Assembly and Characterization of an External Driver for the Generation of Sub-Kilohertz Oscillatory Flow in Microchannels

Summary

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.

Abstract

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.

Introduction

The precise control of liquid flow rate in microchannels is crucial for lab-on-a-chip applications such as droplet production and encapsulation¹, mixing^2,3, and the sorting and manipulation of suspended particles^4,5,6,7. 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 plethora^{6,11,12,13,14,15} 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 cavity^16,17. 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 mixing^19,20 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.

Protocol

1. Rapid prototype mold design and fabrication Open AutoCAD on a PC. Select File on the taskbar, then select Open and browse to and click on a three-dimensional (3D) model file of the channel mold having .dxf or .dwg extension. Select the entire model by clicking and dragging a box around it. Export the design as a .stl file by selecting File | Export, then Other formats and choosing .stl from the dropdown…

Representative Results

To illustrate the capability and performance of the above setup, representative results of oscillatory flow in a simple linear microchannel with a square cross-section are presented. The width and height of the channel are 110 µm and its length is 5 cm. First, we describe the motion of spherical polystyrene tracer particles and how these can be used to check the fidelity of the oscillatory signal as well as the range of oscillation amplitudes achievable. We then discuss the effect of specific fluid properties or mic…

Discussion

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…

Materials

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