12 research outputs found
The Role of Intracellular Interactions in the Collective Polarization of Tissues and its Interplay with Cellular Geometry
Planar cell polarity (PCP), the coherent in-plane polarization of a tissue on
multicellular length scales, provides directional information that guides a
multitude of developmental processes at cellular and tissue levels. While it is
manifest that cells utilize both intracellular and intercellular mechanisms,
how the two produce the collective polarization remains an active area of
investigation. We study the role of intracellular interactions in the
large-scale spatial coherence of cell polarities, and scrutinize the role of
intracellular interactions in the emergence of tissue-wide polarization. We
demonstrate that nonlocal cytoplasmic interactions are necessary and sufficient
for the robust long-range polarization, and are essential to the faithful
detection of weak directional signals. In the presence of nonlocal
interactions, signatures of geometrical information in tissue polarity become
manifest. We investigate the deleterious effects of geometric disorder, and
determine conditions on the cytoplasmic interactions that guarantee the
stability of polarization. These conditions get progressively more stringent
upon increasing the geometric disorder. Another situation where the role of
geometrical information might be evident is elongated tissues. Strikingly, our
model recapitulates an observed influence of tissue elongation on the
orientation of polarity. Eventually, we introduce three classes of mutants:
lack of membrane proteins, cytoplasmic proteins, and local geometrical
irregularities. We adopt core-PCP as a model pathway, and interpret the model
parameters accordingly, through comparing the in silico and in vivo phenotypes.
This comparison helps us shed light on the roles of the cytoplasmic proteins in
cell-cell communication, and make predictions regarding the cooperation of
cytoplasmic and membrane proteins in long-range polarization.Comment: 15 pages Main Text + 8 page Appendi
Theoretical Limits of Energy Extraction in Active Fluids
Active materials form a class of far-from-equilibrium systems that are driven
internally and exhibit self-organization which can be harnessed to perform
mechanical work. Inspired by experiments on synthetic active networks we
examine limits of work extraction from an active viscoelastic medium by
analyzing the transport of a particle. The active viscoelastic material
possesses an equilibrium density where the active and passive forces are
balanced out. In one dimension, a gliding activation front (AF) that converts a
passive to an active medium, provides active energy at a constant rate, which
is injected into the system at one end and propagates to the other. We
demonstrate that there exists a maximum velocity of the AF, above which the
activated region fails to deliver the transport power. We hypothesize, and
intuitively argue based on the limit cases, that the feasibility and the
velocity of transport can be interpreted in terms of the velocity of an
equilibration Domain Wall of the field, which is set by two parameters: a
measure of activity, and the viscoelastic timescale. The phase diagram
comprises Transport and No-Transport sectors, namely for any pair of the two
parameters, there exists a threshold velocity of the AF above which the
particle transport becomes impossible. Constructing the phase diagram we find
that there are regions of the phase diagram for which the threshold velocity of
the AF diverges. Larger viscoelastic timescale makes the transport region more
accessible, and increases the transport velocity therein. Also, we find that
increasing the velocity of AF results in larger extracted power but smaller
transport coefficient; the ratio of the transport velocity and that of the AF.
Our model provides a framework for understanding the energetics of transport
phenomena in biology, and designing efficient mechanisms of transport in
synthetic active materials.Comment: 8+7 pages, 8 figure
Charge/mass dynamic structure factors of water and applications to dielectric friction and electroacoustic conversion
We determine time correlation functions and dynamic structure factors of the
number and charge density of liquid water from molecular dynamics simulations.
Using these correlation functions we consider dielectric friction and electro
acoustic coupling effects via linear response theory. From charge-charge
correlations, the drag force on a moving point charge is derived and found to
be maximal at a velocity of around 300 m/s. Strong deviations in the resulting
friction coefficients from approximate theory employing a single Debye
relaxation mode are found that are due to non-Debyelike resonances at high
frequencies. From charge-mass cross-correlations the ultrasonic vibration
potential is derived, which characterizes the conversion of acoustic waves
into electric time-varying potentials. Along the dispersion relation for
normal sound waves in water, the ultrasonic vibration potential is shown to
strongly vary and to increase for larger wavelengths
Persistent fluid flows defined by active matter boundaries
Biological systems achieve precise control over ambient fluids through the self-organization of active protein structures including flagella, cilia, and cytoskeletal networks. In active structures individual proteins consume chemical energy to generate force and motion at molecular length scales. Self-organization of protein components enables the control and modulation of fluid flow fields on micron scales. The physical principles underlying the organization and control of active-matter driven fluid flows are poorly understood. Here, we apply an optically-controlled active-matter system composed of microtubule filaments and light-switchable kinesin motor proteins to analyze the emergence of persistent flow fields in a model active matter system. Using light, we form contractile microtubule networks of varying shape. We analyze the fluid flow fields generated by a wide range of microtubule network geometries and explain the resulting flow fields within a unified theoretical framework. We specifically demonstrate that the geometry of microtubule flux at the boundary of contracting microtubule networks predicts the steady-state fluid flow fields across polygonal network geometries through finite-element simulations. Our work provides a foundation for programming microscopic fluid-flows with controllable active matter and could enable the engineering of versatile and dynamic microfluidic devices
Polarons and Mobile Impurities Near a Quantum Phase Transition
This dissertation aims at improving the current understanding of the physics of mobile impurities in highly correlated liquid-like phases of matter. Impurity problems pose challenging and intricate questions in different realms of many-body physics. For instance, the problem of ''solvation'' of charged solutes in polar solvents, has been the subject of longstanding debates among chemical physicists. The significant role of quantum fluctuations of the solvent, as well as the break down of linear response theory, render the ordinary treatments intractable. Inspired by this complicated problem, we first attempt to understand the role of non-specific quantum fluctuations in the solvation process. To this end, we calculate the dynamic structure factor of a model polar liquid, using the classical Molecular Dynamics (MD) simulations. We verify the failure of linear response approximation in the vicinity of a hydrated electron, by comparing the outcomes of MD simulations with the predictions of linear response theory. This nonlinear behavior is associated with the pronounced peaks of the structure factor, which reflect the strong fluctuations of the local modes. A cavity picture is constructed based on heuristic arguments, which suggests that the electron, along with the surrounding polarization cloud, behave like a frozen sphere, for which the linear response theory is broken inside and valid outside. The inverse radius of the spherical region serves as a UV momentum cutoff for the linear response approximation to be applicable.The problem of mobile impurities in polar liquids can be also addressed in the framework of the ''polaron'' problem. Polaron is a quasiparticle that typically acquires an extended state at weak couplings, and crossovers to a self-trapped state at strong couplings. Using the analytical fits to the numerically obtained charge-charge structure factor, a phenomenological approach is proposed within the Leggett's influence functional formalism, which derives the effective Euclidean action from the classical equation of motion. We calculate the effective mass of the polaron in the model polar liquid at zero and finite temperatures. The self-trapping transition of this polaron turns out to be discontinuous in certain regions of the phase diagram.In order to systematically investigate the role of quantum fluctuations on the polaron properties, we adopt a quantum field theory which supports nearly-critical local modes: the quantum Landau-Brazovskii (QLB) model, which exhibits fluctuation-induced first order transition (weak crystallization). In the vicinity of the phase transition, the quantum fluctuations are strongly correlated; one can in principle tune the strength of these fluctuations, by adjusting the parameters close to or away from the transition point. Furthermore, sufficiently close to the transition, the theory accommodates ``soliton'' solutions, signaling the nonlinear response of the system. Therefore, the model seems to be a promising candidate for studying the effects of strong quantum fluctuations and also failure of linear response theory, in the polaron problem. We observe that at zero temperature, and away from the Brazovskii transition where the linear response approximation is valid, the localization transition of the polaron is discontinuous. Upon enhancing fluctuations|of either thermal or quantum nature|the gap of the effective mass closes at distinct second-order critical points. Sufficiently close to the Brazovskii transition where the nonlinear contributions of the field are significantly large, a new state appears in addition to extended and self-trapped polarons: an impurity-induced soliton. We interpret this as the break-down of linear response, reminiscent of what we observe in a polar liquid. Quantum LB model has been proposed to be realizable in ultracold Bose gases in cavities. We thus discuss the experimental feasibility, and propose a setup which is believed to exhibit the aforementioned polaronic and solitonic states. We eventually generalize the polaron formalism to the case of impurities that couple quadratically to a nearly-critical field; hence called the ''quadratic polaron''. The Hertz-Millis field theory and its generalization to the case of magnetic transition in helimagnets, is taken as a toy model. The phase diagram of the bare model contains both second-order and fluctuation-induced first-order quantum phase transitions. We propose a semi-classical scenario in which the impurity and the field couple quadratically. The polaron properties in the vicinity of these transitions are calculated in different dimensions. We observe that the quadratic coupling in three dimensions, even in the absence of the critical modes with finite wavelength, leads to a jump-like localization of the polaron. In lower dimensions, the transition behavior remains qualitatively similar to those in the case of linear coupling, namely the critical modes must have a finite wavelength to localize the particle
Persistent fluid flows defined by active matter boundaries
Biological systems achieve precise control over ambient fluids through the self-organization of active protein structures including flagella, cilia, and cytoskeletal networks. In active structures individual proteins consume chemical energy to generate force and motion at molecular length scales. Self-organization of protein components enables the control and modulation of fluid flow fields on micron scales. The physical principles underlying the organization and control of active-matter driven fluid flows are poorly understood. Here, we apply an optically-controlled active-matter system composed of microtubule filaments and light-switchable kinesin motor proteins to analyze the emergence of persistent flow fields in a model active matter system. Using light, we form contractile microtubule networks of varying shape. We analyze the fluid flow fields generated by a wide range of microtubule network geometries and explain the resulting flow fields within a unified theoretical framework. We specifically demonstrate that the geometry of microtubule flux at the boundary of contracting microtubule networks predicts the steady-state fluid flow fields across polygonal network geometries through finite-element simulations. Our work provides a foundation for programming microscopic fluid-flows with controllable active matter and could enable the engineering of versatile and dynamic microfluidic devices
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Author Correction: Stem cell-derived synthetic embryos self-assemble by exploiting cadherin codes and cortical tension.
Funder: Wellcome Trust (207415/Z/17/Z), the ERC advanced grant (669198), the NIH R01 (HD100456-01A1) grant, the NIH Pioneer Award (DP1 HD104575-01), Tianqiao and Chrissy Chen Institute for Neuroscience, and Shurl and Kay Curci Foundatio