A network of polymers can imbibe a large quantity of a solvent and swell, resulting in a gel.  The swelling process can be markedly influenced by a mechanical load and geometric constraint.  When the network, solvent, and mechanical load equilibrate, the gel usually swells by a field of inhomogeneous and anisotropic deformation.  We show that this field in the swollen gel is equivalent to that in a hyperelastic solid.  We implement this theory in the finite-element package, ABAQUS, and analyze examples of swelling-induced deformation, contact, and bifurcation.  Because commercial software like ABAQUS is widely available, this work may provide a powerful tool to study complex phenomena in gels.

Prof. Zhigang Suo and Prof. Frans Spaepen, of the Harvard School of Engineering and Applied Sciences, have just been elected to the National Academy of Engineering.  They are among 65 new members elected to the NAE in 2008.  Update:  Also elected this year is another mechanician, Robert Dodds, of the University of Illinois.

Hydrogels have enormous potential for making adaptive structures in response to diverse stimuli.  In a structure demonstrated recently, for example, nanoscale rods of silicon were embedded vertically in a swollen hydrogel, and the rods tilted by a large angle in response to a drying environment (Sidorenko, et al., Science 315, 487, 2007).  Here we describe a model to show that this behavior corresponds to a bifurcation at a critical humidity, analogous to a phase transition of the second kind.

   A large quantity of small molecules may migrate into a network of long polymers, causing the network to swell, forming an aggregate known as a polymeric gel.  This paper formulates a theory of the coupled mass transport and large deformation.

A (001) surface of silicon consists of terraces of two variants, which have an identical atomic structure, except for a 90° rotation. We formulate a model to evolve the terraces under the combined action of electric current and applied strain. The electric current motivates adatoms to diffuse by a wind force, while the applied strain motivates adatoms to diffuse by changing the concentration of adatoms in equilibrium with each step. To promote one variant of terraces over the other, the wind force acts on the anisotropy in diffusivity, and the applied strain acts on the anisotropy in surface stress. Our model reproduces experimental observations of stationary states, in which the relative width of the two variants becomes independent of time. Our model also predicts a new instability, in which a small change in experimental variables (e.g., the applied strain and the electric current) may cause a large change in the relative width of the two variants.

MECHANICAL AND AEROSPACE ENGINEERING

POSITION: The Department of Mechanical and Aerospace Engineering invites applications for one or more TENURE-TRACK or TENURED FACULTY POSITIONS at the Assistant, Associate or Full Professor levels. Successful candidates will be expected to teach undergraduate and graduate courses in Mechanical and Aerospace Engineering and to establish a vigorous extramurally funded research program.

QUALIFICATIONS: Ph.D. or equivalent degree.

RANK AND SALARY: Level of appointment commensurate with qualifications; salary based on published UC pay scales.

CLOSING DATE FOR APPLICATIONS: November 30, 2006.

Send detailed resume, personal statement summarizing teaching experience and research interests, leadership efforts and contributions to diversity, and names/addresses of 5 professional references to:

The Engineering Science and Mechanics Department at The Pennsylvania State University invites applications for a tenure-track faculty position in computational mechanics at the assistant professor level. Exceptional candidates at the associate or full professor level will also be considered. Candidates are sought with a foundation and research interests in mechanics across all scales from the molecular to the macroscopic, including expertise in: efficient massive and nonlinear computations; molecular and multiscale simulations; innovative and efficient approaches to nonlinear FEM for large deformations, inhomogeneities, and/or inclusions; problems with evolving microstructure such as phase transitions and damage evolution; massively parallel simulations of large systems of equations; novel numerical/empirical approaches to modeling multiscale constitutive behavior of composite, biological or otherwise novel material systems.

We propose a model of persistent step flow, emphasizing dominant kinetic processes and strain effects. Within this model, we construct a morphological phase diagram, delineating a regime of step flow from regimes of step bunching and island formation. In particular, we predict the existence of concurrent step bunching and island formation, a new growth mode that competes with step flow for phase space, and show that the deposition flux and temperature must be chosen within a window in order to achieve persistent step flow. The model rationalizes the diverse growth modes observed in pulsed laser deposition of SrRuO3 on SrTiO3

 Physical Review Letters 95, 095501 (2005)

A vicinal Si (001) surface may form stripes of terraces, separated by monatomic-layer-high steps of two kinds, SA and SB . As adatoms diffuse on the terraces and attach to or detach from the steps, the steps move. In equilibrium, the steps are equally spaced due to elastic interaction. During deposition, however, SA is less mobile than SB . We model the interplay between the elastic and kinetic effects that drives step motion, and show that during homoepitaxy all the steps may move in a steady state, such that alternating terraces have time-independent, but unequal, widths. The ratio between the widths of neighboring terraces is tunable by the deposition flux and substrate temperature. We study the stability of the steady state mode of growth using both linear perturbation analysis and numerical simulations. We elucidate the delicate roles played by the standard Ehrlich-Schwoebel (ES) barriers and inverse ES barriers in influencing growth stability in the complex system containing (SA+SB) step pairs.

Preprint available in the attachment.