Newtonian vs non-Newtonian fluids

Ketchup is called a non-Newtonian fluid because its viscosity depends on the amount of shear stress applied to it.1 This is not “normal” behavior. Most liquids are called Newtonian fluids and behave more like water.

Most Newtonian fluids like soy sauce, grape juice, or a cold brew do not need to be encouraged out of their container. Turn them over and gravity does all the work. If you add pressure, they flow faster (think of water in a garden hose), but they do not change consistency: Water standing in a pool is just as liquid as water from a faucet. However, the viscosity of Newtonian fluids is strongly dependent on temperature, with colder fluids being more viscous than warm ones.

But in undisturbed thixotropic fluids like ketchup, neighboring molecules form weak bonds with each other. These may be hydrogen bonds, van der Waals bonds, or even electrostatic bonds that hold the ketchup together in a gelatinous state. The bonds form over time as long as the fluid remains motionless. But once we break those bonds by shaking or stirring, the flow returns, covering your fries with tomatoey goodness.

The secret in the sauce

To get a thixotropic liquid, something must be present in the mix that encourages weak bonds to form. In food items, it is often a type of protein or other complex molecule such as dextran.2 If one or more of these substances is present, the gel forms. The reaction may be fairly quick or it may take days, depending on a variety of factors.

Non-food items can be thixotropic, too. For example, paint, shampoo, and blood are all thixotropic. Synovial fluid, the lubricant that reduces friction between the bones in our joints, gels at night as we sleep. But as soon as we begin to move our joints the next morning, it quickly reverts to a thin liquid.

The opposite of thixotropy

Much less common is a second type of non-Newtonian fluid that does the opposite. Liquids that get stiffer with agitation are called anti-thixotropic or rheopectic. A common example is inkjet printer ink, which starts as a liquid but gets stiffer as it goes through the printer jets. This thickening allows the ink to dry quickly before it smudges the next sheet.

Another memorable example of rheopecty is Silly Putty.3 Silly Putty does many cool things, including bouncing, stretching, and tearing. Strong bonds hold the polymer together, but weaker bonds form between the molecules. Breaking the weak bonds at different rates produces the unique characteristics kids love. But leave Silly Putty undisturbed long enough and it will puddle like a liquid. Since 1950, parents have warned their children to store the putty in its egg to keep it from melting into the carpet (although many of us failed).4

Thixotropic honey varieties

Some of the most famous honey varietals in the world are highly thixotropic, a characteristic usually attributed to the presence of a certain protein in the nectar collected by the bees. These types of honey can be vexing for beekeepers because the honey won’t easily flow out of the cells, even in a radial extractor. Fortunately, most honey has only small amounts of protein and behaves like a normal Newtonian fluid.5

Because beekeepers are enterprising folks, those with lots of thixotropic honey have clever ways of dealing with it. Many of the extraction techniques, which tend toward messy and time-consuming, were developed in Europe, Australia, and New Zealand where varieties of thixotropic honey are much more common than in North America.

The most famous varieties of thixotropic honey include:

1. Manuka honey, known for its medicinal qualities, comes from the nectar of Leptospermum scoparium, a tree native to New Zealand and Australia. Of all the thixotropic honey varietals, it holds the record for the speed at which it resumes the gelled state after agitation.

2. Ling (or common) heather honey, made from the nectar of the heather plant, Calluna vulgaris, is famously thick, strongly flavored, and dark colored. Common in Europe, heather honey spurred much of the experimentation into alternative extraction methods.6

3. Eucalyptus (Eucalytus spp) honey is the product of various species of eucalyptus trees that grow in Australia. Eucalyptus honey comes in a treasure trove of colors and has a wide range of thixotropic properties that vary from species to species. However, a few species of eucalyptus display no thixotropy at all.7

4. Buckwheat honey derives from the nectar of buckwheat, Fagopyrum esculentum. Buckwheat honey has a memorable dark brown color and a strong, molasses-like flavor. Its thixotropic properties vary, probably as a result of the nectars mixed with it.

5. Grapefruit honey comes from the nectar of grapefruit blossoms, Citrus paradisi. It ranges from light to dark amber and is moderately thixotropic.

The easiest way to sell thixotropic honey is in the comb. When eaten in the comb, the gel breaks down when the honey is spread onto toast or crackers, making it behave like any other comb honey. But if you must extract your crop, beekeepers have devised ways to do it.