Continuous micro-vortex-based nanoparticle manipulation via focused surface acoustic waves

Abstract

Despite increasing demand in the manipulation of nanoscale objects for next generation biological and industrial processes, there is a lack of methods for reliable separation, concentration and purification of nanoscale objects. Acoustic methods have proven their utility in contactless manipulation of microscale objects mainly relying on the acoustic radiation effect, though the influence of acoustic streaming has typically prevented manipulation at smaller length scales. In this work, however, we explicitly take advantage of the strong acoustic streaming in the vicinity of a highly focused, high frequency surface acoustic wave (SAW) beam emanating from a series of focused 6 μm substrate wavelength interdigital transducers patterned on a piezoelectric lithium niobate substrate and actuated with a 633 MHz sinusoidal signal. This streaming field serves to focus fluid streamlines such that incoming particles interact with the acoustic field similarly regardless of their initial starting positions, and results in particle displacements that would not be possible with a travelling acoustic wave force alone. This streaming-induced manipulation of nanoscale particles is maximized with the formation of micro-vortices that extend the width of the microfluidic channel even with the imposition of a lateral flow, occurring when the streaming-induced flow velocities are an order of magnitude larger than the lateral one. We make use of this acoustic streaming to demonstrate the continuous and differential focusing of 100 nm, 300 nm and 500 nm particles.

Experimental Characterization of Acoustic Streaming in Gradients of Density and Compressibility

“…For microparticles below a critical size, * weiqiu@fysik.dtu.dk † bruus@fysik.dtu.dk ‡ per.augustsson@bme.lth.se the motion of microparticles is dominated by acoustic streaming, which in many cases hinders the manipulation of sub-micrometer sized particles. Manipulation below the classical limit has previously been demonstrated by flow vortices generated by two-dimensional acoustic fields [29,30], by acoustically active seed particles [24], by a thin reflector design [31], or in systems actuated by surface acoustic waves [32][33][34].…”

Acoustic Streaming and Its Suppression in Inhomogeneous Fluids

“…Indeed, acoustic streaming has been a major show-stopper in the successful acoustophoretic manipulation of bioparticles such as exosomes, vira, and small bacteria [32], the reason being the unfavorable scalings of the acoustic radiation force and the streaminginduced drag force with smaller particle sizes [22,33]. Already, there have been attempts to suppress acoustic streaming using pulsed actuation [34,35], or to engineer streaming patterns in special geometries that allow nanoparticle manipulation [36][37][38]. In this work, we use the standard chip design sketched in Fig.…”

Focusing surface acoustic waves assisted electrochemical detector in microfluidics

“…The attenuation coefficients in the fluid α is given by where ρ s and c s are the density and leaky SAW velocity of the substrate (ρ s = 4628 kg m −3 , c s = 3931 ms −1 ), ρ and c are the density and leaky SAW velocity of water (ρ = 1000 kg m −3 , c = 1500 ms −1 ), and ρ is the SAW wavelength. For fluid with a length l , the FSAW beam that radiation into the fluid can be given by where E is the energy FSAW, E s is the energy that radiation into fluid. The body force FB is proportion to E s .…”