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

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.

Acoustic streaming in Maxwell fluids generated by standing waves in two-dimensional microchannels

In this work, a semianalytic solution for the acoustic streaming phenomenon, generated by standing waves in Maxwell fluids through a two-dimensional microchannel (resonator), is derived. The mathematical model is non-dimensionalized and several dimensionless parameters that characterize the phenomenon arise: the ratio between the oscillation amplitude of the resonator and the half-wavelength ( $\eta =2A/\lambda _{a}$ ); the product of the fluid relaxation time times the angular frequency known as the Deborah number ( $De=\lambda _{1}\omega$ ); the aspect ratio between the microchannel height and the wavelength ( $\epsilon =2 H_{0}/\lambda _{a}$ ); and the ratio between half the height of the microchannel and the thickness of the viscous boundary layer ( $\alpha =H_{0}/\delta _{\nu }$ ). In the limit when $\eta \ll 1$ , we obtain the hydrodynamic behaviour of the system using a regular perturbation method. In the present work, we show that the acoustic streaming speed is proportional to $\alpha ^{2.65}De^{1.9}$ , and the acoustic pressure varies as $\alpha ^{6/5}De^{1/2}$ . Also, we have found that the growth of inner vortex is due to convective terms in the Maxwell rheological equation. Furthermore, the velocity antinodes show a high dependency on the Deborah number, highlighting the fluid's viscoelastic properties and the appearance of resonance points. Due to the limitations of perturbation methods, we will only analyse narrow microchannels.

Bubble-enhanced ultrasonic microfluidic chip for rapid DNA fragmentation

We present a new bubble-enhanced microfluidic approach for highly efficient DNA fragmentation, suitable for next generation sequencing platforms. Improved on-chip performance arises from acoustic streaming generated by oscillating bubble interfaces.

Outer Acoustic Streaming Flow Driven by Asymmetric Acoustic Resonances

While boundary-driven acoustic streaming resulting from the interaction of sound, fluids and walls in symmetric acoustic resonances have been intensively studied in the literature, the acoustic streaming fields driven by asymmetric acoustic resonances remain largely unexplored. Here, we present a theoretical and numerical analysis of outer acoustic streaming flows generated over a fluid–solid interface above which a symmetric or asymmetric acoustic standing wave is established. The asymmetric standing wave is defined by a shift of acoustic pressure in its magnitude, i.e., S0, and the resulting outer acoustic streaming is analyzed using the limiting velocity method. We show that, in symmetric acoustic resonances (S0 = 0), on a slip-velocity boundary, the limiting velocities always drive fluids from the acoustic pressure node towards adjacent antinodes. In confined geometry where a slip-velocity condition is applied to two parallel walls, the characteristics of the obtained outer acoustic streaming replicates that of Rayleigh streaming. In an asymmetric standing wave where S0 ≠ 0, however, it is found that the resulting limiting velocity node (i.e., the dividing point of limiting velocities) on the slip-velocity boundary locates at a different position to acoustic pressure node and, more importantly, is shown to be independent of S0, enabling spatial separation of acoustic radiation force and acoustic streaming flows. The results show the richness of boundary-driven acoustic streaming pattern variations that arise in standing wave fields and have potentials in many microfluidics applications such as acoustic streaming flow control and particle manipulation.