Said of a semi-solid substance which has the property of becoming more liquid when agitated. Thixotropic substances include ketchup and the ink used in the Astronaut Pen.

Thixotropy also plays an important role in earthquake research. Oftentimes, semi-stabilized thixotrophic muds can liquify during prolonged shaking, resulting in mudslides and collapse of buildings. The technical term for soil mobilization is soil liquefaction.
The 1964 quake in Anchorage, Alaska was the strongest quake to hit North America in the 20th century, and resulted in significant ground flow. Proper city planning should avoid construction on thixotropic soils. However, short term commercial pressures often (usually?) outweigh prudent long term planning.

Thixotropic substances are also used in the oil industry.

When one is drilling for oil, a liquid known as mud is pumped down the borehole. It serves a dual (sometimes triple) purpose: to lubricate the drilling head, and to carry away the debris caused by drilling. The possible third purpose is to actually drive the drilling head.

Consider the case of a inclined or horizontal borehole. If for some reason drilling is interrupted the mud stops circulating and all the debris settle. Not much of a problem if you are drilling vertically, not so good if you are drilling horizontally or at an angle. If however your mud is thixotropic, then when the mud stops circulating it will become less fluid and the debris will stay suspended.

These are quite fun to see for yourself. I remember the first time I saw one of these fluids. The guy doing the demo shook a beaker of the stuff around and sure enough it looked like water. Then he stopped and it solidified. He turned it upside down to prove his point. Not content with that, to further prove the quality of his product he started shaking it around at which point of course it became liquid and splashed all over the floor.

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