with reduced viscosity, and a disperse phase which is no longer continous (not "percolated"), and so the whole thing flows rather easily.

(Btw. the viscosity of mustard is not that high, however it usually shows a yield stress, that is it does not flow at all e.g. under its own weight. You open a mustard glass and the inside looks just the same as it did when you last closed it. Honey however is just a boring Newtonian fluid, no shear thinning, no thixotropy, nothing.)

  • 1
    $\begingroup$ Spot on, except I quibble about the composition of honey: (en.m.wikipedia.org/wiki/Honey): Fructose: 38.2%, Glucose: 31.3%, Maltose: 7.1%, Sucrose: 1.3%, Water: 17.2%, Higher sugars: 1.5%, Ash: 0.2%, Other/undetermined: 3.2% $\endgroup$ Commented Oct 16, 2019 at 14:05
  • $\begingroup$ @JamesGaidis OK, 97.5% ;) But of course it is a natural product, and the numbers vary hugely. While we are on that wp artikel, this en.wikipedia.org/wiki/Honey#Rheology is of course highly relevant. $\endgroup$
    – Karl
    Commented Oct 16, 2019 at 16:32

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