Fabrication, Operation and Flow Visualization in Surface-acoustic-wave-driven Acoustic-counterflow Microfluidics

Summary

In this video we first describe fabrication and operation procedures of a surface acoustic wave (SAW) acoustic counterflow device. We then demonstrate an experimental setup that allows for both qualitative flow visualization and quantitative analysis of complex flows within the SAW pumping device.

Abstract

Surface acoustic waves (SAWs) can be used to drive liquids in portable microfluidic chips via the acoustic counterflow phenomenon. In this video we present the fabrication protocol for a multilayered SAW acoustic counterflow device. The device is fabricated starting from a lithium niobate (LN) substrate onto which two interdigital transducers (IDTs) and appropriate markers are patterned. A polydimethylsiloxane (PDMS) channel cast on an SU8 master mold is finally bonded on the patterned substrate. Following the fabrication procedure, we show the techniques that allow the characterization and operation of the acoustic counterflow device in order to pump fluids through the PDMS channel grid. We finally present the procedure to visualize liquid flow in the channels. The protocol is used to show on-chip fluid pumping under different flow regimes such as laminar flow and more complicated dynamics characterized by vortices and particle accumulation domains.

Introduction

One of the continued challenges facing the microfluidic community is the need to have an efficient pumping mechanism that can be miniaturized for integration into truly portable micro-total-analysis systems (μTAS's). Standard macroscopic pumping systems simply fail to provide the portability required for μTAS's, owing to the unfavorable scaling of the volumetric flow rates as the channel size decreases down to the micron range or below. On the contrary, SAWs have gained increasing interest as fluid actuation mechanisms and appear as a promising avenue for the solution of some of these problems^1,2.

SAWs were show....

Protocol

1. Device Fabrication

1. Design two photomasks, the first for patterning the surface acoustic wave (SAW) layer, and the second for the polydimethylsiloxane (PDMS) microchannel mold.
   1. The first photomask has a pair of opposing interdigital transducers (IDTs)—also known as a SAW delay line—and markers for channel alignment and spatial reference during microscopy. In our standard device we have single-electrode IDTs with a finger width p = 10 μm, aperture of 750 μm, and 25 straight finger pairs. The resulting IDT generates SAWs with a wavelength λ = 4p = 40 μm corresponding to an oper....

Results

Figure 2 shows representative results of device RF testing which were taken prior to bonding the LN layer to the microchannel layer: typical S₁₁ and S₁₂ spectra are reported in panel a) and b) respectively. The depth of the valley at central frequency in S₁₁ spectrum is related to the efficiency of conversion of RF power in SAW mechanical power. Hence, for a fixed number of IDT finger pairs, a reduction in the valley minimum will result i.......

Discussion

One of the greatest challenges faced by the microfluidic community is the realization of an actuation platform for truly portable point-of-care devices. Among the proposed integrated micropumps²³,those based on surface acoustic waves (SAWs) are particularly attractive due to their associated capabilities in fluid mixing, atomization and particle concentration and separation⁴. In this paper we have demonstrated how to fabricate and operate a lab-on-chip device in which fluid is steered in a closed PD.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Double side polished 128° YX lithium niobate wafer	Crystal Technology, LLC
Silicon wafer	Siegert Wafers		We use <100>
IDT Optical lithography mask with alignment marks (positive)	Any vendor
Channel Optical lithography mask (negative)	Any vendor
Positive photoresist	Shipley	S1818
Positive photoresist developer	Microposit	MF319
Negative tone photoresist	Allresist	AR-N-4340
Negative tone photoresist developer	Allresist	AR 300-475
SU8 thick negative tone photoresist	Microchem	SU-8 2000 Series
SU8 thick negative tone photoresist developer	Microchem	SU-8 developer
Hexadecane	Sigma-Aldrich	H6703
Carbon tetrachloride (CCl4)	Sigma-Aldrich	107344
Octadecyltrichlorosilane (OTS)	Sigma-Aldrich	104817
Acetone CMOS grade	Sigma-Aldrich	40289
2-propanol CMOS grade	Sigma-Aldrich	40301
Titanium	Any vendor	99.9% purity
Gold	Any vendor	99.9% purity
PDMS	Dow Corning	Sylgard 184 silicone elastomer kit with curing agent
Petri dish	Any vendor
5 mm ID Harris Uni-Core multi-purpose coring tool	Sigma-Aldrich	Z708895	Any diameter greater than 2 mm is suitable
Acoustic absorber	Photonic Cleaning Technologies	First Contact regular kit
RF-PCB	Any vendor
Spinner	Laurell technologies corporation	WS-400-6NPP	Any spinner can be used
UV Mask aligner	Karl Suss	MJB 4	Any aligner can be used
Thermal evaporator	Kurt J. Lesker	Nano 38	Any thermal, e-beam evaporator or sputtering system can be used
Oxygen plasma asher	Gambetti Kenologia Srl	Colibrì	Any plasma asher or RIE machine can be used
Centrifuge	Eppendorf	5810 R	Any centrifuge can be used
Wire bonder	Kulicke Soffa	4523AD	Any wire bonder can be used if the PCB is used without pogo connectors
Contact Angle Meter	KSV	CAM 101	Any contact angle meter can be used
Spectrum analyzer	Anristu	56100A	Any spectrum or network analyzer can be used
RF signal generator	Anristu	MG3694A	Any RF signal generator can be used
RF high power amplifier	Mini Circuits	ZHL-5W-1	Any RF high power amplifier can be used
Microbeads suspension	Sigma-Aldrich	L3280	Depending on the experimental purpose different suspension of different diameter and different material properties can be used
Optical microscope	Nikon	Ti-Eclipse	Any optical microscope with spatial resolution satisfying experimental purposes can be used
Video camera	Basler	A602-f	Any video camera that has enough frame rate and sensitivity satisfying experimental purposes can be used
Camera acquisition software	Advanced technologies	Motion Box	Any software enabling high and controlled frame rate acquisition can be used