Ultrasound focuser: A multi-cylindrical source configuration and entrapped particles dynamics

We aim to introduce the proof of concept of a 3D ultrasound Focuser with possible advanced applications in living-matter/cell entrapment, particle focusing, transportation through virtual channel and drug, agent or material delivery systems. The proposed mechanism is assumed to be fully submerged in a fluidic environment and composed of three parallel acoustic line sources which are located in such a way that form a triangular right prism. By approximating the wave field of each cylindrical source as a progressive plane wave field whose amplitude decreases with respect to the travelling distance from the source, the acoustic radiation force exerted on a single particle is analytically derived. It is shown that when each source has a π/3 phase different from other sources, an attracting zone around the axis of the triangular prism is formed for wavelengths in the order of the size scale, λ/l∼O(1), where l denotes the distance between each two sources. The optimal operating situation (the largest attracting zone) is found for the case where λ≈l. The theoretical study is supported by stability analysis of dynamics of the entrapped particle which located on the axis of the prism; and validated by computing the trajectories of migration of the test particle. The stability analysis is performed by considering the unsteady solution of Stokes equations and the possible flow of environmental fluid medium. In addition, the required settling time and required length scales to focus the particle to the center line of the prism for different size scale ratios are estimated and discussed. Compared to other 3D focusing techniques, this method is non-invasive, robust, easy to implement, applicable to nearly all types of micro-particles and does not need any specific pre-designed channel for focusing process.

Acoustic radiation force on an air bubble and soft fluid spheres in ideal liquids: example of a high-order Bessel beam of quasi-standing waves

The partial wave series for the scattering of a high-order Bessel beam (HOBB) of acoustic quasi-standing waves by an air bubble and fluid spheres immersed in water and centered on the axis of the beam is applied to the calculation of the acoustic radiation force. A HOBB refers to a type of beam having an axial amplitude null and an azimuthal phase gradient. Radiation force examples obtained through numerical evaluation of the radiation force function are computed for an air bubble, a hexane, a red blood and mercury fluid spheres in water. The examples were selected to illustrate conditions having progressive, standing and quasi-standing waves with appropriate selection of the waves' amplitude ratio. An especially noteworthy result is the lack of a specific vibrational mode contribution to the radiation force determined by appropriate selection of the HOBB parameters.

Theory of the acoustic radiation force exerted on a sphere by standing and quasistanding zero-order Bessel beam tweezers of variable half-cone angles

A rigorous theory is developed to predict the radiation force (RF) exerted on a sphere immersed in an ideal fluid by a standing or quasistanding zero-order Bessel beam of different half-cone angles. A standing or a quasistanding acoustic field is the result of counter propagating 2 equal or unequal amplitude zero-order Bessel beams, respectively, along the same axis. Each Bessel beam is characterized by its halfcone angle beta(l);l = 1, 2 of its plane wave components, such that beta(l) = 0 represents a plane wave. Analytical expressions of RF are derived for a homogeneous viscoelastic sphere chosen as an example. RF calculations for a polyethylene sphere immersed in water are performed. Particularly, the half-cone angle dependency on the RF is analyzed for standing and quasistanding waves. Changing the half-cone angle is equivalent to changing the beamwidth. Potential applications include particle manipulation in microfluidic lab-on-chips as well as in reduced gravity environments.

The mechanism of the attracting acoustic radiation force on a polymer-coated gold sphere in plane progressive waves

Acoustic plane progressive waves incident on a sphere immersed in a nonviscous fluid exert a steady force acting along the direction of wave motion. It is shown here that when an elastic gold sphere is coated with a polymer-type (polyethylene) viscoelastic layer, this force becomes a force of attraction in the long wavelength limit. Kinetic, potential and Reynolds stress energy densities are defined and evaluated with and in the absence of absorption in the layer. Without absorption, the mechanical energy density counteracts the Reynolds stress energy density, which causes a repulsive force. However, in the case of absorption, the attractive force is predicted to be a physical consequence of a mutual contribution of both the mechanical and the Reynolds stress energy densities. This condition provides an impetus for further designing acoustic tweezers operating with plane progressive waves as well as fabricating polymer-coated gold particles for specific biophysical and biomedical applications.