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Experiments show that the simple paddle stirred cell provided an oil drop dispersion that was as monosized as that produced by the controlled shear device, if not better.An analysis indicated that only the section of the membrane close to the radius of the highest shear under the paddle stirred membrane produced oil drops. The membranes used in the experiments contained a regular array of non-tortuous pores uniformly spaced and provided oil injection rates up to 1000 l m -2 h -1 , which is much higher than reported fluxes for the alternative tortuous pore channel membranes made by sintering.keywords emulsification, rheology, sieve-membrane, force balance

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• This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Industrial and En- AbstractWater-in-oil and oil-in-water emulsions were generated using 30 μm pore diameter surface membranes to investigate the factors influencing drop size, and the degree of uniformity of drop size distribution, using a stirred cell employing a simple paddle mounted above a circular disc membrane. The importance of the transitional radius, which is the radius at which the vortex around the unbaffled paddle stirrer changes from a forced vortex to a free vortex and the shear stress at the membrane surface below the stirrer is at its greatest, is demonstrated.Monosized emulsions were produced, with drop size distribution coefficient of variation values of 10% for o/w emulsions and 13.5% for w/o emulsions. These tests demonstrated that a membrane of reduced annular operating area (ringed membrane) produced a more mono-sized o/w emulsion than a membrane where the full area was used to generate the emulsion, without affecting the mean drop size. The improved size distribution was achieved whilst the transitional radius was located within the ringed annular section of the membrane.The force balance model, applied to drops formed at the surface of the membrane during emulsification, predicted the droplet diameter provided further drop break up -1 -within the stirred cell did not occur. Drop break up occurred at Reynolds numbers below 300 for both oil in water and water in oil dispersions. Therefore, for Reynolds numbers greater than this, an annular radial ring membrane can be designed to produce monosized droplets using the stirred cell at known continuous phase viscosities with predictable mean droplet size. This knowledge can be used as a design tool to produce monosized droplets of a specified size for various applications using simple stirred cell emulsification. keywords emulsification, sieve-membrane, force balance

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During membrane emulsification it is shown that the size of the drops formed at the membrane surface may increase with increasing dispersed phase injection rate through the membrane, or it may decrease, depending on the prevailing conditions. This is illustrated using a stirrer positioned above a flat disc membrane with a regular array of pores of 20 μm diameter and a spacing between the pores of 80 μm and another membrane of 200 μm pore spacing. In the former case an additional mechanism for drop detachment is the push-off force, which is determined by the geometry of the drops as they deform at the membrane surface. In the force balance, the push-off force may be added to the shear-drag force to cause drop detachment. In the case of the 200 μm pore spaced membrane this force is much less prominent. The capillary-shear model has been modified to include this push-off force. The study required the use of very low dispersed phase injection rates and very high rates.Hence, two different types of pumps were used to provide these: a peristaltic and syringe pumps. A small study comparing the drop size, and size distributions, showed that the pump type did not influence the drops produced by the membrane emulsification process. keywords emulsification, stirred cell, sieve-membrane, force balance, push-off force 2

Membrane emulsification of sunflower oil in aqueous solutions of 2% (v/v) Tween 20 was performed using a stationary disk membrane with a rotating paddle stirrer and, for comparison, a tubular membrane oscillating normal to the direction of oil flow in an otherwise stationary continuous phase. The oscillation frequency ranged from 10 to 90 Hz. The oil was injected through a sieve-type membrane with a 10 μm pore size and 180 μm between pore spacing at low flux rates to minimize any droplet interference. Using the same membrane material under identical peak shear conditions in both systems, smaller and more uniform drops (30−50 μm median sizes) were produced in the oscillating system. In oscillation, the drop size was modeled by a force balance, including a correction for neck formation at the pore surface, but in the rotating paddle system, neck formation did not appear to be relevant. Drop size was not found to be frequency of oscillation dependent, apart from its influence on the shear stress at the membrane surface.

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