Characterizing diffusion in complex environments created by optical force fields using particle tracking and differential dynamic microscopy

In crowded and complex systems, thermally driven particles may exhibit anomalous subdiffusion, non-Gaussian distributions of displacements, non-ergodic behavior, or Fickian yet non-Gaussian diffusion. These categories of diffusion can be experimentally challenging to distinguish and characterize, and connecting the transport dynamics to properties of the complex environment can also be difficult. This is especially true in experimental studies of diffusion within cells or biomimetic materials. Previous researchers have modeled the complex landscape encountered by a diffusing object using randomized optical force fields. In our experiments, we use a holographic optical tweezers setup to create numerous randomly placed optical traps. Micron-sized colloidal particles are observed diffusing in this heterogenous energy landscape. We quantify their dynamics using both single-particle tracking (SPT) and differential dynamic microscopy (DDM). We determine mean squared displacements, displacement distributions, intermediate scattering functions, non-ergodicity parameters, and confinement length scales. We vary the intensity and spatial patterning of the optical field to determine their effects on the transport dynamics. Our work can be used to help researchers quantify motion in complex environments and understand the advantages of either SPT or DDM in characterizing such systems.