research

Rui Huang and Se Hyuk Im, Physical Review E 74, 026214 (2006).

A stressed thin film on a soft substrate can develop complex wrinkle patterns. The onset of wrinkling and initial growth is well described by a linear perturbation analysis, and the equilibrium wrinkles can be analyzed using an energy approach. In between, the wrinkle pattern undergoes a coarsening process with a peculiar dynamics. By using a proper scaling and two-dimensional numerical simulations, this paper develops a quantitative understanding of the wrinkling dynamics from initial growth through coarsening till equilibrium. It is found that, during the initial growth, a stress-dependent wavelength is selected and the wrinkle amplitude grows exponentially over time. During coarsening, both the wrinkle wavelength and amplitude increases, following a simple scaling law under uniaxial compression. Slightly different dynamics is observed under equi-biaxial stresses, which starts with a faster coarsening rate before asymptotically approaching the same scaling under uniaxial stresses. At equilibrium, a parallel stripe pattern is obtained under uniaxial stresses and a labyrinth pattern under equi-biaxial stresses. Both have the same wavelength, independent of the initial stress. On the other hand, the wrinkle amplitude depends on the initial stress state, which is higher under an equi-biaxial stress than that under a uniaxial stress of the same magnitude.

See an accompanying powerpoint presentation: The aged-rejuvenation-glue-liquid (ARGL) shear band model has been proposed for bulk metallic glasses (Acta Mater. 54 (2006) 4293), based on small-scale molecular dynamics simulations and thermomechanical analysis. The model predicts the existence of a critical lengthscale ~100 nm and timescale ~100 ps, above which melting occurs in shear-alienated glass. Large-scale molecular dynamics simulations with up to 5 million atoms have directly verified these predictions. When the applied stress exceeds the glue traction (computed separately before), we indeed observe maturation of the shear band embryo into bona fide shear crack, accompanied by melting.

The deadline of October 1, 2006 for my program of Mechanics & Structures of Materials was inadvertently omitted in our website. However, at the beginning of our CMS home page there are 2 deadlines listed for all programs. In the meantime any unsolicited proposals for my program, please put in GPG 04-23 as the Program Announcement [1st box]. In the 2nd box put in my program name [Mechanics & Structures of Materials].

A recent discussion here about the effect of surface stress on vibrations of microcantilever has gained some interest from our members. A few years ago, Zhigang and I looked at surface effect on buckling of a thin elastic film on a viscous layer (Huang and Suo, Thin Solid Films 429, 273-281, 2003). Although the physical phenomena (buckling vs vibrations) are different, the conclusion is quite consistent with Wei Hong and Pradeep's comments toward the end of the discussion. That is, surface stress only contributes as a residual stress and thus does not affect the buckling wavelength (frequency in space in analogy to frequency in time for vibrations).

It is well established that the growth of microscopic voids near a crack tip plays a fundamental role in establishing the fracture behavior of ductile metals. Mechanics analyses of plastic void growth have typically assumed the plastic properties of the surrounding metal to be isotropic. However voids are typically of the order of magnitude of one micron so that they exist within individual grains of the metal, or along grain boundaries, at least at the initial growth stage. For that reason, the plastic properties of the material surrounding the void are most properly treated as being anisotropic, rather than isotropic.

In the uploaded preprint, the stress state and deformation state are derived around a cylindrical void in a hexagonal close packed single crystal. The orientation of the cylindrical void and the loading state relative to the crystal are chosen so that the deformation state is one of plane strain. The active slip systems reduce to a total of three slip systems which act within the plane of plane strain. The solution shows that the deformation state consists of angular sectors around the void within which only one slip system is active. Further, it is shown that the stress state and deformation state exhibit self-similarity both radially and circumferentially, as well as periodicity along certain logarithmic spirals which emanate from the void surface.

A possible explanation of the variety of fingerprints comes from the consideration of the mechanics of tissue growth. Formation of fingerprints can be a result of the surface buckling of the growing skin. Remarkably, the surface bifurcation enjoys infinite multiplicity. The latter can be a reason for the variety of fingerprints. Tissue morphogenesis with the surface buckling mechanism and the growth theory underlying this mechanism are presented in the attached notes.

Computational models of the cell nucleus, along with experimental observations, can help in understanding the biomechanics of force-induced nuclear deformation and mechanisms of stress transition throughout the nucleus. Here, we develop a computational model for an isolated nucleus undergoing indentation, which includes separate components representing the nucleoplasm and the nuclear envelope. The nuclear envelope itself is composed of three separate layers: two thin elastic layers representing the inner and outer nuclear membranes and one thicker layer representing the nuclear lamina. The proposed model is capable of separating the structural role of major nuclear components in the force-induced biological response of the nucleus (and ultimately the cell). A systematic analysis is carried out to explore the role of major individual nuclear elements, namely inner and outer membranes, nuclear lamina, and nucleoplasm, as well as the loading and experimental factors such as indentation rate and probe angle, on the biomechanical response of an isolated nucleus in atomic force microscopy indentation experiment.

Microcantilevers have taken much attention as devices for label-free detection of molecules and/or their conformations in solutions and air. Recently, microcantilevers have allowed the nanomechanical mass detection of thin film [1-3], small molecules [4, 5], and biological components such as viruses [6] and vesicles [7] in the order of a pico-gram to a zepto-gram. The great potential of microcantilevers is the sensitive, reliable, fast label-free detection of proteins and/or protein conformations. Specifically, microcantilevers are capable of label-free detection of marker proteins related to diseases, even at a low concentration in solution [8-17]. Microcantilevers, operated in a viscous fluid, have also enabled the real-time monitoring of protein-protein interactions [8, 12-15]. Furthermore, microcantilevers are able to recognize the specific protein conformations [18] and/or reversible conformation changes of proteins/polymers [19, 20].