Surface acoustic wave (SAW) directed droplet flow in microfluidics for PDMS devices

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Original entry: Darren Yang, AP225, Fall 2010

Reference

T. Franke, A.R. Abate, D.A. Weitz, A. Wixforth, “Surface acoustic wave (SAW) directed droplet flow in microfluidics for PDMS devices,” Lab on a Chip, 9, 18 (2009).

Keyworks

Poly(Dimethylsiloxane), microfludics, droplet flow, chip, surface acoustic wave

Summary

The authors of this work developed a technique that can direct the motion of droplets in microfluidic channel using a surface acoustic wave. Because of this method allows individual drops to be direct along separate channel paths at high rate and volume, this technique will be very useful for droplet sorting.

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Background

Often times, to control micro flow and gain functionality of a microfluidic device, additional component such as valves or pumps must be added. There have been many techniques developed to manipulate flow in microfluidic devices, which is based on physical effects, such as hydrodynamic pressure gradients, capillarity, magnetic forces, dielectrophoresis and electrophoresis. Nonetheless, all these techniques depend on some specific property of the object as compared to the bulk carrier fluid.

The authors develop a novel hybrid technique combines the advantages of fast electronic response with hydrodynamic control of droplet flow and redundantize external labelling for manipulation. Moreover, this technique is independent of the properties of the objects to be sorted such as dielectric constant or charge because it actuates a bulk fluid flow. This technique is based a surface acoustic wave device which will be described in the results section.


Results

First, a high-speed fast alternating electric RF-field device, which generates an elliptically oscillating displacement of amplitude in the nm-range on the surface of a piezoelectric substrate, was developed. This wave propagates at the velocity of sound and is known as Rayleigh wave.

When a microfluid channel is placed on top of the RF-field device, the RF wave couples into the fluid transferring momentum and ultimately pushes droplets along the direction of wave propagation (Figure 1).

The electric RF-field device that the authors are presenting in this work directs fluid droplets using acoustic streaming excited by an interdigital transducer (IDT). The IDT consist of two gold electrodes on a piezoelectric substrate, each having a comb-like structure, which are interdigitated at fixed finger. The ratio of the sound velocity in the substrate and twice the finger distance defines the operating frequency of the IDT. The chip presented here has a finger spacing of 13 mm and works at 140 MHz. To demonstrate the ability to direct drops, we produce water drops inHFE-7500 fluorocarbon oil with 5%(vol/vol) 1H,1H,2H,2H-perfluoro-1-octanol (Sigma), stabilized by 1.8 wt% of the fluorosurfactant ammonium carboxylate of DuPont Krytox 157 in a drop making nozzle (Figure 1).


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Figure 1. (a) If the SAW power is switched off all drops flow along the upper channel because of its lower flow resistance. (b) If SAW power is switched on the acoustic streaming induced by the IDT(red arrow) drives the droplets in the lower channel of the branch. (c) When the SAW is switched off all the drops take the upper outlet channel because its cross-section is designed to be slightly larger. (d) Applying a RF signal of approximately 10 dBm to the IDT all drops are pushed into the lower channel. The device is 50 mm high and 100 mm in width right before the branch. Flow rates were 100 ml/h for the dispersed phase and 1000 ml/h for the continuous phase.


In addition to water drops in oil, the author also demonstrate the generality of this method for directing objects, they use this technique to direct polyacrylamide (PAM) particles, which consist primarily of water, dipsersed in a water continuous phase. The diameter of the resulting uniformly sized PAM particles was adjusted to be 20 mm, to roughly match the size of the water drops. In both experiments, this technique could direct the path of water droplets in oil and PAM particles in water respectively into designated channels and thereby demonstrate the generality of our approach of using SAW to direct the drops (Figure 2).


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Figure 2. PAM particles can be also directed and sorted with the PDMS–SAW hybrid device. Similarly to the setup in Fig. 1 particles flow into the upper reservoir when the IDT is switched off and can be collected in the lower reservoir when the RF signal is switched on.