Enhanced Detection in Droplet Microfluidics by Acoustic Vortex Modulation of Particle Rings and Particle Clusters via Asymmetric Propagation of Surface Acoustic Waves

Abstract

As a basis for biometric and chemical analysis, issues of how to dilute or concentrate substances such as particles or cells to specific concentrations have long been of interest to researchers. In this study, travelling surface acoustic wave (TSAW)-based devices with three frequencies (99.1, 48.8, 20.4 MHz) have been used to capture the suspended Polystyrene (PS) microspheres of various sizes (5, 20, 40 μm) in sessile droplets, which are controlled by acoustic field-induced fluid vortex (acoustic vortex) and aggregate into clusters or rings with particles. These phenomena can be explained by the interaction of three forces, which are drag force caused by ASF, ARF caused by Leaky-SAW and varying centrifugal force. Eventually, a novel approach of free transition between the particle ring and cluster was approached via modulating the acoustic amplitude of TSAW. By this method, multilayer particles agglomerate with 20 μm wrapped around 40 μm and 20 μm wrapped around 5 μm can be obtained, which provides the possibility to dilute or concentrate the particles to a specific concentration.

Microfabricated acoustofluidic membrane acoustic waveguide actuator for highly localized in-droplet dynamic particle manipulation

“…The traveling motion of the anti-nodes, akin to the traveling wave propagation with a single IDT, generates a highly localized F drag which pushes the particles to orbit around the node. Other ring-like traps and features have been reported in acoustofluidic experiments before, based on mode-switching in a circular Chladni microplate, 64,77 asymmetric SAWs propagation, 24 bulk vibration of a substrate 85 and SAW microcentrifugation. 86 However, the ring trap mechanism presented here is highly localized, easily controllable, and scalable compared to the earlier works reported.…”

“…This one-of-a-kind transport mechanism differs considerably from the acoustofluidic transport mechanisms reported in the past, leveraging SAWs 24,45 and vibrating structures. 27,79 The latter often occupies a large footprint, presents limited degrees of freedom and, in some cases, cannot be fully realized through microfabrication processes.…”

“…4D fits a parabolic curve, which reflects the second-order velocities' relation with the membrane displacement amplitude, v 2nd-order ∝ A 2 , 65 and confirms the involvement of a second-order streaming-induced force in the forward motion of the water-borne microparticles. This one-of-a-kind transport mechanism differs considerably from the acoustofluidic transport mechanisms reported in the past, leveraging SAWs 24,45 and vibrating structures. 27,79 The latter often occupies a large footprint, presents limited degrees of freedom and, in some cases, cannot be fully realized through microfabrication processes.…”

“…37 IDTs on a bulk piezoelectric substrate can generate surface acoustic waves (SAWs) and Lamb waves and are utilized in various settings, from the generation of streaming inside a sessile droplet to particle separation inside a microfluidic channel. To accurately control the acoustic fields induced by the mechanical deformations originating from the IDTs, several approaches have been investigated, such as shaping the electrodes into focused IDTs, 38,39 slanted IDTs 40,41 and holographic IDTs, 42,43 and pairing IDTs with each other, 15,[44][45][46][47] with microfluidic channels 23,48 or with other structural obstacles such as boundaries 24,49 and phononic crystals. 50,51 Piezoelectric thin film transducers can bring piezoelectric actuation to virtually any substrate, opening a wide array of possibilities for the development of acoustofluidic devices based on CMOS and MEMS processes, which allow for miniaturization of devices and wafer-level batch fabrication.…”

“…technologies have brought significant contributions to the field of microfluidics for precision manipulation techniques, [1][2][3][4] notably due to their relatively low power requirement and superior biocompatibility. 5,6 Ultrasonic actuators have demonstrated the ability to accomplish various microfluidic functions such as separation, 3,[7][8][9][10][11] patterning, [12][13][14][15][16][17] trapping, [18][19][20][21] focusing, 22,23 concentrating, [24][25][26] and guiding [27][28][29] of particles and analytes, which are crucial functions for biomedical applications. To achieve such results, it is often required to enhance and confine the acoustic field near the analytes, which, at this scale, is a challenging endeavor.…”