Dynamics of diffusive and driven nanoparticles in fluids

Abstract

The ability to make measurements and manipulate matter at micrometer and nanometer scales will have far-reaching applications. In the past decade, significant progress has been made in developing microscale and nanoscale motors that can be used for targeted delivery. These advances are not without complications, however, such as those brought about by thermal effects which are more apparent at the nanoscale. Modeling microscale and nanoscale objects that interact with a fluid requires a fluid model that is quantitatively accurate and can capture macroscopically observed quantities without any adjustable parameters to make quantitative predictions of the dynamics. In this thesis, we have modeled diffusive and driven systems through the use of a recently developed numerical multiscale simulation method—a coupled fluctuating lattice-Boltzmann and molecular dynamics (LBMD) method—to study thermal effects on driven systems. The method was chosen because it is quantitatively accurate, it incorporates the full hydrodynamics and correct thermal fluctuations, it uses a consistent particle-fluid coupling scheme where the particle-fluid velocity boundary conditions can be controlled, and it allows simulation of particles with complex geometries. Diffusion of fused colloids and colloidal aggregates, and propulsion of helical shapes are investigated using the LBMD method. We first show that the diffusion coefficients of complex aggregate clusters can be quantitatively described by the LBMD method. Then, the model is used to study the diffusive and driven dynamics of magnetic helices, which have been experimentally demonstrated to be applicable for targeted delivery. The geometrical properties—such as the cross-sectional shape, the number of turns, and the pitch length—are optimized for maximum propulsion and swimming efficiency. The dynamics of magnetic helices that are driven by a rotating magnetic field in the presence of thermal fluctuations is examined for varying propulsive strengths. We show that, even at the nanoscale, helices are a suitable candidate for targeted delivery because spatial and temporal precision can be preserved by meeting a Péclet number criterion. In particular, we show that velocities can be controlled for external field frequencies less than the object's characteristic step-out-frequency and the direction of propulsion can be changed provided that the driving field and the helical rotation are synchronous. Finally, we show that self-propelled systems can also be modeled using this method.