Modernist cooking isn’t just reserved for state-of-the-art kitchens and labs, for culinary ingenuity is found in many surprising places. Street vendors in particular are among the most innovative and resourceful cooks out there. They often combine humble ingredients with science to create extraordinary dishes, often paired with entertaining finesse to stop hungry locals and travelers in their tracks.
For instance, the vendor in this video is preparing fei chang fen, a tasty specialty of Chengdu, in Sichuan province, China. He uses a marvelously simple, yet clever way of making noodles by grabbing handfuls of batter from a large pot and dropping it into a colander, allowing the batter to drain through the holes. The batter at the bottom of the colander flows freely, but the mixture at the top appears much thicker—thick enough, in fact, for the vendor to forcefully slap the surface without submerging his hand. The process repeats as he casually looks around, until the vendor shears off the batter into individual strands of noodles, which he then drops into boiling broth to cook.
Traditionally, elastic fei chang fen noodles are made of sweet-potato starch reserved in a steaming-hot broth with pig intestines. The resulting noodle soup is then garnished with bean sprouts, scallions, peanuts, chili oil, and vinegar.
It is the unique preparation, however, by which the street vendor creates his noodles that caught our attention. By combining starch and liquid, he creates a non-Newtonian fluid: a liquid that does not flow with constant viscosity but with a viscosity that changes in response to shear forces, which are forces that push in opposite directions along two distinct parallel lines. Simply put, non-Newtonian fluids can behave as both a liquid and solid. When you apply shear forces to non-Newtonian fluids, you’re met with resistance—try punching such mixtures, and you might come away with a bruise or two. In contrast, Newtonian fluids (like water) have a relatively constant viscosity, despite shear forces and flow rates, which allow them to flow in predictable ways.
We encounter non-Newtonian fluids every day, a classic example of which is Ketchup, which stubbornly stays in the bottle and acts solid, even if the bottle is inverted. If you shake the bottle, the flow starts and then picks up speed as shear forces reduce the viscosity. Often the result is that too much ketchup dumps on the plate.
There’s no need to shake a bottle of water when you want to fill a cup. When you pour water, you can easily anticipate the trajectory and flow of the fluid; splashes are just a function of the pourer’s clumsiness.
Despite the ubiquity of non-Newtonian fluids, scientists have only recently begun to understand the mechanisms of how they flow.
Researchers at the University of Chicago have uncovered just how the molecules of non‑Newtonian fluids behave. By using high-speed videography and force sensors on mixtures of cornstarch and water (commonly known as Oobleck with an etymology tracing to the pen of Dr. Seuss), the research team observed the so-called “snowplow” effect: when a mixture is compressed, the molecules pack together to create a solid surface.
The snowplow effect partly explains how the noodles in the video can be picked up like a block one second and then flow freely the next. When the vendor smacks the batter in the video (likely done to distribute the batter or for a bit of dramatic flair), the molecules directly under his hand compress into a hard mass that is unable to exit the colander, momentarily exhibiting properties of a solid, just like the packed snow that builds up in front of a snowplow.
But when the vendor lets the batter rest in the colander, the molecules relax, and the mixture flows again. The vendor then shears off the dough into individual strands of noodles, which fall into the boiling broth below and cook. Although scientists now understand how molecules become “jammed” into a solid, they are still trying to understand how molecules relax to take on characteristics of a liquid.
To observe these unique (and often entertaining) properties of non-Newtonian fluids, try creating your own Oobleck by mixing one cup of cornstarch with one-half cup of water. And the next time you find yourself wandering through a market, take a second look at the cooking techniques of street vendors because you might be witnessing a complex scientific process!
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