What is soft matter

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Structured and fluid

In a few words, soft matter is:

  • Things that don’t hurt your hand when you hit them.
  • Synonymous with “complex fluids”
  • Examples: hair gel, mayonnaise, shaving cream, colloidal crystals, polymer solutions and blends

The amazing properties of soft materials come from their a 'subtle balance' between energy and entropy which leads to rich phase behavior and spontaneous (and often surprising) complexity (Jones 2002). They are considered 'structured fluids' because they have the local mobility of liquids, but their constituents are polyatomic structures.

Macosko Fig 5-3-3.gif Homberg Fig 2-1.gif Weitz 339-60-1989.png
Macosko Fig. 5-3-3 Homberg Fig. 2.1 Weitz Nature 339,60,1989.

Comments on Figures: Elaborate HERE! The image on the right shows transmission electron micrographs of clusters of gold, silica and polystyrene. The left column is showing structures in DLCA regime (diffusion-limited colloid aggregation), whereas the right one shows structures in RLCA regime (reaction-limited colloid aggregation). “Diffusion-limited colloid aggregation occurs when there is negligible repulsive force between the colloidal particles, so that the aggregation rate is limited solely by the time taken for clusters to encounter each other by diffusion. Reaction-limited colloid aggregation occurs when there is still a substantial, but not insurmountable repulsive force between the particles, so that the aggregation rate is limited by the time taken for two clusters to overcome this repulsive barrier by thermal activation” (Weitz, Nature 1989). The structures are fractal in both DLCA and RLCA, which means that mass scales proportional to (r/a)^4, where r is radius of gyration and a is radius of particles in the structure. DCLA clusters tend to be more open and thin, understandable considering their fractal dimension is below 2. On the other hand RLCA clusters appear to be more compact with fractal dimension above 2. Still, resemblance between different structures in the same regime is remarkable.

Classes of Structured Fluids


Witten states:

The interaction energy of two colloidal particles in a given solvent is also magnified because of their bulk. Consequently, small changes in the solvent can have a large effect on the interaction energy. This makes it possible to change the interaction between two colloidal particles abruptly from an effective hard-core repulsion to an attraction whose strength is many times the thermal energy kbT. With such an attraction the particles must stick together when they encounter each other. The particles flocculate or precipitate

Does anyone know if this process is reversible? or will the particles typically remained clumped despite reversing the changes in the solvent.



Surfactant assemblies

Associated structures

Properties of soft matter

  • Viscoelasticity
  • Turbidity/opacity
  • Irreversible fragility
  • Temperature sensitivity

Indian boot - de Gennes 1996.gif 2500 years ago, South American Indians take sap from a hevea tree, cover their feet, wait about 20 minutes, and a pair of boots is created. The latex is crosslinked by oxidation only, so is weak. In 1830 Charles Goodyear decides to boil the hevea latex with sulfur (heaven only knows why), the crosslinking is much, much better, and eventually the radial tire is created. (de Gennes, 1996, p.4)

Length scales and order

When studying soft matter, it is important to be aware of the length scales which control the macroscopic behavior. Jones (Soft Condensed Matter, 2002) points out that the length scales of soft condensed matter fall in between atomic and macroscopic scales. This makes course-grained models appropriate for studying these materials. Such models focus on the topological features of the system, rather than specific details of the chemistry. Despite the mesoscopic length scales, fluctuations from Brownian motion are still important; typical bond energies are on the order of thermal energies (kT).

Polymers in solution.png Surfactants in solution.png Particles in dispersion.png
Polymers in solution Surfactant solutions Particle dispersions


De Gennes 1997 p 29.gif De Gennes 1997 Fig 1-1.gif De Gennes 1997 Fig III-3.gif
Structure and size, de Gennes, 1997, p.29 Motion and size, de Gennes,1997, Fig I-1 Structure and concentration, de Gennes, 1993, Fig. III-1
Fractals Random walk eqn.png Polymer cStar eqn.png


Ink making for soft matter physicists

Soot - de Gennes 1996.gif Poor dispersion - de Gennes 1996.gif Good dispersion - de Gennes 1996.gif
de Gennes, 1996, p.29

If you think this is primitive, check out how newpaper ink is make.

Soft matter - Ice cream!

Plain frozen cream is as hard as rock, but the micro-scale structure of ice cream turns it into a wonderful dessert. Ice cream is a three phase mixture of pure water crystals, concentrated cream and sugar, and air pockets. The cream solution can remain liquid, since the sugar lowers the freezing point below 0 C. It coats each of the millions of ice crystals and lightly binds them together. The texture of ice cream is further improved by air pockets introduced during mixing. These air pockets are stabilized by fat molecules from the cream. The air weakens the network of crystals and cream, making the ice cream easier to serve and to eat. Several variations of ice cream have evolved over the centuries. American ice cream traditionally uses a combination of milk and cream, whereas French ice cream uses lower-fat milk, with egg yolks as a stabilizer. Italian gelato also uses egg yolks, but contains less air, resulting in a denser product. Low-fat ice cream utilizes additives such as corn syrup, powdered milk, and vegetable gums. Indian kulfi is based on a recipe from the 16th century, in which milk is boiled down to concentrate the proteins and sugar, then frozen without stirring.
Ice cream - Hamley.gif TEM of a typial ice-cream. (a) Ice crystals, average size 50 nm, (b) air cells, average size 100-200 mm, (c) unfrozen material. (W.S. Arbuckle, Ice Cream, 2nd ed., Avi Publishing, 1972. also, Hamley, Fig. 3.20)
For more information about ice cream, see On Food and Cooking, 2nd ed., Scribner, 2004. by Harold McGee (pp. 39-45), from which this section is based.

From great biology to great physics

Connect these scientists:

  • Thomas Graham (1805-1869)
  • Robert Brown (1773-1858)
  • Michael Faraday (1791-1867)
  • Ludwig Boltzmann (1844-1906)
  • Albert Einstein (1897-1955)
  • Jean Perrin (1870-1942)

Hint: Size dependence of diffusion

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