# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 3 Nanoemulsions
### Preparation of Nanoemulsions  
Stable nanoemulsions can be prepared either by high-or low-energy techniques [93; 95; 119]. High-energy processes use mechanical devices which are capable of generating disruptive forces, mixing and disrupting the oil and water phases, leading to the formation of small oil droplets [93; 96]. These methods utilize mechanical energy devices which create nanoemulsions by means of transferring kinetic energy to the system, generating disruptive forces which eventually break up the phases. The most common equipment used are ultrasonicators, high-pressure homogenizers [97], and microfluidizers (Figure 28). High-pressure homogenization and microfluidization are used for the synthesis of nanoemulsions at a laboratory and industrial scale, whilst ultrasonication is used only at laboratory scale. One disadvantage is that these techniques usually require sophisticated instruments and high energy input. An advantage is that they allow a greater control of particle size and composition, and thus of the stability, rheology, and turbidity of the emulsion. Examples of these techniques include high-pressure microfluidic homogenization, ultrasonic emulsification, flow focusing, and satellite droplets [96; 120; 121; 122; 123; 124].  
Figure 27: Ostwald ripening plots for alkane oil nanoemulsions stabilized using a SDS–polyethylene glycol (PEG) surfactant system. Figure reproduced from Ref. [96].  
Since the energy demand is by far the major disadvantage of the previous methods, a possible solution is to develop techniques based on different physical phenomena, which are less energy-demanding. These low-energy approaches depend on the spontaneous formation of tiny oil droplets when the solution or environmental conditions are altered [95]. They are mainly dependent on the variation of interfacial phenomena/phase transitions as well as the surfactant physicochemical properties and oil to produce nanoemulsions [127]. Low-energy processes were developed taking into account the phase behavior and properties of the involved components, leading to the formation of nanodroplets [128]. These can be achieved either by changing the spontaneous curvature of non-ionic surfactants with changes in the temperature (keeping the composition constant), which is known as the phase inversion temperature (PIT) [129] (Figure 29), or by changing the composition of the system at a constant temperature, which is known as the emulsion inversion point (EIP) method [106]. An advantage of this method is that it uses the available energy of the whole system to form the nanoemulsion. On the other hand, the disadvantages are, among others, the complexity, the precise approach required, and the use of synthetic surfactants [130]. The most widely used low-energy methods include spontaneous emulsification, PIT, the solvent displacement method, the  
Figure 28: **(top)** High-energy processes such as high-pressure homogenization and the ultrasonication break macroemulsion droplets into smaller droplets. **(bottom)** Basic scheme of a microfluidicizer. Figures reproduced from Refs. [125] (top) and [126] (bottom). O/W: oil-in-water.  
phase inversion composition method (i.e., self-nanoemulsification method), membrane emulsification, and liquid-liquid nucleation [127; 131].