Emulsification, microchannel

Figure 1.2. Schematic principle of microchannel emulsification, (a) Top view (b) side view.

More complex geometries have been developed [40] and the influence of the geometrical structure has been examined. Although straight-through microchannel emulsification has been developed [39,41], the production rates are still low compared to those obtained with standard emulsification methods. However, the very high monodispersity makes this emulsification process very suitable for some specific fechnological applicafions such as polymeric microsphere synfhesis [42,43], microencapsulation [44], sol-gel chemistry, and electro-optical materials. 

Membrane emulsification allows a precise control of the droplet size and monodispersity but the scale up of this process is difficult. MicroChannel emulsification is a promising technique but the low production rates restrict its use to highly monodisperse systems intended for high-technology applications. 

S. Sugiura, M. Nakajima, N. Kumazawa, S. Iwamoto, and M. Seki Characterization of Spontaneous Transformation-Based Droplet Formation During MicroChannel Emulsification. J. Phys. Chem. B 106, 9405 (2002). 

T. Kawakatsu, G. Tragardh, C. Tragardh, M. Nakajima, N. Oda, and T. Yonemoto The Effect of Hydrophobicity of Microchannels and Components in Water and Oil Phases on Droplet Formation in MicroChannel Water-in-Oil Emulsification. Colloid and Surfaces A Physicochem. Eng. Aspects 179, 29 (2001). 

I. Kobayashi, M. Nakajima, and S. Mukataka Preparation Characteristics of Oil-in-Water Emulsion Using Differently Charged Surfactants in Straight-Through MicroChannel Emulsification. Colloid Surfaces A Physicochem. Eng. Aspects 229, 33 (2003). 

S. Sugiura, M. Nakajima, and M. Seki Prediction of Droplet Diameter for MicroChannel Emulsification Prediction Model for Complicated MicroChannel Geometries. Ind. Eng. Chem. Res. 43, 8233 (2004). 

S. Sugiura, M. Nakajima, H. Itou, and M. Seki Synthesis of Polymeric Microspheres with Narrow Size Distributions Employing MicroChannel Emulsification. Macromol. Rapid Commun. 22, 773 (2001). 

K. Nakagawa, S. Iwamoto, M. Nakajima, A. Shono, and K. Satoh MicroChannel Emulsification Using Gelation and Surfactant-Free Coacervate Microencapsulation. J. Colloid Interface Sci. 278, 198 (2004). 

Chuah, A.M., Kuroiwa, T., Kobayashi, I., Nakajima, M. (2009). Effect of chitosan on the stability and properties of modified lecithin stabilized oil-in-water monodisperse emulsion prepared by microchannel emulsification. Food Hydrocolloids, 23, 600-610. 

The fast progress in microengineering and semiconductor technology led at the development of microchannels, that Nakajima et al. applied in emulsification technology [12]. 

The distinguishing feature of membrane emulsification technique is that droplet size is controlled primarily by the choice of the membrane, its microchannel structure and few process parameters, which can be used to tune droplets and emulsion properties. Comparing to the conventional emulsification processes, the membrane emulsification permits a better control of droplet-size distribution to be obtained, low energy, and materials consumption, modular and easy scale-up. Nevertheless, productivity (m3/day) is much lower, and therefore the challenge in the future is the development of new membranes and modules to keep the known advantages and maximize productivity. 

Xu, Q.Y., Nakajima, M., and Binks, B.P. (2005). Preparation of particle-stabilized oil-in-water emulsions with the microchannel emulsification method. Colloids Surf. A, Physicochem., Eng. Aspects, 262(1-3), 94-100. 

Dekkers, K.S. (2003). Production of double emulsions by microchannel emulsification, MSc thesis. Wageningen University and Technische Universitat Karlsruhe. 

Kawakatsu, T., Trhgardh, G., Tragardh, C., Nakajima, M., Oda, N., and Yonemoto, T. (2001b). The effect of the hydrophobicity of microchannels and components in water and oil phases on droplet formation in microchannel water-in-oil emulsification. Coll Sutf. A 179(1), 29-37. 

Kobayashi, I., Nakajima, M., Nabetani, H., Kikuchi, Y., Shohno, A., and Satoh, K. (2001). Preparation of micron-scale monodisperse oil-in-water microspheres by microchannel emulsification. JAOCS 78(8), 797-802. 

Significant studies prove the interest in this kind of membrane. Among them we mention the work of Lambricht and Schubert [17] on emulsification with microchannels, and the study of Popat et al. [18] that proves that the bone cell response can be significantly improved using controlled nanoarchitecture (alumina membranes fabricated using a two-step anodization process and which present pore sizes of 30-80 nm). 

In the first demonstration of formation of monodisperse droplets in a microfluidic T-junction [9], on the basis of the experimental results on scaling of the droplet size with the rate of flow of the continuous fluid, it was hypothesized that the droplets are sheared off from the junction by the flow of the continuous fluid, similarly to the classical models of shear-driven emulsification. However, the fact that the break-up occurs in a confined geometry of the microchannels, and that the droplet growing off the inlet of the fluid-to-be-dispersed usually occupies a significant fraction of the cross-section of the main channel, suggest that the pressure drop along a growing droplet may be an important factor in the process. 

This chapter overviews ongoing activities in microchannel process technology development, from single-channel laboratory experiments to industrially-driven, multi-channel and multi-unit development at or near the prototype and pilot level. The non-reactive unit operations covered include heat transfer, mixing, emulsification, phase separation, phase transfer, biological processes, and body force applications. 

Given the two microporous processing options, their comparison can be summarized as in Fig. 7.9 [109]. Generally speaking, membrane emulsification provides higher flux and smaller droplet sizes than the microchannel process. Microchannels, however, are less susceptible to fouling, require little or no... 

Fig. 7.11 Schematic of the straight through" microchannel emulsification system [138].

The microchannel emulsion technique has been extended to the formation of multiple emulsions [158-163], encapsulation [123, 158, 164—166], polymer bead formation [123, 125, 167-169], demulsification [116, 158, 170], and even microbubble formation [171]. New methods of stabilizing emulsions have also been investigated in this realm, including particle-stabilized [172] and protein-stabilized emulsions [173], with some work in emulsification without surfactants [135,146]. In the case of multiple emulsions, microchannel architecture can enable the formation of W/O/W emulsions in which two water droplets of different compositions can be encased in the same oil droplet [163]. 

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