ソフトジャム固体の特性
Hello everyone, SCIEN-SPOT is a podcast that shines a spotlight on the latest scientific
technology from Japan. Your host is REN from SCIEN-TALK. Today we are going to talk about
supplies in connection hidden things that seem completely different in our daily lives.
Mayonnaise in your fridge and a glass cup on your table. It turns out these two are deeply
related in the world of physics according to a joint press release from University of Tokyo and
Kyushu University. Let's explore their secret. First, let's talk about one of today's main
character, soft jammed solids. You can imagine these as substances like mayonnaise, shampoo form
or even coffee form. They are materials where soft particles are packed together
randomly and tightly, like a traffic jam but for particles. They don't flow like a liquid
nor they rock hard like a stone. They exhibit a curious property in between known as viscoelasticity.
For example, honey is highly viscous and a rubber is highly elastic. Soft jammed solid
possesses both stickiness and elasticity. However, their viscoelastic behavior hasn't
been fully understood until now. What was particularly puzzling was a phenomenon called
enormous viscous loss. This is a counterintuitive phenomenon where the viscosity of the material
マヨネーズとガラスの特性
or its stickiness dramatically increases when deformed slowly. Imagine a slime that offers
more resistance the slower you try to move it. Why this happens has been a mystery for a long time.
On the other hand, our second main character today is glass. Window glass or drinking glass known as
silica glass are materials where silicon and oxygen atoms are arranged randomly and fixed
in place. Unlike crystals, their atoms are not arranged in a regular ordered pattern.
If we focus on their structure, both soft jammed solids and glass share a commonality.
They are amorphous solids, meaning their atoms or particles are arranged in this ordered way.
If their structures are similar, do their physical properties also have something in common?
This was a big question in this research. So in this study,
researchers from the University of Tokyo and Kyushu University succeeded for the first time
in the world in quantitatively understanding the viscoelasticity of soft jammed solids.
They focused on highly density emulsions, a typical example of mayonnaise.
For this experiment, they prepared an emulsion made of very tiny oil droplets,
about 0.5 micrometers in diameter. Then, using a special technique called microleology experiments,
柔らかい固体の物理的特性
they measured the emulsion's viscoelasticity. Unlike unconventional rheology which applies
a force to the entire material, microleology involves inserting the micro-sized colloidal
particles into the materials and tracking their movement with lasers to study the material's
viscoelasticity. It's like floating tiny boats in the vast ocean of mayonnaise and studying the
overall properties of the mayonnaise by observing their subtle movements. This method allows for
measurements of samples that are otherwise difficult to test and also enables high-frequency
measurements. Simultaneously, their advanced theoretical approach, they constructed a
mathematical model for high-density emulsions and used it to calculate viscoelasticity.
Surprisingly, these complex theoretical calculations quantitatively matched the
experimental result perfectly. This means they can now precisely predict the viscoelastic
behavior of soft jammed solids and controlled conditions. From this theory, it became clear that
the anomalous, vigorous, viscous loss in soft jammed solids directly leads to the boson peak
vibration, a low-frequency vibration universally found in glass. The boson peak vibration,
which has been studied for many years in glass physics, describes a phenomenon where
アモルファス固体とその動き
atoms and molecules in glass don't vibrate in regular, neat waves but rather exhibit
spatially disordered, localized, shaky vibrations. Imagine a crowd of people randomly stopped
bumping into neighbors and each suddenly snaking in their place on a seemingly inefficient and
chaotic motion. The researchers discovered that in soft jammed solids, a similar low energy,
spatially disordered movement of particles occurs, closely resembling the boson peak
vibration in glass. And it turns out this shaky motion is precisely what causes mayonnaise to
become abnormally sticky when moved slowly. That is, it causes anomalous viscous loss.
In other words, the theory for glass's boson peak can now explain the peculiar stickiness
of mayonnaise. This discovery not only surprised us but also represented a crucial step toward our
major goal in physics, the unified understanding of diverse anomalous solids. Amorphous solids
include not only mayonnaise and glass but also granular materials like sandpiles or grains
composed of particles typically millimeters or centimeters in size. This research provides
a profound insight, suggesting that mayonnaise, glass, and sand, which seem completely different
ソフトジャム固体の物理的弾性
in our daily lives, might actually be governed by the same fundamental physical laws.
The approach established in this study is expected to significantly contribute to
understanding the physical elasticity of various soft jammed solids around us,
such as microgels, cell cytoplasms, and the cell aggregate. Just thinking about a common item
like mayonnaise having such a deep physique hidden within it, isn't the science truly fascinating?
That's all for today's SciencePod. This podcast is broadcasted daily on a weekday morning in both
Japanese and English. I'd love for you to listen to the podcast and post your notes and thoughts
with the hashtag SciencePod. See you next time.
