Pine Research Group
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Glossary
  • Brownian motion: The random movement of small particles suspended in a liquid. Brownian motion is caused by random thermal motion of the liquid molecules, which through their collisions with the small particles, cause the small particles to move about.

  • Colloids: Small particles suspended in a liquid by thermal agitation. That is, colloidal particles are sufficiently small that collisions with the molecules of the liquid in which they are immersed are keep them from sinking or floating under the influence of gravity. This means that colloidal particles are generally smaller than a few micrometers, since larger particles usually sink or float. There is no lower limit to the size of colloids, other than to note that because they are made up of many atoms or molecules, they are generally larger than a nanometer or so (i.e. bigger than atoms or molecules). Many everyday products are colloids, including milk, paint, ink, some bacteria, viruses, as well as globular proteins. Opals are essentially dried out identical spherical colloids, which stack and form an ordered array much like oranges neatly stacked at a grocer′s stand.

  • Complex fluids: Fluids such as colloidal suspensions, emulsions, foams, liquid crystals, polymer solutions, and polymer melts, that contain mesoscopic structures, that is, structures large compared to the size of conventional small molecules such as water but generally too small to be visible to the unaided eye. Complex fluids often exhibit unusual mechanical properties such as transitions between solid-like and liquid-like behavior when subject to modest forces. Shaving gel is a common complex fluid; it is solid at rest but flows under modest shear stress.

  • Emulsions: A mixture of two immiscible liquids, usually consisting of oil droplets suspended in water or of water droplets suspended in oil (an “inverse emulsion”). Droplets are usually stabilized by one or more surfactants, which are molecules with one part that dissolves in water and another part that dissolves in oil. Emulsions of micrometer and sub-micrometer droplets are in effect colloids of liquid droplets suspended in another immiscible liquid. Mayonnaise in an emulsion of olive oil suspended in water with protein (lecithin) from egg yokes serving as a surfactant (the water also comes from the egg yokes – other ingredients are added for flavor).

  • Polymer: A molecule made of one or more repeating molecular units linked together by chemical bonds. A linear polymer is essentially a string of many identical molecules chemically linked together. Naturally occurring biopolymers include proteins (a chain of amino acids) and DNA (an intertwined pair of sugar-phosphate chains linked by organic bases). Plastics are made from artificial polymers such as polystyrene, from which styrofoam and plastic cups are made, and polymethylmethacrylate (PMMA), from which acrylic and plexiglass are made.

  • Rheology: The study of the flow of materials, primarily liquids and complex fluids. The rheology of simple Newtonian liquids like water are characterized by a viscosity. Liquids like water and alcohol flow freely and have a low viscosity. Oils flow more slowly and have a higher viscosity. Liquids like honey flow very slowly and have a still higher viscosity. The rheology of more complex liquids like shampoo or liquid hand soap are characterized by a viscosity and by an elasticity. Such liquids are said to be viscoelastic.

  • Self-assembly: The set of processes by which the components of complex fluids spontaneously organize into larger structures. Self-assembly requires no external intervention; it is the result of a complex interplay among the forces between the components of the complex fluid and their thermal (Brownian) motion.