Hydrodynamic directional control of liquid metal droplets within a microfluidic flow focusing system Berrak Gol, , Michael E. Kurdzinski, Francisco J. Tovar-Lopez, Phred Petersen, Arnan Mitchell, , and Khashayar Khoshmanesh,
Citation: Appl. Phys. Lett. 108, 164101 (2016); doi: 10.1063/1.4947272 View online: http://dx.doi.org/10.1063/1.4947272 View Table of Contents: http://aip.scitation.org/toc/apl/108/16 Published by the American Institute of Physics
APPLIED PHYSICS LETTERS 108, 164101 (2016)
Hydrodynamic directional control of liquid metal droplets within a microfluidic flow focusing system Berrak Gol,1,a) Michael E. Kurdzinski,1 Francisco J. Tovar-Lopez,1 Phred Petersen,2 Arnan Mitchell,1,a) and Khashayar Khoshmanesh1,a) 1
School of Engineering, RMIT University, Melbourne, Victoria 3001, Australia School of Media and Communication, RMIT University, Melbourne, Victoria 3001, Australia
2
(Received 22 January 2016; accepted 4 April 2016; published online 20 April 2016) Here, we investigate the directional control of Galinstan liquid metal droplets when transferring from the high-viscosity glycerol core into the parallel low-viscosity NaOH sheath streams within a flow focusing microfluidic system. In the presence of sufficient flow mismatch between the sheath streams, the droplets are driven toward the higher velocity interface and cross the interface under the influence of surface tension gradient. A minimum flow mismatch of 125 ll/min is required to enable the continuous transfer of droplets toward the desired sheath stream. The response time of droplets, the time required to change the direction of droplet transfer, is governed by the response time of the syringe pump driven microfluidic system and is found to be 3.3 and 8.8 s when increasing and decreasing the flow rate of sheath stream, respectively. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4947272] Gallium based eutectic liquid metals such as GaIn1 (75% gallium, 25% indium) and Galinstan2 (68.5% gallium, 21.5% indium, 10% tin) offer the properties of both liquid and metal. As a liquid, these alloys are highly deformable, have a low viscosity, and a surprisingly high surface tension; while as a metal, these alloys have high electrical and thermal conductivities.1,2 More importantly, these alloys are less toxic than mercury, have an extremely low vapor pressure, and thus can be used safely in research laboratories. These liquid metal alloys have been used both as a carrier fluid3–5 and as the building block of various soft electronics,6–8 reconfigurable devices,9–11 and fluidic actuators.12–15 There is a growing interest in the integration of gallium based liquid metals into microfluidics. Microfluidics enables the generation of precisely sized microscale droplets and offers innovative ways for sorting, patterning, and actuating of droplets, which cannot be achieved using conventional manual techniques.16–18 Continuous production of microscale liquid metal droplets has been demonstrated in microfluidics.19–21 The size of the droplets depends on the ratio of viscous to surface tension forces, and it is quite tunable and predictable with certain channel geometry.16 Liquid metal has a high surface tension, which makes it challenging to produce droplets. The application of high viscosity liquids such as glycerol has been proven as an effective means for pinching off the droplets.20 In this case, the produced liquid metal droplets are surrounded with highly viscous liquids, which might limit the actuation of droplets by means of continuous electrowetting,12,22 electrochemical,23–25 photochemical,26 and self-propulsion27,28 mechanisms, in which the droplets are preferred to be surrounded by nonneutral solutions such as NaOH or H2O2 to facilitate desired reactions at their surface. In order to make functional systems made of a)
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large arrays of liquid droplets, the continuously produced droplets should be selectively transferred to desired secondary liquids, patterned, and energized. We have recently demonstrated the continuous transfer of microscale Galinstan droplets from glycerol (high viscosity liquid) to oxide suppressing NaOH (low viscosity liquid) solution.29 Due to the different properties of glycerol and NaOH, the hydrodynamic lift and the surface tension gradient forces act in favor of transferring of Galinstan droplets across the glycerol-NaOH interface, with the hydrodynamic lift force driving the droplets toward the interface, and the surface tension gradient force enabling them to cross the interface. Despite this, the directional control of transferred Galinstan droplets, especially in the case of more than one liquid-liquid interface, remains a challenge. Liquid metal droplets can be actuated and redirected using a variety of mechanisms, including continuous electrowetting,12,22 electrochemical,23–25 electromagnetic,30 photochemical,26 and self-propulsion.27,28 Despite several merits, the utilization of such mechanisms implies the application of complementary equipment (e.g., signal generators, electrodes, magnets, light sources) or specific nanoparticles. In contrast, the actuation of liquid metal droplets by means of hydrodynamic forces31,32 does not need any complementary equipment as the same experimental setup (e.g., syringe pumps and tubes), which is used for the generation of droplets can be used for controlling the direction of transiting droplets. In this paper, we demonstrate the transition of Galinstan droplets (RG Medical Diagnostics, USA) from the glycerol core stream into two parallel NaOH sheath streams, provided by a microfluidic flow focusing system. A series of experiments, using high-speed imaging, are conducted to investigate the dynamics of droplets. Our experiments reveal that the droplets transit harmonically into the two sheath streams. More interestingly, the flow mismatch between the two sides of the core stream makes the droplets transit into the higher velocity sheath stream. This characteristic is exploited to
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direct the droplets toward the desired sheath stream, and switching the droplets from one sheath stream to another, as discussed here. The experimental setup consists of an inverted microscope (Nikon Eclipse Ti) equipped with a high-speed camera (Phantom Miro M310, 1000 fps) to capture the dynamics of liquid metal droplets, three syringe pumps (Harvard 2000, 2 Harvard PicoPlus) to infuse the desired liquids through the system, and an integrated microfluidic chip to generate and transfer of liquid metal droplets, as explained below. Figure 1 illustrates the plan view of the microfluidic chip, which consists of two main modules: (i) droplet generator and (ii) flow focusing. The droplet generator consists of six components: (1) glycerol inlet, (2) two glycerol inlet channels (W ¼ 200 lm), (3) Galinstan inlet, (4) Galinstan inlet channel (W ¼ 200 lm), (5) orifice (W ¼ 100 lm), and (6) droplet outlet channel (W ¼ 300 lm). The flow focusing module consists of three components: (7) two NaOH inlets, (8) two NaOH channels (W ¼ 300 lm) patterned with an angle of 30 on both sides of the droplet generator outlet channel, and (9) flow focusing channel (W ¼ 700 lm) with a length of 30 mm to enable tracking of Galinstan droplets, with square blocks patterned along the channel every 1 mm. The height of the microfluidic chip is 100 lm. Soft lithography techniques are used to fabricate the microfluidic chip from polydimethylsiloxane (PDMS). The PDMS block is integrated onto a 1 mm thick glass slide and is plasma treated using a Harrick plasma cleaner. The sealed microfluidic chip is post baked at 75 C overnight for permanent bonding. Glycerol is diluted with water at a volumetric ratio of 10:2.9 (glycerol:water) to provide a viscosity ratio of 40 across the glycerol-NaOH interface (lglycerol ¼ 0.052 Pa s, lNaOH ¼ 0.0013 Pa s). The formation of a highly viscous fluid core surrounded by a less viscous fluid through flow focusing microfluidic systems has been extensively studied, with the characteristic regimes,33 stability,34 and folding35,36 of the core flow investigated under various viscosity contrasts. Galinstan droplets with a diameter of 210 lm are continuously generated by applying Galinstan and glycerol through the droplet generator at flow rates of 5 and 25 ll/min,
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respectively. The flow rate of NaOH through each inlet is set to 500 ll/min to facilitate the ordered transition of droplets through the flow focusing system, as discussed below (Figure 2(a)). The Reynolds numbers of core, sheath, and core-sheath flows (defined as Re ¼ qUdhyd =l, where q, U, and l are the density, average velocity, and dynamic viscosity of the fluid and dhyd is the hydraulic diameter of the channel) are obtained as 0.04, 32, and 33, respectively, indicating the laminar characteristics of these flows. The Peclet number of core-sheath flow (defined as Pe ¼ Udhyd =D, where D is the diffusion coefficient) is obtained as 3.6 106, indicating the dominance of convective mixing at the interface of core-sheath flows. Based on the comprehensive characterisations reported in Ref. 34, under these conditions, a stable thread of glycerol is expected to form downstream the flow focusing channel. The dynamics of Galinstan droplets is monitored using high-speed imaging, as presented in Supplementary Movie S1, with snapshot images shown in Figure 2 (Multimedia view). The width of glycerol core flow is 300 lm and is continuously reduced until forming a thin thread at 1000–1300 lm downstream the flow focusing channel, which is in line with the findings reported in Ref. 34. The Galinstan droplets entering the flow focusing channel are drawn toward the glycerolNaOH interface under the hydrodynamic lift force caused by the massive viscosity contrast of interfacing liquids.37 The droplets eventually cross the flow interface at 150 lm downstream the flow focusing channel (Figure 2(b)) under the influence of surface tension gradient force caused by different interfacial tensions between the Galinstan droplet and the surrounding liquids (537 6 13 mN/m for Galinstan-glycerol versus 480 6 12 mN/m for Galinstan-NaOH), as comprehensively discussed in our previous work.29 The transition of droplets leads to pinching off the glycerol core flow (Figure 2(c)). The core flow is then narrowed down to form a thin thread along the middle of the flow focusing channel (Figures 2(c) and 2(d)). The pinched off glycerol is displaced by the transited droplet, before merging with the narrowed thread of glycerol (Figures 2(c) and 2(d)). The transited Galinstan droplet moves toward the sidewall (Figure 2(d)), before returning to the centre of the channel (Figure 2(e)), and surfing along the thin thread of glycerol (Figure 2(f)). Under balanced conditions,
FIG. 1. The plan view of the microfluidic chip consisting of droplet generator and flow focusing modules.
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FIG. 2. Transition of liquid metal droplets from glycerol core flow into NaOH sheath flow captured by high-speed imaging: (a) Applied flow rates, (b) the droplet specified with black arrow approaches the interface of glycerolNaOH, (c) the droplet crosses the interface following which the glycerol core flow is pinched off by NaOH, (d) the droplet moves within the NaOH sheath flow and gets closer to the lower sidewall, while the pinched off glycerol merges with the core flow, (e) the droplet moves toward the interface, and (f) the droplet moves along the interface, and (b0 )–(f0 ) the next droplet goes through the same dynamic cycle but moves toward the upper sidewall. (Multimedia view) [URL: http://dx.doi.org/10.1063/ 1.4947272.1] [URL: http://dx.doi.org/ 10.1063/1.4947272.2] [URL: http:// dx.doi.org/10.1063/1.4947272.3].
the next Galinstan droplet within the glycerol core flow moves toward the opposite glycerol-NaOH interface and undergoes the same dynamic cycle (Figures 2(b0 )–2(f0 )). Figure S1 (Supplementary Material 138) illustrates the variations of droplet lateral location and axial velocity as a function of downstream distance for four consequent droplets. A similar response is observed when increasing the flow rate of NaOH sheath flow to 750 and 1000 ll/min, as presented in Supplementary Movies S2 and S3. However, reducing the flow rate of NaOH sheath flow to less than 500 ll/min leads to disordered transition of droplets, as presented in Figure S2 (Supplementary Material 238). During our experiments, we noticed that the flow rate mismatch between the two sheath streams can lead to constant transition of droplets toward the higher velocity interface. We conduct a series of experiments to investigate if this phenomenon can be employed for the directional control of transiting Galinstan droplets. In doing so, the flow rate of NaOH applied through the lower NaOH inlet channel is set to 750 ll/min, while the flow rate of NaOH applied through the upper NaOH inlet channel is varied (Figures 3(a)–3(a0 )). Our experiments indicate that a minimum flow rate difference of 125 ll/min should be provided between the two NaOH sheath flows to control the direction of transiting droplets. For example, by setting the flow rate of NaOH through the upper inlet channel to 600 ll/min, the droplets
are transited into the lower sheath flow, which has a flow rate of 750 ll/min (Figures 3(a)–3(c)). In contrast, by setting the flow rate of NaOH through the upper inlet channel to 900 ll/min, the droplets are transited into the upper sheath flow (Figures 3(a0 )–3(c0 )). Figures 3(d)–3(d0 ) present the variations of flow velocity and velocity gradient across the width of the flow focusing channel. The results are obtained by solving Navier-Stokes equations through the flow focusing channel in the absence of droplets, as detailed in Supplementary Material 3.38 Considering a flow rate mismatch of 150 ll/min between the two NaOH sheath flows, the velocity profiles are not symmetric. Accordingly, the magnitudes of the two velocity peaks are slightly different, with the location of velocity peak moved toward the slower sheath flow. Likewise, the magnitudes of the two velocity gradient maxima are slightly different. Under these conditions, the droplets are drawn toward the interface with higher velocity gradient under hydrodynamic lift force,37 which is in line with our experimental results. Our previous experiment indicated that changing the flow rate of sheath stream enables us to control the direction of transiting Galinstan droplets. We conduct further experiments to investigate the dynamics of droplets when the flow rate of sheath stream is instantaneously changed. The flow rate of the lower NaOH inlet is initially set to 750 ll/min,
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FIG. 3. Controlling the direction of transiting droplets by varying the flow rate of the two sheath flows. The flow rate of NaOH applied to the lower NaOH inlet channel is set to 750 ll/ min. (a)–(c) By setting the flow rate of NaOH through the upper inlet channel to 600 ll/min, the droplets are transited into the lower sheath flow. (a0 )–(c0 ) In contrast, by setting the flow rate of NaOH through the upper inlet channel to 900 ll/min, the droplets are transited into the upper sheath flow. (d)–(d0 ) The variations of velocity and velocity gradient across the width of the flow focusing channel obtained by numerical simulations in the absence of droplets.
FIG. 4. Transferring of droplets from one sheath flow to another by varying the relative rates of the sheath flows. (a)–(e) The stacked trajectory of droplets when the flow rate of the upper sheath flow is accelerated from 600 to 900 ll/ min, indicating an interim period of 3.3 s before the direction of droplets changes. (a0 )–(e0 ) The stacked trajectory of droplets when the flow rate of the upper sheath flow is decelerated from 900 to 600 ll/min, indicating an interim period of 8.8 s before the direction change occurs. The dashed yellow lines represent the centerline of the flow focusing channel. (Multimedia view) [URL: http://dx.doi.org/10.1063/1.4947272.4].
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while the flow rate of the upper NaOH inlet is set to 600 or 900 ll/min to continuously transfer the droplets to the lower or upper sheath streams, similar to the conditions presented in Figure 3. The dynamics of Galinstan droplets when increasing the flow rate of the upper sheath flow from 600 to 900 ll/min is presented in Supplementary Movie S4. The snapshot images representing the trajectory of a single droplet are stacked in Adobe Photoshop CS6 (Adobe, USA), as shown in Figures 4(a)–4(e) (Multimedia view). The increase of flow rate through the upper sheath stream leads to an increase of pressure in the microfluidic system. The change of pressure does not occur instantaneously and is delayed due to the deformation of plastic syringe, connecting tube, and PDMS microfluidic chip, which acts as a capacitor and determines the “response time” of the microfluidic system, as comprehensively investigated in Ref. 39. This, in turn, opens a window of time before the directional change of droplets happen, which is referred to as “interim period.” Our experiments reveal a 3.3 s interim period for the case of increasing flow rate (Figures 4(b) and 4(c)). The transient conditions experienced during the interim period change the trajectory of transiting droplets within the lower sheath flow but do not disturb or halt the transition process. After the interim period, the droplets are continuously switched to the upper sheath flow. The trajectory of the first few transiting droplets slightly differs before reaching the steady-state conditions (Figures 4(d) and 4(e)). In contrast, the dynamics of Galinstan droplets when decreasing the flow rate of the upper sheath flow from 900 to 600 ll/min is presented in Supplementary Movie S5, with the snapshot images shown in Figures 4(a0 )–4(e0 ) (Multimedia view). Our experiments indicate a 8.8 s interim period for the case of decreasing flow rate (Figures 4(b0 ) and 4(c0 )). The longer interim period observed here is due to the longer response time of the microfluidic system when decreasing the flow rate using a syringe pump, as comprehensively investigated in Ref. 39. Towards the end of interim period (from 6.5 s to 8.8 s), the droplets which have been released in the upper sheath flow cross the thin glycerol thread and enter the lower sheath flow (Figure 4(c0 )). This phenomenon can be seen for the first three droplets presented in Supplementary Movie S5 and further discussed in Supplementary Material S4.38 After the interim period, the droplets are continuously switched to the lower sheath flow (Figure 4(d0 )). The transition of droplets reaches the steadystate conditions following the passage of the first few droplets (Figures 4(d0 )–4(e0 )). The directional change of droplets has also been demonstrated by setting the flow rate of NaOH sheath flow to 650 ll/min, as presented in Figure S5 (Supplementary Material S538). In summary, we demonstrated a simple yet practical method for directional control of liquid metal droplets when transiting from the glycerol core to two parallel NaOH sheath streams. This method facilitates the continuous transfer of droplets to desired sheath stream, and switching of transferred droplets from one sheath flow to another, which is required for creation of highly reconfigurable systems made of large arrays of liquid metal droplets.
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