Abstract
Acoustic tweezing of bioparticles has distinct advantages over other manipulation methods such as electrophoresis or magnetophoresis in biotechnological applications. This manipulation method guarantees the viability of the bio-particles during and after the process. In this paper, the effects of sinusoidal boundaries of a microchannel on acoustophoretic manipulation of microparticles are studied. Our results show that while top and bottom walls are vertically actuated at the horizontal half-wave resonance frequency, a large mono-vortex appears, which is never achievable in a rectangular geometry with flat walls and one-dimensional oscillations. The drag force caused by such a vortex in combination with the tilted acoustic radiation force leads to trapping and micromixing of microparticles with diameters larger and smaller than the critical size, respectively. Simulation results in this paper show that efficient particle trapping occurs at the intermediate sinusoidal boundary amplitudes. It is also indicated that in a square-sinusoidal geometry there are two strong vortices, instead of one vortex. Sub-micrometer particles tend to be trapped dramatically faster in such a geometry than in the rectangular-sinusoidal ones.
Highlights
Acoustic tweezing of bioparticles has distinct advantages over other manipulation methods such as electrophoresis or magnetophoresis in biotechnological applications
The results show that the geometry plays a key role in creating a dominant strong vortex of the streaming pattern
If the top and bottom boundaries are actuated in the z direction with the frequency of fh = c0/2w = 5 MHz which is the horizontal half-wave resonance frequency of the microchannel, the creation of acoustic standing waves or acoustic streaming flows are not expected
Summary
In the absence of external body forces and heat sources, there are three important governing equations in the microfluidic systems a s53,54. Considering the external acoustic field as a perturbation of the steady state of a fluid, all the fields in standard first-order approximation can be expanded in the form g = g0 + g1 where g1 is much smaller than g0. The time averaged second-order perturbation approximation of governing equations are[54,55]. A suspending spherical particle with the diameter of a, in the long-wavelength limit, δν , a ≪ (i.e. particle size and viscous boundary layer are much smaller than the wavelength of the imposing acoustic wave, ) encounters with two acoustophoretic forces. The the time-averaged radiation force, which appears as a result of scattering of the sound wave on the particles, is given b y56,57. The time-averaged streaming-induced drag force on a spherical particle with the radius of a, moving with the velocity of u far from the channel walls, is given by. For particles larger the crossover limit, radiation force will be dominant and make particles move towards the pressure nodes or anti-nodes due to their contrast factor
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