Losert Lab Research Projects
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BIOPHYSICS |
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Dictyostelium Dynamics In order to explore how the dynamics of the cytoskeleton is coupled to extermal stimuli, we study a well known unicellular amoeba, Dictyostelium discodeium, known affectionately as Dicty. (See www.dictybase.org). Throughout most of its life cycle, Dicty exists as a single cell organism happily eating bacteria in the soil and rotting tree trunks. Upon starvation, each Dicty cell emits a chemical signal, announcing its presence and hunger to the larger Dicty community. Hungry Dicty cells chemotax, i.e. move towards increasing concentrations of this chemical signal, and eventual aggregate into a fruiting body which disperses Dicty spores with the hope of sending them onto a bacteria-rich enviornment. We use microfluidics and holographic laser tweezers to explore the robustness of Dicty's ability to detect the chemical signal and move towards it. |
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Deformations of inhomogeneous
Actin Networks Actin proteins reversibly self assemble into a semiflexible biopolymer network that is a major component of the scaffolding of the cell. It has been extensively studied using techniques such as microrheology, and the viscoelastic properties of a uniform, network in thermal equilibrium are fairly well-understood. However, recent in situ measurements have shown that the inhomogeneous actin cortex is likely pre-stressed and subject to active forcing. We study how actin networks respond to active local forcing, including actin networks that are polymerized in various gradients (e.g. temperature gradient or gradients of actin binding proteins). |
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Vesicle Deformation and Relaxation The shapes of Giant unilamellar lipid vesicles are driven out of equilibrium by direct forcing with holographic optical tweezers. Vesicles have been studied extensively due to their relevance as a model for the membrane of cells as well as their potential practical uses e.g. for drug delivery or chemical confinement. We use multipoint laser tweezers formed by a spatial light modulator (holographic optical tweezers) to apply forces to such vesicles in several points simultaneously. This results in shape changes that reveal the mechanical properties of forced vesicles. We are currently working with both single lipid component vesicles as well as ternary mixtures with cholesterol in which domain formation can occur. |
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Collagen
Network Dynamics and Structure Type I collagen is a protein that undergoes self-assembly to form fibrils, that in turn aggregate to form larger structures called collagen fibers. This process, fibrillogenesis, is important for purposes of developing devices that can aid in tissue restoration and replacement. Confocal microscopy allows us to image collagen fiber formation in 3D while the network self-assembles. We investigate the network structure and network formation dynamics for various types of healthy and diseased collagen in collaboration with S. Leikin, NIH. |
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GRANULAR |
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3D imaging of slow
granular rearrangements We utilize confocal microscopy, x-ray microtomography, and laser sheet scanning (a tool developed in our lab) to extract the 3D particle positions and slow particule motions due to extermal forces. Particle rearrangments are compared to theoretical models of jamming and glassy dynamics. |
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Shear
localization We investigate how shear strains localizes into shear bands in forced granular materials. We focus on the start of a flow, when shear bands form. |
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Past
Research
Granular Mixtures In a
Rotating Drum |
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Long
Runout Avalanches Understanding of grain avalanche onset, segregation, and pattern formation is especially relevant to the application to long run-out rock-avalanche systems. Earthquakes often trigger large rock avalanches, which may flow at speeds over 200 km/hr and are highly destructive. Many of these deposits show unusually long run-out, in which events flow across a flat surface more than ten times the initial vertical drop height. Controlled laboratory experiments to date have generally neither looked for nor produced such long run-out lengths, and existing geological models of flow are ambiguous, untested, and occasionally contradictory. |
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Dynamics
of Vertically Vibrated Magnetic Spheres We are currently investigating pattern formation in a vertically vibrated monolayer of magnetic spheres. The spheres, of diameter D, encase cylindrical magnetic cores of length, l. For large D/l, we find that the particles form a hexagonal-close-packed pattern in which the particles' dipole vectors assume a macroscopic circulating vortical pattern. For smaller D/l, the particles form concentric rings. The static configurational magnetic energy (which depends on D/l) appears to be a determining factor in pattern selection even though the experimental system is driven and dissipative. |
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Alloy Directional
Solidification Using transparent organic model system (Succinonitrile with 0.01-0.5wt% Coumarin 152 for dilute binary alloy and CBr4-C2Cl6 for eutectic binary alloy), we are investigating the dynamics of crystal growth and control of microstructure in alloys. In a directional solidification setup the sample is put in a thermal gradient and pulled toward the cold side. |
| Grain Boundary
Migration Using transparent organic model system (Succinonitrile with 0.01-0.5wt% Coumarin 152), we are investigating grain structure and the dynamics of grain coarsening in dilute binary alloys. updated 12/6/2006 |