# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 3 Nanoemulsions  
An emulsion consists of at least two immiscible liquids, where one of these liquids is dispersed as small droplets in the other [93; 94]. Standard emulsions usually have droplets with mean radii between 100 nm and 100 \(\upmu\)m. Nanoemulsions are generally defined as emulsions containing droplets with mean radii between 10 to 100 nm [95; 96; 97] or, according to different sources and authors, covering the size range 50 to 500 nm [98]. Nanoemulsions exhibit the typical properties of all emulsions, but with some specific characteristics that distinguish them from standard macroemulsions [95]. The droplet size is smaller than the wavelength of light (\(r\ll\lambda\)), causing them to be transparent or slightly turbid. Furthermore, the droplet size leads to a better stability to gravitational separation and aggregation than conventional emulsions [95; 96]. Gravity forces have a negligible effect, whilst Brownian motion is likely to put the droplets into motion and provide them with a driving force for destabilization processes. Besides, the Laplace pressure excess inside the droplets is quite large due to their size, and the droplets are difficult to deform (i.e., shear, elongation, or break), provided that the IFT is very low. Unlike microemulsions (size range \(<\) 50 nm), which are also transparent or translucent and present thermodynamic stability, nanoemulsions are only kinetically stable. Nanoemulsions are generally stabilized by the use of surfactants, but solid particles suspended in one phase can also stabilize emulsions. Emulsions stabilized by means of solid particles have been studied for a long time, and were named after [PERSON], who discovered that the droplets' coalescence is suppressed when solid particles are adsorbed at the oil-water interface (Figures 16 and 17) [99]. The solid particles adsorb at the interface of the two liquids, thereby creating a physical barrier that hinders coalescence [100; 101]. The Pickering emulsions' stability is significantly affected by several parameters, among them the composition of the organic and aqueous phases, the contact angle, particle size and concentration, and particle/particle interactions at the interface [100]. Figure 16: SEM image of 10 \(\upmu\)m-diameter colloidosome composed of 0.9 \(\upmu\)m-diameter polystyrene spheres. The colloidosome was formed with an oil droplet containing 50 vol% vegetable oil and 50 vol% toluene, and a water phase containing 50 vol% glycerol (**A**). The close-ups show the 0.15-\(\upmu\)m holes that define the permeability (**B**,**C**). Figure reproduced from Ref. [102]. So far, emulsions have been considered in EOR processes as an independent technique, but the combination of these with different fluids might also yield interesting results. One of these combinations is with CO\({}_{2}\). Because of the low density, and more importantly very low viscosity of the supercritical and liquid CO\({}_{2}\), the macroscopic sweep efficiency of CO\({}_{2}\)-related EOR processes is poor. Surfactant-stabilized CO\({}_{2}\) foams is one way to solve the mobility problem, but a different one arises: the long-term stability. The surfactant-stabilized foams require continuous regeneration. Moreover, a fraction of the surfactant is adsorbed on the rock, increasing material-related costs. So, the use of nanotechnology was a novel way to tackle this issue by using nanoparticles in stabilizing CO\({}_{2}\) foams (Figure 18). Some results [104; 105] showed that nanoparticle foams are notably more stable than surfactant ones due to the nanoparticles' high adsorption energy at the gas-liquid interface. Moreover, the attraction between nanoparticles and the gas-liquid interface is believed to help in minimizing the nanoparticle loss to the rock formation, and silica nanoparticles are expected to withstand the harsh reservoir conditions better than standard surfactants [22; 104]. Nanoemulsions containing oilfield chemicals can be used in many applications, such as well treatments (e.g., scale inhibition, fracture acidizing, etc.