In present study, the interaction between Tween20 and Span20, and its effects on bulk-phase property of gelatin solution and film performance produced by casting (CA) or electrostatic spraying (ES) were studied. Isothermal titration calorimetry and fluorescence probe spectrum indicated that Tween20 and Span20 were preferentially combined in solution and synergistically influenced the polarity and wettability. Surfactant aggregation in the resulted film became smaller and more uniform as the proportion of Tween20 increased with average distribution diameter decreasing about 93%, while water sensitivity of the film enhanced by nearly 40%. Correlation analysis further verified that the polarity of the gelatin solution could affect the water vapor permeability, elongation at break and transmittance of the film. The comprehensive performance of coating system was the best at Tween20/Span20 of 1:1, which also had the best preservation effect. ES produced film with better performance than CA, and would be a promisingly edible coating technology.
Surfactants are often used as additives to stabilize the dispersed phase of the water-based multicomponent fluids (such as non-azeotropic emulsion) for enhancing the boiling process, but their effects on the boiling heat transfer of water are still not clear. In this paper, a pool boiling experiment is designed to investigate the specific effects of surfactants (Tween20 (Polysorbate 20), Span20 (Arlasel 20), and Tween20-Span20 mixture) on the subcooled boiling heat transfer characteristics of pure water. The experimental results are analyzed by considering the influences of addition type and concentration of surfactant as well as the applied heat loads. It is found that: (1) Tween20 can achieve excellent boiling enhancing performance (decreasing the wall superheat by up to 10.4 °C and raising the boiling heat transfer coefficient by up to 13.4%) by increasing the number of nucleation sites. And the enhancement is improved with the increase of concentration when the heat flux is below a certain level, while it is weakened at high heat flux for Tween20 with a high concentration (1 mL L -1 ). The reason is that high concentration Tween20 results in an excessive increase of nucleation sites which prevents the return of working fluid and weakens the enhancement of heat transfer. (2) Span20 shows a boiling enhancement only when its concentration is 0.05 mL L -1 (lower than the CMC) among the test ones. Further increment in Span20 concentration (exceeding the CMC) leads to a large increase of wall superheat because it increases the viscosity of working fluid and deteriorates the heat transfer performance. (3) Tween20-Span20 mixture significantly aggravates the aforementioned boiling deterioration effect of Span20 (due to the reduced CMC for the mixture of Tween20 and Span20, weakening the heat transfer), and thus is not suitable for preparing boiling-heat-transfer-enhancing fluids. These results can provide a better understanding of surfactants and guide the practical preparation of multicomponent fluids for boiling heat transfer enhancement.
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