Bryan VanSaders

Processes from crystallization to protein folding to micro-robot self-assembly rely on achieving specific configurations of microscopic objects with short-ranged interactions. However, the small scales and large configuration spaces of such multi-body systems render targeted control challenging. Inspired by optical pumping manipulation of quantum states, we develop a method using parametric pumping to selectively excite and destroy undesired structures to populate the targeted one. This method does not rely on free energy considerations and therefore works for systems with non-conservative and even non-reciprocal interactions, which we demonstrate with an acoustically levitated five-particle system in the Rayleigh limit. With results from experiments and simulations on three additional systems ranging up to hundreds of particles, we show the generality of this method, offering a new path for non-invasive manipulation of strongly interacting multi-particle systems.
Configuration control of non-conservative multi-body systems is challenging. Here, the authors develop a general method using parametric pumping to selectively excite and destroy undesired structures to populate a targeted one, and demonstrate it with acoustically levitated particle systems.

Cohesive granular materials are found in many natural and industrial environments, but experimental platforms for exploring the innate mechanical properties of these materials are often limited by the difficulty of adjusting cohesion strength. Granular particles levitated in an acoustic cavity form a model system to address this. Such particles self-assemble into free-floating, quasi-two-dimensional raft structures which are held together by acoustic scattering forces; the strength of this attraction can be changed simply by modifying the sound field. We investigate the mechanical properties of acoustically bound granular rafts using substrate-free micro-scale shear tests. We first demonstrate deformation of rafts of spheres and the dependence of this deformation on acoustic pressure. We then apply these methods to rafts composed of anisometric sand grains and smaller spheres, in which the smaller spheres have a thin layer of air separating them from other grain surfaces. These spheres act as soft, effectively frictionless particles that populate the interstices between the larger grains, which enables us to investigate the effect of lubricating the mixture in the presence of large-grain cohesion.
Graphical Abstract

Acoustic levitation is frequently used for non-contact manipulation of objects and to study the impact of microgravity on physical and biological processes. While the force field produced by sound pressure lifts particles against gravity (primary acoustic force), multiple levitating objects in the same acoustic cavity interact via forces that arise from scattered sound (secondary acoustic forces). Current experimental techniques for obtaining these force fields are not well-suited for mapping the primary force field at high spatial resolution and cannot directly measure the secondary scattering force. Here we introduce a method that can measure both acoustic forces in situ, including secondary forces in the near-field limit between arbitrarily shaped, closely spaced objects. Operating similarly to an atomic force microscope, the method inserts into the acoustic cavity a suitably shaped probe tip at the end of a long, flexible cantilever and optically detects its deflection. This makes it possible to measure forces with a resolution better than 50 nN, and also to apply stress or strain in a controlled manner to manipulate levitated objects. We demonstrate this by extracting the acoustic potential present in a levitation cavity, directly measuring the acoustic scattering force between two objects, and applying tension to a levitated granular raft of acoustically-bound particles in order to obtain the force-displacement curve for its deformation.

Sound can exert forces on objects of any material and shape. This has made the contactless manipulation of objects by intense ultrasound a fascinating area of research with wide-ranging applications. While much is understood for acoustic forcing of individual objects, sound-mediated interactions among multiple objects at close range gives rise to a rich set of structures and dynamics that are less explored and have been emerging as a frontier for research. We introduce the basic mechanisms giving rise to sound-mediated interactions among rigid as well as deformable particles, focusing on the regime where the particles' size and spacing are much smaller than the sound wavelength. The interplay of secondary acoustic scattering, Bjerknes forces, and micro-streaming is discussed and the role of particle shape is highlighted. Furthermore, we present recent advances in characterizing non-conservative and non-pairwise additive contributions to the particle interactions, along with instabilities and active fluctuations. These excitations emerge at sufficiently strong sound energy density and can act as an effective temperature in otherwise athermal systems.

Going beyond the manipulation of individual particles, first steps have recently been undertaken with acoustic levitation in air to investigate the collective dynamical properties of many-body systems self-assembled within the levitation plane. However, these assemblies have been limited to two-dimensional, close-packed rafts where forces due to scattered sound pull particles into direct frictional contact. Here, we overcome this restriction using particles small enough that the viscosity of air establishes a repulsive streaming flow at close range. By tuning the particle size relative to the characteristic length scale for viscous streaming, we control the interplay between attractive and repulsive forces and show how particles can be assembled into monolayer lattices with tunable spacing. While the strength of the levitating sound field does not affect the particles’ steady-state separation, it controls the emergence of spontaneous excitations that can drive particle rearrangements in an effectively dissipationless, underdamped environment. Under the action of these excitations, a quiescent particle lattice transitions from a predominantly crystalline structure to a two-dimensional liquid-like state. We find that this transition is characterized by dynamic heterogeneity and intermittency, involving cooperative particle movements that remove the timescale associated with caging for the crystalline lattice. These results shed light on the nature of athermal excitations and instabilities that can arise from strong hydrodynamic coupling among interacting particles.

Various biological and synthetic media out of equilibrium can be viewed as many-ratchet systems that rectify environmental noise through local measurements and information processing, like in Maxwell's prototypical demon. These systems pose a challenge to standard coarse-graining approaches because they are better described in terms of decision-making protocols similar to computer programs rather than force laws. Here, we study a many-body generalization of the Maxwell demon problem: a fluid composed of adaptive particles that achieve collective behavior by biasing noise-driven scattering events subject to measurements. Using a combination of information-theoretic, kinetic, and hydrodynamic tools, we elucidate how microscopic decision-making protocols, rather than microscopic forces, generate macroscopic active states sustained by continuous measurements. These include an informational version of flocking whose order parameter is bounded by the information measured, and the onset of which may be viewed as a measurement-induced phase transition. We find that the signature of such microscopic choices is an `informational activity' that selectively compresses phase space, without work, and causes deviations from equilibrium scaling with the magnitude of environmental noise. We envision applications to noise-induced patterning performed by collections of microrobots guided by reinforcement learning or programmable phoretic colloids in turbulent flows that exploit local measurements and control actions to counteract the scrambling of information by chaos.

We investigate a model system for the rotational dynamics of inertial many-particle clustering, in which submillimeter objects are acoustically levitated in air. Driven by scattered sound, levitated grains self-assemble into a monolayer of particles, forming mesoscopic granular rafts with both an acoustic binding energy and a bending rigidity. Detuning the acoustic trap can give rise to stochastic forces and torques that impart angular momentum to levitated objects. As the angular momentum of a quasi-two-dimensional granular raft is increased, the raft deforms from a disk to an ellipse, eventually pinching off into multiple separate rafts, in a mechanism that resembles the breakup of a liquid drop. We extract the raft effective surface tension and elastic modulus and show that nonpairwise acoustic forces give rise to effective elastic moduli that scale with the raft size. We also show that the raft size controls the microstructural basis of plastic deformation, resulting in a transition from fracture to ductile failure.

Crystallography typically studies collections of point particles whose interaction forces are the gradient of a potential. Lifting this assumption generically gives rise in the continuum limit to a form of elasticity with additional moduli known as odd elasticity. We show that such odd elastic moduli modify the strain induced by topological defects and their interactions, even reversing the stability of, otherwise, bound dislocation pairs. Beyond continuum theory, isolated dislocations can self propel via microscopic work cycles active at their cores that compete with conventional Peach-Koehler forces caused, for example, by an ambient torque density. We perform molecular dynamics simulations isolating active plastic processes and discuss their experimental relevance to solids composed of spinning particles, vortexlike objects, and robotic metamaterials.

Self-assembled colloidal crystals can exhibit structural colors, a phenomenon of intense reflection within a range of wavelengths caused by constructive interference. Such diffraction effects are most intense for highly uniform crystals; however, in practice, colloidal crystals may include particles of irregular size, which can reduce the quality of the crystal. Despite its importance in realizing high-quality structural colors, a quantitative relationship between particles of irregular size, crystal quality, and the resultant structural color response remains unclear. This study systematically and quantitatively investigates the effect of adding particles of irregular size on the microstructural quality and structural color reflectivity of colloidal crystals formed by evaporative self-assembly via experiment and simulation. We examine two sizes of irregular particles-those which are 1.9 times larger and 0.4 times smaller than the host crystal. We find that small irregular particles are more detrimental to surface crystal quality and structural color reflectivity than large irregular particles. When incorporated with 10% volume fraction of irregularly sized particles, the reflectivity of crystal films with large (small) irregularly sized particles decreases by 18.4 +/- 5.6% (27.5 +/- 5.8%), and a measure of surface crystal quality derived from Fourier analysis of scanning electron microscopy images reduces by 40.0 +/- 4.5% (48.8 +/- 6.0%). By modeling colloidal films incorporated with irregular particles via molecular dynamics simulation and computing the reflection spectra of the modeled crystals via the finite-difference time-domain method, we find that the peak reflectivity of the assembled structures increases monotonically with overall crystallinity, and that overall crystallinity is correlated with the volume fraction of incorporated irregular particles. The quantitative relationships developed in this study can be applied to predict the level of irregularly sized particles that can be tolerated in colloidal films before significant degradation in crystal quality and reflectivity occurs.