Nanoemulsions have many interesting physical properties that are different from or are more extreme than those of larger microscale emulsions. In this section, we focus on a few of the physical properties that distinguish nanoemulsions from microscale emulsions as an important new class of soft materials. We examine the relative transparency of nanoemulsions, their response to mechanical shear or ‘rheology’ and the enhanced shelf stability of nanoemulsions against gravitationally driven creaming. We do not intend to provide a comprehensive review of all of the possible properties, but these particular properties serve as a few primary examples.Nanoemulsions appear visibly different from microscale emulsions since the droplets can be much smaller than optical wavelengths of the visible spectrum.
By contrast, nanoemulsions can appear nearly transparent in the visible spectrum and exhibit very little scattering despite significant refractive index contrast. Quantitative measurements of the optical transparency of nanoemulsions in the visible and ultraviolet wavelengths are shown through transmission measurements .Nanoemulsions having a = 40 nm at several different f have been loaded into 0.2 mm pathlength quartz cells, and the per cent transmission intensity has been measured as a function of light wavelength. For all f, the transmission in the visible spectrum is near 100%, especially toward red wavelengths, indicating a high degree of transparency. By contrast, in the ultraviolet (UV) part of the spectrum, as the wavelength of light begins to approach the droplet radius, the nanoemulsions scatter light significantly.
By contrast, nanoemulsions can appear nearly transparent in the visible spectrum and exhibit very little scattering despite significant refractive index contrast. Quantitative measurements of the optical transparency of nanoemulsions in the visible and ultraviolet wavelengths are shown through transmission measurements .Nanoemulsions having a = 40 nm at several different f have been loaded into 0.2 mm pathlength quartz cells, and the per cent transmission intensity has been measured as a function of light wavelength. For all f, the transmission in the visible spectrum is near 100%, especially toward red wavelengths, indicating a high degree of transparency. By contrast, in the ultraviolet (UV) part of the spectrum, as the wavelength of light begins to approach the droplet radius, the nanoemulsions scatter light significantly.
As it increases from the dilute regime up to about f ˜ 0.13, the transmission in the ultraviolet drops as the number of scatterers increases, yet at higher f > 0.13, the UV transmission increases again, indicating that more concentrated emulsions scatter less light. This increase and subsequent decrease in the scattering of light by the droplets as f is increased arises from the behaviour of the nanoemulsion’s structure factor in the low q (transmission) limit. In effect, correlations in the droplet positional structure cause the increased transparency at higher f. The smaller the droplet radius, the broader the range of visible wavelengths over which the transparency is found, especially toward the blue and the ultraviolet wavelengths.