Effect of Surfactants on Drop Size Distributions in a Batch, Rotor-Stator Mixer

Title of Dissertation: EFFECT OF SURFACTANTS ON DROP SIZE DISTRIBUTION IN A BATCH, ROTOR-STATOR MIXER Gustavo A. Padron Aldana, Doctor of Philosophy, 2005 Dissertation directed by: Professor Richard V. Calabrese Department of Chemical Engineering Surfactants are often required to stabilize liquid-liquid dispersions produced by rotor-stator mixers. Since drops are deformed rapidly due to the high power input of these devices, the dynamic interfacial properties governed by the surfactant adsorption rate have a significant effect on the resulting drop size. The objective of this work is to develop a fundamental link between surfactant adsorption dynamics, interfacial properties, and turbulent emulsification processes in rotor-stator mixers. The mean drop size and drop size distributions (DSD) of dilute dispersions produced by a batch rotor-stator mixer were studied. Silicone oils of various viscosities were dispersed in aqueous nonionic surfactant and aqueous methanol solutions. The aqueous methanol (clean) systems allowed comparison of surfactant-laden to surfactantfree systems with similar equilibrium interfacial tensions. The DSD were measured via a video microscopy/automated image analysis technique. The equilibrium interfacial tension of clean and surfactant systems was measured, via a pendant drop technique, as a function of methanol and surfactant concentration, respectively. The dynamic surface tension of surfactant solutions was similarly measured. By fitting the data to the Langmuir adsorption isotherm and a long time approximation to the Ward – Tordai equation, the adsorption parameters and surfactant diffusivities were obtained. This information, with an estimate of the drop deformation timescale, allowed estimation of the surface dilational modulus (E). This is a measure of the Marangoni stresses acting on the drop’ surface due to interfacial tension

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