Acoustokinetics: Crafting force landscapes from sound waves

Factoring the pressure field of a harmonic sound wave into its amplitude and phase profiles provides the foundation for an analytical framework for studying acoustic forces that not only provides novel insights into the forces exerted by specified sound waves, but also addresses the inverse problem of designing sound waves to implement desired force landscapes. We illustrate the benefits of this acoustokinetic framework through case studies of purely nonconservative force fields, standing waves, pseudo-standing waves, and tractor beams.


I. INTRODUCTION
Structured sound waves exert forces and torques that can be harnessed to transport, sort and organize insonated objects [1][2][3][4]. Applications include non-contact processing of sensitive [5,6] and hazardous [7] materials, flow focusing for materials analysis and medical diagnostics [8], and automated remote manipulation for research [9]. Rapidly growing interest in harnessing acoustic forces has inspired a fundamental reassessment of the physics of wave-mediated forces. Recent developments in the theory of acoustic forces [10][11][12][13][14] parallel the analogous theory of optical forces [15,16]. Both offer valuable and often surprising insights into the elementary mechanisms of wave-matter interactions. The acoustokinetic framework introduced here addresses the complementary inverse problem: identifying what wave will create a desired force landscape.
The inverse problem for optical forces recently has been rendered more tractable by expressing the electromagnetic field in terms of its real-valued amplitudes and phases along each Cartesian coordinate [17,18]. This approach is called the theory of photokinetic effects and yields useful analytic expressions for the performance of optical traps [19] including design criteria for optical tractor beams [18]. Here, we show that an analogous factorization of the pressure field in sound waves is similarly useful for understanding and implementing acoustic manipulation. We illustrate the utility of this acoustokinetic framework through case studies on nonconservative acoustic force fields, standing and pseudo-standing waves, and acoustic tractor beams.

A. Light: Photokinetic analysis
We develop acoustokinetics by analogy to photokinetics and therefore briefly review the theory of optical forces. A small particle immersed in an electromagnetic wave develops an electric dipole moment proportional to the local field. This induced dipole experiences a timeaveraged Lorentz force in gradients of the field that can be expressed as [16] F e (r) = 1 2 where E j (r) is the j-th Cartesian coordinate of the electric field and α e is the particle's complex dipole polarizability. Expressing the components of the electric field in terms of their real-valued amplitude and phase profiles, yields the surprisingly simple expression [17], where α e and α e are the real and imaginary parts of the polarizability, respectively. The first term on the right-hand side of Eq. (3) is the manifestly conservative intensity-gradient force responsible for single-beam optical traps such as optical tweezers [20]. The second describes a nonconservative force [21] that is directed by phase gradients [22]. Phase-gradient forces tend to drive trapped particles out of thermodynamic equilibrium with their supporting media [22,23], mediate the transfer of light's orbital angular momentum [24][25][26][27], and have been used to create light-driven micromachines such as pumps [28], mixers [29] and optical tractor beams [30]. Even non-absorbing dielectric particles experience nonconservative optical forces because of radiative contributions to the dipole polarizability [31,32].
The dipole-order expression in Eq. (3) accurately describes the forces experienced by particles with radii, a p , that are small enough to satisfy the Rayleigh criterion, ka p < 1, where k is the wavenumber of light. In the Rayleigh regime, the conservative intensity-gradient force generally dominates the light-matter interaction because α e scales as (ka p ) 3 , whereas α e scales as (ka p ) 6 .

B. Sound: Acoustic radiation forces
The analogous dipole-order acoustic radiation force experienced by a small particle in a harmonic sound field arXiv:1910.03670v1 [cond-mat.soft] 8 Oct 2019 may be expressed in terms of the pressure, p(r, t), as [14] F(r) = 1 2 α a p∇p * + β a k −2 (∇p · ∇)∇p * , (4) where the coefficients α a and β a play the role of dipole and quadrupole polarizabilities, respectively. Expressing F(r) in terms of multipole polarizabilities clarifies the analogy with photokinetics. Lengths in Eq. (4) are scaled by the wavenumber, k = ω/c m , where ω is the sound's frequency and c m is its speed in the medium. Equation (4) applies to inviscid fluids, for which the pressure satisfies the scalar wave equation An object's dipole and quadrupole polarizabilities generally depend on its size, shape and composition as well as the frequency of the sound and the properties of the fluid medium. For simplicity and concreteness, we will specialize to the case of a spherical scatterer of radius a p that is composed of a material of density ρ p and sound speed c p . Such an object's response to the sound field is characterized by the polarizabilities [14] where the monopole coupling coefficient, depends on the compressibility mismatch between the particle and the medium, and the dipole coupling coefficient, gauges the density mismatch.

II. ACOUSTOKINETIC FRAMEWORK
Drawing on the analogy with photokinetics, we express the harmonic sound wave's pressure field in terms of its amplitude and phase profiles: The first term from Eq. (4) then yields which is directly analogous to Eq. (3) for the dipole-order force exerted by light. These contributions to the acoustic radiation force are depicted in Fig. 1. As in the optical case, α a scales as (ka p ) 3 and α a scales as (ka p ) 6 , which means that the conservative force generally dominates for small particles. The second term from Eq. (4) arises from the velocitymatching condition at the sphere's boundary and so has no analogue in optical radiation forces. It vanishes for density-matched particles (β α = 0), which therefore behave exactly like dielectric particles in a light field. When expressed in terms of the amplitude and phase profiles, this term separates naturally into a conservative contribution, that augments the conservative intensity-gradient force from F α (r) and a nonconservative contribution, that is directed both by phase gradients and also by amplitude gradients. The combination, captures the sphere's leading-order coupling to the quadrupole components of the incident field. Unlike the optical case, where quadrupolar forces generally are weaker than dipole contributions, the two terms in F β (r) can be comparable in magnitude to their counterparts in F α (r) because β a scales as (ka p ) 3 and β a scales as (ka p ) 6 .
FIG. 2. Intensity of the "picket fence" field in the x y -plane. For particularly selected parameters, the conservative force vanishes everywhere in this field, and the remaining force (indicated by arrows) is purely nonconservative. With the choice of material properties plotted (C2/C1 = 1/3), the direction of the force is also spatially modulated. The sign of the force in theŷ direction is also its sign in theẑ direction.
These density-dependent terms therefore can be used to exert control in ways that are not possible with light. The acoustokinetic framework described by Eq. (11) are the principal contribution of this work. We now demonstrate its value through case studies on realizable sound fields with exceptional properties.

A. Designing purely nonconservative force fields
To illustrate how the acoustokinetic framework can guide the design of acoustic force fields, we describe the conditions required for harmonic sound waves to exert purely nonconservative forces. This is equivalent to requiring the conservative part of the acoustic force to vanish, which Eqs. (11a) and (11b) show to be possible if the particle is not density matched, β a = 0, and the pressure intensity satisfies the inhomogeneous Helmholtz equation, where C is an undetermined constant. Solutions to Eq. (12) must be real-valued and must be paired with real-valued phase profiles that complete the description of the pressure field and satisfy the wave equation, Eq. (5). One interesting set of purely nonconservative solutions has the sinusoidal amplitude profile with spatial frequency The associated phase profile, identifies this field as the superposition of two plane waves, each oriented at angle γ relative toẑ and at angle relative to one another in the (x, y)-plane. Under these conditions, the in-plane component of the radiation pressure exactly cancels the conservative intensity-gradient force. The remaining scattering force is sinusoidally modulated in the transverse plane. The result is a "picket fence" of parallel force lines, which is illustrated in Fig. 2 using the rotated coordinates for clarity. In these coordinates, the net force, is directed alonĝ and has an amplitude that varies with the transverse coordinate as The scale and depth of the force landscape's modulation depend on the object's properties through C 1 = β a (1 + α a /β a ) and C 2 = α a + β a . Picket fence modes for a given type of object are distinguished by the angle of inclination, γ. The range of possible angles is limited by Eq. (13c) to With this constraint, picket fences can be created for objects that satisfy Under some conditions, including those depicted in Fig. 2, the direction of the force can alternate within the fringe pattern. In terms of the standard coupling coefficients, alternating picket fences can be projected for objects satisfying The associated material-dependent flux reversals could have practical applications for sorting objects by density or compressibility.

B. Conservative forces in standing waves
The acoustokinetic framework also is useful for analyzing the force fields created by specified sound waves. Standing waves, for example, can be decomposed into superpositions of counterpropagating plane waves. The phase-dependent terms in F α (r) and F β (r) vanish in such superpositions leaving a manifestly conservative force landscape, Particles satisfying are drawn toward antinodes of the pressure field, as illustrated in Fig. 3. This condition is satisfied by compressible, low-density particles such as bubbles in water. Particles with complementary properties are drawn toward nodes.

C. Nonconservative forces in pseudo-standing waves
Not all zero-momentum waves are standing waves. Some have non-trivial phase profiles and so can exert nonconservative forces. The archetype for such pseudostanding waves is a superposition of three plane waves with equal amplitude, p 0 /3, and wave vectors k n = −k cos n 2π 3 x + sin n 2π 3 ŷ that satisfy 3 n=1 k n = 0 [33]. The pressure intensity for such a three-wave superposition is plotted in Fig. 4(a) and displays a six-fold rotational symmetry similar to that of the corresponding six-fold standing wave in Fig. 3(b). The triangular lattice of antinodes in Fig. 4(a), however, is meshed with a dual hexagonal lattice of nodes, as indicated by dotted circles in Fig. 4(a). Because nonconservative forces tend to be weaker than conservative forces by a factor of (ka p ) 3 , we focus our attention on regions where the sound field forms stable traps, and expand about these points in polar coordinates, r = (r, θ) for small displacements, kr < 1.
For this field, the conservative contributions from Eqs. (11a) and (11b) simplify to Particles satisfying therefore are drawn to pressure antinodes while complementary particles seek out nodes. A node-seeking particle experiences amplitude and phase profiles of the form u node (r) ≈ 1 2 p 0 kr and (24a) The conservative part of the associated force field exerts a Hookean restoring force, that keeps the particle localized near the node. The force field also includes a nonconservative component, that is directed by the azimuthal phase gradient and causes the displaced particle to orbit its node. The pseudo-standing wave therefore transfers orbital angular momentum to particles moving near its nodes, with each node acting as a unit-charge acoustic vortex [1]. The sign of the orbital angular momentum alternates from site to site on the honeycomb lattice of nodes. The array of alternating acoustic vortexes in a pseudo-standing wave therefore carries no net angular momentum. Nodes repel particles satisfying 2α a > β a , which instead seek out the triangular lattice of antinodes. Near an antinode, the sound field's amplitude and phase profiles are For small displacements, the conservative terms from F α (r) and F β (r) exert a Hookean restoring force on an antinode-seeking particle: The nonconservative terms create a sextupole flow that tends to drive the particle from one antinode to another: where F 0 = kp 2 0 (2α a − β a )/32. The conservative part of the pseudo-standing wave's force field vanishes for materials satisfying 2α a = β a , leaving a purely nonconservative force field. This contrasts with the force exerted by a standing wave, which is always conservative. Realizing this condition requires a precise balance of material properties that is unlikely to be achieved in practice. Nevertheless, this observation illustrates that nonconservative forces can emerge along directions where the sound field carries no net momentum. More generally, it shows that radiation pressure can be directed independently of the direction of wave propagation. This independence can be used to craft tractor beams from propagation-invariant Bessel beams [18,30,34].

D. Bessel Beams
Both the standing wave and the pseudo-standing wave require boundary conditions that completely enclose their targets. Long-range manipulation without physical confinement is best achieved with propagation-invariant traveling waves. The natural basis for such single-beam long-range manipulators is the family of Bessel beams [34][35][36], which are non-diffracting solutions to Eq. (5) in cylindrical coordinates, r = (r, θ, z). The amplitude and phase profiles for a Bessel beam propagating alongẑ are u γ,n (r) = p 0 J n (kr sin γ) and (26) ϕ γ,n (r) = kz cos γ + nθ, respectively, where J n (·) is a Bessel function of the first kind of order n. Bessel beams are distinguished by the convergence angle γ that ranges from γ = 0 for conventional plane waves to γ = π/2 for circular standing waves, and the integer n that imposes a helical pitch on the beam's wavefronts.
The conservative part of a Bessel beam's force field is directed radially. Beams with n = 0 have maximum intensity along the axis, r = 0. Those with n > 0 have zero intensity on the axis. The radial component of the acoustic force is linear in kr for |n| ≤ 2 and scales as (kr) 3 , or higher, for |n| > 2. Whether the force attracts the particle to the axis or repels it depends on the particle's properties. For simplicity, we restrict our analysis to |n| ≤ 2, in which case the conditions for particles to be trapped on-axis are 4α a + β a (1 + 3 cos 2γ) > 0, n = 0 (28a) α a + β a cos 2γ < 0, n = ±1 (28b) β a < 0, n = ±2.
As noted recently [37], the condition for stable trapping by an n = 0 Bessel beam differs qualitatively from the analogous condition in Eq. (20) for trapping at an antinode of a standing wave. For example, dense objects with large values of β a are repelled by the antinodes of standing waves and pseudo-standing waves, but tend to be trapped by the central antinode of a Bessel beam with γ < π/4. Similar reversals arise for trapping at the central node of Bessel beams with |n| > 0.
Having established the condition for stable transverse trapping, we next analyze the axial force on a particle localized at r = 0. Any beam satisfying F(r)| r=0 ·ẑ < 0 can be said to act as a tractor beam. It should be noted that optical Bessel beams do not act as tractor beams for small objects because the dipole-order photokinetic force is always repulsive [18].
Axial forces in propagation-invariant Bessel beams are inherently nonconservative. The relevant terms in Eq. (11) yield Unlike optical Bessel beams, therefore, acoustic Bessel beams can act as tractor beams for particles satisfying both the trapping condition from Eq. (28a) and also α a + β a cos 2 γ < 0.
Expressed in terms of material properties, these conditions simplify to (31b) Figure 5 shows the domain of beam shapes and particle compositions for which the n = 0 Bessel beam acts as a tractor beam. These include the optimal condition f 0 /f 1 = −1/2 that was identified in the original discussion of acoustic tractor beams [34]. Density-matched objects (f 1 = 0) can only be trapped if they are compressible enough that f 0 < 0. This means, however, that F ·ẑ > 0, from which we conclude that Bessel beams are not tractor beams for such objects.
The analogous treatment for n = 1 yields Because β a > 0, such beams do act as tractor beams, at least not for objects trapped on the axis. They instead drive trapped objects downstream. The n = 2 beam can trap objects on the axis, but exerts no axial force at all, Such beams might serve as useful conduits for particles that are moved back and forth along the axis by other forces.

IV. DISCUSSION
The theory of acoustokinetic forces expresses the influence of a sound wave on a small object in terms of the amplitude and phase profiles of the pressure field. The resulting analytical framework, which is summarized in Eq. (11), offers a complementary perspective on acoustic forces to the standard development, which explicitly includes the velocity field. Acoustokinetic expressions are particularly useful for designing acoustic force fields because they inherently account for coupling between the pressure and velocity fields and specify amplitude and phase profiles that can be controlled with transmissive, reflective or emissive elements.
The acoustokinetic approach yields expressions that closely resemble results for optical forces in the Rayleigh limit. Exploring the differences and similarities between acoustokinetic and photokinetic forces offers insights into how sound fields couple to material properties to gener-ate useful and interesting force landscapes. The most important difference is that the acoustic dipole polarizability, α a , includes contributions from the incident wave's monopole component. This means that the force arising from the sound field's dipole component includes terms at the same order of magnitude as leading quadrupole contributions whose scale is set by the polarizability, β a . Such mixing does not arise in optical forces. It vanishes for objects that are density matched with the medium, yielding expressions for acoustic forces that are exactly analogous to their optical counterparts.
The acoustokinetic framework naturally accounts for the conservative nature of the force field exerted by standing waves and differentiates it from the influence of pseudo-standing waves that also carry no net momentum yet still exert nonconservative forces. When applied to acoustic Bessel beams, the acoustokinetic framework provides clear guidance on the limits for axial trapping and the conditions under which a zeroth-order Bessel beam can act as a tractor beam. This result contrasts with the equivalent analysis of optical Bessel beams, which do not to act as tractor beams in the Rayleigh regime.
These examples illustrate the value of the acoustokinetic framework for analyzing acoustic force fields. More generally, the expressions from Eq. (11) can be solved either numerically, or in some cases analytically, for the phase and amplitude profiles that correspond to a specified force field acting on a particle of a given size, compressibility and density. As in the optical case, the framework can be extended beyond the dipole approximation. For objects substantially smaller than the wavelength of sound, however, the dipole-order terms are simple enough to yield analytical expressions.

V. ACKNOWLEDGMENTS
This work was supported by the MRSEC program of the National Science Foundation through Award Number DMR-1420073.