How to Deform an Egg Yolk? Soft Matter Deformation in Liquid Environment

The phenomenon of soft matter in a liquid environment widely exists in nature. Some examples include the soft brain matter that is bathed in the cerebrospinal fluid inside the hard skull, a soft egg yolk that is bathed in the fluidic egg white inside the eggshell, and red blood cells in our circulation system, etc. Deformability is one of the most important features of these soft matters. For a soft matter bathed in a liquid environment and enclosed in a rigid container, an interesting yet fundamentally important question is that how one could deform or even damage the soft matter without breaking the container. This answer to this question could shed some light on the understanding of such problems as concussive brain injury. It also helps one to develop cell separation and processing equipment without affecting its viability.

In this study, we developed a novel experimental method to examine the response of a soft capsule bathed in a liquid environment to sudden external impacts. A translational and a rotational impact system have been developed to investigate the deformation of the egg yolk, in which a transparent container holding both the egg yolk and egg white is used to replace the eggshell. We found that the soft matter is not sensitive to translational impacts, but is very sensitive to rotational, especially decelerating-rotational impacts. During the decelerating rotational impacts, the yolk experienced a dramatic deformation.

To reveal the fundamental mechanism of the soft matter deformation observed in the experiment, a theoretical model has been developed. In our model, it shows that pressure distribution around the yolk experienced a sever change during the deceleration impact. It is the pressure distribution, the centrifugal force, and the shape of the membrane together play a critical role in causing the deformation of the soft object. The existence of the membrane separates the space into the fluid region outside and the inner soft matter. The momentum transferred from the outer shell to the membrane and then to the liquid yolk inside. Since the travel distance is different, the pressure field of the fluid is disturbed, leading to the deformation of the membrane-bound egg yolk and the fluid flow outside of the membrane.

This finding, as the first study of its kind, reveals the fundamental physics behind the motion and deformation of a membrane-bound soft object, e.g., egg yolk, cells, soft brain matter, etc., in response to external impacts.