Appearance and Emulsion Type

Not all emulsions exhibit the classical milky opaqueness with which they are usually associated. A tremendous range of appearances is possible, depending on the droplet sizes and the difference in refractive indices between the phases. 

An emulsion can be transparent if either the refractive index of each phase is the same, or alternatively, if the dispersed phase is made up of droplets that are sufficiently small compared with the wavelength of the illuminating light. Thus, an O/W microemulsion of even a crude oil in water may be transparent. If the droplets are of the order of 1 pm diameter, a dilute OfW emulsion will take on a somewhat milky-blue cast if the droplets are very much larger, then the oil phase will become quite distinguishable and apparent. Physically the nature ofthe simple emulsion types can be determined by the following methods [75]  

Texture The texture of an emulsion frequently reflects that of the external phase. Thus, OP emulsions usually feel watery or cream/ while W/O emulsions feel oily or greasy . This distinction becomes less evident as the emulsion viscosity increases, so that a very viscous O/W emulsion may feel oily. 

Mixing An emulsion readily mixes with a liquid that is miscible with the continuous phase. Thus, milk or mayonnaise (O/W) can be diluted with water while margarine (W/O) can be diluted with vegetable oil. Usually, an emulsion that retains a uniform and milky appearance when greatly diluted is more stable than one that aggregates upon dilution [76]. 

Dyeing Emulsions are most readily and consistently coloured by dyes soluble in the continuous phase (e.g. methylene blue for water or fuschin for oil) [6]. 

O/W emulsions usually have a very high specific conductance, like that of the aqueous phase itself, while W/O emulsions have a very low specific conductance. A simple test apparatus is described in [96]. 

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