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An emulsion is made from immiscible hydrophobic and hydrophilic fluids, or most likely in the kitchen, oil and water. The problem with making a stable emulsion is the two fluids tend to unmix after some time. Due to the surface tension, it is energetically favourable to form a single surface when two drops meet. Adding an emulsifier like lecithin to the solution will help the oil droplets to become stable by lowering the surface tension and hence they can become much smaller and more evenly dispersed through the water without coagulating. Still, heat can affect the emulsion by inverting the phases.
The word emulsion stems from the latin verb mulgere which means to milk. Hence it would not surprise you to know that milk is an emulsion, but also butter and mayonaise are emulsions which differ in the proportions of fat and water. Emulsions where the size of the granules of the dispersed phase is near or just below that of visible light have another interesting property. Those familiar with Rayleigh scattering know that blue smoke or the colour of the sky is a result of this. Smaller wavelengths are scattered more easily in every direction while the lower wavelength light continues on its way. When the sun is low on the horizon the light traverses more of the atmosphere hence scattering the blue light and one is left with only the red and yellow hues that are visible at a sunset or sunrise. Emulsions will do something analogous in what is called the Tyndall effect. Milk contains fat and casein globules, though the fat globules are much larger and will tend to scatter all wavelengths in equal amounts, yielding a white suspension. When the fat is largely taken away, one is left with the much smaller casein globules that do display the Tyndall effect. Skimmed milk therefore has a slightly blueish tinge.
Nanoemulsions
If the droplets become smaller than about 600 nm one is said to have a nanoemulsion. For a small radius droplet the pressure is very large, because it is energetically favourable to minimise the interface surface area to volume ratio. This can be seen from the Young-Laplace equation that was reviewed earlier in the episode about foam. Nanoemulsions are made by applying a large amount of shear forces adding a lot of potential energy to obtain the small droplets. These are thermodynamically unstable. The surface tension must become very low to sustain such small droplets.
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| The ouzo effect shown in pastis |
Microemulsions are basically the same as nanoemulsions with the notable difference that they are the product of spontaneously emulsified liquids by the addition of surfactant. The nomenclature is a bit misguiding in this respect. For microemulsions the interface (surface) tension is too low to play any significant role, and hence shear forces are not necessary to form smaller droplets. Sometimes a co-surfactant, like an alcohol, is used as well. The Ouzo effect is a form of spontaneous emulsification creating a microemulsion. The ethanol soluble anethole in the pastis is converted in an emulsion by adding water in which it has a low solubility, thereby making it cloudy like milk. Other anis flavoured beverages that display this behaviour are pastis, absinthe and raki for instance.
Aerosols and granular systems
An aerosol is a system with a gaseous continuous phase, and a liquid or a solid in the dispersed phase. Due to gravity aerosols are not stable, they decay and eventually one gets phase separation or even inversion. When the grains of a substance are large enough for them to be barely touching there will be neither gas nor solid walls. The system will consist of solid granules interspersed by air pockets.
Granular materials form a much more complex collective behaviour than can be deduced from their single constituents alone. These kinds of systems behave differently in different energy regimes, and can be attributed properties given to gasses, fluids as well as solids. The dynamic and static properties can viewed at macroscopic level (viscosity e.g.) as well as at the level of the grain (collisions). It is a vast subject that I do not wish to enter here, though tell you a few interesting properties.
There is a lot of energy dissipation during granular flow due to inelastic collisions between the particles. In order to see the dynamic properties one has to supply energy constantly. Therefore the properties differ from fluids in various ways as the following effects might demonstrate.
The Brazil nut effect or granular convection occurs when particles of different sizes undergo motion and the larger sizes end up on the top (independent of individual density).
Sand piles undergo a change from the static regime to the flow regime by avalanching at a certain critical angle of the inclined sand. This is called self-organized criticality and is a very profound phenomenon of this class of systems. There is no special tuning parameter to reach a critical point (like a specific temperature or pressure). At the critical point or phase transition there is a tendency for complex (non-linear) systems to display self-similar fluctuations at all scales, hence to be scale invariant.
There are also interesting acoustic effects belonging to granular matter. Some sand can 'sing' when it is moved. There are examples of singing or booming dunes in the desert.
So much for the physics. On to the food. Edible granular systems... that sounds rather appetising, doesn't it? However, there is a type of sand that you can eat. An example is made here with hazelnuts, maltodextrin and honey. Of course one can use all kinds of finely ground things to make sands like this, however the nice thing about maltodextrin is it dissolves immediately in the mouth so it gives a extra 'airy' perception.
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| Hazelnut sand |
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