Transparent droplets refractive index

Kizling and coworker [21] suggested that salts in the aqueous phase stabilised w/o HIPEs by two means. First, the Ostwald ripening process is inhibited due to the decreased solubility of the aqueous solution in the continuous oil phase. Secondly, the attractive forces between adjacent aqueous droplets are lowered, as a result of the increase in refractive index of the aqueous phase towards that of the oil phase. When the refractive indices of the two phases are matched, the attractive forces are at a minimum and highly stable, transparent emulsions are formed. The attractive force, A, is given by ... 

Not all emulsions exhibit the classical milky opaqueness with which they are usually associated. A tremendous range of appearances is possible, depending upon the droplet sizes and the difference in refractive indices between the phases. An emulsion can be transparent if either the refractive index of each phase is the same, or alternatively, if the dispersed phase is made up of droplets that are sufficiently small compared with the wavelength of the illuminating light. Thus an O/W microemulsion of even a crude oil in water may be transparent. If the droplets are of the order of 1 pm diameter a dilute O/W emulsion will take on a somewhat milky-blue cast if the droplets are very much larger then the oil phase will become quite distinguishable and apparent. Physically the nature of the simple emulsion types can be determined by methods such as [95] ... 

PDLCs comprise LC droplets dispersed in a polymer matrix. Random director orientation leads to the scattering of light due to refractive index mismatch between LC and polymer. A field applied across the film orients the LC so that the refractive indices match and the scattering is reduced, rendering the film transparent. 

Transparent particles may be distinguished between those having a refractive index larger or smaller than the surrounding medium. Liquid droplets or glass beads in air have a relative refractive index m which is larger than unity, typically in the range 1.3 to 1.5, and water droplets in oil or bubbles in liquid have a relative refractive index below unity. 

Due to reasons like scattering of visible light at the droplet-bulk interface (the nature of which is dependent on factors like the droplet size as also difference in refractive index of the droplet and the bulk phase), absorption of light in presence of colored matter etc. emulsions are not always clear to the naked eye, but can be opaque to translucent to transparent according to their nature. Table 1.1 gives a... 

MLCs also have their place in optical applications, mainly as so-called polymer-dispersed LCs (PDLCs). A PDLC constitutes a microemulsion of an MLC in a film of a conventional (nonPLC) polymer. In the switched off state the MLC and the polymer have different refractive indices, dispersed MLC droplets (not unlike to the islands in PLCs) scatter light quite effectively, and the film is opaque. Then an external electric field is applied, for instance across a capacitor-like metal coating on both sides of the film. The director in all MLC droplets becomes the same. One can choose the MLC + polymer pair so that the refractive index along the director is the same as that of the host polymer. In that case the film in the electric field becomes transparent. Switching the field off and on, one has a light valve with a fairly large area. 

Systematic experimental studies of direct and indirect photophoretic mechanisms of particle and droplet motion in fluids have not been conducted yet. However, some data are available on characteristic velocities and their dependence on the size, refractive index, and absorption coefficient of irradiated objects.For fluid droplets, both negative (in the case of high absorption ) and positive (for transparent droplets ) photophoresis has been observed. This is in agreement with the theory of indirect photophoresis reviewed above. For nine oil-in-water emulsion systems with droplet size 1-5 p,m studied in, the positive photophoretic velocity of a droplet was found to be linearly proportional to the droplet radius, and almost linearly dependent on the refractive index of the droplet substance in the range njtio = 1.42-1.59. Typical velocities were 5-20 p,m/s at a CW Nd YAG (Aq = 1064 nm) laser power of 100-300 mW focused into the beam with Wo 20 p,m. 

These facts are very relevant to paint films. They can explain, for example, why a solution of two transparent resins of different refractive index can dry to give a milky film, if the resins are incompatible. Resins that are incompatible are simply resins that will not mix. The solvents help them to mix in solution, but as the liquids evaporate, the molecules of the two types of resin tend to segregate in separate groups. Complete separation into two layers does not occur, because the viscosity is, by this time, too high. Instead, we have a separation into droplets of one resin inside a film of the other. 

The classification of microemulsions based on size is not adequate. Whether a system is transparent or translucent depends not only on the size but also on the difference in refractive index between the oil and the water phases. A microemulsion with small size (in the region of 10 nm) may appear translucent if the difference in refractive index between the oil and the water is large (note that the intensity of light scattered depends on the size and an optical constant that is given by the difference in refractive index between oil and water). Relatively large microemulsion droplets (in the region of 50 nm) may appear transparent if the refractive index difference is very small. The best definition of microemulsions is based on the application of thermodynamics, as discussed below. 

Field-induced reorientation of the director with attendant optical changes has recently been used in a novel application with the potential for large-area LCDs polymer dispersed LCs (PDLCs). A PDLC is a microemulsion of MLC dispersed in a conventional transparent polymer film. In the off state there is a mismatch between the refractive index of the MLC and that of the host polymer film. Hence the dispersion of MLC droplets scatters light very effectively, giving an optically opaque film (Fig. 5.14, left-hand side). On application of an external electric field (across a capacitor-like transparent coating of tin oxide on both sides of the polymer film), the director assumes the same orientation in all of the microdroplets. If the... 

Nanoemulsions are transparent or translucent systems in the size range 20-200 nm [35]. Whether the system is transparent or translucent depends on the droplet size, the volume fraction of the oil and the refractive index difference between the droplets and the medium. Nanoemulsions having diameters < 50 nm appear transparent when the oil volume fraction is < 0.2 and the refractive index difference between the droplets and the medium is not large. With increasing droplet diameter and oil volume fraction the system may appear translucent, and at higher oil volume fractions the system may become turbid. 

Second-order refraction Transparent droplets. Scattering angle should be chosen carefully to yield a linear diameter-phase relationship. Accurate knowledge of the refractive index is required. Should be used only when physical restrictions, such as limited optical access, make it mandatory. 

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