Droplet group production in an AC electro- flow focusing microdevice

Microfluidics and Nanofluidics manuscript No. (will be inserted by the editor) Droplet group production in an AC electro-flow focusing microdevice Elena Castro-Hern´andez ·Pablo Garc´ıa-S´anchez ·Alfonso Velencoso-G´omez ·Antonio Silas-Jurado ·David Fern´andez Rivas · Antonio Ramos Received: date / Accepted: date Abstract We report the production of droplet groups with a controlled number of drops in a microfluidic electro-flow focusing device under the action of an AC electric field. This regime appears for moderate voltages (500-700 V peak-to-peak) and signal frequencies between 25 and 100 Hz, much smaller than the droplet production rate (∼500 Hz). For this experimental conditions the production frequency of a droplet package is twice the signal frequency. Since the continuous phase flow in the microchannel is a Hagen-Poiseuille flow, the smaller droplets of a group move faster than the bigger ones leading to droplet clustering downstream. Keywords AC electric field ·flow focusing ·microfluidics ·drops 1 Introduction The accurate control of the volume and number of bubbles and droplets has many practical applications as, Elena Castro-Hern´andez ·Alfonso Velencoso-G´omez ·Antonio Silas-Jurado ´ Area de Mec´anica de Fluidos, Departamento de Ingener´ıa Aeroespacial y Mec´anica de Fluidos, Universidad de Sevilla, Avenida de los Descubrimientos s/n 41092, Sevilla, Spain. E-mail: [email protected] Pablo Garc´ıa-S´anchez ·Antonio Ramos Departamento de Electr´onica y Electromagnetismo, Facultad de F´ısica, Universidad de Sevilla, Avenida de Reina Mercedes s/n, 41012 Sevilla, Spain. E-mail: [email protected] David Fern´andez Rivas Mesoscale Chemical Systems and MESA+Institute of Nanotechnology, P.O. Box 217, 7500 AE Enschede, The Netherlands. for example, in the chemical engineering, advanced materials science and biological fields (Song et al., 2006; Burns and Ramshaw, 2001; Xu et al., 2005; Garstecki et al., 2006; Ma et al., 2017; Belloul et al., 2013; Churski et al., 2012; Frenz et al., 2008). Droplet generation methods can be classified as passive or active, depending on the use of external actuation (Zhu and Wang, 2017). Techniques such as cross-flow, co-flow, flow-focusing (Ga˜n´an- Calvo, 1998), and step emulsification are grouped as passive. The active techniques encompass the use of electric, magnetic, or centrifugal fields, just to name a few. External forces are not only used to generate droplets (Gu et al., 2008; Ga˜n´an-Calvo et al., 2006), but also to manipulate them downstream as, for example, droplet coalescence, splitting and mixing (Ray et al., 2017; Budden et al., 2013; Chokkalingam et al., 2014). Droplet cluster formation has been achieved using passive methods with a potential to serve as building blocks for new materials (Shen et al., 2016). In this work, we generate drop clusters by combining a passive technique to produce droplets and an active one to modulate the droplet size within clusters. The former is achieved with a planar microfluidic flowfocusing junction (Anna et al., 2003), whereas the latter is accomplished by the application of an AC electric field. The advantages of using an electric field are that control is almost instantaneous and the features of drop production might be changed without altering the flow parameters (flow rate, viscosity, surface tension, etc). Furthermore, the production of droplets packages is not possible without the application of an external field. Droplet size modulation in a flow-focusing geometry has been demonstrated by using an electrical signal of triangular shape at a frequency of 10 Hz (He et al., 2010). In order to produce different groups of droplets we employ sinusoidal signals with frequen- 2 Elena Castro-Hern´andez et al. w h PDMS Glass w V Qi Q /2 o a) b) Q /2 o Fig. 1 a) Cross-section of the device at the level of the outlet microchannel (w= 100 µm, h= 35 µm). b) Sketch of a microfluidic electro-flow focusing device under an AC electric field. The electrodes are in black and the dispersed phase in blue. cies ranging from 25 to 100 Hz and moderate voltages (500-700 V peak-to-peak). Clustering of droplets is carried out downstream by size-dependent velocity dispersion in the microchannel. AC electro-flow focusing devices have been previously used to control the production of droplets (Tan et al., 2014) and long jets (Castro-Hern´andez et al., 2015, 2016). In these works, the signal frequency was greater than the production droplet rate. Also, the existence of an unstable droplet generation regime was reported for values of the signal frequency below a threshold and sufficient voltage amplitude. Here, we have found a new regime where generation of droplet groups with a different drop number is observed. 2 Experimental setup Soft lithography techniques are used to fabricate a microfluidic electro-flow focusing device by replica molding in polydimethylsiloxane (PDMS, Dow Corning, relative permittivity εr,PDMS = 2.5). Figure 1 shows a schematic view of the device with a rectangular crosssection w= 100 µm wide and h= 35 µm tall. Two sets of electrodes are patterned on both sides of the junction and they are produced using the microsolidics technique (Siegel et al., 2006). The PDMS device is plasma bonded to a glass slide. A water-in-oil (W/O) emulsion is produced by focusing an inner aqueous stream (dispersed phase) with two outer oil streams (continuous phase). The inner and outer flow rates, Qiand Qorespectively, are controlled by means of a double syringe pump (Model 33, Harvard Apparatus) and for all our experiments they are fixed to Qi= 50 µl/h and Qo= 400 µl/h. The dispersed phases are aqueous solutions of KCl in Milli-Q water with viscosity ηi= 1 cP and electrical conductivities κ= 3×10−4S/m, κ= 3×10−3S/m and κ= 3 ×10−2S/m. The continuous phase is mineral oil (RTM14, Sigma Aldrich) with a viscosity of ηo= 100 cP. The relative permittivity of mineral oil is εr,o= 2.1 and its electrical conductivity is negligible (κo<10−10 S/m), being considered from now on as a perfect insulator. A 5 % (w/w) of a non-ionic surfactant (Span 80, Sigma Aldrich) is added to the continuous phase lowering the surface tension of the liquid to liquid interface from σ= 40 mN/m to σ= 5 mN/m, being this value independent of the KCl concentration (Tan et al., 2014). The high-voltage is applied to the downstream pair of electrodes while the others are grounded, which guarantees that the incoming liquid has zero potential (Castro- Hern´andez et al., 2015, 2016). As a consequence, there is an applied AC potential difference between this incoming liquid and the downstream electrodes. A sinusoidal voltage with frequencies varying from f= 1 Hz to f= 1000 Hz (TGA1244, TTi) is amplified from Vpp = 0 V to Vpp = 1000 V (PZD700A, Trek). The microfluidic device is placed on an inverted microscope (Eclipse Ti-U, Nikon) connected to a high-speed camera (Phantom v7.3) with a resolution of 800 ×256 px2when operated at an acquisition rate of 104fps. 3 Results and discussion Figure 2 depicts the effect of decreasing the signal frequency on the number of droplets per group for a given water conductivity (κ= 3 ×10−4S/m) and voltage amplitude (Vpp = 650 V). The number of droplets per group was determined by visual inspection of the videos. We performed two sets of experiments: (i) Starting from zero frequency and increasing the signal frequency; (ii) Starting from 100 Hz and decreasing the signal frequency. In both cases we found the same droplet groups, confirming the reproducibility and the absence of hysteresis. A greater number of droplets per group is found for smaller frequencies, and a package of N= 10 is obtained when f= 25 Hz. Reducing the signal frequency below this value, produces groups with a higher number of drops and an extent that exceeds the channel length. Figure 2 also shows that for f= 90 Hz the drop number is N= 2 while for f= 65 Hz the drop number is N= 3. For intermediate frequencies (65−90 Hz) groups of 2 and 3 droplets are alternatively produced. Such an intermittent production of droplet Droplet group production in an AC electro-flow focusing microdevice 3 90Hz 65Hz 47Hz 40Hz 35Hz 30Hz 25Hz a) c) d) e) f) b) g) 100 mm Fig. 2 Series of images showing droplet groups with a different drop number, N, for Qi= 50 µl/h, Qo= 400 µl/h, ηo= 100 cP, κ= 3 ×10−4S/m and Vpp = 650 V: a)f= 90 Hz, N= 2; b)f= 65 Hz, N= 3; c)f= 47 Hz, N= 4; d)f= 40 Hz, N= 5; e)f= 35 Hz, N= 6; f)f= 30 Hz, N= 7; g)f= 25 Hz, N= 10. groups have been noticed for all in-between signal frequencies. This multi-drop regime is observed for voltage amplitudes between Vpp = 500 V and Vpp = 700 V and signal frequencies between f= 25 Hz and f= 100 Hz. For moderate values of the voltage and signal frequencies above f= 100 Hz the unstable regime appears and with further increment a jet is observed (Tan et al., 2014; Castro-Hern´andez et al., 2015, 2016). For Vpp > 700 V the unstable regime is present for all tested frequencies, while for Vpp <500 V drop diameters in a group are more uniform. For fixed inner and outer flow rates, the droplet production rate is nearly constant fdrop ∼500 Hz, only slightly dependent on signal frequency. The same trends are found for all tested water conductivities (0.3,3 and 30 mS/m). Nevertheless, the number of droplets per group is less clear for the two highest water conductivities, presumably, because the unstable regime is more pronounced for those values. In order to check the similarity between droplet packages, we measured the drop sizes and compared them. We analyzed 1000 images for each experimental condition by using an image processing code in MATLAB. From this analysis we obtained the droplet size and calculate the polydispersity index (PDI) as in the following example: In the experimental conditions shown in Figure 2a), i.e. packages with two droplets, we obtain a PDI around 5% for the larger drops; and for the smaller       8. U2 ( 8. 8 .  . / . .i .i .i .i 8 Fig. 3 Droplet radius versus relative droplet number in a package, ni/N, for Qi= 50 µl/h, Qo= 400 µl/h, ηo= 100 cP, κ= 3 ×10−4S/m and Vpp = 650 V. drops PDI is around 11%. For all experimental conditions we obtained a PDI below 14%. Figure 3 depicts the droplet radius as a function of the relative droplet number in a package, ni/N, where nilabels the droplet in the sequence of a group and N is the number of droplets per group. In this figure, notice that ni/N = 0 is equivalent to ni/N = 1 due to the periodicity of droplet production. As discussed below, large droplets are produced when the signal amplitude reaches a value around zero and their size is mainly determined by the microchannel dimensions. On the other hand, the size of smaller droplets is given by the competition between electrical and capillary pressures. For the signal frequencies of this work, the meniscus should be an equipotential (at zero potential as the upstream electrodes). The charge relaxation time of our water solutions is of the order of ε/κ ∼10−6s. This time is much shorter than the signal period (of the order of 0.01 s) and than the droplet breakup time (estimated from experiments as 1/fdrop = 1/500 s). Therefore, there is enough time for charge to relax and to make the meniscus surface equipotential. The electrical stress on the surface is normal (no tangential stress) and given by q2 s/2ε, where qsis the induced surface charge density. This induced charge is proportional to the instantaneous applied voltage, i.e. qs(t)∝V0cos(ωt). Therefore, the electrical normal stress on the meniscus is pE=q2 s(t) 2ε∝V2 0 2(1 + cos(2ωt)) ,(1) where we have neglected the effect of the issuing drops. Otherwise, other terms should be added to the electrical 4 Elena Castro-Hern´andez et al. IJURXSH7        IH7        Fig. 4 Group frequency, fgroup, versus signal frequency for Qi= 50 µl/h, Qo= 400 µl/h, ηo= 100 cP, κ= 3 ×10−4S/m and different voltages. The inset shows the number of droplets per group, N, as a function of voltage amplitudes and signal frequencies. pressure. Presumably, this is the case for voltage amplitudes greater than 700 V. Figure 3 also shows that the drop radius in a group mirrors the oscillating electrical pressure. The competition between electrical and capillary pressures suggests that smaller drops are detached for higher electrical pressure while the bigger ones are detached around zero pressure and voltage. This is in agreement with experiments using electrical signals of triangular shape (He et al., 2010). Figure 4 displays measurements of the group frequency, fgroup, versus the applied signal frequency, f, for κ= 3 ×10−4S/m and a voltage amplitude between Vpp = 500 V and Vpp = 700 V. The group frequency is defined as the frequency production of a droplet package. The figure also shows that the group frequency is twice the signal frequency and, thus, the droplet production rate can be determined as fdrop = 2Nf. This proves that the droplet groups follow the electrical pressure given by Equation 1, consistently with the assumption of negligible electrical interaction between the drops and the meniscus. The inset in Figure 4 shows that, for our experimental conditions, the droplet number, N, is independent of the voltage amplitude. We have also determined the spatial periodicity, λ, of the droplet groups for κ= 3×10−4S/m and Vpp = 650 V. This period is measured as the distance between the largest drops of two consecutive groups. Figure 5 shows a linear increase of the spatial periodicity with respect to the period of the electric forcing, 1/(2f). This seems to indicate that all groups travel at the same velocity m /sI2 -/2 . .. ... sI-/2 .0.. .0. .0. .0. Fig. 5 Spatial periodicity versus electric forcing time for Qi= 50 µl/h, Qo= 400 µl/h, ηo= 100 cP, κ= 3 ×10−4S/m and Vpp = 650 V. and the spatial periodicity is given by the duration of the forcing cycle. Thus, we can infer the velocity of the groups from a linear fit to the data. We have obtained vgroup ≈42 mm/s, between the average and the maximum velocities within the channel, as shown below. As the Reynolds number of the continuous phase is small, the flow in the channels is a Hagen-Poiseuille flow, with zero velocity at the walls and maximum velocity at the center of the channel. For a total flow rate of Qo+Qi= 450 µl/h and a rectangular channel with h= 35 µm and w= 100 µm, the maximum flow velocity is 67.1 mm/s and the average flow velocity is 35.7 mm/s (Bruus, 2007). As a first approximation, the velocity of a drop is the average of the flow velocity within the volume occupied by the drop. Since drops move mainly along the axis of the channel, the smaller droplets of a group flow faster than the bigger ones and they assemble downstream of the channel. This phenomenon has been previously used to produce pairs of droplets with different sizes (Ahn et al., 2006). In our experiments, the difference in velocities between the largest and the smallest drop within a group is ∼5 mm/s, an order of magnitude smaller than the group velocity. Consequently, in a traveled distance of 1 mm the group shortens around 0.1 mm. 4 Conclusions In summary, we report the production of droplet groups with a controlled number of drops in an AC electroflow focusing microdevice. This new regime appears for Droplet group production in an AC electro-flow focusing microdevice 5 moderate values of the voltage amplitude and signal frequencies much smaller than the droplet production rate. The production of droplet groups is only possible with the action of an external field. Also, the number of droplets per group is controlled by tuning the signal frequency, and the production rate of droplet packages is twice the signal frequency. The production of droplet groups is achieved by electrohydrodynamic means and droplet clustering is carried out by size-dependent velocity dispersion in the microchannel. Acknowledgements The authors would like to acknowledge financial support from Spanish Government Ministry MEC under Contracts DPI2013-46485-C3-1-R and FIS2014-54539- P and Regional Government Junta de Andaluc´ıa under Contract P11-FQM-7919. They would also like to acknowledge the technical assistance of S. Schlautman in the fabrication of the microfluidic devices. 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