*The content in the third column separated by / is the respective order in the cases two or more kinds of nanoparticles are included. first reported on solid-like Ag films at the interface between two immiscible liquids, liquid-liquid interfacial self-assembly has become a well-established robust procedure. The main advantage of the liquid-liquid interfacial assembly is that hydrophilic nanoparticles dispersed in a polar phase can be directly self-assembled without the requirement of any ligand functionalization. Conversely, as shown by Lin et al., hydrophobic nanoparticles like CdSe can also be directed to self-assemble at the water/toluene interface. In liquid-liquid interfacial assembly, the two liquids should have sufficient difference in polarity to remain immiscible, like water and oil. As shown in Figure (a), the contact angle, i.e., the wettability, of the nanoparticles with respect to the oil and water determines whether self-assembly is possible or not. Normally, an intermediate contact angle (θ ~ 90 o ) results in the assembly of the nanoparticles at the interface. The intermediate conditions between hydrophilic and hydrophobic limits characterized by θ in Figure (a) can in fact be measured if the particles are not too closely packed. Isa et al. demonstrated that the contact angle for a single nanoparticle at the interface can be measured using freeze fracture shadow-casting cryo-scanning electron microscopy, Figure ). Reincke et al. showed that by gradually varying the surface charge density of the nanoparticles, the contact angle can be varied, Figure ). Without adding ethanol, the nanoparticles remained in the water phase, due to their high surface charge, when a separate heptane phase was added and vigorously mixed. They showed despite being a stable colloid after ethanol addition, when the non-polar heptane phase was introduced on the top of the water-ethanol surface, the nanoparticles immediately assembled as a film at the interface. A similar approach was also adopted by the same authors via pH reduction from 9 to 2 by 1 M HCl addition to water that induced migration of Au nanoparticles to the liquid-liquid interface. Interestingly, upon the restoration of the pH by NaOH addition, large 16 nm nanoparticles dissolved back to the water phase but the smaller 9 nm particles remained at the interface. It was proposed that 4-mercaptobenzoic acid capped nanoparticles smaller than 10 nm stably attach to the oilwater interface while larger nanoparticles attach reversibly. When the hydrophobization of the nanoparticles by surface ligands is not strong enough for the nanoparticles to be able to cross the barrier from polar to non-polar medium, the nanoparticles get trapped at the interface. Thus, alkanethiol coated Au nanoparticles can also be self-assembled at the oil-water interface with a high packing density, As PVP is a water soluble polymeric stabilizer, it creates an intermediary surface potential required for interfacial trapping. Other materials, for instance sulphonated graphene sheets (GSs) can be self-assembled at the hexane/water interface in-spite of their thin sheet-like structure, Figure (f). While nanoparticles spontaneously migrate from the bulk to the liquid-liquid interface, the direct injection of nanoparticles at the interface has been shown to yield more efficient trapping of nanoparticles. As shown in Figure (g), this requires the free end of the capillary carrying the nanoparticles only to touch the interface by capillary forces. This direct injection makes the self-assembly of nanoparticles irrespective of size. As larger nanoparticles have larger energy barriers to cross to get trapped at the interface, direct injection gets past the barrier by introducing the nanoparticles directly at the interface. While it is a promising technique, the main challenge faced by experimentalists while performing liquid-liquid interfacial assembly is the difficulty to transfer the self-assembled film onto a substrate. The summary of different significant reports on liquid-liquid interfacial assembly is provided in Table . Au nanoparticle film at the heptane/water interface by ethanol addition © 2011 CC-BY-NC-SA. (c) Selfassembled monolayer of alkanethiol coated Au nanoparticles at water-hexane interface. Reprinted with permission © 2004 John Wiley & Sons. (d) Low-magnification image of amidine latex particles at the n-decane/water interface, measured the single nanoparticle contact angle at the interface by freeze fracture shadow-casting cryo-scanning electron microscopy. Reprinted with permission © 2007, American Chemical Society. (e) cyclohexane/water Interfacial self-assembly of PVP coated Au nanorods. Reprinted with permission © 2018 John Wiley & Sons. (f) AFM image of sulphonated graphene sheets (GSs) self-assembled at the hexane/water interface. Reprinted with permission © 2011 Royal Society of Chemistry. (g) Girault et al showed that direct injection of the nanoparticles by a capillary tube at the interface leads to efficient trapping of the nanoparticles. The free end of the capillary only touched the interface by capillary forces. The Figure shows assembly of nanoparticles of three different sizes by this method. Reprinted with permission © 2015, American Chemical Society, © 2013, American Chemical Society.