Signatures of soft matter

What is soft matter? 

Soft Matter: A Very Short Introduction by Tom McLeish has just been published.

McLeish identifies six characteristics of soft matter.

1. Thermal motion 

They exhibit large local spatial rearrangements of their microscopic constituents under thermal agitation. In contrast,  "hard" materials experience only small distortions due to thermal motion.

2. Structure on intermediate length scales

There are basic units ("fundamental" structures), typically involving a very large number (hundreds to thousands) of atoms, that are key to understand soft matter behaviour. These basic units are neither macroscopic nor microscopic (in the atomic sense), but rather mesoscopic (meso from the Greek word for middle). The relevant scales range from several nanometres up to a micrometer. An example of these length scales is those associated with (topological) defects in liquid crystals, such as those shown below.

Image is from here.

3. Slow dynamics

The mesoscopic length scales and complex structures lead to phenomena occurring on time scales of the order of seconds or minutes.

4. Universality

The same physical properties can arise from materials with quite different underlying chemistries.
This characteristic is of significant practical relevance. Solving a problem for one specific material can also solve it for whole families of materials. This universality is also of deep conceptual significance as understanding a general phenomenon is usually more powerful than just a specific example. 

5. Common experimental techniques

The dominant tools are microscopy, scattering (light, x-rays, neutrons), and rheometers which measure mechanical properties such as viscosity (rheology).

6. Multi-disciplinarity

Soft matter is studied by physicists, chemists, engineers, and biologists. 

The chapter titles in the book are 

Milkiness, muddiness, and inkiness [Colloids]

Sliminess and stickiness [Polymers]

Gelification and soapiness [Foams and Self-assembly]

Pearliness [Liquid crystals]

Liveliness [Active matter]

I highly recommend the book. Hopefully, later I will write a review.