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Abstract Ultrasound‐directed self‐assembly (DSA) uses ultrasound waves to organize and orient particles dispersed in a fluid medium into specific patterns. Combining ultrasound DSA with vat photopolymerization (VP) enables manufacturing materials layer‐by‐layer, wherein each layer the organization and orientation of particles in the photopolymer is controlled, which enables tailoring the properties of the resulting composite materials. However, the particle packing density changes with time and location as particles organize into specific patterns. Hence, relating the ultrasound DSA process parameters to the transient local particle packing density is important to tailor the properties of the composite material, and to determine the maximum speed of the layer‐by‐layer VP process. This paper theoretically derives and experimentally validates a 3D ultrasound DSA model and evaluates the local particle packing density at locations where particles assemble as a function of time and ultrasound DSA process parameters. The particle packing density increases with increasing particle volume fraction, decreasing particle size, and decreasing fluid medium viscosity is determined. Increasing the particle size and decreasing the fluid medium viscosity decreases the time to reach steady‐state. This work contributes to using ultrasound DSA and VP as a materials manufacturing process. 

Ultrasound directed self-assembly (DSA) utilizes the acoustic radiation force (ARF) associated with a standing ultrasound wave to organize particles dispersed in a fluid medium into specific patterns. The ARF is a superposition of the primary acoustic radiation force, which results from the incident standing ultrasound wave, and the acoustic interaction force, which originates from single and multiple scattering between neighboring particles. In contrast with most reports in the literature that neglect multiple scattering when calculating the ARF, we demonstrate that the deviation between considering single or multiple scattering may reach up to 100%, depending on the ultrasound DSA process parameters and material properties. We evaluate a theoretical case with three spherical particles in a viscous medium and derive operating maps that quantify the deviation between both scattering approaches as a function of the ultrasound DSA process parameters. Then, we study a realistic system with hundreds of particles dispersed in a viscous medium, and show that the deviation between the ARF resulting from single and multiple scattering increases with decreasing particle size and increasing medium viscosity, density ratio, compressibility ratio, and particle volume fraction. This work provides a quantitative basis for determining whether to consider single or multiple scattering in ultrasound DSA simulations. 

Vat photopolymerization (VP) additive manufacturing involves selectively curing low‐viscosity photopolymers via exposure to ultraviolet light in a layer‐wise fashion. Dispersing filler materials in the photopolymer enables tailored end‐use properties, but also increases the viscosity and the timescale associated with interparticle network structural recovery postshear. These rheological properties influence self‐leveling and recoating of the liquid photopolymer mixture during VP. Herein, viscosity of photopolymer and rigid spherical glass microparticles (filler) is experimentally determined as a function of filler fraction, filler size distribution (mono‐ and polydisperse), shear rate, and temperature, which are important VP process parameters. Employing existing viscosity models for mono‐ and polydisperse polymer mixtures demonstrates that particle–particle interactions and the formation of nonspherical clusters of particles strongly affect the viscosity of both monodisperse and polydisperse mixtures with particle volume fractions > 0.05 due to agglomeration/deagglomeration of clusters at elevated shear rates. Consequently, unmodified viscosity models, which assume uniformly dispersed, rigid, spherical particles, are applicable only for mixtures with particle volume fractions < 0.05. It is shown that modifying model parameters such as the fluidity limit and intrinsic viscosity, which explicitly account for nonspherical clusters of particles, improves agreement between viscosity models and experiments, in particular when using a fractal approach. 

We model near-field thermal emission from metasurfaces structured as two-dimensional arrays of ellipsoidal SiC particles. The modeling approach is developed from fluctuational electrodynamics and is applicable to systems of ellipsoidal particles within the dipole limit. In all simulations, the radial lengths of particles are restricted to the range of 10–100 nm, and interparticle spacing is constrained to at least three times the particle characteristic length. The orientation and dimensions of constituent ellipsoidal particles are varied to tune localized surface phonon resonances and control the near-field energy density above metasurfaces. Results show that particle orientation can be used to regulate the relative magnitude of resonances in the energy density, and particle dimensions may be changed to adjust the frequency of these resonances within the Reststrahlen band. Metasurfaces constructed from particles with randomized dimensions display comparatively broadband thermal emission rather than the three distinct resonances seen in metasurfaces made with ellipsoidal particles of equivalent dimensions. When the interparticle spacing in a metasurface exceeds about three times the particle characteristic length, the spectral energy density above the metasurface is dominated by individual particle self-interaction and can be approximated as a linear combination of single-particle spectra. When interparticle spacing is at the lower limit of three times the characteristic length, however, multiparticle interaction effects increase and the spectral energy density above a metasurface deviates from that of single particles. This work provides guidance for designing all-dielectric, particle-based metasurfaces with desired near-field thermal emission spectra, such as thermal switches.