Department of Physics
University of Oregon

Solid mechanics is an old but highly underdeveloped field. After decades of research, we still have no understanding of instabilities in crack propagation comparable to theories of pattern formation in crystal growth. Similarly, the leading experts in solid mechanics assert that strain hardening in dislocation-mediated plasticity is an intractable problem, more difficult even than turbulence. As an heretical counterexample to this assertion, I will outline a thermodynamic theory of dislocations based on the assumption that any such complex, externally driven system must be moving toward states of increasing probability. In other words, some form of the second law of thermodynamics must be applicable. The relevant thermodynamic theory, however, must be couched in terms of an effective disorder temperature, and not just the ordinary kinetic temperature. With only a small number of physics-based parameters, this theory fits experimental data for Cu for temperatures ranging from 300K to about one third of the melting point, and for fifteen decades of strain rate. It also provides an accurate account of strain hardening.

An overview is given on recent progress in the study of avalanches in a number of experimental systems and in models having disorder. In particular, we show how recent studies on magnetizing avalanches Barkhausen noise in magnets (driven by a slowly increasing magnetic field) shed light on modeling damage avalanches in stressed materials, the plastic depinning of charge density waves, the statistics of earthquakes in irregularly shaped fault zones, and other systems characterized by crackling noise. In particular, we focus on the universal, i.e., detail independent, effects of disorder in these cases.Unexpected connections between nonequilibrium and equilibrium avalanches reveal a surprisingly large universality class of systems that all show the same scaling behavior on long length scales. This universality class includes driven far-from-equilibrium behavior (for various histories), and the equilibrium behavior of some of these systems. The studies draw on methods from the theory of phase transitions, the renormalization group, and numerical simulations.

The membranes that form the boundaries of every cell and every organelle inside every cell are remarkable materials – flexible, two-dimensional, self-assembled fluids. I'll describe two projects from my lab that explore the physical characteristics of membranes and illuminate their role in guiding biological function. One relates to the trafficking of cargo in cells, which involves dramatic changes in membrane shape whose physical underpinnings remain poorly understood. Measuring the stiffness of membranes by tugging on them with optical tweezers, we’ve found that a key trafficking protein can drastically alter membrane rigidity, suggesting a new mechanistic picture for intracellular transport. The other project relates to the fluidity of membranes – phenomenologically well-established yet fundamentally poorly characterized – which we probe by carefully examining the Brownian motion of membrane-anchored nanoparticles. Surprisingly, we have discovered that membranes are not simple “Newtonian” fluids, like water, but rather are viscoelastic – two-dimensional analogues of the entertaining grade-school staple of corn-starch and water. This finding impacts not only membrane biophysics but also our basic understanding of low-dimensional fluid states. I'll also very briefly describe some of my group’s other projects involving soft and biological materials, at scales spanning molecules and whole organisms.

NANOGrav is a consortium of radio astronomers and gravitational wave physicists whose goal is to detect gravitational waves using an array of millisecond pulsars as clocks. Whereas interferometric gravitational wave experiments use lasers to create the long arms of the detector, NANOGrav uses earth-pulsar pairs. The limits that pulsar timing places on the energy density of gravitational waves in the universe are on the brink of limiting models of galaxy formation and have already placed limits on the tension of cosmic strings. Pulsar timing has traditionally focused on stochastic sources, but most recently I have been investigating the idea of detecting individual gravitational wave bursts wherein there are some interesting advantages. I will also demonstrate how the array can be used to reconstruct the waveform and obtain its direction.

It is well established that nanodevices, especially those based on nanowires, can provide tremendous improvements in performance and functionality compared to larger devices. The assembly techniques required to produce these devices are, however, either expensive, or require time-consuming and complex manipulation of nanoscale building blocks. Hence “self-assembly” techniques are potentially very attractive.

Over the last few years my group has focused on self-assembly of atomic clusters to form contacted electronic devices. Methods for assembly of clusters into nanowires will be described, including those based on 1) percolation, 2) silicon V-groove templates, and 3) polymer-patterned surfaces. I will describe some of the devices fabricated using these techniques (gas sensors, transistors and interconnects) as well as some of the fundamental science behind them (percolation theory, bouncing of nanoparticles).

Steady progress in cooling and trapping technology has enabled the realization of degenerate alkali atomic gases. In this setting of ultra-cold temperatures, despite their extreme diluteness, these trapped gases exhibit a rich behavior akin to that found in conventional condensed matter systems, albeit in very different regimes and probed with very different techniques. Starting with a demonstration of Bose-Einstein condensation in bosonic atoms and Fermi sea in fermionic ones, the fields has seen an explosion of activity and discoveries ranging from vortex lattices to resonant paired superfluidity. I will present an overview of the milestones in this burgeoning field,discussing recent developments and speculating about future ones.