MATERIALS RESEARCH SCIENCE AND ENGINEERING CENTER
Theoretical Physics of Biopolymers
Charged colloids - Emulsions - Granular materials
Yield stress. All solids will flow and behave as fluids if a sufficiently large shear stress is applied, a problem of considerable practical interest. There has been a growing realization that near the yielding transition collective effects are important not only in crystals with defects, but in fully amorphous solids as well. Acoustic emission in
crystals supports the idea that plasticity proceeds via avalanches of dislocation rearrangements that are power-law distributed in size. Recent numerical investigations indicate that avalanches occur in
amorphous solids as well. Although no defects can be defined, there are locations in the sample, so-called shear transformation zones,
where the system is about to fail. Such local failures perturb the stress in the sample, and can trigger new plastic events that organize into elongated microfractures. Rheological properties are controlled by the spatiotemporal organization of these avalanches. Understanding their organization and dynamics remain a challenge for both crystals and amorphous solids.
Optically responsive materials - Rheotaxis- Active assembly
Crystals typically form under the influence of inter-particle potentials and thermal motion. A whole new set of questions arises when order emerges in a non-equilibrium system of self-propelled active particles. IRG1 investigators will focus on colloidal swimmers that exhibit some behaviors common to flocking systems, such as birds, fish, or bacteria, which brings to the fore a new kind of material that is dynamic, with reconfigurable properties.
Dry granular materials - Cycling dislocations and work hardening in 2D - Correlated disordered materials
The initial discovery of random organization was for neutrally buoyant non-Brownian particles dispersed in a highly viscous liquid periodically
sheared at low Reynolds number. The underlying equations are time reversible, which means that an absorbing state, where the system ceases to explore new microstates, can be obtained when the particles
organize in such a way that there are no interparticle contacts or collisions. IRG 1 will investigate random organization in an entirely new context: granular systems that are dominated by gravity and that are not fluidized, so that particles are always in contact. Under these conditions, the potential and frictional forces between particles make the underlying equations decidedly not time reversible.
IRG1: Random Organization of Disordered Materials
Senior Investigators and Research Projects
From powders to fluids- From fluids to more
visocus and back
Beyond cyclic shear, the IRG 1 team will investigate the self-organizing mechanisms that arise when a continuous shear strain is applied to amorphous materials. A remarkable example occurs in many dense suspensions, where the system suddenly self-organizes into highly dissipative states as the strain rate is increased past a threshold. The viscosity can jump by several orders of magnitude, a phenomenon called discontinuous shear thickening (DST). While such rheological properties are of considerable importance for the processing and properties of foods (e.g. chocolate), ceramics, and cements, the microscopic origin of DST is still highly debated.
Nadrian C. Seeman
Structural DNA Nanotechnology