Generation of Geometrically Confined Droplets Using Microchannel Arrays: Effects of Channel and Step Structure Isao Kobayashi,* ,† Marcos A. Neves, †,‡ Tomoyuki Yokota, †,‡ Kunihiko Uemura, † and Mitsutoshi Nakajima* ,†,‡ Food Engineering DiVision, National Food Research Institute, 2-1-12 Kannondai, Tsukuba, Ibaraki 305-8642, Japan, and Graduate School of Life and EnVironmental Sciences, UniVersity of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8572, Japan The aim of this study was to investigate the generation characteristics of geometrically confined droplets using microchannel (MC) arrays made of single crystal silicon. Twelve MC array devices, each consisting of four MC arrays, were used in this study. Each MC array consists of rectangular MCs (5 μm in height) with or without a step. This study focused on the effects of the MC width and step height. Refined soybean oil was used as a dispersed phase, and a Milli-Q water solution containing 1.0 wt % sucrose monolaurate was used as a continuous phase. When rectangular MCs with a step height of 4.8 μm were used, geometrically confined droplets with a height of 9.8 μm were obtained, and their diameter and volume gradually increased with increasing MC width. In particular, highly uniform discoid droplets with coefficients of variation below 4% were obtained using the rectangular MCs with an appropriate width range. In contrast, droplets could not be generated from rectangular MCs without a step. When rectangular MCs with a width of 27.6 μm were used, the step height affected the resultant droplet shape. Highly uniform discoid droplets were generated via rectangular MCs with step heights below a critical value of ∼13 μm. Further increase in the step height resulted in the generation of highly uniform spherical droplets. The volume of the discoid droplets was somewhat larger than that of the spherical droplets. 1. Introduction Emulsion droplets (i.e., droplets dispersed in an immiscible bulk continuous phase) have a spherical shape, since they have the smallest interfacial area and minimal interfacial tension energy among droplets with the same volume. Emulsions are commonly produced using conventional emulsification devices (e.g., colloid mills or high-pressure homogenizers) that apply mechanical force and/or cavitation force to rupture droplets. 1 These devices can produce emulsions in a wide range of average droplet size; however, the resultant emulsions generally have wide droplet size distributions. Several techniques of directly generating uniform emulsion droplets have been developed within the past two decades. Membrane emulsification can generate quasi-uniform emulsion droplets with average sizes of 0.3-30 μm and a lowest coefficient of variation (CV) of 10% using microporous membranes with narrow pore size distributions. 2-6 Emulsification using a rotating membrane has generated uniform emulsion droplets with a smallest average size of 80 μm and a CV typically below 10%. 7 Microchannel (MC) emulsification can generate highly uniform emulsion droplets with average sizes of 1-90 μm and a CV typically below 5% using MC arrays, each consisting of highly uniform grooved MCs or straight-through MCs of unique geometry. 8-12 These emulsification techniques also enable precisely controlling the resultant droplet size. Droplets dispersed in a microfluidic space can be geo- metrically confined when droplet size is larger than at least one of the space dimensions. Within the past decade, microfluidic techniques using T-junction and flow-focusing devices have been used to generate geometrically confined droplets. 13-16 The resultant droplets, with a smallest dimension of 10 μm, are highly uniform under appropriate flow conditions of the two phases. Their shape and size can be varied in a microfluidic device, since droplet generation via microfluidic techniques is basically driven by shear stress due to a variable forced flow of the continuous phase. 13,16,17 In contrast, microfluidic devices usually have one droplet generation unit, resulting in very low droplet throughput. We recently developed novel MC array devices for generating geometrically confined droplets. 18 This technique enables generating highly uniform discoid droplets with smallest dimensions below 10 μm, via rectangular MCs with a step (Figure 1). A potential advantage of MC array devices is the mass production of geometrically confined droplets of highly uniform size, since many MCs can be positioned in such devices. Droplet generation via a rectangular MC with a step is normally conducted as follows. 19 The pressurized dispersed phase starts to pass through an MC at a break- through pressure (ΔP d,BT ), which corresponds to the Laplace pressure between two phases in an MC as shown in the following equation: 20 ΔP d,BT ) 4γ cos θ/d MC (1) where γ is the interfacial tension between the two phases, θ is the interface contact angle with the MC wall, and d MC is the MC diameter. Following that, the dispersed phase rapidly expands in the well with a deformed shape and subsequently transforms into a discoid droplet. This droplet generation process does not require any forced flow of the continuous phase, which is analogous to spontaneous-transformation-based droplet gen- eration for MC emulsification. 21 Our previous paper reported the effect of the dispersed-phase flow in a rectangular MC on droplet generation. 19 Highly uniform discoid droplets were generated below a critical dispersed-phase velocity, which was * To whom correspondence should be addressed. Tel.: +81-29-838- 8025. Fax: +81-29-838-8122. E-mail: isaok@affrc.go.jp (I.K.). Tel./ Fax: +81-29-838-4703. E-mail: mnaka@sakura.cc.tsukuba.ac.jp (M.N.). † National Food Research Institute. ‡ University of Tsukuba. Ind. Eng. Chem. Res. 2009, 48, 8848–8855 8848 10.1021/ie8018998 CCC: $40.75  2009 American Chemical Society Published on Web 04/09/2009