Milliseconds mixing in microfluidic channel using focused surface acoustic wave

Abstract

A feasible approach of acoustic wave based ultra-fast homogeneous mixing in microfluidic channel is reported. After comparing simulation models of energy distribution between parallel interdigital transducers (IDTs) and concentric circular type focused interdigital transducers (F-IDTs), the F-IDTs were designed and built into microfluidic device to generate focused surface acoustic wave in a specific region of the microchannel. In the acoustic enhanced mixing region, continuous laminar flow was mixed efficiently in milliseconds by concentrated acoustic radiation. The active enhanced ultra-fast mixing was optimized and analyzed experimentally. This method could be developed for fast chemical or biochemical reactions and assays.

Introduction

Microfluidic systems or micro-total-analysis systems are widely used in chemical synthesis, biochemistry analysis, drug delivery, sequencing or synthesis of nucleic acids, etc. [1], [2], [3]. Because of the low Reynolds number of most microfluidic devices, rapid homogeneous mixing is difficult to achieve [4], [5]. Many approaches, such as multi-laminating, chaotic, electrical fields, ultrasound, electrowetting based droplet shaking, have been developed to enhance the micromixing for chemical or biological studies in microfluidics [6]. Particularly, the acoustic enhanced mixing has attracted a great interest based on the advantages such as easy control, compactness, high frequency response, and high precision [7], [8].

Sound wave propagating into liquid induces acoustic streaming effect which could act as an active internal stirrer of liquid [9]. This effect has been recently applied to enhance mixing in lab-on-a-chip systems [1], [10], [11], [12]. Yang and Guo generated acoustic wave by piezoelectric lead–zirconate–titanate (PZT) ceramic to induce mixing in microfluidic systems [13], [14]. Liu and Ahmed developed bubble-based acoustic mixers to improve mixing [1], [15]. Especially, a method of enhancing mixing based on surface acoustic wave (SAW) arouses much attention for its advantages like simple fabrication, large disturbance force, and fast operation. Sritharan introduced mixing in millimeters length in microfluidic channel using parallel SAW which propagated perpendicular to the flow direction [16]. Shilton reported increased efficiency in generating intense micromixing in microliter drops in an open device by the focused SAW (F-SAW) [7]. Similarly, using focused type IDTs, Luong demonstrated high-throughput micromixers based on 13 MHz SAW and got a better mixing efficiency comparing to the straight one [17]. The advantages of F-SAW such as the capacity to efficiently induce internal stirring in small liquid volume and concentrate acoustic radiation at a certain region were revealed. It presents a great potential for using F-SAW to achieve fast mixing in milliseconds scale (even less) and precisely monitor the mixing position in microfluidic system.

In this article, simulation models were built to investigate the theoretical distribution of acoustic radiation on the substrate which revealed the active mixing region. F-IDTs were built into the microfluidic device to generate F-SAW, and the mixing channel was placed and half-bonded on the desired area of the substrate. Then two different solutions were injected into the Y-type microchannels and mixed homogeneously at the F-SAW propagation region. Furthermore, after optimizing conditions, the ultra-fast mixing was achieved in milliseconds and analyzed experimentally.