# Plenty of Room at the Bottom: Nanotechnology as Solution to an Old Issue in Enhanced Oil Recovery
## 2 Nanofluids  
Nanofluids can be defined as dilute liquid colloidal suspensions of nanoparticles (with volumetric concentrations ranging from 0.0001% to 10%) when at least one of their principal dimensions is smaller than 100 nm [42]. Since [PERSON] [43] coined the term, numerous publications and research have dealt with the applications of nanofluids in different fields [44; 45]. Broadly speaking, nanoparticles have a core/shell structure, with different geometric configurations and material combinations that allow them to fulfill a broad spectrum of requirements (Figure 2).  
Decreasing the particle size leads to a change in the interatomic spacing. This can be related to compressive strains induced by the internal pressure due to the nanoparticles' small radius of curvature [32]. Nanoparticles and nano-dimensional layers can adopt different crystal structures than those shown by normal bulk material. There are several techniques or processes capable of creating these nanostructures with different degrees of quality, speed, and cost. These approaches can be classified into two categories: bottom-up and top-down [35; 47; 48]. Top-down methods are those in which nanoparticles are prepared from bulk materials, generating isolated or atom clusters using distribution techniques involving physical methods such as laser beam processing, milling or grinding, or photolithography. As advantages, top-down methods offer reliability and device complexity, but they generally imply relatively high energy consumption and produce more waste. On the other hand, bottom-up methods involve molecular components as starting materials linked to each other by means of chemical reactions as well nucleation and growth processes so as to promote the formation of nanoparticles [49; 50; 51]. These latter techniques can be split into three different categories: chemical synthesis, self-assembly, and positional assembly. So far, according to the literature [52], positional assembly (even though it presents drawbacks as a manufacturing tool) is the only technique allowing single atoms or molecules to be placed deliberately one-by-one in the lattice.  
The potential benefits of nanotechnologies have been welcomed as far as health and environmental aspects are concerned (e.g., in water sanitation). Nevertheless, some concerns have been expressed since the features being exploited might also have a negative impact on human health and the environment. The former could generate possible long-term side effects associated with medical applications and the biodegradability of nanomaterials [53]. One of these examples was reported with nanoparticles as possible adsorbents/catalysts for heavy oil recovery and upgrading [35; 36]. These were injected in a porous medium where a percentage remained deposited. They will persist in-situ for many years, but so far no study has been carried out on the long-term environmental effect of these nanocatalysts. A sustainable nanocatalyst should present--besides a higher activity, durability, and selectivity--recyclability in a cost-effective process [35].