Science Can’t Figure Out Why Some Liquids Flow

Fluids are essential for daily life. Without water, survival is impossible. And without chocolate syrup, desserts would lack joy. Not all liquids behave the same way. Some can act quite strangely under certain conditions. Scientists have studied these odd fluids for decades. Yet, their mysteries remain largely unsolved. Recently, a breakthrough might have occurred. It involves glass beads and laser beams.

Fluids fall into two categories: Newtonian and non-Newtonian. Newtonian fluids are straightforward. Water is a good example. It can quickly slip from your hand. Surprisingly, chocolate syrup also belongs to this group. It flows slowly, making a mess if you're not careful. Remember to lick your palm before it drips away!

Viscosity measures a fluid's flow resistance. It indicates how easily one molecule can move past another. Higher viscosity means slower flow. Newtonian fluids follow Newton's law of viscosity. Their viscosity remains constant, no matter the force applied.

Non-Newtonian fluids behave differently than regular fluids. They can change their viscosity in surprising ways. There are various subcategories of these fluids. One type is called dilatant. Dilatant fluids increase in viscosity when force is applied. Examples include quicksand, silly putty, and oobleck. Oobleck is a mixture of cornstarch and water. If you dip your fingers in gently, it flows easily. However, if you hit it hard, it becomes solid. With enough cornstarch, you could even run across a pool of it. But remember, that could lead to trouble!

On the flip side, some fluids like ketchup are called pseudoplastics. When at rest, ketchup sits still. But give the bottle a tap, and it flows out easily. At a molecular level, long chains of atoms called polymers get tangled. When you shake or hit the bottle, they stretch and align. This change allows the ketchup to slide out smoothly. Hopefully, it goes on your fries, not on your clothes. Non-Newtonian fluids have many quirky behaviors, and they can be fascinating to explore!

Scientists may have found answers to a problem that has puzzled them for over 50 years. This issue first appeared in the 1960s. Engineers were trying to extract oil using fluids that contained long chain polymers. These fluids had a specific behavior based on the pumping rate. When pumped slowly, they worked fine. But when pumped faster, they became very viscous, similar to oobleck.

The key to this behavior lies in how these fluids flow through tiny spaces in the soil. When not confined, their viscosity actually decreases under more force, much like ketchup. For a long time, scientists believed the polymers might be clogging the soil's pores. Yet, this did not explain why the fluids flowed easily when the rate dropped.

A significant breakthrough came in late 2021. Researchers struggled with studying soil and other porous materials because they are not transparent. To address this, they created a medium using glass beads and made a polymer solution with the same refractive index. This ensured that both the liquid and solid bent light in the same way.

To visualize the fluid's movement, they added a red dye that emitted a specific wavelength of light when exposed to a laser. They also included tracer particles to emit a different color when excited by another laser. With this complex setup, they observed the fluid flowing at various rates.

They discovered that as the fluid moved faster, the long polymers began to tumble. This tumbling action pushed nearby molecules and created a phenomenon known as elastic turbulence. This led to the formation of eddies that slowed the entire flow.

The researchers believe this new understanding could aid in purifying groundwater. It may help develop new polymer solutions that push water through rocks, trapping contaminants. However, more research is needed since elastic turbulence is not fully understood. Perhaps next, they will solve why some people enjoy ketchup on scrambled eggs. While unrelated to non-Newtonian properties, it's a mystery worth pondering.