Active Jammed Systems
May 26-28 2012, New York City, USA

May 26, Saturday (NYAS)
Driven Granular Matter Chair: Paul Chaikin   (jointly held with the ICAM Annual Meeting)
8:45 -9:40am George Whitesides
"Dynamic self-assembly"
Complexity, emergence, and dynamical self-assembly are related, and controversial, subjects. Many important systems-from the living cell to power distribution systems to the weather-are complex, and the importance of complex systems-systems of components interacting non-linearly-is unarguable. It is, however, unclear if there is a science of complexity, or if different types of complex systems share common characteristics. Complex systems may show also show so-called emergent (that is, "new" or simply, perhaps, unexpected) behavior. This seminar will illustrate some of the issues in complexity using studies in "synthetic complexity"-that is, studies of systems of multiple components interacting with one another by relatively simple rules. Designing these systems is a challenge in its own right, but once designed, they are both illuminating in their relevance to complexity, and stimulating in their propensity to show unexpected behaviors. The talk will emphasize mm-scale objects interacting electrostatically, but will also discuss other systems.
9:40-10:00am Coffee Break
10:00-10:40am Olivier Pouliquen
"Rheology of very dense suspensions"
The rheology of suspensions of rigid particles close to the jamming transition is experimentally investigated using a non conventional rheological method inspired by recent studies in dry granular media. A shear cell has been developed, in which the granular pressure is controlled, the volume fraction being free to adjust. Using this new setup, we show that suspensions can be described by a visco-plastic frictional rheology, providing a link with the rheology of dry granular media. This configuration also circumvents the divergences observed in volume fraction imposed rheology at the jamming transition and provides precise measurements of the constitutive equations close to the jamming transition. The link between this point of view and the classical empirical models proposed for suspensions will be discussed.
10:40-11:20am Bruno Andreotti
"Sand ripples and dunes"
An erodible bed sheared by a fluid flow, gas or liquid, is generally unstable, and bedforms grow. The following questions are discussed, in the light of the recent literature: What are the relevant dynamical mechanisms controling the emergence of bedforms? Do they form by linear instability or nonlinear processes like pattern-coarsening? What determines their time and length scales, so different in air and water? What are the similarities and differences between aeolian and subaqueous patterns? What is the influence of the mode of transport: bedload, saltation or suspension? Can bedforms emerge under any hydrodynamical regime, laminar and turbulent? What determines the maximum size of sand dunes? What is the link between river meanders and bedforms? Guided by these questions, a unified description of bedform growth and saturation is proposed, with emphasis on the hydrodynamical regime in the inner layer, the relaxation phenomena associated with particle transport and finite size effects.
11:20-12:00pm David Pine
"Reversibility in sheared suspensions"
We explore the connection between reversibility and shear-induced ordering in non- Brownian suspensions of spheres or rods that are subjected to periodic strain. We find a number of remarkable results. First, we find that the shear-induced random collisions that usually produce diffusive dynamics can also lead to a non-equilibrium phase transition from a fluctuating (diffusing) state to a self-organized quiescent state (no diffusion). We also measure a time-dependent elastic component in the viscoelastic response even in the absence of Brownian motion. Interestingly, the time-dependent rheological data exhibit a universal scaling that is associated with a well-known class of non-equilibrium phase transitions associated with directed percolation. In suspensions of rods, we find that oscillatory shear flow tends to align rods perpendicular to the flow along the vorticity direction rather than aligning the system along the flow direction as is more commonly observed in shear flows. The orientational degree of freedom associated with the rods produces a time dependent control parameter that can drive the system through the non-equilibrium phase transition and can thus delay the ultimate transition to a fluctuating state. The basic features of these different behaviors can be understood in terms of a simple model in which hydrodynamics do not play an essential role.
12:00-12:30pm ICAM Soft Matter Panel Discussion
12:30-2:00pm LUNCH: 121 Fulton Street (Menu)
  • Mimosas
  • Bloody Marys
  • Coffee

Entree Choices (pick 1):
  • Banana French Toast: Caramelized Bananas / Cinnamon and Sugar
  • Buttermilk Pancakes: Vermont Maple Syrup / Fresh Berries
  • Belgian Waffles: Caramelized Bananas / Fresh Cream
  • Scrambled Eggs with Potato Hash
  • Breakfast Burrito: Bacon, Egg & Cheese / Salsa / Sour Cream & Guacamole
  • Chopped Vegetable Salad: Red Onion / Avocado / Black Olives / Tomato / Cucumber / Corn / Feta / Hearts of Palm / Creamy Black Truffle Vinaigrette
Synchronized Swimming Chair: Leif Ristroph
2:00-2:40pm John Brady
"Chemical swimming"
The design of nanoengines that can convert stored chemical energy into motion is an important challenge of nanotechnology, especially for engines that can operate autonomously. Recent experiments have demonstrated that it is possible to power the motion of nanoscale and microscale objects by using surface catalytic reactions - so-called catalytic nanomotors. The precise mechanism(s) responsible for this motion is(are) still debated, although a number of ideas have been put forth. Here, a very simple mechanism is discussed: A surface chemical reaction creates local concentration gradients of the reactant (the fuel) and product species. As these species diffuse in an attempt to re-establish equilibrium, they entrain the motor causing it to move. This process can be viewed either as osmotic propulsion or as self-diffusiophoresis - or more figuratively as 'chemical swimming.' The concentration distributions are governed by the ratio of the surface reaction velocity to the diffusion velocity of the reactants and/or products. For slow reactions the reaction velocity determines the self-propulsion. When surface reaction dominates over diffusion the motor velocity cannot exceed the diffusive speed of the reactants. The implications of these features for different reactant concentrations and motor sizes are discussed and the predictions are compared with Brownian Dynamics simulations. We also show that chemically active particles can attract or repel each other through long-range 'Coulomb-like' interactions. And suspensions of active particles can exhibit Debye-like screening and phase behaviors analogous to those of a one-component plasma.
2:40-3:20pm Lydéric Bocquet
"The (non-) equilibrium structure of active colloidal suspensions"
Assemblies of self-propelling organisms, from bacteria to birds or fishes, generically exhibit a wide variety of collective behaviors, structures and patterns, such as flocks, swarms, schools, … The beauty of these patterns does not reflect the difficulty to understand these strongly out of equilibrium systems, which has been the object of an intense theoretical and numerical work over the last years. Following a different perspective, we explore the experimental behavior of this so-called 'active' matter on the basis of artificial (abiotic) swimmers, made of self-propelling colloids. In this talk, I will present various results obtained in our group in this context. While some of their behaviors can be understood on the basis of extensions of "equilibrium" statistical descriptions -- with the activity of the system described in terms of an effective temperature --, new phenomena are observed which depart from such equilibrium expectations. I will discuss in particular the observation of dynamic clustering occuring at moderate concentrations of the active suspension. This surprising behavior can be interpreted in the context a chemotactic aggregation scenario first introduced by Keller and Segel to account for bacterial aggregation, and accounting here for chemical interactions between colloidal swimmers. It suggests that chemical interactions between self-propelled colloids can mimic, on a purely physical basis, chemoattractivity and its consequences.
3:20-4:00pm Coffee Break
4:00-4:40pm M. Cristina Marchetti
"Jamming and phase separation of dense active matter"
Recent experiments on confluent layers of epithelial cells and dense vibrated granular media have motivated interest in the behavior of active systems at high density, where the interplay of steric repulsion and activity can yield active glassy and solid states. In this talk I will discuss the behavior of dense collections of self-propelled particles in two dimensions. Two specific results will be highlighted: (i) the suppression of self-propulsion due to steric repulsion yields phase separation of the dense active fluid in solid-like and gas phases in the absence of any aligning or attractive interaction; and (ii) confinement yields glassy active states where the dynamics is controlled by the low-frequency modes of the jammed packing.
  1. S. Henkes, Y. Fily and M. C. Marchetti, Phys. Rev.E 84, 040301(R) (2011).
  2. Y. Fily and M. C. Marchetti, to appear in Phys. Rev. Lett.
4:40-5:20pm Jun Zhang
"Interaction between moving structures in a fluid: drafting and inverted drafting"
Flying birds and swimming fish are familiar to everyone though their remarkable locomotion abilities remain poorly understood. Even less understood is their group behavior -- birds form flocks and fish form schools. Do they benefit energetically by structuring such groups? Are these groups formed due to physical interactions through the surrounding fluid, or simply by social or behavioral decisions?
Puzzled, and also inspired by these questions, we have performed some laboratory experiments on interaction between open flows and moving boundaries. The results from these experiments offer a few surprises that contest our commonsense. I will discuss the relevant physics and biological implication.
5:20-5:30pm Closing Remarks
5:30-7:00pm Poster Session / Welcome Reception
May 27, Sunday (NYU)
Dynamics Near Jamming Chair: Matthieu Wyart
9:00 -9:40am Sylvie Cohen-Addad
"Impact of disorder and surfactants on the relaxation dynamics of foams"
Aqueous foams are jammed packings of gas bubbles in a surfactant solution. For small applied shear, they behave like viscoelastic solids showing a rich spectrum of relaxation times reminiscent of soft glassy materials. Upon large shear or coarsening, bubble rearrangements are triggered and foams unjam. Their dynamics present many similarities with that of other soft sphere packings in a viscous fluid such as emulsions, pastes or even wet granular media, raising the question whether generic collective mechanisms may be involved. In addition, surfactant monolayers adsorbed on the liquid–gas interfaces confer to foams specific interfacial rheology that tunes the local dynamics at the film and the bubble scales. I will discuss experimental evidences of the coupling between surface rigidity and 3D foam relaxations and, present recent experiments with ordered foams that allow to delineate the impact of disorder and surfactant on relaxations. These findings help to outline the landscape of the generic analogies among soft sphere packings.
9:40-10:20am Doug Durian
"Unsteadiness and dynamical heterogeneities in dense granular materials"
The physics of granular flow is of widespread practical and fundamental interest, and is also important in geology and astrophysics. One challenge to understanding and controlling behavior is that the mechanical response is nonlinear, with a forcing threshold below which the medium is static and above which it flows freely. Furthermore, just above threshold the response may be intermittent even though the forcing is steady. Two familiar examples are avalanches on a heap and clogging in a silo. Another example is dynamical heterogeneities for systems brought close to jamming, where intermediate-time motion is correlated in the form of intermitted string-like swirls. This will be briefly reviewed in the context of glassy liquids and colloids, and more deeply illustrated with experiments on three different granular systems. This includes air-fluidized beads, where jamming is approached by density and airspeed; granular heap flow, where jamming is approached by depth from the free surface; and dense suspensions of NIPA beads, where jamming is approached by both density and shear rate. Emphasis will be given to measurement and analysus methods for quantifying heterogeneities, as well as to unified scaling of the size of heterogeneities with distance to jamming.
10:20-11:00am Coffee Break
11:00-11:40am Craig Maloney
"Diffusion in soft particle suspensions near jamming"
Suspensions of soft particles exhibit a remarkable bifurcation at the random close packing volume fraction, ϕc [1]. There is a yield stress above ϕc but not below. We perform numerical simulations of soft-particle suspensions under shear near ϕc. We find that above ϕc at sufficiently low shearing rates, the effective diffusion constant, Deff, scales with length of the simulation cell in the flow-gradient direction, Ly. This is in agreement with the size-dependent diffusion constant observed in low-temperature Lennard-Jones glasses. Furthermore, the value of Deff can be understood in terms of organized lines of slip and is independent of the form of the repulsive interactions between the particles and the precise way in which the viscous drag of the suspending fluid is modeled in the particle-scale simulations.
[1] Microfluidic Rheology of Soft Colloids above and below Jamming. K. N. Nordstrom, E. Verneuil, P. E. Arratia, A. Basu, Z. Zhang, A. G. Yodh, J. P. Gollub, and D. J. Durian. Phys. Rev. Lett. 105, 175701 (2010) -- Published October 21, 2010
11:40-12:20pm Elisabeth Bouchaud
"Bulk fingering instabilities in a soft solid and in a complex fluid"
Instabilities in viscous liquids confined in Hele-Shaw cells have attracted considerable attention during the past. On the contrary, instabilities arising in soft elastic materials have started to be studied much more recently. In most cases, fingering in confined elastomers arises at the interface with an elastic plate: Interfacial crack fronts lose their stability due to a competition between bulk and surface effects.
We present here a bulk fingering instability arising in a hyperelastic polyacrylamid gel.
Experiments are performed in two types of Hele-Shaw cells: One is the classical setup used for studying liquids, and the other one has mobile sides which can be pulled at a prescribed velocity. In both cases, an instability is observed when the strain exceeds a critical value which is independent of the gel shear modulus. By colouring the material, we were able to show that fingers grow within the thickness of the cell, leaving behind layers of gels sticking to the upper and lower glass plates. It is shown that the wavelength (width of the fingers) increases non linearly with the spacing of the latter. Finally, at a later stage, when these layers are stretched beyond the fracture threshold, an interfacial crack nucleates and propagates, at a strain which depends on the shear modulus.
Similar instability is observed in a Maxwell liquid constituted by oil-in-water droplet microemulsion where the drops are connected to each other by a telechelic polymer. In this case, the critical strain depends on the flow rate.
12:20-12:30pm Closing Remarks
12:30-2:00pm LUNCH: Le Pain Quotidien (Menu)
Box lunches:
  • Baguette sandwich: choice of Cheddar Cucumber, Chicken Mozzarella, Paris Ham or Turkey Avocado
  • Mesclun Salad
  • Taboule Salad
  • Cookie
  • Brownie
  • Lemonade
Active Dynamics of Cells Chair: Jasna Brujic
2:00-2:40pm Virgile Viasnoff
"Cell doublets and Janus particles"
Cell-cell adhesion is usually studied with cell plated on a rigid substrate. Cell-cell contacts consecutively form by random encounter of their filipodia or lamelipodia. Traction and tension forces exerted at the cell interface are thus hard to measure due to the dominant cell-substrate interaction. We designed an experimental system that allows to follow in real time with high spatial and time resolution the formation of a junction in a cell doublet in suspension. We will present the peculiar structure adopted by actin cytoskeletton and cadherins at the cell cell contact and we will show how the contact area react to force application. We will then discuss semiquantitatively how the cell doublet interaction is similar to the case of Janus emulsions leading to a possible quantitative description of the differential Adhesion Theory for cell sorting.
2:40-3:20pm Thomas Gregor
"Emergence of collective behavior in developing cell populations of social amoebae"
Collective dynamics are widely observed during development of multicellular bodies and emerge as a result of communication among individual cells via signaling molecules. Little is known experimentally of the fundamental features that describe how the highly nonlinear spatio-temporal dynamics at the single-cell level can give rise to coherent dynamics at the population level. Here we use a FRET-based sensor protein, combined with live-imaging, to monitor cytosolic levels of cAMP which serves as the messenger molecule in developing cells of social amoebae Dictyostelium discoideum to allow individual cells to aggregate to form fruiting bodies. Timelapse recordings of cell populations during the first 10 hours of development reveal the very onset of periodic spike-like signaling and sequential changes in the frequency at single cell level resolution. Collective cAMP oscillations in populations of cells under perfusion reveal a sharp phase transition between a decoupled state and collective behavior for a range of cell densities and dilution rates. These observations suggest that the intact population is able to drive itself to this transition spontaneously during development.
Focusing on how single cell dynamics influence, and give rise to, the behavior of the aggregate, we develop a simple model of the single cell response to time-dependent pulses of the extracellular signaling molecule cAMP, characterized by a particular type of excitable system. We then use this model to study collective multicellular dynamics mediated by diffusion coupling. We first consider the mean-field case where we find an intriguing ``dynamical quorum sensing'' transition in which all cells simultaneously transition from quiescent to oscillating across the phase boundary. Then we include spatial dynamics and study pattern formation, both with and without the cells capable of chemotactic response to signal gradients. Finally, we highlight how modification of single cells can alter the collective dynamics.
3:20-4:00pm Coffee Break
4:00-4:40pm Daniel Fletcher
"Coupling mechanics and biochemistry in filaments and membranes"
Cell shape changes require the coordinated assembly of proteins in the cytoplasm and on the membrane. In processes such as motility and endocytosis, cells must organize cytoskeletal filaments and membrane proteins to generate controlled forces for protrusion and invagination. While many of the essential proteins involved in motility and endocytosis are known, basic physical questions remain about the molecular mechanisms that control cell shape. How does spatial organization of membrane proteins affect their ability to generate force? And how does the presence of force on a filament affect assembly of cytoskeletal networks? This talk will describe recent in vitro reconstitution experiments that suggest physical boundary conditions are essential regulators of cell shape change.
4:40-5:20pm Cecile Sykes
"Mimicking systems of cell shape changes"
In order to unveil generic mechanisms of cell movements, we design stripped-down experimental systems that reproduce cellular behaviours in simplified conditions. Actin- based motility is mimicked using beads or oil droplets placed in an appropriate in vitro system that contains the actin machinery. Cortices of cells and their contractility are mimicked using liposomes covered with growing actin filaments that are straight or branched, in the presence of myosin. We will explain how the efficiency of contraction or motility depends on the concentrations of proteins, the length of the filaments, , and how they are attached to the liposome membrane. Moreover, the mechanics of bio-mimicking liposomes can be characterized using tube pulling experiment, and we will present an unexpected result, that membrane dynamics is greatly affected by membrane composition and how liposomes are prepared.
5:20-5:30pm Closing Remarks
7:00pm Banquet at Kafana
May 28, Monday (NYU)
Self-Organization in Complex Systems Chair: L. Mahadevan
9:00 -9:40am Tom Duke
"Lamellipod propulsion by a jammed mesh of growing actin filaments"
Many cells spread on surfaces and crawl across them by extending an actin-rich lamellipod – a thin section at the perimeter of the cell that is typically several micrometres broad, but less than 200nm high. The precise mechanism by which propulsive force is generated within the lamellipod is unknown. One popular model posits that protrusion operates by a Brownian ratchet mechanism, as actin filaments polymerize behind the fluctuating cell membrane and support its forward motion. I will present an alternative idea, in which hard-body interactions between growing actin filaments generate the majority of the propulsive force. Branched actin filaments are nucleated at the leading edge, where the Arp2/3 complex is activated by proximity to highly-curved membrane. As the filaments polymerize, they form a dense, glass-like gel and because longer filaments pack less efficiently than shorter ones, excluded volume effects cause the gel to expand. If some of the actin filaments are bound by adhesions to the surface, this expansion results in forward motion of the leading edge.
9:40-10:20am Yoel Forterre
"Dense granular flows as a self-activated process"
Athermal systems encompass a wide variety of disordered soft materials such as foams, granular media or emulsions. In all these materials, the media is composed of densely packed macroscopic particles such that thermal fluctuations are negligible, yielding peculiar flow behaviors intermediate between a solid and a liquid. Recently, the idea that mechanical fluctuations in athermal systems could play the role of a temperature and be a source of non-locality for the rheology have motivated a large number of studies. In this talk, we study how a shear band in a granular medium dramatically changes the mechanical behavior of the material further in the non-sheared region. To this end, we carry out a micro-rheology experiment, where a constant force F is applied to a small rod immersed outside the shear band. In the absence of a shear band, a critical force Fc is necessary to move the intruder. When a shear band exists, the intruder moves even for a force F less than the critical force Fc. We systematically study how the velocity V of the rod varies with Fc -F and with the distance to the shear band, and show that the behavior can be described by an Eyring-like process activated by stress fluctuations. This self-activated process could play an important role in the rheology of dense granular flows close to the jamming transition, as discussed in a simple model.
10:20-11:00am Coffee Break
11:00-11:40am Chase Broedersz
"Active stresses in fibrous networks near the edge of mechanical stability"
The mechanical properties of cells are regulated in part by internal stresses generated actively by molecular motors in the cytoskeleton. On a larger scale, collective motor activity allows the cell to contract the surrounding extracellular matrix, consisting also of biopolymer networks. Experiments show that such active contractility dramatically affects the networks' elasticity, both in reconstituted intracellular F-actin networks with myosin motors as well as in extracellular gels with contractile cells. We provide insight into this remarkable behavior with a model for the mechanics of contractile disordered networks of fibers with simple linear bending and stretching elasticity. We find that these networks exhibit a low-connectivity rigidity threshold governed by fiber-bending elasticity and a high-connectivity threshold that controls a crossover between bending and stretching dominated network elasticity. Owing to their low connectivity, typical biopolymer networks fall below this upper threshold and their mechanical stability thus relies on the fibers' bending rigidity. Furthermore, we show that motors can stabilize floppy networks and control the elasticity in an anomalous fashion close to the point of marginal stability by coupling to critical network fluctuations. Interestingly, in addition to the geometrical critical points controlled by network connectivity, the network can also be bought into a critical state by varying motor density.
11:40-12:20pm Andrea Cavagna
"Maximum entropy approach to bird flocks"
Interactions among neighboring birds in a flock cause an alignment of their flight directions. I will show that the minimally structured (maximum entropy) model consistent with these local correlations correctly predicts the propagation of order throughout entire flocks of starlings, with no free parameters. Such model is mathematically equivalent to the Heisenberg model of magnetism, and defines an "energy" for each configuration of flight directions in the flock. Comparing flocks of different densities, the range of interactions that contribute to the energy involves a fixed number of (topological) neighbors, rather than a fixed (metric) spatial range. I will finally show that a complete maximum entropy approach needs to go fully dynamical in order to capture the active matter aspects of flocking.
12:20-12:30pm Closing Remarks
12:30-2:00pm LUNCH: Apple Restaurant & Bom Bar (Menu)
Buffet Lunch Menu:
  • Vegetable Lasagna
  • Jasmine Rice
  • Sesame Chicken: Sautéed with onions & bell Peppers
  • Steamed Vegetables: Carrots, broccoli, cauliflower, zucchini, celery, squash, and kale
  • Fried Vietnamese Dumplings: Dumplings stuffed with ground pork, shrimps, coriander and onions. Served with fish sauce
Cellular Tissues as Active Jammed System Chair: Dov Levine
2:00-2:40pm Emmanuel Farge
"Mechanotransduction in early embryogenesis and evolution: mechanisms and origins of primary invagination and endo-mesoderm differentiation in early muti-cellular epithelia."
The modulation of developmental biochemical pathways by mechanical cues is a recently established feature of animal development, with, among other cases, critical involvement required in the triggering of mesoderm invagination or in vital anterior midgut differentiation at Drosophila embryo gastrulation. However, the role of mechanotransduction in evolution has been unexplored so far.
First we describe the role of mechanotransduction biochemical pathways controlling Myo-II behaviour in the coordination of the collective apical constriction of mesoderm cells that is necessary for mesoderm invagination. We suggest Myo-II mechanosensitivity involved during invagination at the origin of Haeckel Gastrae first organism emergence from multicellular colonies of cells.
Second, we present experimental data showing that a common mechanosensitive pathway involving the beta-catenin specifies mesodermal identity at gastrulation in both zebrafish and Drosophila. We find that mechanical strains developed by zebrafish epiboly and Drosophila mesoderm invagination trigger the phosphorylation of beta-catenin that impairs its interaction with the E-cadherin in adherens junctions into the future mesoderm. This leads to the release of b-catenin into the cytoplasm and nucleus, where it triggers and maintains, respectively, the expression of the zebrafish brachyury homologue notail and of Drosophila twist, both crucial transcription factors for early mesoderm differentiation and development.
The search of a common biochemical pathway leading to mesoderm formation across Bilateria has so far proved to be difficult. Here the conserved role of the beta-catenin mechanosensitive pathway in the establishment of mesoderm identity at such large evolutionary distances converges to mesoderm speciation as mechanically induced by gastrulation morphogenetic movements back to the last common Protostome-Deuterostome ancestor.
2:40-3:20pm Josef Kas
"Are fundamental changes in a cell's material properties necessary for tumor progression?"
With an increasing knowledge in tumor biology an overwhelming complexity becomes obvious which roots in the diversity of tumors and their heterogeneous molecular composition. Nevertheless in all solid tumors malignant neoplasia, i.e. uncontrolled growth, invasion of adjacent tissues, and metastasis, occurs. Recent results indicate that all three pathomechanisms require changes in the active and passive cellular biomechanics. Malignant transformation causes cell softening for small deformations which correlates with an increased rate of proliferation and faster cell migration. The tumor cell's ability to strain harden permits tumor growth against a rigid tissue environment. A highly mechanosensitive, enhanced cell contractility is a prerequisite that tumor cells can cross its tumor boundaries and that these cells can migrate through the extracellular matrix. Initial tumor growth is limited to the developmental compartments from which the tumor cells originate. The observation that compartmentalized cell growth is not merely found during development but throughout tumor progression does not only radically redefine how tumors have to be resected, it also has critical impact on how a tumor progresses and what the target cells must be when screening for new cytostatics. It is the cells that can cross compartment boundaries and thus are not restricted to local tumor growth that have to be fought by chemotherapy. Therefore, passive and active biomechanical behavior of tumor cells, cell jamming, cell demixing and surface tension-like cell boundary effects are investigated as key factors to stabilize or overcome compartment boundaries. Insights into changes of these properties during tumor progression may lead to selective treatments. Such drugs would not cure by killing cancer cells, but slow down tumor progression with only mild side effects and thus may be an option for older and frail patients.
3:20-3:40pm Coffee Break
3:40-4:20pm Lisa Manning
"Emergent mechanical behavior in embryonic tissues"
Biological tissues often behave like elastic solids on short time scales and fluids on long time scales. Different tissue types exhibit different characteristic macroscopic mechanical properties such as surface tension and viscosity, and cell rearrangements in developing animal tissues are often governed by these "material" properties. But individual cells are not equivalent to molecules in a fluid; cells resist shape changes, modulate adhesive contacts, and exert active tension on their neighbors in tightly packed, disordered structures. By exploiting analogies with foams and supercooled fluids, we develop two models for the emergent mechanical behavior in zebrafish tissues. The first "dynamic" model treats cells as individual units and introduces interactions between cells to capture intracellular degrees of freedom. We show that this minimal model, which is contains only three parameters and is carefully calibrated using experimental data, makes predictions for bulk structural and dynamical properties of tissues which we have quantitatively verified. It shows that dynamics of cells in the interior of a tissue are similar to the dynamics of molecules in supercooled fluids near a glass transition. A second "steady-state" model studies ensembles of mechanically stable "jammed" cell packings, and makes predictions for cell shapes that we have also verified experimentally. It also specifies how the collective property of surface tension emerges from properties of individual cells such as cell-cell adhesion and "cortical tension". Together, our results suggest that embryonic tissues are a strange viscoelastic "material": while the bulk properties are fairly generic, the surface properties are different from ordinary materials because individual cells at the surface polarize and change their shapes.
4:20-5:00pm L. Mahadevan
"Motion and stasis in cellular suspensions and self-propelled swarms"
I will discuss two problems, one motivated by disease, and the other by toys. (i) Sickle cell disease was the first disease to have its molecular cause identified - a single point mutation that renders hemoglobin susceptible to polymerization, and thence allow cells to change their stiffness and jam in capillaries; the resulting hemostasis is the pathophysiological precursor of all symptoms of the disease. I will tell you about our attempts to integrate experiments and theory to understand the biophysics of the disease, along with some implications for clinical medicine. (ii) The patterns generated by collective motion of self-propelled objects has led to a large, primarily theoretical, literature. I will discuss our experiments and theory to probe some of the questions that these studies raise, with an emphasis on the role of confinement and topography on the collective motion and stasis of bristlebots - toothbrushes driven by cell phone motors.
5:00-5:10pm Closing Remarks
5:10-6:00pm Wine & Cheese