Nanoscale Dancing: Photophoretic Swimmers and their Stochastic Movement

Abstract

Micro- and Nano-swimmers, converting energy from their surrounding environment into propulsion, can explore complex environments, and have been analyzed for their potential applications in studying areas such as out-of-equilibrium physical systems and the potential to act as controllable nanorobots. At such small length scales, thermal fluctuations between particle and solvent materials dominate motion, a process known as Brownian Motion. Swimmers, contrary to their passive Brownian particle counterparts, can move along a direction corresponding to a local axis of asymmetry related to the symmetry breaking of the system. Previously, microswimmers have been synthesized and their motion has been characterized using the Mean Squared Displacement (MSD) and Velocity Autocorrelation (VAC). This Thesis is divided into two components. First, the distribution of step sizes was analytically derived following a Finite Difference discretization scheme, and a novel distribution-based least squares minimization scheme to recover particle swimming velocity due to their self-propulsion was developed. Contrary to previous methods of velocity determination, which assume that the particle’s velocity vector exists only in two dimensions, this method assumes that the movement of the particle and its velocity vector explores the whole three-dimensional space and that the motion can only be tracked in two dimensions (representing a projection onto lower dimensional space). The performance of the scheme is characterized by testing it with simulated swimmer trajectories and the balance between spatial and temporal accuracy is demonstrated. Second, a first proof-of-principle experiment for truly scalable nano-scale swimming through photophoresis is performed, using the distribution-based method for velocity determination from Chapter 1 to analyze swimmer trajectories. This work will serve as a starting point for various potential paths of further inquiry, such as the control of nanoswimmers, nano-scale swarming behavior, and the further optimization of swimmer geometry to reach smaller length scales. By demonstrating nano-scale swimming behavior and developing a full analysis scheme to look at swimmer trajectories, the doors to nanoscale applications have been opened.