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Molecules


Soft Matter

Soft matter is a subfield of condensed matter that is most imtimate to our daily life. Its predominant physical behaviors occur at an energy scale comparable with room temperature and therefore easily deformed by thermal stresses or thermal fluctuations. Soft matter is a misnomer because
Soft Matter Length Scale many materials in this class are not soft to the touch. It is better defined as "mesoscopic matter" since the size of the basic constituents ranges from a few nano-meters to about one micro-meters (Fiugre 12-35a) - at the boundary between quantum and classical objects (Figure 12-35b). It is the properties and interactions of these mesoscopic structures that determine the macroscopic behavior of the material; the quantum

Figure 12-35a Soft Matter
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Figure 12-35b Length Scale
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effect in the atomic or molecular scale are unimportant. Some examples for the soft matter are listed below.
Vesicle Emulsion
  • Micelles, Vesicles, and Emulsions -
    • Micelle is an aggregate of surfactant molecules dispersed in liquid. Surfactant is a class of organic chemicals call amphiphiles, which has a hydrophilic (water loving) head and a hydrophobic (water hating) tail (see surfactant for more detail).

Figure 12-35e Vesicle
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Figure 12-35f Emulsion
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    • Vesicle is a double layers sphere made from surfactant molecules with the hydrophilic heads facing the aqueous solutions on the outside and inside (Figure 12-35i).
Most vesicles have specialized functions depending on what materials they contain. For example, the living cell contains vesicles called Endoplasmic reticulum, Golgi complex, and Lysosomes for processing foods and removing wastes. The cell membrane is also a form of vesicle.
Emulsion Examples
    • Emulsion is formed from two immiscible liquids that do not separate readily as shown in the middle of Figure 12-35f. It is created when energy is added to the mixture (by shaking for example). Some emulsions like milk are quite stable and will take a long time to separate since the nano-size particles is small enough to be suspended by brownian movement. Others may part quite quickly like the salad dressing of oil and vinegar (figure 12-35g). The emulsion itself consists of small droplets of one liquid within the body of a second liquid. Emulsions can fail in four basic ways: (i) Coalescence is the forming of bigger spheres until the two liquid are separated completely. (ii) Flocculation is the forming of clumps instead of larger spheres. (iii) Creaming is for the less dense liquid to float to the top. (iv) Breaking is the combined action of coalescence and creaming (see Figure 12-35g).

Figure 12-35g Emulsion Examples [view large image]

Liquid Crystal Phase Liquid Crystal Examples
  • Liquid Crystals - Liquid crystals are in the "meso-phase" between solid and liquid. The transition can be induced by temperature (thermotropic) as shown in Figure 12-35h or by dissolving the material in a liquid. Liquid crystals have the ability of in a form of orderly orientation but yet are in dynamic motion. Such special property is the result of structural anisotropy, e.g., rod-shape (calamatic), disc-shaped (discotic), and board-like (sanidic), and forced alignment by interaction

Figure 12-35h Liquid Crystal Phase
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Figure 12-35i Liquid Crystal Examples [view large image]

between the molecules. Actually, in the meso-phase the 3-dimensional ordering has been reduced to 1 or 2-dimensional ordering (Figure 12-35h). Figure 12-35i shows the various types of liquid crystals.

LC Display Liquid crystals find wide use in liquid crystal displays, which rely on an electric field to control its orientation. Polarization of the light depends in turn on the orientation so that it is either blocked or transmitted by a second polarization filter (polarizer) perpendicular to the first one (Figure 12-35j). The liquid crystal in this application is usually the nematic type (rod-shaped) in twisted form at relaxing state.

Figure 12-35j LC Display

Legend for Figure 12-35j: L - light, P1 - horizontal polarizer, P2 - vertical polarizer, E1, E2 - electrodes, LC - liquid crystals, G -grid, V - voltage supply, S - switch, I - image.

Biomolecules Organic Compounds
  • Biomolecules - The biomolecuels of life is made from organic compounds, but not all organic compounds are related to life. Figures 12-35k and 12-35l happen to show the same thing with different perspective. In general the biomolecules in a living system are more complicated. For examples:

    1. In the case of protein, it is not simply a polymer but a more complicated polymer makes from 20 different kinds of monomers (the amino acids). Every specific amino acid sequence gives a unique shape, and this in turn gives the molecule a single, well-defined function.

Figure 12-35k Bio-molecules

Figure 12-35l Organic Compounds [view large image]

2. DNA and RNA are also polymers made from nucleotides (the monomers). In a biological system the combination of the mono-mers cannot be at random. It has to be very specific and coiled up into chromosomes, otherwise the organism would not survive.
3. The photosynthesis in plants generates a simple CH2O molecule, which joins together to form the 5-carbon fructose (ribose) and 6-carbon glucose. These two kinds combine to give us the table sugar - the sucrose or disaccharide. More linkages of the glucose molecules would produce polysaccharides. There are three common polysaccharides in organisms: starch, glycogen, and cellulose. Glucose is the basic energy source in cells. Starch and glycogen are storage form of glucose in plant and animal cells, respectively, and cellulose is found in plant cell walls. Plants also possess other pathways to turn glucose into protein, lipid, and nucleotide.

4. The monomers in a cell's membrane are lipids, which form a kind of vesicle as mentioned earlier. However all kinds of opening (each guarded by special protein) are inserted over the surface for transporting materials in and out of the cell.

5. Polymer or surfactant solutions can form liquid crystal structures too (called lyotropic). Tobacco Mosaic Virus is an example of a biological material forming lyotropic liquid crystals at rather low concentration because the virus has a very elongated shape. See a long list of biomolecules on a Wikipedia website.

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