Quantifying Transport in Crowded, Complex Systems Modeled by Holographic Optical Tweezers through Differential Dynamic Microscopy

Drug delivery, intracellular transport, and the movement of bacteria in polymer solutions often rely on thermally driven motion through complex, crowded systems. Investigations of particle transport behavior in crowded and complex systems can be experimentally challenging, and it can be difficult to quantitatively describe the degree and type of anomalous diffusion. Here, we model diffusive transport through a heterogenous landscape by using micron-sized colloidal particles and holographic optical tweezers. We use optical microscopy to observe the motion of a two-dimensional layer of colloidal particles as they interact with a varying number of randomly placed optical traps across a range of trapping laser powers. By using differential dynamic microscopy and single particle tracking, we can characterize both ergodic and nonergodic transport dynamics of our model system and determine characteristic confinement length scales. The methods we have developed to quantitatively tease apart the dynamics of those particles trapped by the randomly place optical tweezers and those particles that are diffusing freely could be applied to studying transport dynamics in other complex systems.