Introduction to Soft Matter

Thread Content

If you were in a room full of mosquitoes, where would you hide? The answer is to stay against the wall, hiding in a corner; this way, there will be no mosquitoes behind you, and you only need to deal with those in front. An interesting experiment conducted by A. D. Dinsmore, D. T. Wong, Philip Nelson, and A. G. Yodh from the Department of Physics and Astronomy at the University of Pennsylvania was published in Physical Review Letters in 1998. This experiment has some similarities to our mosquito incident. In the experiment, several scientists first placed a \"large ball\" with a diameter of only 0.474 micrometers into a miniature pear-shaped container. Through long-term, repeated optical imaging, we observed that this large sphere could appear anywhere within this container. Then they added many smaller spheres (with a radius of 0.042 micrometers), and in the photographs, it appeared that the large sphere could basically only stay on the edges. How did this result occur? First, we know that both the large balls and the small balls inside the container are in constant random motion, and at the same time, the small balls keep colliding with the large balls from various directions. At any given moment, the large ball generally experiences different amounts of force from the impacts of the small balls in various directions; as a result, it moves in a certain direction due to these varying forces. When the large ball hits the wall of the container, it realizes that nothing will hit it on the side against which the container is positioned; all impacts come from the other side. So these impacts forced it to lean against the edge of the container. The actual process is more complex than what we’ve analyzed here, but that’s roughly how it works: the big ball hides against the wall in order to avoid being \"stung\" by the small balls, the \"mosquitoes\". 　From a physical analysis perspective, such a result is due to the effect of entropy. Entropy is a physical quantity that characterizes the degrees of freedom of a system (or, in more layman’s terms but not entirely accurate, the range within which large and small particles can move freely). Since our balls are hard and do not deform, there are always some areas around the large ball and the walls that the small balls cannot reach; in other words, those areas are not accessible to the small balls. And when the large ball is next to the wall, there is overlap in the areas where these small balls cannot go. Accordingly, the area where the small ball can go becomes a bit larger. Thus, the degrees of freedom of the system increase as well. Physical laws state that a closed system always tends toward maximum entropy, meaning that such systems prefer states with higher degrees of freedom. As a result, the large ball, constrained by this rule, moves to stay at the edge of the container. 　The physical system discussed in the experiment mentioned above belongs to the category of physical systems known as soft matter. In 1991, Nobel laureate and French physicist P. G. De Gennes used the term \"soft matter\" as the theme of his speech at the Nobel Prize ceremony to describe substances such as complex liquids, and this term gained widespread acceptance. From then on, the term \"soft matter\" gradually replaced what Americans call \"complex fluids\", thereby fostering the development of an interdisciplinary field that spans physics, chemistry, and biology. 　　　Soft materials such as liquid crystals, polymers, colloids, membranes, foams, particulate matter, and biological systems are widely present in nature, living organisms, daily life, and industry. De Gennes gave an important characteristic of soft matter: weak forces cause large changes. In his popular science book \"Soft Matter and Hard Science,\" he uses rubber as an example to illustrate the properties of soft matter. Natural rubber latex oxidizes to form solidified rubber, but this rubber is very weak and can easily break down due to further oxidation by air. Natural rubber becomes very durable after being vulcanized, and is not prone to breaking. Sulfur, which is in the same group as oxygen, has only slightly less chemical reactivity than oxygen, yet the effects it produces are quite different. This is what is known as the weak force causing major changes. De Gennes wrote in his book: “If you count the number of carbon atoms that react with sulfur, you will find that they account for only 1/200, which is a representative figure.” However, this extremely weak chemical reaction is sufficient to cause the physical state of the substance to change from liquid to solid: the fluid turns into rubber. This proves that the state of matter can be changed through slight external influences, just as a sculptor can alter the shape of clay by gently pressing it with his thumb. This is the core and fundamental definition of soft matter. The state of a physical system can be described jointly by its internal energy and the product of entropy and temperature. Since the change in internal energy is related to the forces acting on the system, at a constant temperature, for soft materials, if the forces applied are not strong, then the change in their internal energy will also be small. However, if such weak forces are to cause significant changes in the system, then its entropy must change drastically. In other words, in soft matter, the changes in the system are primarily caused by entropy, or rather, entropy plays a dominant role. Such soft matter can thus be called entropy-driven matter. Under this influence, soft matter systems exhibit many novel behaviors; for example, originally chaotic microscopic systems become orderly, and complex protein molecules fold into specific structures on their own. By utilizing these properties, we can create many soft materials with special characteristics that are difficult to replace by hard materials.