Droplet Manipulation Using Acoustic Streaming Induced by a Vibrating

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Droplet manipulation using acoustic streaming induced by a vibrating membrane Hoang Van Phan, Tuncay Alan, and Adrian Neild Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b04481 • Publication Date (Web): 27 Apr 2016 Downloaded from http://pubs.acs.org on April 28, 2016

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Analytical Chemistry

Droplet manipulation using acoustic streaming induced by a vibrating membrane Hoang Van Phan, Tuncay Alan, and Adrian Neild∗ Laboratory for Micro Systems, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia [email protected]

Abstract We present a simple method for on-demand manipulation of aqueous droplets in oil. With numerical simulations and experiments, we show that a vibrating membrane can produce acoustic streaming. By making use of this vortical flow, we manage to repulse the droplets away from the membrane edges. Then, by simply aligning the membrane at 45o to the flow, the droplets can be forced to follow the membrane’s boundaries, thus steering them across streamlines, and even between different oil types. We also characterize the repulsion and steering effect with various excitation voltages at different water and oil flow rates. The maximum steering frequency we have achieved is 165 Hz. The system is extremely robust and reliable: the same membrane can be reused after many days and with different oils and/or surfactants at the same operating frequency.

Introduction Microfluidics has become an important tool for many biological and chemical applications. 1–3 Among its many subcategories, droplet-based microfluidics, or droplet microfluidics, has 1 ACS Paragon Plus Environment


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drawn considerable research interest thanks to the advantages it offers over a continuous flow system: minimal cross-contamination and highly controlled local environment while providing high throughputs. For instance, such two-phase systems have been successfully used for kinetic measurements, 4 nanoparticle synthesis, 5 single-cell screening 6 and polymerase chain reaction (PCR). 7,8 As droplet microfluidics continues to find new applications, there is an increasing need for a flexible, simple method for on-demand/active droplet manipulation. The most popular handling mechanisms so far are electric forces 9–15 and optics, 16–19 each of which has its own benefits and drawbacks. An appealing alternative is acoustic energy, which gives rise to acoustofluidics. 20–25 Acoustics is attractive because it operates at low power levels (compared to electrophoretic methods), is biocompatible 26–29 and easily integratable on-chip. Acoustic signals are generally introduced in two different ways: by resonating interdigitated transducers (IDTs) or by vibrating piezoelectric plates. IDTs are pairs of metal electrodes deposited on a piezoelectric substrate (such as LiNbO3 or AlN). At the resonance frequency, which is dictated by the finger pairs’ width and spacing, the electrodes generate surface acoustic waves (SAWs). Previous studies have utilized SAW for particle manipulation, 30–34 on-demand droplet generation, 35 fusion 36 and steering/sorting. 37–40 In contrast, piezoelectric plates are usually used to resonate a fluid volume (which leads to bulk acoustic waves, BAWs), or a localised structure (which is primarily used to generate acoustic streaming). This excitation method has been extensively used for manipulating particles, 41–47 but rarely for manipulating droplets. A recent study has shown how droplets can be merged, sorted and washed by utilizing BAW-induced acoustic radiation force. 48 In short, a standing wave field exerts a force on a droplet that attracts it to the pressure nodes. While the method is versatile, it is relatively challenging to operate because BAWs rely on the resonance of the fluid chamber. Hence, firstly, the operating frequency is extremely sensitive to even a slight change in the device’s

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geometry or the contained fluid; and secondly, spatially localized effects are challenging to achieve. 44 Taking the case of acoustic streaming, it has not been widely used in droplet microfluidics because of issues in excitation. Micro air bubbles, the most commonly employed feature to generate strong streaming, 42,46 are obviously not suitable for droplet manipulation due to the complications of three-phase interactions and bubble instabilities. Fabricated features that are protrusions from the fluid chamber, 49,50 another source of streaming, are also not a good candidate: their resultant streaming field is not sufficiently strong compared to the viscosity of oil. In this study, we show that a vibrating silicon nitride (SiN) square membrane 51 is capable of efficient droplet manipulation. This can be achieved by utilizing the acoustic streaming field resulting from excitation of the membrane via a piezoelectric plate. Firstly, we show the streaming pattern of the membrane with simulations and particle visualization experiments. Then, we demonstrate how the vortices can be used to repulse water-in-oil droplets away from the membrane edges. Effectively, the membrane becomes a virtual obstacle to the droplets. When bulk oil flow is perpendicular to a side of the membrane, we observe a “stagnation” point close to the upstream edge: the point at which the droplet becomes momentarily stationary. The strength of the repelling effect is characterized based on the location of this “stagnant” droplet at different flow rates and excitation voltages. Next, we employ the repulsion effect to steer the droplets within a H-channel from one outlet to another by forcing them to move along the membrane edges. The steering effect is similarly characterized based on various flow rates and excitation voltages. We find that the maximum steering frequency achieved in the experiments is approximately 165 Hz. Finally, we remark the flexibility and simplicity of our design by showing that the same membrane can be reused for different oils and after many days at the same operating frequency and without any reduction in performance.



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Principle of Operation Acoustic streaming is the term used to describe vortical flows that are caused by vibration of a fluid volume. 52 Some examples include a standing wave inside a rectangular channel, 53,54 streaming around an air bubble (also known as cavitation microstreaming), 55–57 and SAWinduced streaming. 39 The vibrating membrane works in a similar manner to the bubble’s vibration, but without the instabilities posed by the air inclusion. The nature of acoustic streaming is a second-order effect caused by the first-order velocity field. Nyborg 52 shows that the gradient of the first-order velocity field creates a body force F: F = −ρ0 h(v1 · ∇)v1 + v1 (∇ · v1 )i

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where ρ0 is the fluid’s equilibrium density, v1 is the first-order velocity field and h i is the time-average operator. This force acts as the driving force behind the streaming field vs , which is mathematically the time-averaged second-order velocity field (vs = hv2 i). The streaming field can be found from the stationary Navier-Stokes equation: 1 ρ0 (vs · ∇)vs = −∇p2 + µ∇2 vs + (µB + µ)∇(∇ · vs ) + F 3

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where p2 is the second-order pressure field, µ and µB is the dynamic and bulk viscosity of fluid respectively, and ∇2 is the Laplacian operator.

Results and Discussion Numerical Simulation We simulate the system in COMSOL Multiphysics software to show that the membrane vibrations do generate acoustic streaming. (Please refer to Table 1 for the relevant information of the simulations, and Figure S-1 for the boundary conditions.) Figure 1 shows the stream-


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ing field, vs , in 2 different oils, each at 2 vibration modes 58 at 150 kHz, which is assumed to be constant for the simulated scenarios. Firstly, we can clearly see the regions that the streamlines do not pass. These regions are instead occupied by streaming vortices (closed vortical streamlines, compare Figure 1(b)i to (b)iii for instance). Secondly, the strength of the vortices depends on the oil type: it is higher for the less viscous oil Novec 7500. This can be observed from both the area of the vortices (most easily seen from the rightmost vortex in the second vibration mode, Figure 1(a)ii and (b)ii) and the maximum velocity amplitude (that of Novec 7500 oil is at least 1.6 times as high as FC-40 oil’s). This is to be expected as it is more difficult for the membrane to drive a more viscid fluid at a specific excitation amplitude a0 . Thirdly, the vortex patterns change substantially when one switches from the first to the second mode shape. However, the vortices do not extend beyond the membrane’s edges in all cases. Flow

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Figure 1: Numerical results of acoustic streaming at 150 kHz in (a) FC-40 oil and (b) Novec 7500. The figures show the streaming velocity field obtained from the Laminar Flow module at 2.5 µl min-1 flow rate. The surface plots of (a) and (b) represent the second-order velocity magnitude in mm s-1 . The white lines in figures i and ii represent the streamlines starting from the inlet. The streaming vortices can be seen in figures (b)iii and iv, which plot the uniformly distributed streamlines. Flow is from right to left. The horizontal red bars in (a) and (b) mark the surface of the membrane. When the droplets fill the height of the channel such as those in this study, we can expect that the streaming vortices can act as a blockage that stops the droplets as they flow close to the membrane. A previous study shows that with a vibrating liquid-air interface in water, 5 ACS Paragon Plus Environment


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Table 1: Important model parameters. The membrane’s parameters are typical of our experiments. The oils’ properties are obtained from the manufacturer’s datasheet and literature. 59 Membrane Side length Vibration amplitude Excitation frequency FC-40 oil Density Dynamic viscosity Novec 7500 oil Density Dynamic viscosity

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particles can be trapped in those vortices based on the interplay between the average flow velocity and streaming velocity. 42 On the contrary, droplets are expected to flow around the membrane’s edge, i.e. flowing in the neglected out-of-plane direction of the simulations.

Particle Visualization Now, we show experimentally the existence of acoustic streaming with the help of particle visualization. Figure 2(a) and (b) show the streaming pattern in a water-filled channel under two imposed flow conditions (static and with flow, respectively). We can distinguish two types of streaming vortices: one lying in the plane parallel to the membrane surface (xy-plane, Figure 2(b)), and one in the perpendicular plane (xz-plane, which is the one investigated and predicted by the simulations in Figure 1). Firstly, with no imposed net flow, the butterfly-wing pattern in Figure 2(a) corresponds to a pair of counter-rotating xy-plane vortices, which is similar in nature to the streaming vortices obtained by a vibrating discontinuous membrane. 51 On the otherhand, the xz-vortices can be observed based on the zigzag motion of the particles at the membrane boundaries (marked by dashed oval in Figure 2(a)). The maximum velocity of the vortices in the xyplane is measured (by particle tracking velocimetry 60 ) to be 0.4-0.5 mm s-1 . We cannot measure the velocity of the xz-vortices because of their out-of-plane nature. Moreover, some


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particles are collected on the membrane surface as shown by the “Concentration spot” in Figure 2(a). We note that these particles are not fixed in an exact location, but rather move around within that region, also as a result of streaming. Figure 2(b) shows the streaming field when there is an imposed bulk flow. Here, the actuation is such that the vortices parallel to the membrane are no longer observed. More importantly, the particles’ zigzag motion at the downstream membrane’s edges becomes more visible. This zigzag motion is effectively the projection on the xy-plane of the helix-trajectory of the particles (Figure 2(c)). The different effects on the particles between Figure 2(a) and (b) can be explained by those membranes being not identical. (a)

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Figure 2: Particle visualization of acoustic streaming. (a) Superimposed picture of streaming in static water at 119 kHz. The red squares mark the 2 membranes. The maximum particle velocity due to streaming vortices in xy-plane is approximately 0.4 − 0.5 mm s-1 . (b) Superimposed picture of streaming in presence of flow (maximum velocity of 7.6 mm s-1 at the channel cross-sectional center) at 137.4 kHz. (c) Relationship between numerically predicted streaming vortices and the experimentally observed zigzag motion. The white dashed lines mark the channel walls. The channel height is 21 µm and 34 µm in (a) and (b) respectively. Each scale bar represents 500 µm. Comparing the numerical results of the second vibration mode (Figure 1(a)ii and (b)ii) to the experimental data, we must take into account that the simulation xz-plane is perpendicular to the experimentally recorded xy-plane. The simulations predict two outer vortices along the edges of the membrane; these appear as the zigzag motion of the particles in the image plane (Figure 2(b)). The bright spots within the membrane area in Figure 2(b) 7 ACS Paragon Plus Environment


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correspond to the particles trapped in the inner vortices. Interestingly, experiments show that streaming also occurs inside the water droplets when they flow over the membrane surface (Figure S-4 and Supporting Video 1).

Droplet Repulsion Effect We now move to utilizing acoustic streaming for water droplet manipulation. Since the droplets are large compared to the vortices (depending on the flow rates, their radii range from 50-90 µm) and there is a net flow, they do not circulate like the particles in Figure 2. Instead, the droplets flow along the edges of the membrane as seen in Figure 3, because they would rather avoid the vortices by following a different streamline than disrupting the streaming field. We notice that there is a “stagnation” point: the point at which an incoming droplet is brought to stationary due to the interaction between the fluid flow and the streaming. To characterize the strength of the repulsion effect, we have calculated the normalized distance (by the membrane side length) between the downstream edge of the membrane (i.e. the left edge) and the leading edge of the “stagnant” droplet. (A higher distance implies that the effect is stronger.) We choose to base the calculation on the droplet’s leading edge (instead of, for example, its center) because it is the part that comes into contact with the vortices first. Unsurprisingly, the data show that at lower excitation voltages, the effect is weaker, as the membrane’s vibration is not strong enough to disturb the flow sufficiently. Figure 3(a) also shows that for the same oil flow rate, a higher water flow rate results in lower stopping distance. This can be attributed to the droplet size and population density: at higher water flow rate, larger droplets are produced more frequently. Collisions between droplets thus weaken the effectiveness of the streaming vortices in resisting the droplets. The droplet detachment downstream of the membrane (Figures 3iii-iv) suggests a combination effect of the droplet size and laminar flow past an obstacle (which is similar to 8 ACS Paragon Plus Environment

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Figure 3: Effects of the applied voltage on droplet repulsion from a single small membrane at 139 kHz can be quantified based on the normalized distance from the membrane’s downstream edge (left edge) to the stagnant droplet. (a) The repulsion effect in diluted Novec 7500/Pico-Surf 1 oil at different flow rates. (b) The repulsion effect for different oils at a given flow rates combination. (c) Channel dimension and the sign convention of the normalized distance (the zoom shows an example of calculating the location of the droplet’s edge). In (b), the first data set (red solid line) is the same as that in (a). The second and third data set (red dashed and cyan solid line) compare repulsion behavior between Novec 7500/Pico-Surf 1 and FC-40/Pico-Surf 1, both of which are obtained by reusing the device after 19 days (at the same operating frequency of 139 kHz). The flow rates of water and oil are given as (Q1 ,Q2 ), respectively (figure (c)). Figures i-iv are marked in (a) and (b). Each scale bar represents 500 µm. pinched flow fractionation 61 ). Suppose when a droplet reaches the hindrance structure, the hindrance will force the droplet to change to the streamlines further away from the obstacle surface (i.e. the membrane edges). These outer streamlines do not reattach to the downstream side of the hindrance, which explains the lack of droplets immediately downstream of the membrane. 9 ACS Paragon Plus Environment


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Whilst this observation supports the role of acoustic streaming in diverting the droplets, we still cannot definitely conclude so. It could be contended that the droplets are being affected by the acoustic radiation force (ARF) 62 similarly to the previous study by Leibacher et al. 48 . Indeed, similar to microparticles, a transition regime might exist in which the droplet are influenced by both streaming and radiation forces. 32 We now argue that acoustic streaming is the primary mechanism at play. Firstly, the zigzag motion of particles along the membrane’s edges (Figure 2) concurs with the motion of the droplets: the xz-plane vortices that direct the particles in a zigzag manner are responsible for repulsing the droplets. Secondly, we can see that the oil’s viscosity affects how effectively the droplets are stopped. From Figure 3(b), at the same excitation voltage and flow rates, the stopping distance is always higher for the less viscous oil (Novec 7500, kinematic viscosity is 0.77 mm2 s-1 ) than for the more viscous one (FC-40, kinematic viscosity is 1.8 mm2 s-1 ). A higher stopping distance indicates stronger streaming, which the simulations suggest to occur with less viscous oil. Thirdly, the calculated value of the ARF is one order of magnitude lower than the strength of streaming effect, which is expected because the effect of ARF weakens with lower frequency. 32,63–65 And lastly, simulations predict that streaming even prevents the droplets from entering the regions under the strongest influence of ARF. (For more details on the comparison of acoustic streaming and radiation forces, please refer to the SI.) We also observe that a droplet can be trapped at the membrane center, as seen in Figure 3iii (FC-40) and 3v (Novec 7500). The vortices responsible for droplet trapping is similar to the inner vortices of the left membrane in Figure 2(b) (the C-shaped particle trajectories). Trapping also points to the effect of viscosity on streaming strength: at the same flow rate combination, trapping begins to occur at 43 Vrms and above for Novec 7500 oil, while for the more viscous FC-40 oil, trapping only occurs at the maximum applied voltage 64 Vrms . Lastly, Figure 3 illustrates the reliability and flexibility of the membrane: the same


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membrane can be used for different oils at the same operation frequency. This would not be possible for BAW-based devices, because the resonance frequency depends on the fluid’s speed of sound. Furthermore, the membrane can be reused after 19 days at the same frequency with virtually unchanged performances (solid and dashed red lines in Figure 3(b)).

Droplet Steering Having shown that the droplets only go around the boundaries of the membrane, it is reasonable to expect that if we position the membrane at an angle to the flow field, we can move droplets cross-streamline. We set this experiment up by making use of an H-channel and aligning the membrane at 45o to the flow, as shown in Figure 4. Although the membrane has effectively become triangular (half of the membrane is now bonded to PDMS, making that part fixed), the repulsion effect still exists. We also find that the membrane size is important to steering effectiveness: a 450-µm membrane performs extremely poorly compared to a 270-µm membrane. As a result, we characterize the steering effect using only the latter membrane type. Please refer to SI for a discussion on steering with the 450-µm membrane. Figure 5 shows how steering depends on flow rates and excitation voltages. (Supporting Video 2 shows steering at 197 kHz (1,4,5) µl min-1 and 64 Vrms .) We characterize how well droplets are steered based on the distance of the droplet centers to the channel wall, and normalize it by the expansion chamber’s width (150 µm). Droplets exit through the cross outlet only if the normalized displacement distance is greater than 0.5. We use the location of the droplets’ centers in this case as it is more representative of the trajectory of the droplets. Overall, the steering behavior becomes stronger at higher voltage and lower flow rates of the oil streams. This highlights the interaction between acoustic streaming and bulk flow rate. It is worth noting that the normalized displacement distance cannot take into account different droplet sizes. For example, the steering behavior of (5,30,50) µl hr-1 appears to be weaker than that of (1,30,50) µl hr-1 simply due to the fact that the droplets of the former are larger, thus the distance from their centers to the bottom wall is lower even though 11 ACS Paragon Plus Environment

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Figure 4: 3D view schematic of the acoustofluidic device for droplet steering and the membrane fabrication process. The membrane is a 270-µm square. The PDMS H-channel is 45 µm high. The T-junction’s width is 30 µm up to the connection to the H-chamber, then it changes to 60 µm (inset i). Q1 denotes the flow rate of water, Q2 and Q3 the flow rate of oil in the T-junction and the sheath flow, respectively. The flow rate combination for steering experiments is written as (Q1 ,Q2 ,Q3 ). Inset i shows the dimensions of the PDMS channel. Inset ii shows the fabrication steps of the membrane. a and t denotes the membrane’s side length and thickness respectively. complete steering is achieved in both scenarios (Figure 5v and vi). We also note that the production rate of droplets also affects the steering behavior. At a high rate, the spacing between two adjacent droplets is so small that the first one does not have sufficient time to cross streamline. As a result, the second droplet can collide with the first one, as shown by the clump of three droplets in Figure 5iii (dashed oval). Occasionally, these droplets still exit to the desired (top) outlet; otherwise they start hindering one another after leaving the membrane’s surface due to their sizes: the extra droplet (dashed arrow in Figure 5iv) is prevented from entering the top outlet by the preceding droplet (solid arrow in Figure 5iv). This process is captured in Supporting Video 3. The device achieves the highest steering frequency of approximately 165 Hz at the flow rate combination (1,4,5) µl min-1 . We believe that this is not necessarily the maximum throughput of the systems: the lack of a spacing oil flow prevents us from increasing the flow rates further (at high flow rates, the droplets are too packed to be effectively deflected). We also note that the membrane can fail to manipulate the droplets if they are too large:


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Crossing Oil Streams We now show that our device is also capable of moving droplets between different oil streams (or medium exchange). This might be useful when one wants to bring a droplet in an oil stream with surfactant (for easier, more stable droplet production) to one without surfactant for droplet fusion and content recovery. 13 ACS Paragon Plus Environment


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Figure 6: Image sequence of a droplet crossing from the stream of oil with surfactant to the stream of oil without surfactant. The oil of both streams is FC-40, and the oil of the bottom stream has 30 vol% perfluoro-octanol surfactant. The flow rate is (10,30,50) µl hr-1 . The arrow indicates the droplet of interest. Figure 6 shows the image sequence of a droplet migrating from the stream of oil with surfactant to a stream of the same oil with no surfactant. The steering behavior also occurs in the same manner even if the oil of each stream is different. For example, we have successfully reproduced this between Novec 7500 with Pico-Surf and pure Novec 7500, Novec 7500 with Pico-Surf and pure FC-40, and between oil with Pico-Surf and pure Pico-Break (the chemical used to dissolve the Pico-Surf surfactant). We also note that the membrane does not cause significant mixing of the two oils, which agrees with our previous study. 51 Firstly, the acoustic streaming effect is strong enough to affect the droplets, but not enough to mix the 2 oils. Secondly, to achieve good mixing, the streaming vortices have to lie on a plane which crosses through both oil streams (i.e. on the xy-plane); instead, the vortices in our device predominantly lie in the xz-plane. The interface between two oils seen in the expansion channel above the membrane (Figure 6) is only disturbed by the motion of the droplets. We have used the membrane to steer droplets between 2 streams of different compositions. 14 ACS Paragon Plus Environment

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It is possible to force the steered droplets back to the original stream with another membrane located downstream. Compared to other mechanisms of on-demand medium exchange, 48,66,67 our method is simpler to operate and control. The droplets will travel across as many different media as required, provided that the membrane edge is long enough. Compared to the BAW method by Augustsson et al. 66 , we can move the droplets from one stream to another at once without the need of multiple fluid injections. SAW has been used in previous studies for droplet manipulation, either through acoustic radiation force 38 or acoustic streaming 37,39 (which is physically the same as streaming around a vibrating membrane). In comparison, our actuation method offers high localization: the effect only exists above the membrane, whilst SAW-induced streaming occurs along the line of wave propagation. The membrane is also more compact than IDTs, thus many membranes can potentially be integrated on a single small chip at different orientations to achieve other manipulation techniques (e.g. merging and trapping) in a highly localized manner.

Conclusions We have presented a flexible, simple and effective device to manipulate aqueous droplets. By vibrating a continuous, uniform silicon nitride square membrane, we can repulse the droplets away from its edges. The underlying mechanism is shown numerically and experimentally to be acoustic streaming. The strength of this repelling effect is stronger with higher excitation voltage and less viscous oil. We then utilize this phenomena to steer droplets cross streamlines at different flow rates of water and oil, as well as various applied voltages. Again, droplets are steered more strongly at higher voltage, and at lower flow rates combinations. Our current design can reach a sorting frequency of 165 Hz, a reasonably good value considering that this membrane streaming mechanism is still nascent compared to, for instance, SAWs or dielectrophoresis. As such, there is much room for improvement. We propose that for future studies, a spacing oil flow can be used so that the flow rates can be adjusted



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more flexibly, which allows for investigation of the effects of droplet size and the maximum achievable steering frequency. We remark the device’s robustness and reliability: the resonance frequency does not change after 19 days, or when used with different types of oil or surfactant. Our design is the first to utilize streaming induced by a vibrating membrane for droplet manipulation. As a result, the system is biocompatible and has straightforward, intuitive operation. Since the piezoelectric plate is located remotely from the SiN chip, unwanted heating of the working fluids is negligible, and the device can be easily integrated for different purposes. The membrane is fabricated with common and scalable techniques, which further increases its practicality.

Methods and Materials Device Fabrication Our devices consist of a poly-dimethyl-siloxane (PDMS) channel bonded onto a SiN chip. For the droplet repulsion system (Figure 3(c)), the expansion channel is 1500 µm wide and 48.6 µm high, and the T-junction’s width is 30 µm. For the steering system (Figure 4), the H-channel’s height is 45 µm, the inlet on one side has a T-junction (30 µm wide) upstream to generate the droplets. Each H-channel has a rectangle column outside of the main channel, which is designed for 45o -alignment (“Alignment mark” in Figure 5 and 6). PDMS channels are made by soft lithography, 68 and the SiN membrane is fabricated using standard methods: photolithography, reactive-ion etching (RIE) and KOH wet etching (Figure 4ii). 69 A photograph and a sketch of the assembled device are shown in Figure S-2 (details on the assembly are given elsewhere 51 ). The SiN chips are made from low-stress low pressure chemical vapor deposition (LPCVD) silicon nitride (100) wafers (thickness t = 997 ± 7 nm, Else Kooi Laboratory, TU Delft, The Netherlands). The square membranes we use have side lengths a ≈ 270 µm. In SI, we show 16 ACS Paragon Plus Environment

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droplet steering with a larger membrane type (a ≈ 450 or 530 µm).

Materials R The fluids used in the experiments are Milli-Q water and different fluorinated oils. In

droplet repelling and particle visualization experiments, we use Novec 7500 with 5% PicoSurf 1 (further diluted at 20 vol% in pure Novec 7500) and FC-40 with 2% Pico-Surf 1. Pico-Surf is the proprietary surfactant of Sphere Fluidics. Fluorescent particles of 1.1, 5.1 and 6.6 µm (Magsphere Inc.) are mixed in 2 wt% poly-ethylene glycol (PEG, CAS: 9003-116, Sigma-Aldrich) in water solution at 5 vol% and 50 vol%, respectively, for particle tracing. (PEG is used to prevent particle adhesion to the walls). For droplet steering, we use FC-40 with different surfactants: with 2% Pico-Surf 1 or with 30 vol% 1H,1H,2H,2H-perfluoro1-octanol (Sigma-Aldrich). For clarity, the oil used will also be mentioned in the figure captions.

Experiment Set-up The SiN membrane is actuated by a piezoelectric disk (Pz26, Ferroperm). The applied power is controlled by changing the peak-to-peak voltage of the signal generator (Stanford Research Systems DS345) while keeping the gain of the signal amplifier (T&C Power Conversion, Inc. AG 1006) constant. The output root-mean-square voltage Vrms in our experiments ranges between 9 and 64 Vrms . We use syringe pumps (New Era NE-1000) to pump the fluids. The experiments are recorded using a high speed camera (Photron FASTCAM-ultima1024) at 250, 500, 1000 or 2000 frames per second depending on the flow rates, and are subsequently analyzed in MATLAB. Because analyzing a single frame does not give a good indication of the droplets’ behavior, we have to use the ‘imfuse’ built-in function of MATLAB to composite multiple frames (Figure S-3). ‘imfuse’ provides two options for images composition: ‘false color’ and ‘diff’. ‘falsecolor’ colors the differences of two images, and ’diff’ subtracts the image intensities. We find that to characterize the repulsion effect, ‘false color’ is more 17 ACS Paragon Plus Environment


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suitable whilst both methods are needed to analyze droplet steering behavior.

Acknowledgement This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). The authors thank Professor Kristian Helmerson for kindly lending us the high speed camera. The authors thank the Australian Research Council (No. DP110104010) for their kind support of this research. The authors declare no competing financial interest.

Supporting Information Available Numerical simulation set-up, photograph of the device, an example of the ‘imfuse’ MATLAB function, internal streaming of droplets, acoustic radiation force calculation, negative control experiments and droplet steering with a large membrane. This material is available free of charge via the Internet at http://pubs.acs.org/. Membrane

Figure 7: For TOC only.

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Membrane


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Droplet Manipulation Using Acoustic Streaming Induced by a Vibrating

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