Difference between revisions of "What is soft matter"
(→Structured and fluid)
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Revision as of 16:51, 21 September 2008
- 1 Structured and fluid
- 2 Properties of soft matter
- 3 Length scales and order
- 4 Ink making for soft matter physicists
- 5 Soft matter - Ice cream!
- 6 From great biology to great physics
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||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 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
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 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.
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)
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.
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.
Properties of soft matter
- Irreversible fragility
- Temperature sensitivity
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||Surfactant solutions||Particle dispersions|
|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|
Ink making for soft matter physicists
|de Gennes, 1996, p.29|
If you think this is primitive, check out how newpaper ink is make.
Soft matter - Ice cream!
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