Emulsification in turbulent flow: 1. Mean and maximum drop diameters in inertial and viscous regimes

Systematic experimental study of the effects of several factors on the mean and maximum drop sizes during emulsification in turbulent flow is performed. These factors include: (1) rate of energy dissipation, ε; (2) interfacial tension, σ  ; (3) viscosity of the oil phase, ηDηD; (4) viscosity of the aqueous phase, ηCηC; and (5) oil volume fraction, Φ. The emulsions are prepared by using the so-called “narrow-gap homogenizer” working in turbulent regime of emulsification. The experiments are performed at high surfactant concentration to avoid the effect of drop–drop coalescence. For emulsions prepared in the inertial   turbulent regime, the mean and the maximum drop sizes increase with the increase of ηDηD and σ, and with the decrease of ε. In contrast, Φ   and ηCηC affect only slightly the mean and the maximum drop sizes in this regime of emulsification. These results are described very well by a theoretical expression proposed by Davies [Chem. Eng. Sci. 40 (1985) 839], which accounts for the effects of the drop capillary pressure and the viscous dissipation inside the breaking drops. The polydispersity of the emulsions prepared in the inertial regime of emulsification does not depend significantly on σ and ε  . However, the emulsion polydispersity increases significantly with the increase of oil viscosity, ηDηD. The experiments showed also that the inertial turbulent regime is inappropriate for emulsification of oils with viscosity above ca. 500 mPa s, if drops of micrometer size are to be obtained. The transition from inertial to viscous turbulent regime of emulsification was accomplished by a moderate increase of the viscosity of the aqueous phase (above 5 mPa s in the studied systems) and/or by increase of the oil volume fraction, Φ>0.6Φ>0.6. Remarkably, emulsions with drops of micrometer size are easily formed in the viscous   turbulent regime of emulsification, even for oils with viscosity as high as 10,000 mPa s. In this regime, the mean drop size rapidly decreases with the increase of ηCηC and Φ (along with the effects of ε, σ  , and ηDηD, which are qualitatively similar in the inertial and viscous regimes of emulsification). The experimental results are theoretically described and discussed by using expressions from the literature and their modifications (proposed in the current study).

Transition from inertial to viscous regime of turbulent emulsification is induced by increasing the viscosity of the continuous phase and/or the oil volume fraction. Much smaller drops are obtained in the viscous regime of emulsification.Figure optionsDownload as PowerPoint slide