A growing number of professional football players have been diagnosed with a neurodegenerative disease called chronic traumatic encephalopathy (CTE), likely the result of suffering repeated concussions or similar repetitive brain trauma over the course of their careers. It’s also common in other high-contact sports like boxing, Muay Thai, kickboxing, and ice hockey. We might find clues about the underlying physics by studying the deformation of egg yolks, according to a new paper published in The Physics of Fluids. This in turn could one day lead to better prevention of such trauma.

Egg yolk submerged in liquid egg white encased in a hard shell is an example of what physicists call “soft matter in a liquid environment.” Other examples include the red blood cells that flow through our circulatory systems and our brains, surrounded by cerebrospinal fluid (CBR) inside a hard skull.  How much a type of soft matter deforms in response to external impacts is a key feature, according to Villanova University physicist Qianhong Wu and his co-authors on this latest study. They point to red blood cells as an example. It’s the ability of red blood cells to change shape under stress (“erythrocyte deformability“) that lets them squeeze through tiny capillaries, for instance, and also triggers the spleen to remove red blood cells whose size, shape, and overall deformability have been too greatly altered.

In the case of traumatic brain injury, it is linked to how much the brain deforms in response to impact. The precise cause of CTE is still a matter of ongoing research, but the prevailing theory holds that repetitive brain trauma can damage blood vessels in the brain, causing inflammation and the growth of clumps of a protein called Tau. Eventually those clumps spread throughout the brain killing off brain cells. Those suffering from CTE often experience memory loss, depression, and in severe cases, dementia, among other symptoms.

Prior studies have shown that deformation of soft matter in a liquid environment occurs in response to sudden changes in the fluid field, such as shear flow or a sudden change of the flow pathway. Wu et al. were interested in the specific case of soft matter in a liquid environment that is also enclosed in a rigid container—like the yolk of an egg, surrounded by liquid egg white, all encased in a shell. They wondered if it was possible to break the yolk without breaking the shell, since it’s the case with most concussions that the brain can be damaged without cracking the skull.

To answer that question, Wu et al. set up a simple preliminary experiment with a Golden Goose Egg Scrambler, a novel kitchen device that enables users to scramble an egg right in the shell. Wu’s team applied rotational forces to scramble the egg and were intrigued by how the egg yolk deformed and broke while the shell remained intact. That inspired them to conduct additional experiments to glean insight into the fundamental flow physics behind the effect.

They purchased fresh eggs from a local grocery store, removed the yolks and egg whites, and then placed them in a transparent rigid container, the better to monitor the deformation by recording the entire process with high-speed cameras. They built two separate apparatus. One administered so-called “translational impact”—i.e., striking the container directly—via a small hammer falling from a vertical guide rail (see Fig 1A in gallery), with a spring at the bottom enabling the container to move vertically. They used an accelerometer to measure the container’s acceleration.

For the second setup (see Fig 1B in gallery), they connected the container to an electric motor to study two types of rotational impact: accelerating rotational impact and decelerating rotational impact (i.e., when the outer contained is speeding up or slowing down as it rotates). They also peeled off the membranes surrounding the fresh yolks and suspended them in petri dishes filled with water, the better to study how those membranes, too, respond to stress.

Wu et al. were somewhat surprised to find that, in the case of translational impact, there was almost no deformation of the yolk. Instead, the entire container (and its contents) moved as a single rigid body. In the case of accelerating rotational impact, the team found that the yolk would start out in a spherical shape and then begin to stretch horizontally to form an ellipsoid. The yolk could maintain a stable ellipsoid shape for several minutes if the angular velocity was kept constant.

The most intriguing results occurred in the case of decelerating rotational impact. Here, the yolk began deforming significantly almost immediately, expanding horizontally and increasing its radius at the center—sufficient deformation to severely damage the yolk under sustained stress.

“We suspect that rotational, especially decelerational rotational, impact is more harmful to brain matter.”

To make sure this wasn’t primarily an effect of the yolk as a biomaterial, Wu et al. conducted the same experiment with synthesized soft capsules submerged in a calcium lactate solution, enclosed by a thin membrane of calcium alginate. They got similar results, confirming that “the dominant mechanism leading to the deformation of soft matter in a liquid environment is a result of mechanical forces instead of biological responses,” they wrote.

Based on this, “We suspect that rotational, especially decelerational rotational, impact is more harmful to brain matter,” said Wu, and that centrifugal force likely plays a critical role. “The large deformation of brain matter during this process induces the stretch of neurons and causes the damage.” This could explain why a boxer can get knocked out by a sharp blow to the chin. “Considering the chin is the farthest point from the neck, hitting on the chin could cause the highest rotational acceleration/deceleration of the head,” the authors concluded.

“Critical thinking, along with simple experiments within the kitchen, led to a series of systematic studies to examine the mechanisms that cause egg yolk deformation,” Wu said of the implications of their findings. “We hope to apply the lessons learned from it to the study of brain biomechanics as well as other physical processes that involve soft capsules in a liquid environment, such as red blood cells.”

DOI: Physics of Fluids, 2021. 10.1063/5.0035314  (About DOIs).

Listing image by Ji Lang/Qianhong Wu

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