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Relation between Structure and Dynamics in Glass-Formation

When cooled too rapidly, liquids fail to crystallize and fall into glassy states. Both liquids and glasses are amorphous, and structural differences between them are hard to observe. However, in a liquid the mobility of particles is dramatically larger than in the glass. Thus, although overall the structure is amorphous, in glasses there are local structures that inhibit the movement of particles inside them. One way to characterize these structures is to consider the minimum number of particles that need to move before each particle can move. In discrete kinetically-constrained models on a lattice, this structural property, the mean... Read more

Combinatorial Design of Textured Mechanical Metamaterials

[source] We show how to assemble an astronomical number of distinct smart materials that each recognize and morph into a unique shape. To design these, we stack copies of one oriented building block, and crack the three-dimensional jigsaw puzzle that governs the placement of these blocks. We illustrate our strategy by 3D printing a metacube, which reveals a smiley pattern when compressed, and which can recognize other patterns. These novel materials bridge the gap between matter and machine, and may radically change the way in which materials can mechanically actuate and sense information, in e.g., prosthetics, wearable tech and robots.... Read more

Nonlinear Elasticity in the Interaction of Living Cells with their Mechanical Environment

[source] The elastic cytoskeleton of biological cells contains molecular motors that produce mechanical forces by which cells attach to and pull on their surroundings. This mechanical interaction is responsible for many aspects of cellular function, from cell spreading and proliferation to stem-cell differentiation and tissue development. Both the cytoskeleton and the extracellular matrix comprise cross-linked, semi-flexible polymeric filaments, and as such they exhibit very nonlinear viscoelastic behavior that includes a power-law stiffening of the elastic moduli with increasing stress. Read more

Box Shape Determines Jamming in Confined Geometries

An important aspect of dense particulate matter is jamming. As the particle density increases, some particles become stuck because others block their movement. The jamming transition from a state in which almost all particles can move to a state in which almost none can move is currently attracting much interest. We theoretically investigate the effects of confinement and in particular of the system's shape on this jamming transition in granular matter and glasses, whereas previous work in the field concentrated on the behavior of particles in square or cubic systems, unconfined systems or even infinite systems. Read more

Thermodynamic Analogies in Granular Materials

[source] We used molecular dynamics simulations to investigate energy distributions and spatial clustering in granular gases. Scaling arguments and mean-field calculations we performed for the granular temperature in these systems helped us gain insight on the nature of energy flow in more general driven dissipative systems. We then introduced a solvable stochastic model for dissipative interactions in generic systems, including granular materials, foams, colloidal suspensions and bacterial baths. Read more

Non-Equilibrium Statistical Mechanics of Dividing Cell Populations

[source] We suggested a model describing the dynamics of protein distributions in a proliferating cell population, motivated by chemostasis experiments on yeast in steady-state growth. Protein variation in our model is affected by a stochastic source internal to the cells and variation in division and inheritance at the population level, enabling us to assess the contribution and character of each of these components separately. We drew an analogy between the dynamics of protein distributions along cell generations and that of stress in layers of granular material. Read more

Identifying the Fingerprints of Molecular Motors in the Active Fluctuations of the Red Blood Cell Membrane

[source] The mechanical rigidity of cells has an important effect on their biological function, especially for red-blood cells (RBC) that have to be flexible and resilient at the same time, in order to pass unharmed through narrow capillaries in the body. The metabolic activity of the RBC controls its rigidity, and generates strong undulations of the cell membrane. The microscopic properties of the molecular motors that generate the RBC membrane activity are poorly understood. Read more

Deformable Triangular-Lattice Antiferromagnet Models Buckled Colloidal Monolayers

[source] Disorder in glassy systems usually originates from geometric frustration, which arises when competing interactions may not be satisfied simultaneously. In dense arrangements of spheres, each quadruplet of particles can close-pack to form a tetrahedron. However, it is geometrically impossible to fill space with tetrahedra. For this reason particulate systems often fall into metastable disordered states (which minimize energy only locally) and fail to find the crystalline ground state (which minimizes the energy globally but not locally). Read more

Attraction Controls the Inversion of Order by Disorder in Buckled Colloidal Monolayers

[source] Geometrically frustrated systems cannot satisfy all local constraints, and thus, they remain disordered down to zero temperature, with a highly degenerate ground state. If entropy of fluctuations about each ground-state configuration slightly varies, then the configuration with the highest entropy will be thermodynamically selected in an order by disorder effect. When the packing density of a buckled colloidal monolayer approaches close packing, the free-volume-dominated free energy of the system causes neighboring spheres to prefer touching opposite walls, giving rise to effective antiferromagnetic interactions. Read more

Shape Regulation and Elastic Interactions between Living Cells

[source] Live cells exert contractile forces, which result in elastic deformations of their environment. Cells in live tissue measure the resultant forces and displacements created on their surfaces by neighboring cells and in response correct their own forces in some preprogrammed way. To analytically calculate the interaction energy between live cells we model them as spheres surrounded by infinite material with linear elastic behavior. Each sphere in our model creates a radial deformation on its surface. If each sphere generates an isotropic displacement, the interaction energy vanishes. Thus to model real cells Read more

 

Yair Shokef | School of Mechanical Engineering, Tel Aviv University, Tel Aviv 69978, Israel | Office: Wolfson 334 | Phone: +972-3-640-8393 | Email: This email address is being protected from spambots. You need JavaScript enabled to view it.