Stress Fibers (red)

Biophysics and cell mechanics

Cells throughout our body constantly interact with their microenvironment. While biochemical communication has been extensively studied for a long time, the importance of mechanical interactions (i.e. cells' ability to apply, sense and respond to forces) has been recognized only recently. Precise mechanical conditions, from the subcellular level and up to the organ scale, are critical for tissue development, function, remodeling and healing. Nevertheless, understanding the precise nature of the mechanisms and processes underlying the response of living cells to mechanical cues - cellular mechanosensitivity - remains largely a basic open problem. 


Dynamic fracture

The ability of solids to withstand mechanical forces is one of their fundamental properties. When a solid is loaded externally, there reaches a point where its global energy can be reduced by breaking into pieces, i.e by creating new free surfaces instead of continuing to store mechanical energy. The major vehicle for these failure processes are cracks, which are non-equilibrium propagating dissipative structures. Cracks are "natural laboratories" for probing material behavior under extreme conditions as their tips concentrate stresses and strains that approach a mathematical singularity. Moreover, crack propagation involves many interacting time and length scales ranging from the linear elastic forcing on large scales to the strong non-linearities and dissipation on the small scales near the crack's tip.

Physics of sliding friction

The physics of frictional interfaces is central to a wide range of physical, biological, engineering and geophysical systems, ranging from crawling cells to earthquake faults. Yet, a basic understanding of the interfacial friction constitutive law and the spatiotemporal dynamics that emerge when two deformable macroscopic objects move one relative to the other is currently missing. In addition, novel laboratory and geophysical observations revealed new frictional phenomena, such as slow rupture, which are not yet well-understood. At a more fundamental level, frictional interfaces raise basic questions about strongly out-of-equilibrium physics and the roles played by lower dimensional objects in the macroscopic response of physical systems.

Irreversible plastic deformation

The ability of solids to deform irreversibly, i.e. plastically, is of enormous importance to human kind. Yet, we have a rather limited fundamental understanding of the physics of plastic deformation in amorphous and heavily dislocated solids. The ultimate theoretical challenge in these fields is to develop dynamic equations of motion - the analog of the Navier Stokes equations - for amorphous and heavily dislocated solids. Building on advances in understanding amorphous materials, through detailed experiments and computer simulations, we have recently extended the original Shear-Transformation-Zones (STZ) theory to conform with the internal-variable, effective-temperature non-equilibrium thermodynamics discussed above. Furthermore, we have recently explored the implications of this thermodynamic framework to strain hardening theory of heavily dislocated polycrystalline solids

Visualization of the composition of a computer simulated amorphous silicon in terms of solid-like (blue) and liquid-like (green) atomic environments

Nonequilibrium thermodynamics of driven glassy systems

Glassy systems are usually regarded as the paradigmatic examples of non-equilibrium systems because they exhibit an anomalously slow relaxation behavior and a broken time-translation invariance (ergodicity breaking). These systems are driven even further from equilibrium when deformed by externally applied forces. Under such conditions, standard equilibrium thermodynamics is inapplicable. Therefore, a basic theoretical question is whether there exists a generalized thermodynamic formalism for such systems.

Biomaterials and nano-composites

Biological nano-composites — such as bone, tooth and nacre — exhibit exceptional mechanical properties. In particular, they combine stiffness and fracture resistance that is not yet achieved by man-made composites of similar composition. Hence, much recent effort has been devoted to exploring the basic physical principles underlying the heterogeneous micro-structures and deformation mechanisms of these materials in order to provide guidelines for the development of novel synthetic nano-composites. This scientific strategy is known as biomimetics, where design principles identified in well-adapted biological materials — which evolved over enormously long time scales — are mimicked in man-made materials