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

They are materials that can be easily deformed by external stresses, electric or magnetic fields, or by thermal fluctuations. These materials have structures much larger than atomic or molecular scale. 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 left demonstrates the "Weissenberg effect". A rod is ratated at a speed of 20<math>pi</math>/s in Oil(a) and Polyisobutylene(PIB)(b). Picture(a) shows what is observed for a simple Newtonian fluid where centrifugal forces created by rotating the rod deplete fluid from the region immediately adjacent to it and create a vortex in the liquid near the rod. Picture (b), on the other hand, the polymer solution is drawn towards the rod and climbs up. This is a typical elastic fluid response which rises from the existence of normal stress differences in shear, in other words, entanglements of long-chained polymers. (Rodriguez, Cohen, Ober, Archer, Principles of Polymer Systems 5ed,Taylor&Francis)

The image in the middle depicts the important types of soft matters we humans are mostly concerned with. They are a) Spherical micelle, b) Cylindrical micelle, c) Lamellar phase, d) Reversed micelle, e) Bicontinuous structure, and f) Vesicle. In figure a) and b) and e) are the three kinds of differently shaped micelles. Micelle is an aggregate of surfactant molecules dispersed in a liquid colloid.

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.

The reason that the colloids flocculate is due to the Van der Waals forces that now allow them to stick together (http://en.wikipedia.org/wiki/Colloid). I would imagine that simply changing the solvent will not weaken these forces (once the colloids were allowed to interact). The only way to deflocculate the suspension would be to mechanically agitate it.


Aggregation occurs between colloidal particles in a fluid when the particles have a strong enough attraction to permanently attach to one another. Aggregated colloids are known to increase the viscosity of the fluid through the same methods as dispersed colloids and through a different mechanism called screening. When a fluid with aggregates is subjected to shear flow the aggregates screens the fluid or forces the fluid to go around the aggregate therefore increasing the viscosity of the fluid. This is due to their fractal structure.


The word "Polymer" is originally Greek. Poly means 'many' and mer comes from merous which roughly means 'parts'.

Polymers are large molecules with Molecular Weights that are high enough to allow for chain entanglements. Typically, Molecular Weight of polymer is greater than 5000 g/mole. These materials may be organic, inorganic, or organometallic, and synthetic or natural in origin. Polymers are essential materials for almost every industry as adhesives, building materials, paper, cloths, fibers, coatings, plastics, ceramics, concretes, liquid crystals, photoresists, and coatings. Natural inorganic polymers include diamonds, graphite, sand, asbestos, agates, chert, feldspars, mica, quartz, and talc.

Natural organic polymers include polysaccharides (or polycarbohydrates) such as starch and cellulose, nucleic acids, and proteins. Synthetic inorganic polymers include boron nitride, concrete, many high-temperature superconductors, and a number of glasses. Siloxanes or polysiloxanes represent synthetic organometallic polymers. See also Silicone resins.

Synthetic polymers used for structural components weigh considerably less than metals, helping to reduce the consumption of fuel in vehicles and aircraft. They even outperform most metals when measured on a strength-per-weight basis. Polymers have been developed which can also be used for engineering purposes such as gears, bearings, and structural members.

Single Polymer Chains AFM.jpg

This is how real linear polymer chains "look" like as recorded using an atomic force microscope under liquid medium. Chain thickness is 0.4 nm. (Attribution should be given to the work: Y. Roiter and S. Minko)

Thermal Transition

Thermal trans.gif (http://www.uwsp.edu/chemistry/polyed/images/thermal_trans.gif)

In the solid state many polymers are amorphous rather than crystalline. However, many polymers, especially those high ordered structures do crystallize. These polymers are known as semi-crystalline materials, because typically only part of the macromolecular chain is involved in crystallization. The glass transition temperature (Tg) is the temperature at which an amorphous polymer undergoes a change from a rigid solid to a more flexible rubbery material. This temperature marks the onset of segmental motion in amorphous polymer samples. In semi-crystalline polymers, both the glass and malt transition temperature (Tm) may be observed since both amorphous and crystalline domain exist in the polymer structure. In this material, the glass transition temperature lies below the melt transition temperature. Another important thermal transition in semi-crystalline polymers is the crystalline transition temperature (Tc). This is the temperature at which the polymer sample undergoes crystallization. This crystallization happens between the glass and the melt transition temperature.

Surfactant assemblies

Associated structures

Witten: Association is a temporary joining together of the structures. The structure thus joined can transmit forces and thus alter mechanical properties strongly, but at the same time they are weak enough to break and reform over the time of an experiment. Thus the associations alter themselves in response to the local stress or flow in the liquid. Typical examples are micelles. Gk2x20.gif

Properties of soft matter

  • Viscoelasticity

Some materials exhibit properties of both elastic and viscous materials. When strain is applied to viscoelastic material, its viscosity results in a strain rate that depends on time. However, one the strain is removed, the material will slowly return to its original configuration. Examples of such materials include amorphous polymers, semicrystalline polymers, and biopolymers.

  • Turbidity/opacity

Turbidity is the cloudiness or haziness of a liquid due to particles suspended in the liquid. Turbidity is one of the many standards placed upon drinking water in the USA. The upper limit on turbidity for drinking water is 0.3 NTU or Nephelometric Turbidity Units measured by a nephlometer. Fun names!

  • Irreversible fragility

When you say irreversible fragility, do you mean a form of hysteresis? If so, there is some interesting mention of some of the thermodynamically irreversible propoerties found in surface interactions in Israelachvili's book (like with adhesion on p 323). --BPappas 05:12, 22 September 2008 (UTC)

  • 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)
Video: Cholesteric Liquid Crystals Changing Color responding to changes in temperature. [1]

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.

Most newspapers in the United States now use soy based inks rather than petroleum based inks. Since these inks take a longer time to dry than other types of ink, typically they are used on papers that absorb most of the ink. When a person reads a newspaper, the ink has often not yet dried and will still rub off on their hands. Until recently, all soy-based inks still contained some petroleum resin. One recently patented ink does not contain any soy. I cannot find any information on what the challenges were production and chemical-wise for switching entirely from petroleum. Does anyone know why petroleum products (especially resins) were still needed in the soy-based inks?

Comment: Apparently the main problem with soy-only inks is that they do not dry and harden properly, and that the only way to produce a soy-only ink would be to use oxidative polymerization of the unsaturated fatty acids contained in the soy oil. This does not produce as absorbent or as spreadable an ink as is needed for paper production today, but according to this piece (I just put the HTML version here, not the pdf), almost all inks were vegetable-only before the 1950s. In order for the ink to harden using today's printing techniques like heatset, there needs to be a resin present. Also, some papers won't absorb the soy oil if they are not porous enough, and thus need lower viscosity petroleum additives to draw the ink into the printing material [2] --BPappas 04:58, 22 September 2008 (UTC)

This is a quote from a US department of agriculture document on the relative benefits of soy verses petroleum inks: "The newspaper industry is a large user of soy inks. Soy inks account for more than 90 percent of all colored inks and about one-third of black inks used by U.S. newspapers. Soy inks produce better colors and provide greater clarity with reduced rub-off on readers' hands. The lighter color of soybean oil makes it ideal for color inks because the true color of the pigments can show through. Newspaper pictures are composed ofa pattern of dots, which with petroleum-based inks increase in size during the press run, reducing the clarity of the picture. With soy inks, the dots remain relatively the same size, keeping picture clarity constant throughout the press run. Moreover,soy inks can be used for printing newspapers without a change in equipment or printing methods. Soy ink has been found to be a better carrier of pigments, driers, and other agents than other ink vehicles, which can result in less press time, lower cost, and higher quality results." (http://www.bioplastic.org/industrial-use-1997.html)

(http://en.wikipedia.org/wiki/Soy_ink, http://pubs.acs.org/cen/whatstuff/stuff/7646scit2.html)

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.

An interesting method to achieve creamier ice cream is using liquid nitrogen during the primary freezing of the ice cream. This method has been adopted by brands like Dippin' Dots and Blue Sky Creamery. The rapid freezing makes the crystal grains smaller without needing as much milkfat and therefore improves the creaminess of the cream. This is also a popular demonstration for showing phase changes.

Comment from rshankar: Ice cream is a good example of the use of polymeric chains to keep colloidal particles separated and stabilized in a non-polar solvent (such as oil). Since the solvent is non-polar, the colloids cannot ionize and repel one another. To get around this, long polymeric chains are added to the colloids. Due to entropy, the chains remain bunched up, which does not allow other colloidal particles to approach. This is the reason that egg yolks and cheaper stabilizers like guar gum are added to ice cream.

Source: http://www.mrl.ucsb.edu/~seshadri/SoftMatter.pdf

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

Along the lines of moving from great biology to great physics, there is some fascinating work being done around the world on biological materials, which are comprised of many repeats of a same molecule or protein. These materials are rightfully called "biopolymers". Typical biopolymers you can find in the modern scientific literature are:

  • actin: one of the major proteins that confers structure to the cell (the cytoskeleton).
  • tubulin or microtubules: another major intracellular protein, known to serve as highways for cellular transport and for motion of chromosomes during mitosis.
  • vimentin: another major intracellular protein, which fits into the category of intermediate filaments (intermediate because they aren't quite as persistent as microtubules, but not quite as flexible as actin filaments either). In the category of intermediate filaments fit many proteins, which we have only begun to understand in recent years.
  • fibrin: an extracellular protein responsible for the solidification of blood clots. One can rightfully expect that the dynamics of fibrin polymerization and its strength are fascinating topics to study, just by thinking of how these little monomers present in the blood have to quickly assemble into filaments to block the flow of blood!
  • collagen: probably the most abundant extracellular matrix protein in mammals, this triple-helix (comprised of three tropocollagen molecules) self-assembles into fibrils and those in turn can form higher hierarchical structures. Collagen is one of the major components of skin, tendons, ligaments and cartilage.

Many biopolymer networks in the body have a common dual purpose: provide sensitivity (i.e. compliance) to small deformations and stresses, while maintaining integrity under high stress. Typical elastic materials have a constant Young's modulus at low strains and then weaken or flow at higher stresses or strains. Biopolymer networks are known to stiffen at strains on the order of 10%; this is rightfully called "strain-stiffening". What this means is that the harder you pull on the material (without breaking it), the more it resists. And this resistance is not proportional to the amount of pull; in rheological terms, stress increases nonlinearly (supralinearly) with strain.

Scientists around the world are trying to understand theoretically, experimentally and numerically the origin of these nonlinear mechanical properties. They are getting closer to finding the answer, which is certain to be system-dependent. It is believed to be a combination of non-affinity, collective fiber reorganization or realignment and network connectivity.

Pierre-Gilles de Gennes and his explanation of Soft Matter

Pierre-Gilles de Gennes was born in Paris, France. A picture of him is below:


He started out as a researcher working on topics like neutron scattering and magenetism and got his PhD in 1957. In 1961, he was an assistant professor in Orsay and started their group on superconductors. He later switched to studying liquid crystals in 1968. From 1976 on he was the director of the École Supérieure de Physique et de Chimie Industrielles de la Ville de Paris (ESPCI). It wasn't until 1980 when he started focusing on interfactial problems.

According to nobelprizeorg.com, "After receiving the Physics Prize in 1991, de Gennes decided to give talks on science, on innovation, and on common sense, to high school students. He visited around 200 high schools during 1992-1994." The research he was focused on before his death was in respect to cel adhesion and particularly brain function and memory objects.

He has long been thought as the founding father of Soft Condensed Matter. After a life time of contribution to scientific research on soft matter, he was awarded the Nobel Prize in physics in 1991 for "discovering that methods developed for studying order phenomena in simple systems can be generalized to more complex forms of matter, in particular to liquid crystals and polymers."

In his Nobel Lecture, he humorously and accurately described what he has learnt and discovered regarding soft matters. Below are the first few paragraphs of the lecture:

What do we mean by soft matter? Americans prefer to call it “complex fluids”. This is a rather ugly name, which tends to discourage the young students. But it does indeed bring in two of the major features: I) Complexity. We may, in a certain primitive sense, say that modern biology has proceeded from studies on simple model systems (bacterias) to complex multicellular organisms (plants, invertebrates, vertebrates...). Similarly, from the explosion of atomic physics in the first half of this century, one of the outgrowths is soft matter, based on polymers, surfactants, liquid crystals, and also on colloidal grains. 2) Flexibility. I like to explain this through one early polymer experiment, which has been initiated by the Indians of the Amazon basin: they collected the sap from the hevea tree, put it on their foot, let it “dry” for a short time. And, behold, they have a boot. From a microscopic point of view, the starting point is a set of independent, flexible polymer chains. The oxygen from the air builds in a few bridges between the chains, and this brings in a spectacular change: we shift from a liquid to a network structure which can resist tension - what we now call a rubber (in French: caoutchouc, a direct transcription of the Indian word). What is striking in this experiment, is the fact that a very mild chemical action has induced a drastic change in mechanical properties: a typical feature of soft matter.
Of course, with some other polymer systems, we tend to build more rigid structures. An important example is an enzyme. This is a long sequence of aminoacids, which folds up into a compact globule. A few of these aminoacids play a critical role: they build up the “active site” which is built to perform a specific form of catalysis (or recognition). An interesting question, raised long ago by Jacques Monod, is the following: we have a choice of twenty aminoacids at each point in the sequence, and we want to build a receptor site where the active units are positioned in space in some strict way. We cannot just put in these active units, because, if linked directly, they would not realise the correct orientations and positions. So, in between two active units, we need a “spacer”, a sequence of aminoacids which has enough variability to allow a good relative positioning of the active sites at both ends of the spacer. Monod’s question was; what is the minimum length of spacers?
It turns out that the answer is rather sharply defined(l). The magic number is around 13-14. Below 14 units, you will not usually succeed in getting the desired conformation. Above 14, you will have many sequences which can make it. The argument is primitive; it takes into account excluded volume effects, but it does not recognise another need for a stable enzyme - namely that the interior should be built preferably with hydrophobic units, while the outer surface must be hydrophilic. My guess is that this cannot change the magic number by much more than one unit. Indeed, when we look at the spacer sizes in a simple globular protein like myosin, we see that they are not far from the magic number.

  • NOTE: The full lecture can be found at [3]

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