Phyiscs Faculty Candidate, "Forces in Collective Cell Motion," by Dr. Thomas Angelini, Harvard University Friday, 2/12/2010, 4:00 PM-5:00 PM
Individual living cells generate forces and direct their motion in well known ways. For example, planktonic bacteria swim through fluids by rapidly turning their flagella, and individual tissue cells migrate across surfaces in a cyclic process of expansion, adhesion, and retraction. These canonical types of motion, however, are not characteristic of cells within large, dense aggregates, such as bacterial colonies or the tissues of complex organisms. In this talk I will discuss tools and concepts of condensed matter physics that I have adapted to study the collectively generated forces that control multi-cellular motion within enormous cell aggregates. I will present research on bacterial biofilms, showing how they can spread by generating molecular gradients throughout the colony. I will also discuss collective motion within two-dimensional confluent sheets of mammalian tissue cells, showing how sub-cellular motions as well as multi-cellular forces, transmitted across long distances, each influence collective migration in different ways.
Refreshments will be served in Olin Hall 118 at 3:30 p.m.
Sponsored by: WPI Physics Department, Dr. Erkan Tuzel
Physics Faculty Candidate, "Collective motion and density fluctuations in bacterial colonies" by Dr. Hepeng Zhang, University of Texas at Austin Monday, 2/15/2010, 4:00 PM-12:00 AM
The emergence of collective motion such as in bird flocks, fish schools, and insect swarms is a ubiquitous self-organization phenomenon. Such collective behavior plays an important role in a range of problems, such as spreading of diseases in animal or fish groups. Current models have provided a qualitative understanding of collective motion, but progress in quantitative modeling is hindered by the lack of experimental data.
Here we examine a model microscopic system, where we are able to measure simultaneously the positions, velocities, and orientations of up to a thousand bacteria (wild-type Bacillus subtilis) in a colony. The motile bacteria form closely-packed dynamic clusters within which they move cooperatively. Physical dimensions of clusters scale with the square-root of their sizes, defined as the number of the constituent bacteria. Cluster size exhibits a power-law distribution truncated by an exponential tail, and the probability of finding large clusters grows markedly as bacterial density increases. Mobile clusters cause anomalous fluctuations in bacterial density, as found in mathematical theories and numerical models. Our results demonstrate that bacteria are an excellent system to study general phenomena of collective motion.
Refreshments will be served in Olin Hall 118 at 3:30 P.M.
Sponsored by: WPI Physics Department, Dr. Erkan Tuzel
Physics Faculty Candidate, "Motion of Particle Suspensions and Swimming Microorganisms at Low Reynolds Number," by Dr. Jeffrey Guasto, Department of Physics, Haverford College Friday, 2/19/2010, 4:00 PM-5:00 PM
The transport of particulate matter in fluids is important to our understanding of many biological and physical phenomena. Here, I discuss two problems concerning the random motion of particles at small Reynolds numbers (Stokes Flows) where viscous forces dominate over particle inertia.
(i) The time reversibility of these flows is a fundamental principle in hydrodynamics. However, recent studies have shown that many-body interactions between particles in sheared suspensions give rise to chaotic dynamics. When the strain is small, the system can self-organized into reversible state, but sufficiently large strain leads to an abrupt transition to irreversibility similar to a phase transition. This phenomenon is studied experimentally in a system with spatially varying strain (i.e. channel flow).
(ii) Understanding the mixing produced by single-celled swimming microorganisms is important in marine ecology (e.g. suspension feeding, biogenic mixing) and reveals interesting microscale physics. To gain a better understanding of the interplay between Brownian motion and flows produced by swimmers, we study the green alga Chlamydomonas reinhardtii. In the absence of swimmers, passive tracer particles exhibit purely Brownian motion. However, passing swimmers give intermittent kicks to the tracers, which result in complex particle trajectories and enhanced diffusion.
Refreshments will be served at 3:30 P.M. in Olin Hall 118
Sponsored by: WPI Physics Department, Dr. Erkan Tuzel
Physics Faculty Candidate, "The soft glassy behavior of particle suspensions and oxide films , by Dr. Ryan Larsen, Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana- Champaign Friday, 2/26/2010, 4:00 PM-5:00 AM
Soft glassy materials are important in a wide variety of applications and industries, including food science, construction, mining, waste management,pharmaceutical, and biotechnology. In many applications the balance between solid-like and liquid-like properties is essential to the function of the material. Many glassy materials exhibit yield stress behavior: they transition from solid-like to liquid-like behavior as the stress is increased. However, in some glassy materials, stress induces the opposite behavior, causing materials to become more solid-like. This unusual behavior is not confined to a single type of glassy material. I will present case studies of shear-induced solid-like behavior in two completely different glassy materials: particle suspensions and the oxide films of liquid metals. In particle suspensions, the solid-like behavior is associated with a jamming transition, which can be characterized with an apparent non-linear elasticity. In oxide metals, the solid-like behavior is associated with shear-induced wrinkling of the oxide surface. Despite the differences in the mechanisms responsible for these effects, both materials share an important characteristic: deformation induces internal stresses that persist even after the stress is applied. These results suggest that the engineering of favorable flow properties requires control of the strain energy that is stored in glassy materials while they are flowing.
Refreshments will be served in Olin Hall 118 at 3:30 P.M.
Sponsored by: WPI Physics Department, Dr. Erkan Tuzel
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Phyiscs Faculty Candidate, "Forces in Collective Cell Motion," by Dr. Thomas Angelini, Harvard University Friday, 2/12/2010, 4:00 PM-5:00 PM
Individual living cells generate forces and direct their motion in well known ways. For example, planktonic bacteria swim through fluids by rapidly turning their flagella, and individual tissue cells migrate across surfaces in a cyclic process of expansion, adhesion, and retraction. These canonical types of motion, however, are not characteristic of cells within large, dense aggregates, such as bacterial colonies or the tissues of complex organisms. In this talk I will discuss tools and concepts of condensed matter physics that I have adapted to study the collectively generated forces that control multi-cellular motion within enormous cell aggregates. I will present research on bacterial biofilms, showing how they can spread by generating molecular gradients throughout the colony. I will also discuss collective motion within two-dimensional confluent sheets of mammalian tissue cells, showing how sub-cellular motions as well as multi-cellular forces, transmitted across long distances, each influence collective migration in different ways. Refreshments will be served in Olin Hall 118 at 3:30 p.m. Sponsored by: WPI Physics Department, Dr. Erkan Tuzel
Physics Faculty Candidate, "Collective motion and density fluctuations in bacterial colonies" by Dr. Hepeng Zhang, University of Texas at Austin Monday, 2/15/2010, 4:00 PM-12:00 AM
The emergence of collective motion such as in bird flocks, fish schools, and insect swarms is a ubiquitous self-organization phenomenon. Such collective behavior plays an important role in a range of problems, such as spreading of diseases in animal or fish groups. Current models have provided a qualitative understanding of collective motion, but progress in quantitative modeling is hindered by the lack of experimental data. Here we examine a model microscopic system, where we are able to measure simultaneously the positions, velocities, and orientations of up to a thousand bacteria (wild-type Bacillus subtilis) in a colony. The motile bacteria form closely-packed dynamic clusters within which they move cooperatively. Physical dimensions of clusters scale with the square-root of their sizes, defined as the number of the constituent bacteria. Cluster size exhibits a power-law distribution truncated by an exponential tail, and the probability of finding large clusters grows markedly as bacterial density increases. Mobile clusters cause anomalous fluctuations in bacterial density, as found in mathematical theories and numerical models. Our results demonstrate that bacteria are an excellent system to study general phenomena of collective motion. Refreshments will be served in Olin Hall 118 at 3:30 P.M. Sponsored by: WPI Physics Department, Dr. Erkan Tuzel
Physics Faculty Candidate, "Motion of Particle Suspensions and Swimming Microorganisms at Low Reynolds Number," by Dr. Jeffrey Guasto, Department of Physics, Haverford College Friday, 2/19/2010, 4:00 PM-5:00 PM
The transport of particulate matter in fluids is important to our understanding of many biological and physical phenomena. Here, I discuss two problems concerning the random motion of particles at small Reynolds numbers (Stokes Flows) where viscous forces dominate over particle inertia. (i) The time reversibility of these flows is a fundamental principle in hydrodynamics. However, recent studies have shown that many-body interactions between particles in sheared suspensions give rise to chaotic dynamics. When the strain is small, the system can self-organized into reversible state, but sufficiently large strain leads to an abrupt transition to irreversibility similar to a phase transition. This phenomenon is studied experimentally in a system with spatially varying strain (i.e. channel flow). (ii) Understanding the mixing produced by single-celled swimming microorganisms is important in marine ecology (e.g. suspension feeding, biogenic mixing) and reveals interesting microscale physics. To gain a better understanding of the interplay between Brownian motion and flows produced by swimmers, we study the green alga Chlamydomonas reinhardtii. In the absence of swimmers, passive tracer particles exhibit purely Brownian motion. However, passing swimmers give intermittent kicks to the tracers, which result in complex particle trajectories and enhanced diffusion. Refreshments will be served at 3:30 P.M. in Olin Hall 118 Sponsored by: WPI Physics Department, Dr. Erkan Tuzel
Physics Faculty Candidate, "The soft glassy behavior of particle suspensions and oxide films , by Dr. Ryan Larsen, Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana- Champaign Friday, 2/26/2010, 4:00 PM-5:00 AM
Soft glassy materials are important in a wide variety of applications and industries, including food science, construction, mining, waste management,pharmaceutical, and biotechnology. In many applications the balance between solid-like and liquid-like properties is essential to the function of the material. Many glassy materials exhibit yield stress behavior: they transition from solid-like to liquid-like behavior as the stress is increased. However, in some glassy materials, stress induces the opposite behavior, causing materials to become more solid-like. This unusual behavior is not confined to a single type of glassy material. I will present case studies of shear-induced solid-like behavior in two completely different glassy materials: particle suspensions and the oxide films of liquid metals. In particle suspensions, the solid-like behavior is associated with a jamming transition, which can be characterized with an apparent non-linear elasticity. In oxide metals, the solid-like behavior is associated with shear-induced wrinkling of the oxide surface. Despite the differences in the mechanisms responsible for these effects, both materials share an important characteristic: deformation induces internal stresses that persist even after the stress is applied. These results suggest that the engineering of favorable flow properties requires control of the strain energy that is stored in glassy materials while they are flowing. Refreshments will be served in Olin Hall 118 at 3:30 P.M. Sponsored by: WPI Physics Department, Dr. Erkan Tuzel
Powered by the Social Web - Bringing people together through Events, Places, & Common Interests