The protocol demonstrates a convenient method to produce harmonic oscillatory flow from 10-1000 Hz in microchannels. This is performed by interfacing a computer-controlled speaker diaphragm to the microchannel in a modular manner.
Microfluidic technology has become a standard tool in chemical and biological laboratories for both analysis and synthesis. The injection of liquid samples, such as chemical reagents and cell cultures, is predominantly accomplished through steady flows that are typically driven by syringe pumps, gravity, or capillary forces. The use of complementary oscillatory flows is seldom considered in applications despite its numerous advantages as recently demonstrated in the literature. The significant technical barrier to the implementation of oscillatory flows in microchannels is likely responsible for the lack of its widespread adoption. Advanced commercial syringe pumps that can produce oscillatory flow, are often more expensive and only work for frequencies less than 1 Hz. Here, the assembly and operation of a low-cost, plug-and-play type speaker-based apparatus that generates oscillatory flow in microchannels is demonstrated. High-fidelity harmonic oscillatory flows with frequencies ranging from 10-1000 Hz can be achieved along with independent amplitude control. Amplitudes ranging from 10-600 µm can be achieved throughout the entire range of operation, including amplitudes > 1 mm at the resonant frequency, in a typical microchannel. Although the oscillation frequency is determined by the speaker, we illustrate that the oscillation amplitude is sensitive to fluid properties and channel geometry. Specifically, the oscillation amplitude decreases with increasing channel circuit length and liquid viscosity, and in contrast, the amplitude increases with increasing speaker tube thickness and length. Additionally, the apparatus requires no prior features to be designed on the microchannel and is easily detachable. It can be used simultaneously with a steady flow created by a syringe pump to generate pulsatile flows.
The precise control of liquid flow rate in microchannels is crucial for lab-on-a-chip applications such as droplet production and encapsulation1, mixing2,3, and the sorting and manipulation of suspended particles4,5,6,7. The predominantly used method for flow control is a syringe pump that produces highly controlled steady flows dispensing either a fixed volume of liquid or a fixed volumetric flow rate, often limited to entirely unidirectional flow. Alternative strategies for producing unidirectional flow include using gravitational head8, capillary forces9, or electro-osmotic flow10. Programmable syringe pumps allow for a time-dependent bidirectional control of flow rates and dispensed volumes but are limited to response times greater than 1 s due to the mechanical inertia of the syringe pump.
Flow control at shorter time scales unlocks a plethora6,11,12,13,14,15 of otherwise inaccessible possibilities due to qualitative changes in flow physics. The most practical means of harnessing this varied flow physics is through acoustic waves or oscillatory flows with time periods ranging from 10-1– 10-9 s or 101 -109 Hz. The higher end of this frequency range is accessed using bulk acoustic wave (BAW; 100 kHz-10 MHz) and surface acoustic wave (SAW; 10 MHz-1 GHz) devices. In a typical BAW device, the entire substrate and the fluid column are vibrated by applying a voltage signal across a bonded piezoelectric. This enables relatively high throughputs but also results in heating at higher amplitudes. In SAW devices, however, the solid-liquid interface is oscillated by applying voltage to a pair of interdigitated electrodes patterned on a piezoelectric substrate. Due to the very short wavelengths (1 µm-100 µm) particles as small as 300 nm can be precisely manipulated by the pressure wave generated in SAW devices. Despite the ability to manipulate small particles, SAW methods are limited to local particle manipulation since the wave rapidly attenuates with distance from the source.
At the 1-100 kHz frequency range, oscillatory flows are usually generated using piezo-elements that are bonded to a polydimethylsiloxane (PDMS) microchannel above a designed cavity16,17. The PDMS membrane above the patterned cavity behaves like a vibrating membrane or drum that pressurizes the fluid within the channel. At this frequency range, the wavelength is larger than the channel size, but the oscillation velocity amplitudes are small. The most useful phenomenon in this frequency regime is the generation of acoustic/viscous streaming flows, which are rectified steady flows caused due to non-linearity inherent in the flow of liquids with inertia18. The steady streaming flows typically manifest as high-speed counter-rotating vortices in the vicinity of obstacles, sharp corners, or micro-bubbles. These vortices are useful for mixing19,20 and separating 10 µm sized particles from the flow stream21.
For frequencies in the range of 10-1000 Hz, both the velocity of the oscillatory component and its associated steady viscous streaming are considerable in magnitude and useful. Strong oscillatory flows in this frequency range can be used for inertial focusing22, facilitate droplet generation23, and can generate flow conditions (Womersley numbers) that mimic blood flow for in vitro studies. On the other hand, streaming flows are useful for mixing, particle trapping, and manipulation. Oscillatory flow in this range of frequencies can also be accomplished using a piezo-element bonded to the device as described above23. A significant hurdle to implementing oscillatory flows through a bonded piezo element is that it requires features to be designed beforehand. Furthermore, the bonded speaker elements are not detachable, and a new element must be bonded to each device24. However, such devices present the advantage of being compact. An alternative method is using an electromechanical relay valve20. These valves require pneumatic pressure sources and custom control software for operation and therefore increase the technical barrier to testing and implementation. Nevertheless, such devices enable the application of set pressure amplitude and frequency.
In this article, the construction, operation, and characterization of a user-friendly method to generate oscillatory flows in the frequency range of 10-1000 Hz in microchannels is described. The method offers numerous advantages such as cost-effective assembly, ease of operation, and ready to interface with standard microfluidic channels and accessories such as syringe pumps and tubing. Additionally, compared to previous similar approaches25, the method offers the user selective and independent control of oscillation frequencies and amplitudes, including the modulation between sinusoidal and non-sinusoidal waveforms. These features allow users to easily deploy oscillatory flows and, therefore, facilitate widespread adoption into a broad range of currently existing microfluidic technologies and applications in the fields of biology and chemistry.
We have demonstrated the assembly (see protocol critical steps 3 and 4) and operation (see protocol critical steps 5 and 6) of an external speaker-based apparatus for the generation of oscillatory flow with frequencies in the range of 10 to 1000 Hz in microfluidic devices. Particle tracking of suspended tracer particles is required to determine the fidelity of the harmonic motion as well as for calibrating the range of oscillation amplitudes achievable over the range of operating frequencies. The amplitude-frequency curv…
The authors have nothing to disclose.
We would like to acknowledge the support given and facilities provided by the Department of Mechanical Science and Engineering Rapid Prototyping Lab at the University of Illinois to enable this work.
Oscillatory Driver Assembly | |||
Alligator-to-pin wire | Adafruit | 3255 | Small alligator clip to male jumper wire (12) |
Aux cable | Adafruit | 2698 | 3.5 mm Male/Male stereo cable 1 m |
Controller chip | Damgoo | TPA3116 | 50w+50w 2 channel audio amplifier (bluetooth and AUX) |
DC adapter | Adafruit | 798 | 12 V DC 1A regulated switching power adapter |
Micro-pipette tip | VWR Signature | 37001-532 | 200 ul micropipette tip |
Silicone sealant | Loctite | 908570 | Clear silicone waterproof sealant (80 ml) |
Speaker | Drok | 6843996 | 4.5 inch 4 Ohm 40 W speaker |
Speaker mount | 3D printed from 'speakermount.stl' in supplementary files | ||
Speaker-to-tube adapter | 3D printed from 'speaketubeadapter.stl' in supplementary files | ||
Microchannel Manufacture | |||
Biopsy punch | Miltex | 15110 | Biopsy punch with plunger (1 – 4 mm) |
Degasser | |||
Disposable cup | |||
Disposable spoon | |||
Glass Slides | VWR Signature | 16004-430 | 3" x 1" pre clean 1 mm thick |
Mold | Si – SU-8 or 3D printed | ||
Oven | Fischer Scientific | Isotemp | |
PDMS resin and cross-linker | Dow Chemical | 4019862 | Sylgard 184 PDMS resin and crosslinker (500 g) |
Polyethylene tubing | Becton Dickinson Intramedic | 427440 | Polyethylene tubing (PE 60 – PE 200) |
Razor blades | VWR | 55411-050 | Single edge industrial razor blades |
RF plasma generator | Electro-Technic Products | BD – 20 | High frequency generator |
Silicone Mold Release | CRC | 03301 | Food Grade Silicon Mold release (16 oz) |
Observation and Characterization | |||
Camera | Edgertronic | SC2+ | |
Lens | Nikon | Plan Fluor 10x | |
Microscope | Nikon | Ti Eclipse manual stage | |
Needles | Becton Dickinson | 305175 | PrecisionGlide 20G |
Syringe | Becton Dickinson | 1180100555 | Monoject 1 ml |
Syringe pump | Harvard Apparatus | Dual syringe programmable syringe pump | |
Tracer Particles | Spherotech | PP-10-10 | Polystyrene tracer particles 1 um |