**3.3 Visualization techniques**

Many researchers have used imaging and visualization techniques to study jet flows, atomization, and droplets; a number of systems are reviewed in the literature (Bell et al., 2005; Chigier et al.,1991). Jet lengths and spray widths ranging to milimeters and drop and particle sizes ranging to micrometers must be taking into account in order to select imaging system components.

Several studies used particle and droplet visualization in supercritical fluids (Badens et al., 2005; Gokhale et al.,2007; Kerst et al.,2000; Lee et al.,2008; Mayer & Tamura,1996; Obrzut et al.,2007; Randolph, et al., 1993; Shekunov et al., 2001).

The optical technique described in these works provides the ability to visualize mixing occurring between two fluids with different refractive indices. For instance, shadowgraphy is an optical method to obtain information on non-uniformities in transparent media, independently if they arise by temperature, density or concentration gradients. All of these inhomogeneities refract light which causes shadows.

Although for SAS precipitation, microscopy-base imaging offers the advantage of examining the dynamic process that leads to particle formation, the presence of particles smaller than two microns complicates an already difficult task of imaging an injection process.

disintegration regimes and observed the formation of uniform PLA microparticles (Lee et al., 2008). Other authors (Chang et al., 2008; Gokhale et al., 2007; Obrzut et al., 2007; Reverchon et al., 2008) did not find relevant differences in the various precipitates obtained. Particularly, PLA morphologies showed to be insensitive to the SAS processing conditions (Randolph et al., 1993). This characteristic fact could be assigned to the high molecular weights and the tendency to form aggregated particles because of the reduction of the glass

At subcritical conditions the interfacial tension between the injected liquid and the bulk phase never goes to zero and a supercritical mixture is not formed between the liquid solvent and CO2. The droplets formed during atomization are subjected to a very fast internal formation of a liquid/CO2 mixture. Due to a high solubility of CO2 in pressurized organic liquids and a very poor evaporation of organic solvents into the bulk CO2, the droplets expand. During these processes, the interfacial tension allows the droplets to maintain its spherical shape, even when the solute is precipitated within the droplet. Saturation occurs at the droplet surface and solidification takes place with all solutes progressively condensing on the particle internal

This kind of particles has also been observed in other SAS works (Reverchon et al., 2008). It has been also obtained expanded hollow particle at same conditions. The different surface morphologies can depend on different controlling mass transfer mechanisms, as suggested

Operating conditions above the MCP, from a thermodynamic point of view, are characterized by zero interfacial tension. But, the liquid injected into the precipitator, before equilibrium conditions are obtained, experiences the transition from a pure liquid to a supercritical mixture. Therefore, interfacial tension starts from the value typical of the pure liquid and progressively reduces to zero. This fact means that droplets formed after jet break-up (whose presence indicates in every case the existence of an interfacial tension) are formed before the disappearance of the interfacial tension. In other words, the time of equilibration is longer than the time of jet break-up and spherical microparticles instead of

Many researchers have used imaging and visualization techniques to study jet flows, atomization, and droplets; a number of systems are reviewed in the literature (Bell et al., 2005; Chigier et al.,1991). Jet lengths and spray widths ranging to milimeters and drop and particle sizes ranging to micrometers must be taking into account in order to select imaging

Several studies used particle and droplet visualization in supercritical fluids (Badens et al., 2005; Gokhale et al.,2007; Kerst et al.,2000; Lee et al.,2008; Mayer & Tamura,1996; Obrzut et

The optical technique described in these works provides the ability to visualize mixing occurring between two fluids with different refractive indices. For instance, shadowgraphy is an optical method to obtain information on non-uniformities in transparent media, independently if they arise by temperature, density or concentration gradients. All of these

Although for SAS precipitation, microscopy-base imaging offers the advantage of examining the dynamic process that leads to particle formation, the presence of particles smaller than

two microns complicates an already difficult task of imaging an injection process.

transition temperature in SC-CO2.

by Duhkin et al. (Duhkin et al., 2005).

nanoparticles can be obtained.

**3.3 Visualization techniques** 

system components.

surface. The final result is the formation of a solid shell.

al.,2007; Randolph, et al., 1993; Shekunov et al., 2001).

inhomogeneities refract light which causes shadows.

The ability to identify and characterize these small formations drives future system improvements, including lighting enhancements laser-induced fluorescence, and higher spatial resolution cameras. In this way Reverchon et al. used light scattering technique to clearly differentiate between an atomized very droplet laden spray and a dense "gasplume", limitation which cannot be gained by applying optical techniques due to the fact that both the droplet laden spray and the dense "gas-plume" result in a dark shadow (Reverchon et al., 2010).

 On the other hand, extensive research has been done using scanning electron microscopy (SEM) to evaluate the size and morphology of particles formed under supercritical conditions (Armellini& Tester, 1994; Bleich et al., 1994; Mawson et al. 1997; Randolph et al., 1993; Shekunov et al., 2001;). A limitation of SEM analysis is that it is applied to particles after they have been removed from the dynamic system.
