Directed self-assembly of three-dimensional user-specified patterns of particles using ultrasound

Abstract

Particles dispersed in a fluid medium are organized into three-dimensional (3D) user-specified patterns using ultrasound directed self-assembly. The technique employs standing ultrasound wave fields created by ultrasound transducers that line the boundary of a fluid reservoir. The acoustic radiation force associated with the standing ultrasound wave field drives the particles into organized patterns, assuming that the particles are much smaller than the wavelength, and do not interact with each other. A direct solution method is theoretically derived to compute the ultrasound transducer operating parameters required to assemble a user-specified pattern of particles in any 3D simple, closed reservoir geometry with any arrangement of ultrasound transducers. This method relates the ultrasound wave field and its associated radiation force to the ultrasound transducer operating parameters by solving a constrained optimization problem that reduces to eigendecomposition. Experimental validation of the method is accomplished by assembling 3D patterns of carbon nanoparticles in a cubic water reservoir lined with four ultrasound transducers. Additionally, the versatility of the method is demonstrated by simulating ultrasound directed self-assembly of complex 3D patterns of particles in cubic and noncubic reservoir geometries lined with many ultrasound transducers. This method enables employing ultrasound directed self-assembly in a variety of engineering applications, including biomedical and materials fabrication processes.