The materials that surround us are far from perfect. The polymer molecules that make up the plastics we use to contain our drinks, to mould computer casings and to formulate hair sprays are irregular in both size and shape. Similarly, the latex particles in house paints have a broad distribution of size, surface charge and chemical composition. Even our automotive fuels and lubricants are not simple fluids, but complicated mixtures of linear, branched and aromatic hydrocarbon molecules. When these molecular, chemical and structural irregularities are slight, such ‘complex fluids’ usually have a single phase, which looks homogeneous and is often transparent. Greater irregularity, however, makes a fluid separate into more than one phase, such as the oil and water in salad dressings. Reporting in Physical Review Letters, Evans, Fairhurst and Poon have now derived equations that describe how the disorder in complex fluids partitions among the phases1. Their equations are applicable to fields as diverse as petroleum refining and toiletries, as well as to the design of paints, coatings and plastics.
The fundamental problem can be nicely illustrated in a polymer-science context: ethylene-propylene rubber2 (EPR). This commercial synthetic rubber consists of linear polymer chains that have a more or less random sequence of chemically bonded ethylene (E) and propylene (P) monomer units. Because each position along a chain of N ≈ 1,000 monomer units can be occupied by either an E or a P monomer, there are roughly 2N ≈ 10300 possible molecules that can be made by random linkings of the two monomer types. So a macroscopic sample of EPR, containing small multiples of Avogadro's number of particles (that is, of order 1024), clearly does not contain all of these possible molecules, but it does provide a representative sampling of the composition distribution of the polymer chains.
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