Apparent hysteresis in a driven system with self-organized drag

Interaction between extended defects and impurities lies at the heart of many physical phenomena in materials science. Here we revisit the ubiquitous problem of the driven motion of an extended defect in a field of mobile impurities, which self-organize to cause drag on the defect. Under a wide range of external conditions (e.g. drive), the defect undergoes a transition from slow to fast motion. This transition is commonly hysteretic: the defect either moves slow or fast, depending on the initial condition. We explore such hysteresis via a kinetic Monte Carlo spin simulation combined with computational coarse-graining. Obtaining bifurcation diagrams (stable and unstable branches), we map behavior regimes in parameter space. Estimating fast-slow switching times, we determine whether a simulation or experiment will exhibit hysteresis depending on observation conditions. We believe our approach is applicable to quantifying hysteresis in a wide range of physical contexts.Comment: 11 pages (preprint format), 4 color figures in separate file

Crossover Scaling of Wavelength Selection in Directional Solidification of Binary Alloys

We simulate dendritic growth in directional solidification in dilute binary alloys using a phase-field model solved with an adaptive-mesh refinement. The spacing of primary branches is examined for a range of thermal gradients and alloy compositions and is found to undergo a maximum as a function of pulling velocity, in agreement with experimental observations. We demonstrate that wavelength selection is unambiguously described by a non-trivial crossover scaling function from the emergence of cellular growth to the onset of dendritic fingers, a result validated using published experimental data.Comment: 4 pages, four figures, submitted to Physical Review Letter

Dynamics of Spreading of Chainlike Molecules with Asymmetric Surface Interactions

In this work we study the spreading dynamics of tiny liquid droplets on solid surfaces in the case where the ends of the molecules feel different interactions with respect to the surface. We consider a simple model of dimers and short chainlike molecules that cannot form chemical bonds with the surface. We use constant temperature Molecular Dynamics techniques to examine in detail the microscopic structure of the time dependent precursor film. We find that in some cases it can exhibit a high degree of local order that can persist even for flexible chains. Our model also reproduces the experimentally observed early and late-time spreading regimes where the radius of the film grows proportional to the square root of time. The ratios of the associated transport coefficients are in good overall agreement with experiments. Our density profiles are also in good agreement with measurements on the spreading of molecules on hydrophobic surfaces.Comment: 12 pages, LaTeX with APS macros, 21 figures available by contacting [email protected], to appear in Phys. Rev.

Hierarchical Self-Assembly of Halogen-Bonded Block Copolymer Complexes into Upright Cylindrical Domains

Self-assembly of block copolymers into well-defined, ordered arrangements of chemically distinct domains is a reliable strategy for preparing tailored nanostructures. Microphase separation results from the system, minimizing repulsive interactions between dissimilar blocks and maximizing attractive interactions between similar blocks. Supramolecular methods have also achieved this separation by introducing small-molecule additives binding specifically to one block by noncovalent interactions. Here, we use halogen bonding as a supramolecular tool that directs the hierarchical self-assembly of low-molecular-weight perfluorinated molecules and diblock copolymers. Microphase separation results in a lamellar-within-cylindrical arrangement and promotes upright cylindrical alignment in films upon rapid casting and without further annealing. Such cylindrical domains with internal lamellar self-assemblies can be cleaved by solvent treatment of bulk films, resulting in separated and segmented cylindrical micelles stabilized by halogen-bond-based supramolecular crosslinks. These features, alongside the reversible nature of halogen bonding, provide a robust modular approach for nanofabricatio

Modeling Elasticity in Crystal Growth

A new model of crystal growth is presented that describes the phenomena on atomic length and diffusive time scales. The former incorporates elastic and plastic deformation in a natural manner, and the latter enables access to times scales much larger than conventional atomic methods. The model is shown to be consistent with the predictions of Read and Shockley for grain boundary energy, and Matthews and Blakeslee for misfit dislocations in epitaxial growth.Comment: 4 pages, 10 figure

Molecular ordering of precursor films during spreading of tiny liquid droplets

In this work we address a novel feature of spreading dynamics of tiny liquid droplets on solid surfaces, namely the case where the ends of the molecules feel different interactions to the surface. We consider a simple model of dimers and short chain--like molecules which cannot form chemical bonds with the surface. We study the spreading dynamics by Molecular Dynamics techniques. In particular, we examine the microscopic structure of the time--dependent precursor film and find that in some cases it can exhibit a high degree of local order. This order persists even for flexible chains. Our results suggest the possibility of extracting information about molecular interactions from the structure of the precursor film.Comment: 4 pages, revtex, no figures, complete file available from ftp://rock.helsinki.fi/pub/preprints/tft/ or at http://www.physics.helsinki.fi/tft/tft_preprints.html (to appear in Phys. Rev. E Rapid Comm.

Deciduous wood chips as bedding material: Estimation of dust yield, water absorption and microbiological comparison

Dust yield, water absorbing capacity and airborne microbes of laboratory animal beddings made of deciduous wood were compared. Three different bedding materials, aspen, alder and birch, two chip sizes and effects of a softening treatment were studied. Overall dust yield from Chips was relatively low. Aspen should, however, be preferred to alder, because it was less dusty. Small (1 X 2 X 1 mm) aspen Chips yielded less inhalable dust than the large chips (4 X 4 X 1 mm). Water absorbing capacity was better in small chips than in large chips and could also be increased with a softening procedure. In this study softening of small aspen chips resulted in unacceptable high dust yield. Though alder had a better water absorbing capacity, aspen seems to soak up urine well enough during a normal change cycle

Applying individual level data on children’s care periods to microsimulation models

Paper presented at the 4th General Conference of the International Microsimulation Association, 11–13 December 2013, Australi