Design characteristics in spray processing for complex multiphase structure preservation

Abstract

Present studies showed the ability of spray processing to tailor the morphology of simple or double emulsion based fluid systems as investigated for two air assisted nozzle geometries under various process conditions. For the air assisted nozzle, the spray process parameters were varied concerning air to liquid flow ratio (GLR), spraying pressure, and total flow rate. The results depicted that the emulsion flow inside the nozzle (liquid-cap) as well as in the spray (outside nozzle) have distinct big impact on the resulting product structure due to the respective flow stresses acting. Increasing flow stresses to a supercritical level can lead to an additional significant dispersing impact of the dispersed fluid phases. Besides the process parameters, the material properties of the sprayed emulsion system such like viscosity ratio (λ) of dispersed to continuous phase and interfacial tension have been varied. The results showed that the stability of the dispersed phases increased with the investigated viscosity ratio λ from 0.32 to 30. Critical representative dimensionless numbers the ‘liquid Weber number (Wel,Drop,cr)/λ’ and the ‘gas Weber number (Weg,Drop,cr)/λ’ were defined to describe the effects of liquid-cap-tip flow and air-assisted spray flow respectively for preserving microstructure of the treated emulsion. Above these critical Weber numbers, the dispersed phase drop sizes were exponentially decreased due to the stresses acting during liquid flow inside the nozzle or in the spraying. Viscosity and dynamic moduli (G , G) of emulsions increased with decreasing droplet size of the dispersed phase(s) thus altering the spraying performance but as well the properties of the liquid product systems reconstituted from resulting powders. A critical gas Weber number, Weg,Nozzle,cr, for spray droplet (tertiary droplet) has also been defined, which gave information on the smallest spray droplets attained with the dispersed droplets remaining unchanged. The impact of the gas Weber number (Weg,Nozzle) on the spray (tertiary) droplet using internal (INMIX) and external (EXMIX) mixing nozzles was studied. High speed videography and laser shadowgraphy were applied to visualize the liquid spray filament stretching and breakup, as well as the velocity distribution in the spray, especially in the Rayleigh to fiber-type filament breakup domain. The spraying regime, in which the secondary droplet are kept mostly unchanged, was defined. It was found that the Rayleigh to membrane-type filament breakup provided emulsions with unchanged structure. During CaBER measurement, the