Acoustic radiation force on a cylindrical particle near a planar rigid boundary II. – Viscous fluid cylinder example and inherent radiation torque F.G. Mitri Santa Fe, NM, 87508, United States ARTICLE INFO Handling editor: Zhongqing SU Keywords: Acoustic radiation force Acoustic radiation torque Multiple scattering Viscous fluid cylinder Modal expansion method Addition theorem ABSTRACT Objective: This study extends the scope of a previous analysis on the time-averaged acoustic radiation force on a rigid (sound impenetrable) cylinder near a flat boundary [F.G. Mitri, J. Phys. Commun. 2 (2018) 045019] to the case of a viscous compressible fluid (sound penetrable) particle, and determine the time-averaged acoustic ra- diation torque as well. Motivation and novelty: Previous analytical formalisms did not consider the case of a sound penetrable cylindrical particle insonified at an arbitrary angle of incidence (in the polar plane) near a reflecting boundary. This work fills this gap, and provides exact expressions and computations for the acoustic radiation force and torque components. Method: The partial-wave series expansion method, in conjunction with the method of images and the trans- lational addition theorem of cylindrical wave functions are used to derive the analytical expressions for the longitudinal and transverse acoustic radiation force components. Moreover, the emergence of a radiation torque that causes the particle to rotate around its center of mass is computed using an exact partial-wave series expression. Results, key conclusion and some perspectives: Attractive (pulling), repulsive (pushing) and neutral (zero) forces arise depending on the particle-boundary distance, the cylinder size parameter as well as the angle of incidence (in the polar plane) of the insonifying waves. Emphasis is also given on the emergence of an acoustic radiation torque (that vanishes for a rigid or non-viscous circular cylinder). Computations for the axial radiation torque efficiency anticipate the generation of positive radiation torque, its reversal, in addition to a zero efficiency, leading, respectively, to counter-clockwise, clockwise or lack of particle rotation as the angle of the incident waves de- viates from normal incidence with respect to the boundary surface. The extension to the case of an elliptical/oval cylinder near a boundary is mentioned, and replies to some misleading and obtuse comments on the paper [F.G. Mitri, Phys. Fluids 28 (2016) 077104] are provided. 1. Introduction A submerged particle in a nonviscous fluid medium located near a boundary and illuminated by acoustical waves, experiences a steady- state (quadratic) repulsive or attractive radiation force [1–4]. The behavior of the radiation force is dictated by multiple scattering effects occurring between the particle and boundary, and described accurately using the translational addition theorem [5]. The previous work [4] considered a rigid (sound impenetrable) cylindrical particle, in which numerical predictions for the radiation force components showed that particle repulsion, attraction or neutrality toward the boundary (or alternatively, toward the source) can arise depending on the source-object distance, the angle of incidence of the insonifying waves (in the polar plane) and the size parameter of the cylindrical object. Notice, however, that in some cases, sound penetrable particles such as organ- elles, cells, and fluid-like cylinders [6] are considered in numerous ap- plications in particle manipulation and acousto-fluidics [7,8]. Therefore, to predict the behavior of the force for liquid particles, an improved formalism is needed, taking into account the compressibility of the particle. This analysis fills this gap, and provides a rigorous formalism for the acoustic radiation force components for a (viscous) liquid cylindrical particle, located near a rigid reflective boundary (Fig. 1). Particularly, the present analysis emphasizes the emergence of an axial radiation torque, in addition to the radiation force, which causes particle spinning/rota- tion around its center of mass. It is important to note that the radiation E-mail address: F.G.Mitri@ieee.org. Contents lists available at ScienceDirect Physics Open journal homepage: www.journals.elsevier.com/physics-open https://doi.org/10.1016/j.physo.2020.100029 Received 21 May 2020; Received in revised form 15 June 2020; Accepted 10 July 2020 Available online 15 July 2020 2666-0326/© 2020 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Physics Open 4 (2020) 100029