沙漠研究 19-1, 97- 100 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Development of Microchannel Emulsification Technology for Monodispersed
Soybean and Olive Oil-in-Water Emulsions
Marcos A. NEVES1, 2, Isao KOBAYASHI2, Mitsutoshi NAKAJIMA1
Abstract: Emulsions are widely used in food industry, among others. Monosized droplets inemulsions have advantages for control of their properties, and application fields. We havedeveloped microchannel emulsification (MC) technology for monosized droplets formation.
The grooved type MC and module were used to characterize MC emulsification, andstraight-through MC and module were developed for higher productivity. Both monodisperse
oil-in-water and water-in-oil emulsions with average droplets size ranging 2-200 μm andcoefficients of variation (CV) lower than 10% were successfully obtained. Emulsification wassignificantly affected by MC size and structure. A straight–through MC was highly applicableto emulsification. MC emulsification can also be applied to production of solid-lipid andpolymeric microparticles, as well as microcapsules for food and bioprocesses.
Keywords: Microchannel emulsification, Monodisperse droplets, Olive oil, Soybean oil

1. Introduction
Recent advances in micro and nanotechnology have enabled their application to food industry, while
processing raw materials into valuable products. This may contribute to better nutrition and human health,meanwhile more sustainable processes can be forecasted. Amongst these technologies, emulsification byprecisely manufactured microchannels has been developed most recently.
Emulsions are commonly produced using conventional emulsification devices, which apply
mechanical shear force to disrupt the dispersed phase into smaller droplets, resulting in emulsions ofconsiderable polydispersity, with a typical C.V. over 20% (McClements, 2004). Nakashima et al. (1991)
proposed direct production of quasi-monodisperse emulsions with a minimum C.V. of 10% using a
microporous membrane. Meanwhile, it is difficult to produce highly monodisperse emulsions owing to
the pore size distribution of the membranes used.
Employing microfabrication technology, we have recently developed microchannel (MC)emulsification technology for monosized oil droplets formulation (Kobayashi et al., 2005a, 2005b, 2008).
Microchannels consist of grooves and/or through-holes ranging between 1-1000 μm in size,microfabricated on a chip, as depicted in Figure 1.
(a)

(b)

Fig. 1. Scanning electron microscopy (SEM) images of silicon arrays of microchannels: (a) grooved MC; (b)oblong straight-through MC.

1Alliance for Research on North Africa (ARENA), University of Tsukuba, Tsukuba, Ibaraki, 305-8572, Japan; Fax nº.
+81-29-853-3981; Tel. nº. +81-29-853-3981; E-mail: mnaka@sakura.cc.tsukuba.ac.jp
2
National Food Research Institute, NARO, Tsukuba, Ibaraki, 305-8642, Japan;

98(a)(c)

Co ntinuous
phase

(b)

Dispersed
oil phaseContinuouswater phase

Microscope
video system(x500)

Continuous phase

Emulsion
Dispersed phase

SpacerGlass plate
ModuleStraight-throughMC plate

Fig. 2. Schematic illustration of droplet generation: (a) grooved MC; (b) asymmetric straight-through MC; (c)typical experimental setup image for straight-through MC emulsification.

(a)
(c)

(b)

Fig. 3. Schematic illustration of MC: (a) Top view of alternate horizontal-vertical asymmetric straight-throughMC plate WMS 1-4; (b) Cross-sectional view of MC WMS 1-4; (c) SEM micrograph of grooved MC plate
MS 308, depicting its dimensions.

For most of emulsification technologies using microdevices, the dispersed phase that passed throughthe channel is directly breakup into droplets. Microchannels with precisely controlled size and size
distribution are useful for generation of monodisperse emulsion droplets with a desired size.
In this paper, we review emulsification technologies using microchannels, and show the results ofproduction of emulsions using soybean and olive oils as oil phase.
2. Materials and Methods
The silicon-based grooved type MC was used to characterize MC emulsification, and straight-throughMC was also used for high throughput. Soybean and olive oils were tested as dispersed oil phase, inseparate. A typical experimental setup for MC emulsification is depicted in Figure 2(c). This

99

emulsification setup consists of an MC plate, a module, apparatuses for supplying the two phases, and a
microscope video system. The MC plate is fixed into the module, which has been initially filled with thecontinuous phase. The pressurized dispersed phase reaches the channel entrance, and then is pushed out
into the continuous phase via the channels, to generate emulsion droplets, represented schematically inFigure 2(a) for grooved MC, and Figure 2(b) for asymmetric straight-through (AST) MC.
The AST MC plate WMS 1-4 has the dimensions 24 mm × 24 mm × 0.5 mm, and consists of about
23,400 MC placed within an area of 10 × 10 mm2 located at the centre of the plate, as depicted in Figure3(a). The circular channels have the dimensions detailed in (Fig. 3(b)). Due to the oxidation treatment,
performed using an oxygen plasma reactor, MC plate surface is hydrophilic, enabling the formulation of
O/W emulsions. The grooved MC plate MS 308 has the dimensions 8 mm × 22.5 mm × 0.5 mm, andconsists of a microfluidic channel array of uniform size, comprising about 600 channels, which have 10 μmin width, 70 μm in length and 2 μm in depth. Details on the grooved MC dimensions are depicted in Fig.
3(c).
3. Results and Discussion
Using the grooved type MC, successful emulsification was achieved when soybean oil was used asdispersed phase, as shown in Figure 4. Emulsification behavior was significantly affected by MC sizeand structure. Oblong straight–through MC and modified straight–through MC, which were fabricated bydeep-reactive-ion etching, were highly applicable to emulsification. Both monosized soybean and olive
oil (Fig. 6) droplets were obtained. The aspect ratio of the oblong channels considerably affects thedroplet generation process in straight-through MC emulsification.
Continuous outflow of the dispersed phase from the channel exit was observed using the channels
with a small aspect ratio of 1.9. On the other hand, monodisperse emulsion droplets were stably
generated from the channels with aspect ratio of 3.8. The studies in straight-through MC emulsification
revealed that monodisperse emulsions were successfully produced using the oblong straight-through MCs
with a threshold aspect ratio of approximately three (Kobayashi et al., 2004a).
The droplet diameter was independent of the applied flow rate of the continuous phase. Theinterfacial-tension-based droplet generation, proposed in grooved MC emulsification, is applicable tosuccessful droplet generation from the oblong channels. Droplets size could be controlled ranging 2-200
μm, and coefficients of variation lower than 10% were successfully obtained.
The oil droplets generation process using the straight-through MCs was also analyzed using thecomputational fluid dynamics (CFD) method (Kobayashi et al., 2004b). Using the channels over a
threshold aspect ratio of 3-3.5, droplets formation was obtained, which was confirmed by bothexperimental and simulation results (Fig. 5).
(a) 15.0 ms(b) 36.7 ms

WaterChannelexit

SoybeanoilFig. 4. Microscopic image of emulsification behavior
from a grooved MC; regular droplet
generation at low dispersed phase flux.

(c) 38.7 msNeckZYXFig. 5. Emulsification behavior using straight-throughMC, calculated by computational fluiddynamics. Successful formulation of a droplet
with a diameter of about 35 μm from an elliptic
straight-through MC (longer line, 40 μm;
shorter line, 10 μm).

10050 μm
Fig. 6. Microscopic image of emulsification behavior:
Olive oil was used as dispersed phase.

30 μm
Fig. 7. Microscopic image of olive oil droplets
obtained by MC emulsification.

The diameter of the droplets generated by MC emulsification is primarily controlled by the Figure 6
depicts the emulsification behavior using olive oil as dispersed phase, by AST MC, and Figure 7 shows amagnification of the monodispersed olive oil droplets generated. The resultant droplet diameter wasindependent of the applied flow rates of the continuous phase. The interfacial-tension-based droplet
generation has been proposed in grooved MC emulsification (Sugiura et al., 2001).
The diameter of the droplets generated by MC emulsification is primarily controlled by the channelgeometry. The resultant droplet diameter was greatly influenced by the channel depth, and wassignificantly influenced by the terrace length (Sugiura et al., 2002a), but being independent of the channel
width and channel length (Sugiura et al., 2002b). The droplet diameter of the resultant monodisperse
emulsions was almost constant below the critical flow rate of the dispersed phase in the channel, which isproportional to the capillary number (Ca [-]) (Sugiura et al., 2002c). Ca is defined as the ratio of theviscous force to the interfacial tension force. The critical Ca was also independent of the channel size andthe interfacial tension. Above the critical Ca, a drastic increase in the droplet diameter was observed.
Besides the silicon MCs, stainless steel MCs and acrylic MCs were also used for production ofmonodisperse O/W and W/O emulsions, respectively. Monodisperse emulsion droplets generated by MC
emulsification have been applied to monodisperse lipid microparticles, polymeric microparticles, naturalpolymeric microbeads, coacervate microcapsules, and W/O/W emulsions.
The formulation ofmicrobubble by MC emulsification has been investigated as well.

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