663 research outputs found
Chloroplasts in plant cells show active glassy behavior under low-light conditions
Plants have developed intricate mechanisms to adapt to changing light
conditions. Besides photo- and helio- tropism -- the differential growth
towards light and the diurnal motion with respect to sunlight -- chloroplast
motion acts as a fast mechanism to change the intracellular structure of leaf
cells. While chloroplasts move towards the sides of the plant cell to avoid
strong light, they accumulate and spread out into a layer on the bottom of the
cell at low light to increase the light absorption efficiency. Although the
motion of chloroplasts has been studied for over a century, the collective
organelle-motion leading to light adapting self-organized structures remains
elusive. Here we study the active motion of chloroplasts under dim light
conditions, leading to an accumulation in a densely packed quasi-2D layer. We
observe burst-like re-arrangements and show that these dynamics resemble
colloidal systems close to the glass transition by tracking individual
chloroplasts. Furthermore, we provide a minimal mathematical model to uncover
relevant system parameters controlling the stability of the dense configuration
of chloroplasts. Our study suggests that the meta-stable caging close to the
glass-transition in the chloroplast mono-layer serves a physiological
relevance. Chloroplasts remain in a spread-out configuration to increase the
light uptake, but can easily fluidize when the activity is increased to
efficiently re-arrange the structure towards an avoidance state. Our research
opens new questions about the role that dynamical phase transitions could play
in self-organized intracellular responses of plant cells towards environmental
cues
Stokesian Dynamics simulation of Brownian suspensions
The non-equilibrium behaviour of concentrated colloidal dispersions is studied by Stokesian Dynamics, a general molecular-dynamics-like technique for simulating particles suspended in a viscous fluid. The simulations are of a suspension of monodisperse Brownian hard spheres in simple shear flow as a function of the Péclet number, Pe, which measures the relative importance of shear and Brownian forces. Three clearly defined regions of behaviour are revealed. There is first a Brownian-motion-dominated regime (Pe ≤ 1) where departures from equilibrium in structure and diffusion are small, but the suspension viscosity shear thins dramatically. When the Brownian and hydrodynamic forces balance (Pe ≈ 10), the dispersion forms a new ‘phase’ with the particles aligned in ‘strings’ along the flow direction and the strings are arranged hexagonally. This flow-induced ordering persists over a range of Pe and, while the structure and diffusivity now vary considerably, the rheology remains unchanged. Finally, there is a hydrodynamically dominated regime (Pe > 200) with a dramatic change in the long-time self-diffusivity and the rheology. Here, as the Péclet number increases the suspension shear thickens owing to the formation of large clusters. The simulation results are shown to agree well with experiment
Experimental and computational investigation of flow of pebbles in a pebble bed nuclear reactor
The Pebble Bed Reactor (PBR) is a 4th generation nuclear reactor which is conceptually similar to moving bed reactors used in the chemical and petrochemical industries. In a PBR core, nuclear fuel in the form of pebbles moves slowly under the influence of gravity. Due to the dynamic nature of the core, a thorough understanding about slow and dense granular flow of pebbles is required from both a reactor safety and performance evaluation point of view.
In this dissertation, a new integrated experimental and computational study of granular flow in a PBR has been performed. Continuous pebble re-circulation experimental set-up, mimicking flow of pebbles in a PBR, is designed and developed. Experimental investigation of the flow of pebbles in a mimicked test reactor was carried out for the first time using non-invasive radioactive particle tracking (RPT) and residence time distribution (RTD) techniques to measure the pebble trajectory, velocity, overall/zonal residence times, flow patterns etc. The tracer trajectory length and overall/zonal residence time is found to increase with change in pebble\u27s initial seeding position from the center towards the wall of the test reactor. Overall and zonal average velocities of pebbles are found to decrease from the center towards the wall. Discrete element method (DEM) based simulations of test reactor geometry were also carried out using commercial code EDEM and simulation results were validated using the obtained benchmark experimental data. In addition, EDEM based parametric sensitivity study of interaction properties was carried out which suggests that static friction characteristics play an important role from a packed/pebble beds structural characterization point of view. To make the RPT technique viable for practical applications and to enhance its accuracy, a novel and dynamic technique for RPT calibration was designed and developed. Preliminary feasibility results suggest that it can be implemented as a non-invasive and dynamic calibration methodology for RPT technique which will enable its industrial applications. --Abstract, page iii
Jamming modulates coalescence dynamics of shear-thickening colloidal droplets
Recent investigations into coalescence dynamics of complex fluid droplets
revealed the existence of sub-Newtonian behaviour for polymeric fluids (elastic
and shear thinning). We hypothesize that such delayed coalescence or
sub-Newtonian coalescence dynamics may be extended to the general class of
shear thickening fluids. To investigate this droplets of aqueous corn-starch
suspensions were chosen and its coalescence in sessile pendant configuration
was probed by high-speed real time imaging. Temporal evolution of the neck
(growth) during coalescence was quantified as a function of suspended particle
weight fraction \phi_w. The necking behaviour was found to evolve as the
power-law relation where R is neck radius with exponent \b\le0.5
implying it is a subset of the generic sub-Newtonian coalescence. Second
significant delay in the coalescence dynamics is observed for particle
fractions beyond the jamming fraction {\ \phi}_w>\ \phi_J\geq0.35}. Our
proposed theoretical model captures this delay implicitly through altered
suspension viscosity stemming from increased particle content
Forced back into shape: Mechanics of epithelial wound repair
Wound repair, the closing of a hole, is inherently a physical process that requires the change of shape of materials, in this case, cells and tissues. Not only is efficient and accurate wound repair critical for restoring barrier function and reducing infection, but it is also critical for restoring the complex three-dimensional architecture of an organ. This re-sculpting of tissues requires the complex coordination of cell behaviours in multiple dimensions, in space and time, to ensure that the repaired structure can continue functioning optimally. Recent evidence highlights the importance of cell and tissue mechanics in 2D and 3D to achieve such seamless wound repair
Unit cell geometry of multiaxial preforms for structural composites
The objective of this study is to investigate the yarn geometry of multiaxial preforms. The importance of multiaxial preforms for structural composites is well recognized by the industry but, to exploit their full potential, engineering design rules must be established. This study is a step in that direction. In this work the preform geometry for knitted and braided preforms was studied by making a range of well designed samples and studying them by photo microscopy. The structural geometry of the preforms is related to the processing parameters. Based on solid modeling and B-spline methodology a software package is developed. This computer code enables real time structural representations of complex fiber architecture based on the rule of preform manufacturing. The code has the capability of zooming and section plotting. These capabilities provide a powerful means to study the effect of processing variables on the preform geometry. the code also can be extended to an auto mesh generator for downstream structural analysis using finite element method. This report is organized into six sections. In the first section the scope and background of this work is elaborated. In section two the unit cell geometries of braided and multi-axial warp knitted preforms is discussed. The theoretical frame work of yarn path modeling and solid modeling is presented in section three. The thin section microscopy carried out to observe the structural geometry of the preforms is the subject in section four. The structural geometry is related to the processing parameters in section five. Section six documents the implementation of the modeling techniques into the computer code MP-CAD. A user manual for the software is also presented here. The source codes and published papers are listed in the Appendices
Deconstructing the glass transition through critical experiments on colloids
The glass transition is the most enduring grand-challenge problem in
contemporary condensed matter physics. Here, we review the contribution of
colloid experiments to our understanding of this problem. First, we briefly
outline the success of colloidal systems in yielding microscopic insights into
a wide range of condensed matter phenomena. In the context of the glass
transition, we demonstrate their utility in revealing the nature of spatial and
temporal dynamical heterogeneity. We then discuss the evidence from colloid
experiments in favor of various theories of glass formation that has
accumulated over the last two decades. In the next section, we expound on the
recent paradigm shift in colloid experiments from an exploratory approach to a
critical one aimed at distinguishing between predictions of competing
frameworks. We demonstrate how this critical approach is aided by the discovery
of novel dynamical crossovers within the range accessible to colloid
experiments. We also highlight the impact of alternate routes to glass
formation such as random pinning, trajectory space phase transitions and
replica coupling on current and future research on the glass transition. We
conclude our review by listing some key open challenges in glass physics such
as the comparison of growing static lengthscales and the preparation of
ultrastable glasses, that can be addressed using colloid experiments.Comment: 137 pages, 45 figure
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