The cell nucleus as active matter

Chromatin is a multiscale dynamic architecture that acts as a template for many processes such as transcription and replication. We use single-walled carbon nanotubes (SWNTs) as probes to create a dynamic view of chromatin organization and the transport mechanism of the nuclear pore complexes. With novel image reconstruction algorithms and point spread function engineering, we develop fluorescence microscopy techniques for high resolution dynamic tracking of 3D position and orientation of SWNTs. 

High resolution mapping of non-equilibrium state transitions in acto-myosin cortices

The cortical actin cytoskeleton is a quasi 2D active material in which dynamics are dominated by rapid actin turnover and myosin-driven contractility. We map the dynamics of in vitro reconstituted actin cortices in emulsion droplets using IR fluorescent SWNTs. These reconstituted cortices exhibit different dynamic steady states with a change in connectivity. At critical connectivity, the 2D cortices form small clusters and locally break rotational symmetry. Beyond the critical connectivity, the system transitions to a system-wide contraction and global breaking of translational symmetry. We apply the new dynamic mapping technique to in vivo actin cortices in live starfish oocytes and drosophila embryos in various developmental stages.

Detection of non-equilibrium fluctuations in active gels

Active force generation at the molecular scale in cells can result in stochastic non-equilibrium dynamics on mesoscopic scales. Molecular motors such as myosin can drive steady-state stress fluctuations in cytoskeletal networks. We develop a non-invasive technique to probe non-equilibrium fluctuations in an active gel using micron-long SWNTs. SWNTs behave essentially as semiflexible polymers. Both thermal and active motor-induced forces in the network induce transverse fluctuations of SWNTs. Active driven shape fluctuations of the SWNTs exhibit dynamics that reflects the non-equilibrium activity, in particular the emergence of correlations between the bending modes and observation of breaking of detailed balance in this configurational space of the SWNT probes.