Crowding and confinement act in concert to slow DNA diffusion within cell-sized droplets.

Dynamics of biological macromolecules, such as DNA, in crowded and confined environments are critical to understanding cellular processes such as transcription, infection, and replication. However, the combined effects of cellular confinement and crowding on macromolecular dynamics remain poorly understood. Here, we use differential dynamic microscopy to investigate the diffusion of large DNA molecules confined in cell-sized droplets and crowded by dextran polymers. We show that confined and crowded DNA molecules exhibit universal anomalous subdiffusion with scaling that is insensitive to the degree of confinement and crowding. However, effective DNA diffusion coefficients D e f f decrease up to 2 orders of magnitude as droplet size decreases-an effect that is enhanced by increased crowding. We mathematically model the coupling of crowding and confinement by combining polymer scaling theories with confinement-induced depletion effects. The generality and tunability of our system and models render them applicable to elucidating wide-ranging crowded and confined systems.

Surface plasmon polariton scattering by subwavelength silicon wires.

Surface plasmon polariton scattering from 2D subwavelength silicon wires is investigated using the finite-difference time-domain method. It is shown that coupling an incident surface plasmon polariton to intercavity modes of the particle can dramatically change transmitted fields and plasmon-induced forces. In particular, both transmission and optical forces are highly sensitive to the particle size that is related to the excitation of whispering gallery modes or standing wave modes depending on the particle shape and size. These features might have potential sensing applications.

Optical trapping and control of a dielectric nanowire by a nanoaperture.

We demonstrate that a single sub-wavelength nanoaperture in a metallic thin film can be used to achieve dynamic optical trapping and control of a single dielectric nanowire. A nanoaperture can trap a nanowire, control its orientation when illuminated by a linearly polarized incident field, and rotate the nanowire when illuminated by a circularly polarized incident field. Compared to other designs, this approach has the advantage of a low-power driving field entailing low heating and photodamage.