Browsing by Author "Fakhri, Nikta"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Self-organized stress patterns drive state transitions in actin cortices
    (AAAS, 2018) Tan, Tzer Han; Malik-Garbi, Maya; Abu-Shah, Enas; Li, Junang; Sharma, Abhinav; MacKintosh, Fred C.; Keren, Kinneret; Schmidt, Christoph F.; Fakhri, Nikta; Center for Theoretical Biophysics
    Biological functions rely on ordered structures and intricately controlled collective dynamics. This order in living systems is typically established and sustained by continuous dissipation of energy. The emergence of collective patterns of motion is unique to nonequilibrium systems and is a manifestation of dynamic steady states. Mechanical resilience of animal cells is largely controlled by the actomyosin cortex. The cortex provides stability but is, at the same time, highly adaptable due to rapid turnover of its components. Dynamic functions involve regulated transitions between different steady states of the cortex. We find that model actomyosin cortices, constructed to maintain turnover, self-organize into distinct nonequilibrium steady states when we vary cross-link density. The feedback between actin network structure and organization of stress-generating myosin motors defines the symmetries of the dynamic steady states. A marginally cross-linked state displays divergence-free long-range flow patterns. Higher cross-link density causes structural symmetry breaking, resulting in a stationary converging flow pattern. We track the flow patterns in the model actomyosin cortices using fluorescent single-walled carbon nanotubes as novel probes. The self-organization of stress patterns we have observed in a model system can have direct implications for biological functions.
  • Thumbnail Image
    Item
    Single-Walled Carbon Nanotube Dynamics Simple and Complex Media
    (2011) Fakhri, Nikta; Pasquali, Matteo
    Understanding the dynamics of single-walled carbon nanotubes (SWNTs) in simple and complex environments is crucial for establishing potential application of nanotube architectures for materials and biosciences. In this thesis we employ the visualization and analysis tools to image and quantify and the Brownian bending and diffusion of SWNTs in different media in order to understand and eventually to tailor nanotube mobility in confined environments. We image Brownian bending dynamics of SWNTs in water using Near-infrared (NIR) fluorescence microscopy. The bending stiffness of each chirality-assigned SWNT is extracted from the variance of the curvature fluctuations. Relaxation times of the bending fluctuations are measured from the autocorrelation of SWNT shapes. We find that the bending stiffness scales as the cube of the nanotube diameter, in agreement with an elastic continuum model. The measured shape relaxation times are in excellent agreement with the semiflexible chain model, showing that SWNTs may truly be considered as the ideal model semiflexible filaments. The motion of stiff objects in crowded environments has been investigated for more than three decades in polymer science and biophysics; yet, theory and experiments have not established whether a minute amount of flexibility affects the mobility of stiff slender filaments. We image the Brownian motion of SWNTs in a network by NIR fluorescence microscopy. We show direct evidence of SWNTs reptating in the network, and confirm that their small flexibility enhances significantly their rotational diffusion. Our results establish the reptation dynamics of stiff filaments and provide a framework to tailor SWNTs mobility in confined media. By varying SWNT surface modifications, we can selectively tune the sensitivity of the carbon nanotubes to the different physical properties of the porous media for sensing applications. We introduce a simple procedure for dispersion of SWNTs in aqueous solutions using triblock copolymer, PS-b-P2VP-b-PEO. This process yields stable dispersions of individual SWNTs without a need for ultracentrifugation, thus increasing nanotube yield. We show that the SWNT suspension is stable under a wide pH range as well as high salinity environments. These stable suspensions can be used in a wide range of applications in different media where stability is crucial.