Aplicações do método de elementos finitos na obtenção da força e torque de radiação acústica em partículas anisotrópicas

Abstract

The mean acoustic fields of acoustic radiation torque and force result from the nonlinear interaction of a harmonic acoustic wave in time with a particle. These phenomena are widely used in acustofluidics, in the manipulation of microparticles such as cells and microorganisms. Here we will highlight the contributions of numerical models developed through the finite element method used, firstly, in the validation of analytical models of nonlinear interaction with spheroidal particles of subwavelength, and later, in the composition of a semi-analytical model that describes the fields acoustic averages on any axisymmetric particle. In the first work, the analytical expressions for the force and torque of acoustic radiation on a sub-wavelength rigid spheroid are obtained in the scattering dipole approximation, where the force has a gradient component of a scattering component, while torque is given in terms of a contribution due to momentum and another due to acoustic spin. We compared the numerical and analytical results, using an acoustic field composed of two plane waves with crossed wave vectors.. Another work that also involves the acoustic radiation torque on a subwavelength spheroid will be presented, however with an approach that highlights the relationship between the acoustic spin and the torque due to the interaction of the particle with a first order Bessel beam, where we will also see the comparison between analytical and numerical results. In the following work, a semi-analytical method of remarkable efficiency was proposed in obtaining the means-acoustics radiation force and torque fields exerted on any subwavelength axisymmetric particle, in an ordinary acoustic field. Bypassing both the limitations of analytical methods, restricted to problems with symmetry in relation to a given coordinate system, and the limitations of purely numerical methods, which demand a lot of time and computational power to simulate the behavior of the particle along the acoustic field. In this work, the mean-acoustic fields are given as a function of scattering coefficients referring to the monopole and dipole modes, which are numerically calculated in a given simplistic configuration, and then are used to generalize the results to any incident field. We compare our method with an exact result for a rigid sphere of subwavelength in water, as well as demonstrate the potential of our methodology, presenting a more realistic example of application using a red blood cell immersed in blood plasma under the action of a stationary ultrasonic wave.