Influence of flow rate on the droplet generation process in a microfluidic chip

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1 Influence of flow rate on the droplet generation process in a microfluidic chip Florian Lapierre, Nan Wu, Yonggang Zhu * CSIRO Materials Science and Engineering, PO Box 56, Highett, Melbourne, VIC 3190, Australia ABSTRACT Droplet generation in microfluidics has attracted a great deal of attention due to the potential applications in many areas of science and technology. The understanding of the generation mechanism is still unsatisfactory and proposed models lack generality for different microchips with flow conditions and channel geometries. In this paper, we present new results of droplet generation in a PMMA microchip using flow-focusing technique. The current data are compared with existing published data obtained with similar generation microchip method. The dependence of the droplet size/slug length on the capillary number and ratio of the continuous phase and dispersed phase flow rates is investigated. A model has been proposed which explains well the data from several similar studies. Keywords: water in oil emulsion, droplet generation, microfluidics, flow focusing, capillary number 1. INTRODUCTION The emergence of new technologies using microfluidic devices has given recent momentum to droplet formation studies using micro-channels. The generation of fine emulsions in which aqueous micro-droplets are used as a container for DNA in an organic medium has a great potential in pharmaceutical and bio-chemical applications. [1] Chemical and biochemical reactions have also been successfully performed in microfluidic systems where the generated droplets-size could be controlled to a nano and picolitre scale. [2] Emulsion has been the subject of numerous studies in microfluidic devices. The commonly used geometry to generate fine droplets has been the T-junction type channels. The T-junction device could be designed by different methods as shown by Hung et al. [3] and Nisisako et al. [4] In studies of size controlled droplets, Tice et al. [5, 6] investigated the conditions to form nanoliter-sized droplets as plugs. In particular, they investigated the effect of viscosity on the droplet formation regimes at the junction of a T-channel type system. Flow focused devices have also shown that two streams of the organic phase between which an aqueous phase stream in injected is a reliable way to generate droplets of aqueous in organic with different sizes. Cramer et al. [7] have proved that specially designed capillary flows with co-flowing continuous phases permitted drop formation. They have shown that the tendency of the dispersed phase to generate a liquid jet rises with increasing velocity of the continuous phase along with higher flow rates, viscosity of the disperse phase and lower interfacial tensions. They concluded that interfacial tension force was the only conservative force which drives the droplets to adopt a spherical shape at the formation location. The above discussed concept of flow focusing in which the drop liquid flows as a stream in the middle of a channel and surrounded by the continuous liquid flow has received the attention of many works both numerically and experimentally. Kuksenok et al. [8] have shown numerically that in flow focusing formation of droplets of phase A in phase B or vice versa B in A are the result of morphological instabilities occurring in the microchannel. In a flow focusing system Pulido-de-Torres et al. [9] studied droplet formation in an emulsion of water-sunflower oil and investigated the effect of surfactant on drop size. They identified different droplet generation regimes of mono-dispersed and poly-dispersed emulsions. All cited studies still confirm the importance of the design of the flow focused system, yet optimized geometrical configurations have to be established. The motivation of the present study is to investigate a flow focusing system designed in the form of a specific cross microchannel network for the generation of fine water-in-oil (W/O) droplets which can be employed to control precisely the size of the formed droplets. A particular focus is given to the effects of Smart Nano-Micro Materials and Devices, edited by Saulius Juodkazis, Min Gu, Proc. of SPIE Vol. 8204, 82040H 2011 SPIE CCC code: X/11/$18 doi: / Proc. of SPIE Vol H-1

2 the continuous phase flow conditions on the droplet generation at the specifically designed junction in the microchannel network. 2. EXPERIMENTAL DETAILS 2.1 Configuration and fabrication of the microchips The PMMA microchip uses a flow focusing geometry with channels thatt were rectangular in cross-section. Two aqueous streams were merged before they were forced to flow through a smalll orifice located downstream of the inlet channels. Oil added through two symmetric perpendicular channels exerted pressure and viscous stress on the aqueous fluid and forced the mixture into a narrow thread, which then broke into droplets inside the orifice. The width of the orifice was 50 µm and the depth of all the channels was 70 µm. A large reservoir (10 µm 1 µm 70 µm) was connected downstream of the droplet generator. The microchip was fabricated in a polycarbonate slide with dimensionss of 75 mm 25 mm and a thickness of 2 mm. The microchip was fabricated in the CSIRO Microfabrication Laboratory, Clayton, VIC 3169, Australia. The microfluidic channel pattern was designed using a commercial layout tool and fabricated as a film mask. The mask pattern was exposed into a 70 µm layer of Laminar 5083 resist/stainless steel plate using a collimated UV exposure system. The exposed pattern was developed in 1% Na 2 CO3 solution and the resulting profile was replicated as a Ni shim by a 2 stage process. Firstly, a thin layer of Ni was sputter deposited onto the patterned surface followed by a thicker electrodeposited layer (100 µm) in a Ni sulphamate bath. The nickel shim was subsequently used as a die to hot emboss the pattern of channels into polycarbonate chips. The embossing was performed in a planar hydraulic presss at a temperature of 155ºC. During embossing, the pressure was maintained for two minutes and then released after cooling to ~50ºC. The accesss holes were then drilled through the backside of the polycarbonate plate. Before laminating the capping layer, the PC chip was treated with oxygen plasma for 15 minutes. To access the microchannels in the chip, a nano-ports assembly was carefully glued to each liquid port. A schematic view of the microchip is presented in Figure 1. Figure 1. A schematic illustration of the arrangement of microfluid channels for droplet generation. The nozzle size D is 50 µm. and all channels have a depth of 70 µm. Proc. of SPIE Vol H-2

3 2.2 Experimental details The experiments were carried out in CSIRO Microfluidics Laboratory at Highett, Melbourne VIC 3190, Australia. Mineral oil (Sigma) was used as the continuous phase with the addition of 4% (w/w) Abil EM90 (Gold schmidt GmbH, Germany) and 1% (w/w) Span 80 (Fluka) as surfactants. The dynamic viscosity of the oil phase was mpa.s at 23 C. Deionised water was used as the disperse phase, which has a viscosity of 1 mpa.s. The interfacial tension of the two phases was 5.83 mn/m. A nemesys high pressure pump system (Cetoni, Germany) with four independent pumps was used to pump the fluids from the SGE syringes (Supelco) ranging from 50 µl to 250 μl capacity to the microchip. For the current study, the each aqueous flow rate (Q w ) was fixed at 40 µl/h while the oil flow rate (Q o ) varied from 10 µl/h to 160 µl/h. The resulting capillary number Ca = μ U / σ (here µ and U are the dynamic viscosity and flow velocity of the continuous phase, σ is the interfacial tension) varies from 0.1 to The microchip was connected to the syringes with polytetrafluoroethylene (PTFE) tubing (i.d. 0.30mm, Cole- Parmer). The microchip was placed on a microscope (Nikon Eclipse TE2000-U) stage for visualization and measurement. A high speed video camera (MotionPro X3, Redlake) equipped with a de-magnification lens (Nikon C ) was attached to one microscopic port to record pictures of droplets at 1000 frames per second. A 4 objective (Plan Fluor, Nikon) was used. The droplet size was measured from the pictures taken. 3. RESULTS AND DISCUSSIONS 3.1 Droplet sizes Figure 2 shows a series of images taken from the droplet formation experiments, with each image corresponding to different oil flow rate. The total aqueous flow rate was fixed at 80 µl/h. Figure 2. Images of droplet formation for different oil phase rates. The total aqueous phase was kept at 80 l/h. while the oil phase changed from 10 to 160 l/hour. Proc. of SPIE Vol H-3

4 The slug length (L) was measured from the images using an imaging processing technique. However, even through the droplet formation was carried out at an oil flow rate of 10 µl/h, the slug length was much larger than the nozzle length and thus it is impossible to estimate the length. Only data in the range of l/h are shown in Figure 3. It can be seen that the increase of oil flow reduced the drop size. At small oil flow rates, the aqueous phase formed a slug and the slug length was larger at smaller oil flow rate. Figure 3. Aqueous phase slug length as a functionn of the oil flowrate. The total aqueous phase was kept at 80 µl/h. 3.2 Drop generation mechanism The droplet formation in flow-focusing device is mainly due to the break-up of the dispersedd phase in the carrier fluids by the exerted shear force. The simplified versions of the jet break-up problem were investigated by Rayleigh (1879) [10] (water jet in air), Taylor (1934) [11] and Tomotika (1935) [12] (liquid jet in another viscous fluid). For example, Tomotika took into account the effect of viscosities of both phases and calculated the maximum instabilities of the jet. For a viscosity ratio μ D /μ C of 50 whichh matches the value of current experiments (here the subscripts D and C refer to dispersed and continuous phases, respectively), the instability wavelength is about 5.7 times of the jet diameter. This value was significantly larger than the measured values shown in figures 2 and 3. This is not surprising since the numerical calculations were based on the assumption of small external fluid motions and the domination effect was the viscosity ratio for jet break-up. In general case, the droplet formation is dependent on the flow rates, fluid properties of both phases, interfacial tensionn and nozzle geometry, as summarised in Cristini and Tan. [13] A simple relation was proposed between droplet size and flow rate, which is essentially simplified as, 1 d Ca (1) As pointed out by Zhu and Power [14], such a relation is inappropriate to explain the drop size dependence. As shown in Figure 4, the two sets of data measured in two flow-focusincapillary numbers. At lower numbers, there is a larger departure from the slope. Moreover, there is a large difference in normalised droplet size between the two sets of data. This implies the capillary number alone is not sufficient to explain the dependence of droplet microfluidic devices indicate that the data only follow the scaling law in Eq. (1) at relatively large size. Proc. of SPIE Vol H-4

5 Figure 4 Normalised drop slug length as a function of capillary number. The open square symbols correspond to data collected from the mineral oil/water system (μ D /μ C = 50) while the cross symbols are data from a vegetable oil/water system (μ D /μμ C =1000). More recently, several groups have investigated the influence of the flow rate and the geometrical parameters of the flow-focusing microchip device on the droplet generation. For example, Ward et al. [15] proposed the following model without incorporating directly the capillary number, i..e. L D Q = α( Q d c 0.25 ) (2) where α is a constant dependant of the geometrical parameters and fluid properties. This relation represented well the measured data for capillarity numbers from 0.1 to 1. Tan et al [16] included capillary number explicitly in their equation, i.e. L D Qd 0.2 = α( ) Ca Q c 0.2 (3) The empirically derived constants appeared to provide a good fitting across the whole range of their experimental dataa and predictedd accurately the dispersed phase slug length in their microfluidic device. More recently, Liu and Zhang [17] proposed in a new model to predict the droplet generation in microchannel, i.e., L Q = d ( α + β ) C D Q c Ca m (4) where α, β and m are the fitting parameters depending on the channel geometry. The value of m was suggested to vary from 0.2 to 0.3 and from their numerical modellings m was found to be Figure 5 replotted the two sets of data measured from our microchips. Also plotted in the figures are the published dataa from similar flow-focusing devices. [15, 18, 19] The droplet size or slug length is normalised by the channel width. Equations (2) and (3) did not provide a reasonable fit to the current data and we thus used Equation (4) as the preferred model. A value of 0.2 was chosen for m following the findings from Tan et al. and Liu and Zhang. The current data of Proc. of SPIE Vol H-5

6 (L/D)Ca -m show an approximately linear dependencee on Q D /Q C, with α = 0.2 and β = This linear fitting is also plotted in the figure. It can be seen that the data from published work also follow the fitted model reasonably well. Figure 5 Normalised drop slug length as a function of ratio of flow rates. 4. CONCLUSIONS The droplet generation was investigated in a flow-focusing microfluidic device. Light mineral oil was used as the carrierr phase and DI water was used the dispersed phase. Surfactants weree added to both phases. At a given water flow rate, the slug length from the jet break-up decreases with increasing oil flow rate. The current data shows that the droplet size is dependent on the ratio of the flow rates and capillary, and can be represented by the model, with fitted constants, L Qd 0.2 = ( ) Ca. This model also represented reasonable well the published data measured from different D Qc flow focusing devices. REFERENCES 1. Wu, N., et al., A PMMA microfluidic droplet platform for in vitro protein expression using crude E. coli S30 extract. Lab Chip, (23): p Huebner, A., et al., Microdroplets: A sea of applications? Lab Chip, (8): p Hung, L.-H., et al., Alternating droplet generation and controlled dynamic droplet fusion in microfluidic device for CdS nanoparticle synthesis. Lab Chip, : p Proc. of SPIE Vol H-6

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