The role of channel height and actuation method on particle manipulation in surface acoustic wave (SAW)-driven microfluidic devices

Abstract

Surface acoustic wave (SAW) micromanipulation offers modularity, easy integration into microfluidic devices and a high degree of flexibility. A major challenge for acoustic manipulation, however, is the existence of a lower limit on the minimum particle size that can be manipulated. As particle size reduces, the drag force resulting from acoustic streaming dominates over acoustic radiation forces; reducing this threshold is key to manipulating smaller specimens. To address this, we investigate a novel excitation configuration based on diffractive-acoustic SAW (DASAW) actuation and demonstrate a reduction in the critical minimum particle size which can be manipulated. DASAW exploits the inherent diffractive effects arising from a limited transducer area in a microchannel, requiring only a travelling SAW (TSAW) to generate time-averaged pressure gradients. We show that these acoustic fields focus particles at the channel walls, and further compare this excitation mode with more typical standing SAW (SSAW) actuation. Compared to SSAW, DASAW reduces acoustic streaming effects whilst generating a comparable pressure field. The result of these factors is a critical particle size with DASAW (1 $$\upmu$$ μ m) that is significantly smaller than that for SSAW actuation (1.85 $$\upmu$$ μ m), for polystyrene particles and a given $$\lambda _{\text {SAW}}$$ λ SAW = 200 $$\upmu$$ μ m. We further find that streaming magnitude can be tuned in a DASAW system by changing the channel height, noting optimum channel heights for particle collection as a function of the fluid wavelength at which streaming velocities are minimised in both DASAW and SSAW devices.

From rectangular to diamond shape: on the three-dimensional and size-dependent transformation of patterns formed by single particles trapped in microfluidic acoustic tweezers

“…Although the acoustic landscape depicted in Fig. 1 is only on the lateral plane, several numerical studies 20,31,32 showed that the relative location of the nodes, antinodes and midpoints would not vary with height. Hence, either the particle trapping locations at the bottom or at the top level shall coincide with the lateral locations of the antinodes, according to Fig.…”

Enhanced acoustic streaming effects via sharp-edged 3D microstructures

“…50 Acoustic streaming arises from the attenuation and transfer of acoustic energy into the bulk fluid flow, impacting particle motion through Stokes drag. 51,52…”

“…50 Acoustic streaming arises from the attenuation and transfer of acoustic energy into the bulk fluid flow, impacting particle motion through Stokes drag. 51,52 Whereas both acoustic radiation forces and acoustic streaming are periodic and wavelength-scale in typical acoustofluidic systems, including those driven by bulk acoustic wave (BAW) 24 and surface acoustic wave (SAW) 52,53 actuation, the use of sharp-edged microstructures here enables the generation of high streaming velocities and complex streaming vortices whose locations are determined by the local presence of these structures, rather than acoustic anti/nodal positions. Here acoustic streaming drives vortical motion around sharp tips, and the Gor'kov acoustic potential results in further particle motion in response to spatial variation in time-averaged acoustic fields, where this is calculated using a combination of the first-order pressure and velocity fields.…”

Biomedical Applications of Microfluidic Devices: A Review

“…By taking advantage of the size differences of the particles, large and small particles are subjected to greater or lesser force by the focused acoustic pressure point [ 122 ]. In acoustic separations, a pair of electrodes is slotted vertically on two sides of the microchannel, and a surface acoustic wave moves towards the microchannel [ 123 ]. Acoustic waves radiated from two opposite directions interfere with each other and generate standing waves that create pressure points at specific points depending on the wavelength inside the linear microchannel [ 124 ].…”