What is Soft Matter?

This blog will be about soft matter, a branch of physics concerned with small, but not quantum-small matter.

There seems to be an awful lot of blogs (and general media coverage) about particle physics, cosmology, astronomy, evolution and climate change. I really like to read these, but what I am really interested in is soft matter, and I always feel sad that this interesting topic is so left out. The easiest way to change that is to try to write about it myself. I am by no means an in-depth expert of the field, so please don’t hold me responsible for any stupid things I might write.

So, what exactly is soft matter?

The easiest way to understand this is to look at your house, and think about which things are soft, so that you can deform them with your hands: Your toothpaste, liquids, gels,… while metals and diamonds are not soft.

Soft matter usually has a structure at a length scale of 100 nanometers to a couple of micrometers. Then the material is also called a colloid (though with that term you often mean solid bodies of that size in a liquid). The energy of these structures is about the same as the thermal energy at room temperature, so a small change in energy – for example by pressing the material with your hand – can make a lot of difference.

On the other hand, metal and diamond are crystals, so they have an atomic periodic ordering on a much smaller length scale, with much higher energy. So you have to press a lot harder, or make the temperature much higher to change these materials.

For soft matter, the main properties do not come from the material used, but rather from the structure that is formed. So a big part of research is to look at model systems of very simple objects, for example hard spheres, with that size and look at the behaviour of a system of many of the spheres. But I will talk more about hard spheres as model in one of the next posts.

So just for now: Thinking of your house, and how many things in it you can deform with your hands, should make it clear to you that soft matter actually plays a huge role in our lives. So if you are interested in physics to understand how our world works and where all the interesting things we see come from, you should not only be interested in quantum physics or cosmology, but also in soft matter.

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