Generation of Geometrically Confined Droplets Using Microchannel

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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 geometrically 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 * To whom correspondence should be addressed. Tel.: +81-29-8388025. Fax: +81-29-838-8122. E-mail: [email protected] (I.K.). Tel./ Fax: +81-29-838-4703. E-mail: [email protected] (M.N.). † National Food Research Institute. ‡ University of Tsukuba.

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 breakthrough pressure (∆Pd,BT), which corresponds to the Laplace pressure between two phases in an MC as shown in the following equation:20 ∆Pd,BT ) 4γ cos θ/dMC

(1)

where γ is the interfacial tension between the two phases, θ is the interface contact angle with the MC wall, and dMC 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 generation 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

10.1021/ie8018998 CCC: $40.75  2009 American Chemical Society Published on Web 04/09/2009

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Figure 1. Schematic diagrams of the generation of discoid droplets via rectangular MCs with a step positioned in a silicon MC array device.

dominated by the interfacial tension force. In addition, the surface properties of MC arrays greatly affect the droplet generation behavior. The MC surface must be preferentially wetted by the continuous phase to successfully generate discoid droplets. Hydrophilic and hydrophobic MC arrays were needed to generate highly uniform oil-in-water (O/W) and W/O droplets, respectively.18 Nonspherical micromaterials such as microparticles and microcapsules can be produced using emulsion droplets; however, special kinds of emulsion composition and process are needed. In contrast, the use of geometrically confined droplets enables producing nonspherical micromaterials via commonly used processes such as gelation, crystallization, and polymerization. Nonspherical micromaterials have a shorter distance between their center and interface and a larger interface area than spherical ones with the same volume. Thus, components inside nonspherical micromaterials can be utilized more effectively, due to enhanced diffusion, mixing, and reaction via and/or inside the micromaterials. Highly uniform nonspherical micromaterials are also expected to have potential applications

as light diffusers for optical films; for precisely controllable delivery of functional food ingredients and drugs; and for immobilizing enzymes, microorganisms, and cells. The geometrically confined droplets generated using microfluidic techniques have enabled the production of highly uniform nonspherical microparticles.22-25 Although our previous MC array devices could generate highly uniform discoid droplets, their height was restricted to twice the MC height.18 Droplet dimensions are assumed to be controlled by the dimensions of the rectangular MC and step. Therefore, in this study we designed new MC array devices consisting of many pairs of rectangular MCs and steps with different dimensions. The purpose of this study was to investigate the effects of the MC and step structure on the generation phenomena of geometrically confined soybean oil-in-water droplets via new MC arrays and on the size, size distribution, and shape of the resultant droplets. This work focused on MC width and step height.

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Figure 2. (a) Schematic diagrams of the surface view of a silicon MC array device and an MC array. (b) Optical micrograph of the fabricated MC array (MSX-2). (c) Schematic diagram of the droplet generation setup used in this study.

2. Experimental Section 2.1. MC Array Devices. Figure 2a schematically illustrates an MC array device. An MC array device made of single-crystal silicon was fabricated through a two-step photolithography and silicon dry etching process, which formed rectangular MCs and wells.18 Each MC array in the device consisted of 25 rectangular MCs and a well, and a step was formed at each MC outlet (Figure 2a). Two series of MC array devices were used to investigate the effects of the MC width (wMC) and step height (hstep). Table 1 presents the dimensions of the MC arrays used in this study. All the MC arrays were designed to have a height (hMC) of 5 µm and a length (lMC) of 200 µm. Figure 2b depicts an optical micrograph of the rectangular MCs fabricated in MSX-2, demonstrating that their size was highly uniform. Rectangular MCs fabricated in MC array devices had very narrow size distributions of typically r2). Here, r1 and r2 can be approximated by dav,dr/2 and hdr/2 for the curved interface of the geometrically confined droplet. Substituting these approximated variables into eq 6 yields ∆PLap ) 2γ(1/dav,dr + 1/hdr)

(7)

Since γ and hdr were constant for droplet generation using MSX devices described in this section, ∆PLap in eq 7 was regarded

(9)

The data in Figure 4 demonstrated that the diameter and volume of the resultant geometrically confined droplets can be estimated as the functions of wMC. The geometrically confined droplets generated using MSX-2 to 7 were highly uniform due to their very low CV (below 4%). The droplet size distribution of the resultant droplets generated using MSX-1 somewhat broadened with their CV of 6.0%. In contrast, using the MSX-8 device resulted in generating geometrically confined droplets with a wide size distribution (CV ) 13.1%). The MSX-1 device consisted of narrow rectangular MCs with a cross-sectional aspect ratio (RMC ) wMC/hMC) of 2.5. We previously reported that uniform spherical droplets were stably generated using rectangular straight-through MCs with cross-sectional aspect ratios exceeding a threshold (approximately 3).29,30 A key phenomenon for this successful droplet generation was that sufficient space for the continuous phase must remain at the MC outlet during droplet generation. Thus, the CV value for MSX-1 would have been due to relatively unstable droplet generation caused by the small RMC of rectangular MCs. When the MSX-8 device was used, the dispersed phase expanded with random configurations in the well during droplet generation. This behavior led to very unstable droplet generation and a large CV value of the resultant droplets. The results described in this section demonstrated that the size, size distribution, and shape of the droplets generated using the MSX devices were significantly affected by wMC. Highly uniform discoid droplets, which were precisely controlled in size, were obtained using MSX devices consisting of rectangular MCs with appropriate wMC. 3.2. Effect of Step Height. We used five MC arrays (MSX3_0 to 3_45, Table 1) with wMC of 27.6 µm, hMC of 5.0 µm, and hstep of 0-44.6 µm. Figure 6 depicts typical examples of droplet generation using MSX-3_5 to 3_15 at ∆Pd,BT. The dispersed phase that passed through a rectangular MC without a step continuously expanded in the well (data not shown). Although a neck formed inside the MC in this case, droplets were not generated. Uniform oil droplets were periodically generated via rectangular MCs with a step without applying a forced crossflow of the continuous phase (Figure 5a and b), demonstrating that the step at the MC outlet is prerequisite for successfully generating droplets using MSX devices. Twodimensional lattices of the resultant droplets were formed in

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Figure 5. Optical micrographs of droplet generation from rectangular MCs with a constant MC width. (a) MSX-3_5. (b) MSX-3_10. (c) MSX-3_15. The scale bars in a-c are 50 µm.

Figure 6. (a) Effect of step height on the average diameter and CV of the oil droplets generated using MSX-3 devices. (b) Effect of step height on the average droplet volume.

the well using MSX-3_5 to 3_15, whereas the droplets generated using MSX-3_45 packed three-dimensionally in the well. The resultant droplets contacted two-dimensionally and/or three-

dimensionally but did not coalesce during the droplet generation experiments. The size and size distribution of the droplets generated using MSX-3 devices are presented in Figure 7a. The droplets generated using MSX-3_5 and 3_10 had dav,dr greater than hdr and a circular configuration (Figures 5a and b and 6a), proving that they were geometrically confined with a discoid shape in the well. The value of dav,dr was smaller than hwell for MSX-3_15 and 3_45 (i.e., spherical droplets (dav,dr ) hdr) were generated using the MSX-3 devices) (Figures 5c and 6a). In the range where discoid droplets were generated, dav,dr decreased with increasing hstep, and hdr increased linearly with increasing hstep (Figure 6a). The ratio dav,dr/hdr (i.e., the oblateness of the discoid droplets) was 2.2 for MSX-3_5 and 1.2 for MSX-3_10. In contrast, dav,dr was almost constant in the hstep range where spherical droplets were generated. The two dotted lines (Figure 6a) that intersect at hstep of ∼13 µm indicate that discoid droplets are generated using MSX-3 devices with an hstep of less than ∼13 µm. The resultant droplets with a discoid or spherical shape had a CV of below 4%, confirming their size uniformity. Figure 6b illustrated that Vav,dr of the discoid droplets gradually decreased with increasing hstep and was somewhat larger than that of the spherical droplets. These trends in Vav,dr tentatively explained by the droplet generation process via a rectangular MC with a step, schematically illustrated in Figure 7. The detachment process started when the tip of the dispersed phase advancing inside the MC reached the MC outlet (Figure 7a and b). In the initial stage of the detachment process, part of the dispersed phase gradually expanded in the well (Figure 7c), since the internal pressure of the dispersed phase inside the MC (Pd,MC) became lower than that in the well (Pd,well) (Pd,MC < Pd,well). In the middle stage of the detachment process, the dispersed phase inside the MC (near the MC outlet) flowed rapidly into the well, leading to the formation of a neck inside the MC (Figure 7d and e). This stage was driven by the following internal pressure balance: Pd,MC > Pd,well. In the final stage of the detachment process, pinch-off of the neck generated a geometrically confined droplet in the well (Figure 7f). The driving force in this stage was considered to be the following

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Figure 7. Schematic diagram of the droplet generation process via a rectangular MC with a step.

internal pressure balances: Pd,neck > Pd,MC and Pd,neck > Pd,well. Our previous paper reported that the pinch-off process occurred instantaneously, taking one-tenth of the total detachment time.19 If the dispersed phase that expanded in the well had a specific volume, ∆PLap and Pd,well for the expanding dispersed phase with a quasi-discoid shape became higher than those for the expanding dispersed phase with a quasi-spherical shape in the middle stage. The resistance to rapid expansion of the dispersed phase in the well was assumed to decrease with decreasing Pd,well. The quasi-discoid dispersed phase in the well may have had the resistance to its rapid expansion lower than that of the quasispherical dispersed phase of the same volume in the well. We assumed that the lower resistance to rapid expansion of the quasi-discoid dispersed phase in the well promoted the rapid flow of the dispersed phase into the well, leading to the generation of discoid droplets with larger Vav,dr. As demonstrated in this section, hstep at the MC outlets in the MSX-3 devices was a key parameter affecting droplet generation behavior, as well as the shape and volume of the resultant droplets. MSX-3 devices consisting of rectangular MCs with appropriate hstep were needed to generate highly uniform discoid droplets with precisely controlled dimensions. Although the effect of hstep was investigated using the MSX-3 devices with a specific wMC in this section, wMC is also a parameter affecting the hstep range in which geometrically confined droplets are generated. The volume of the resultant geometrically confined droplets increased with increasing wMC (Figure 4b), suggesting that their equivalent diameters also have a similar trend. Their average equivalent diameter (deq,av,dr) can be calculated by the following equation: deq,av,dr ) (6Vav,dr /π)1/3

(10)

deq,av,dr ranged from 13.8 to 39.0 µm, which is dependent on wMC. The resultant droplets are geometrically confined if hstep is lower than a critical value, which can be approximated by deq,av,dr minus hMC. The critical hstep ranged from 8.8 to 34.0 µm, gradually increasing with increasing wMC. We therefore

concluded that both wMC and hstep have to be appropriately controlled to generate highly uniform discoid droplets via rectangular MCs with a step. 4. Conclusions This study demonstrated that highly uniform discoid droplets were generated using MC arrays where MC and step dimensions were appropriately controlled. Here, wMC and hstep were important parameters affecting the generation phenomena of soybean oil-in-water droplets, as well as their size, size distribution, and shape. The diameter and volume of geometrically confined droplets increased with increasing wMC. The resultant droplets had a discoid shape when their ∆PLap was higher than a threshold. The step at the outlet of rectangular MCs was prerequisite for generating highly uniform droplets using MC arrays, since no droplets were generated using rectangular MCs without a step. The geometrically confined droplets were generated when hstep was lower than a critical value. The threshold hstep would increase with increasing wMC. Further investigation using MC arrays with different scales is required to generalize the effect of the MC and step structure on the generation of geometrically confined droplets. Acknowledgment This work was supported in part by the Food Nanotechnology Project of the Ministry of Agriculture, Forestry and Fisheries of Japan. I.K. would like to express appreciation for a grant from the Kurata Foundation. Nomenclature dav,dr ) average droplet diameter [m] ddr ) droplet diameter [m] deq,av,dr ) average equivalent droplet diameter [m] dMC ) MC diameter [m] g ) acceleration due to gravity [m s-2]

Ind. Eng. Chem. Res., Vol. 48, No. 19, 2009 ∆hd ) height of an oil chamber [m] hdr ) droplet height [m] hMC ) MC height [m] hstep ) step height [m] hwell ) well height [m] lMC ) MC length [m] Pd,MC ) internal pressure of a dispersed phase inside an MC [Pa] Pd,neck ) internal pressure of a dispersed phase at a neck [Pa] Pd,well ) internal pressure of a dispersed phase in a well [Pa] ∆Pd ) pressure applied to a dispersed phase [Pa] ∆Pd,BT ) breakthrough pressure [Pa] ∆PLap ) Laplace pressure [Pa] r ) curvature radius of an interface [m] RMC ) cross-sectional aspect ratio of an MC [-] Vav,dr ) average droplet volume [m3] Vdr ) droplet volume [m3] wMC ) MC width [m] Greek Symbols θ ) interface contact angle with the MC wall Fd ) dispersed phase density [kg/m3] σ ) standard deviation [µm]

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ReceiVed for reView December 9, 2008 ReVised manuscript receiVed March 16, 2009 Accepted March 25, 2009 IE8018998