COLIN DENNISTON

Teaching

Theoretical Condensed Matter Physics (AM 579a)
   

Research Interests

In microfluidic devices the volumes are small and surface forces at solid-fluid interfaces start to become comparable or even larger than bulk forces, such as pressure gradients. Interfacial effects in complex fluids are poorly understood and often frustrate designs based on macroscopic models.  We have constructed a quantitiative model of some of these effects and applied it to manipulate variations in interfacial properties, both spatially and temporally. In the example illustrated to the left (click on the figure to see the movie) the walls have different interactions with the two constituents of a miscible binary fluid. This coupled with a concentration gradient can drive fluid flows between the walls and rotate the green wheel in the middle of the channel.

As suggested by their name, liquid crystals, are fundamentally liquid and hence their hydrodynamics are important.  Shear flow can act like a non-equilibrium analogue to a magnetic field, inducing an isotropic to nematic transition.  Despite these interesting effects, difficulties in incorporating hydrodynamics have often resulted in their effects being ignored.  We have developed a modified lattice Boltzmann algorithm to simulate rigid rod suspensions which includes hydrodynamics.  Currently we are examining how hydrodynamics affects the phase diagram, such as in shear banding, and the phase ordering dynamics after quenches to different parts of the phase diagram.

The state of matter in dense granular flow is neither fluid nor solid.  Neighboring particles cage each other so that rearrangements are strongly suppressed.  Nevertheless, grains do vibrate strongly within their individual cages and, occasionally, are able to break free.  This results in an odd two-temperature picture, similar to that used to describe spin glasses.  Collisions within a cage are not isotropic and can transfer momentum and energy coherently, resulting in a network of collision chains.  The resulting model of the stress tensor is very similar to fixed principal axes models recently suggested for static granular material and suggests the origin of this structure may be found in the dynamics.

The frustrated XY model provides a framework to study two-dimensional arrays of Josephson junctions and superconducting wire networks.  A perpendicular magnetic field induces a finite density of circulating supercurrents, or vortices, within the array.  The interplay of two length scales - the mean separation of vortices and the period of the underlying physical array - gives rise to a wide variety of interesting physical phenomena.  Many of these effects influence the finite temperature superconducting phase transitions.  Recent experiments have measured the critical exponents of these transitions, opening the opportunity to do careful comparison of theory and experiment.  My Ph.D. thesis focused on the critical properties of the frustrated XY model at both commensurate and incommensurate fields and in the presence of disorder.

Movies

Publications

Collaborators
 

Mark Robbins, Johns Hopkins, Baltimore
Julia Yeomans, Theoretical Physics, Oxford
Chao Tang, NEC Research Institute, Princeton
Enzo Orlandini, INFM-Dipartimento di Fisica, Universita di Padova
Hao Li, University of California at San Francisco

People who have done experments directly related to my work
 

Paul Chaikin, Princeton University, Princeton
Sean Ling, Brown University, Providence
Shobo Bhattacharya, NEC Research Institute, Princeton
Steve Elston, University of Oxford, Oxford
Jerry Gollub, Haverford College, Haverford, PA
Narayanan Menon, University of Massachusetts, Amherst
Doug Durian, University of California, Los Angeles

NSERC Summer Student Projects
 

Nehal Al Tarhuni, Simulations of granular flow including some nice graphics and movies