What is an emulsion?
Salad dressings, milk, and hand lotion are all familiar examples of emulsions. At their simplest, emulsion are just a mixture of two immiscible liquids: one liquid forms a continuous phase in which the other liquid is dispersed in the form of small drops. Commonly, the continuous phase is water or another polar solvent and the dispersed phase is an oil. However, other possibilities exist including inverted emulsions (water dispersed in oil) and double emulsions (water in oil in water). If no special measures are taken, the drops in the dispersed phase will revert to a separate continuous phase - a familiar behavior to those who enjoy oil and vinegar dressings. Phase separation occurs via three mechanisms: coalescence in which drops merge to form larger drops, coarsening in which larger drops grow and smaller drops shrink due to the larger internal pressure of the smaller drops, and creaming or sedimentation in which drops sink or rise out of the bulk. Surfactants (surface active agents) are additives that inhibit coalescence and coarsening by keeping drops from touching (steric or electrostatic repulsion) and/or reducing the interfacial tension between the liquids. Soap is a common surfactant used to stabilize small drops of grease (oil) in water. Sedimentation and creaming occur when buoyancy forces are large compared to the forces associated with Brownian motion. Raw milk develops a layer of cream on the top which consists of large oily droplets. Homogenized milk, on the other hand, is processed to reduce the size of these large droplets so that no creaming occurs. Typically, drops must be less than approximately 1 mm to remain suspended.
Why are physicists interested in emulsions?
Emulsions are typically used as a means of storing and handling sensitive materials (e.g. flammable or reactive with air). However, emulsions have a great potential for use in many other areas, including high tech optical applications. For example, Fig. 2 shows an emulsion made with a nematic liquid crystal. The amount of light that each drop transmits can be varied by placing the drops in a strong electric or magnetic field.
liquid crystal emulsion
Fig. 2. Liquid crystal in water emulsion (5 mm drops)
viewed between crossed polarizers.
In our lab, we are interested in the following properties and uses of emulsions:
* Effects of particle size and size distribution on viscosity and elasticity
* Role of particle ordering and internal structure on macroscopic flow properties
* Optical band gap materials
* Light switches
* Seed particles for novel flow visualization
How are emulsions made?
Most emulsions are formed by ripping droplets apart with shear forces (usually by stirring) or inertial forces (usually by impact). This is typically not a well controlled process and consequently the emulsions that result have a wide drop size distribution (poly-disperse). For reasons discussed above, we are interested in producing "mono-disperse" emulsions. If a true mono-disperse emulsion could be made, it would consist of identical droplets. However, since we live in the real world this is not possible and "mono-dispersity" is a matter of definition. For our purposes, a mono-disperse emulsion consists of drops which when packed together form an ordered hexagonal lattice as shown in Fig. 1. For this to occur, the standard deviation of the size distribution must be less than ~6%.
There are a number of techniques to make mono-disperse emulsions.
* Depletion force fractionation : This technique relies on the fact that for identical materials large objects rise faster than small objects (the buoyancy force is proportional to the volume while the drag force goes like the drop diameter). To fractionate a poly-disperse emulsion, additional particles are added to the continuous phase. These particles constantly bang into the emulsion drops and if the emulsion drops are sufficiently large, produce an effective attractive force between emulsion drops. Then, these larger drops cluster together and rise relatively quickly to the top of the emulsion while the smaller emulsion drops remain suspended. Continued modification of the size and density of the depletion particles is then used to size segregate the emulsion. The advantage of this technique is it needs only a density mismatch to work, while the disadvantage is it can be quite slow and has small effective yields.
* Rayleigh jet breakup : Because of surface tension, a cylinder of fluid is unstable and will break up into drops. However, in the absence of external perturbations, the width of the drop distribution is usually quite wide. To make a fluid cylinder, liquid is forced through a nozzle or other small opening at a relatively high speed which forms a jet. To make mono-disperse the jet diameter is modulated by a number of techniques which either involve moving the nozzle (both transverse and longitudinal motions are effective) or varying the flow rate. With these techniques droplets as small as 5 mm have been produced. In order to form an emulsion, the drops must be added to a continuous phase.
* Dripping drop technique : A drop hanging from the end of a tube is held up by interfacial tension and pulled down by gravity. If fluid is being added to the drop, a point will be reached where the gravitational force exceeds the interfacial force and the drop is pulled away from the tube. For small drops (micron size) gravity is much too weak in comparison with interfacial tension so another force must be found. By placing the tube in an axially flowing fluid, the drop will feel a drag force proportional to the drop size and, again, when this force is large enough the drop will break away from the tube.