Research Article Drop Detachment from a Micro-Engineered Membrane Surface in a Dynamic Membrane Emulsification Process Drop size distribution is an important characteristic of emulsions, probably the most crucial one for their use in various applications. Here, a pilot-scale appara- tus with a cone-shaped flow geometry is introduced. The plate contains a micro- engineered membrane manufactured from silicon allowing for the production of emulsions with narrow drop size distributions. The process is characterized by producing model emulsions of the oil-in-water type under laminar rheometric flow conditions and by accessing the regime of drop detachment as a function of the wall shear stress applied, by means of high-speed imaging in a separate flow cell. Furthermore, clear evidence is given of the crucial influence of the membrane wetting properties on the emulsification results, by comparing the performance of micro-engineered membranes composed either of silicon, silicon nitride, or nickel, for pore diameters from 1 to 12 lm, in the flow cell. Keywords: Drop formation, Drop size, Emulsions, Membrane processes, Pilot scale Received: April 18, 2013; revised: June 06, 2013; accepted: June 24, 2013 DOI: 10.1002/ceat.201300256 1 Introduction Emulsions are widely used in the food, pharmaceutical, chemi- cal and cosmetics industries. The goal is to tailor the emulsion microstructure such that the product quality is optimized with respect to, e.g., texture, rheology, color, and stability. The most prominent applications in the context of our emulsion re- search relate to the use of emulsions as carriers for functional components [1], or as templates to generate more complex microstructures [1, 2], like multiple emulsions and related en- capsulation systems. The mean drop size and the drop size distribution are the major characteristics of emulsions, with direct impact on their stability [3, 4] and functionality. When the emulsion drops carry functional components, a narrow drop size distribution is mandatory to control the release [5]. Only in such a case, one can claim tuning the release kinetics by tailoring the drop size. Accordingly, the development of processes enabling to control drop size and distribution is of prime interest. Membrane emulsification is generally expected to deliver emulsions with narrow drop size distributions. In this process, which was first introduced by Nakashima et al. in 1988 [6], a disperse liquid phase is pressed through the pores of a mem- brane into a continuous cross-flowing fluid phase, with the two liquids being at least partially immiscible. Drops form at the pore outlet and are detached by the hydrodynamic force induced by the flow of the continuous liquid phase. Several works have reported spontaneous drop formation and detach- ment from Shirasu porous glass (SPG) membrane pores [7, 8]. The main advantages of membrane emulsification from the processing perspective are (i) low energy input resulting in a gentle treatment of the components and (ii) good scalability, and, concerning the material characteristics, the adjustability of (iii) the mean drop size and (iv) the narrow drop size distri- bution width. Peng and Williams [9] and Schröder et al. [10] used micro- porous ceramic membranes to study emulsion formation on the laboratory and pilot scale, and they also studied drop de- tachment from capillaries to better understand the mecha- nisms of drop formation and detachment under cross-flow conditions. Both introduced force balance models, taking into account the interfacial force and the drag force as the main in- teracting forces. In accordance with this analytical momentum conservation model and related experiments, the drop size was controlled by the velocity of the continuous fluid phase. Con- sequently, the drop size decreased with increasing velocity of the continuous fluid phase. Flow cells of different degrees of complexity were introduced to study the mechanisms of formation and detachment of drops Chem. Eng. Technol. 2013, 36, No. 10, 1785–1794 © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com Sebastian Holzapfel Elisabeth Rondeau Pascal Mühlich Erich J. Windhab Laboratory of Food Process Engineering, ETH Zurich, Zurich, Switzerland. Supporting Information available online – Correspondence: S. Holzapfel (contact@sebastian-holzapfel.de), Prof. Dr. E. J. Windhab (erich.windhab@hest.ethz.ch), Laboratory of Food Process Engineering, ETH Zurich, Schmelzbergstr. 9, LFO E21, 8092 Zurich, Switzerland. Membrane processes 1785