Fabrication of Hydrogel-Based Composite Fibers and Computer Simulation of the Filler Dynamics in the Composite Flow.

Fibrous structures with anisotropic fillers as composites have found increasing interest in the field of biofabrication since they can mimic the extracellular matrix of anisotropic tissues such as skeletal muscle or nerve tissue. In the present work, the inclusion of anisotropic fillers in hydrogel-based filaments with an interpenetrating polymeric network (IPN) was evaluated and the dynamics of such fillers in the composite flow were analyzed using computational simulations. In the experimental part, microfabricated rods (200 and 400 µm length, 50 µm width) were used as anisotropic fillers in extrusion of composite filaments using two techniques of wet spinning and 3D printing. Hydrogels such as oxidized alginate (ADA) and methacrylated gelatin (GelMA) were used as matrices. In the computational simulation, a combination of computational fluid dynamics and coarse-grained molecular dynamics was used to study the dynamics of rod-like fillers in the flow field of a syringe. It showed that, during the extrusion process, microrods are far from being well aligned. Instead, many of them tumble on their way through the needle leading to a random orientation in the fiber which was confirmed experimentally.

Demixing and ordering in Ni(Ti,Zr)(Sb,Sn) half-Heusler materials.

We employed density functional theory, Monte Carlo simulations and a mean field model to study phase separation in thermoelectric Ni(Ti,Zr)(Sb,Sn) half-Heusler materials, simultaneously alloyed in the (Ti,Zr)- and (Sb,Sn) sublattices. We found that the material shows demixing and ordering phenomena as the temperature is lowered. Below a critical temperature, which depends on the overall stoichiometry, demixing occurs within the sublattices. Typically, a strong demixing in the (Ti,Zr) sublattice is accompanied by a weaker demixing in the (Sb,Sn) sublattice stoichiometry. In the coexistence region, the Sb concentration in the Ti-rich phase is higher than that in the Zr-rich phase. For a Sn : Sb ratio of 1 : 3, we find a second phase separation in the Sb/Sn sublattice at about 200 K. Here, a phase without Sn coexists with a phase that has a 1 : 1 ratio of Sb and Sn. In the latter phase Sb and Sn alternate within the sublattice forming a highly ordered structure. The results provide new insights into the pattern formation in Ni(Ti,Zr)(Sb,Sn) which helps in creating nano-structuring strategies to improve the figure of merit in this class of thermoelectric materials.

Analyzing spinodal decomposition of an anisotropic fluid mixture.

Spinodal decomposition leads to spontaneous fluctuations of the local concentration. In the early stage, the resulting pattern provides explicit information about the material properties of the mixture. In the case of two isotropic fluids, the static structure factor shows the characteristic ring shape. If one component is a liquid crystal, the pattern is typically anisotropic and the structure factor is more complex. Using numerical methods, we investigate how structure factors can be used to extract information about material properties like the diffusion constant or the isotropic and the anisotropic contributions to the interfacial tension. The method is based on momenta taken from structure factors in the early stage of the spinodal demixing. We perform phase field calculations for an isotropic and an anisotropic spinodal decomposition. A comparison of the extracted results with analytic values is made. The calculations show that linear modes dominate in the beginning of the growth process, while non-linear modes grow monotonously in the region of the k-space for which damping is predicted by the linearized theory. As long as non-linear modes are small enough, linearized theory can be applied to extract material properties from the structure factor.

Molecular dynamics study of colloidal quasicrystals.

Colloidal quasicrystals have received increased interest recently due to new insight in exploring their potential for photonic materials as well as for optical devices [Vardeny et al., Nat. Photonics, 2013, 7, 177]. Colloidal quasicrystals in aqueous solutions have been found in systems of micelles with impenetrable cores [Fischer et al., Proc. Natl. Acad. Sci. U. S. A., 2011, 108, 1810]. A simple model potential for micelle-micelle interaction is the step potential, which is infinite for core overlaps and constant for shell overlaps. Dotera et al. performed Monte Carlo simulations of the step potential model and found quasicrystals for specific values of the packing fraction η and the shell-core ratio λ [Dotera et al., Nature, 2014, 506, 208 ]. However, the overlap of real micelles causes repulsive forces, which increase with decreasing core distance. We consider this by introducing a novel model potential with repulsive forces depending on a third parameter α. In a systematic manner we study this more realistic potential with two-dimensional molecular dynamics simulations. For α = 0 the model is similar to the step potential model. For the first time, we provide a comprehensive overview of crystalline, quasicrystalline, and disordered structures as a function of η and λ. Simulations performed with α > 0 show the impact of the repulsive forces. We find that quasicrystalline structures at high densities vanish while new quasicrystalline structures appear at intermediate densities. Our results help to tailor colloidal systems for today's advanced applications in photonics and optical devices.

Mean field theory for a reversibly crosslinked polymer network.

We present a mean field theory for melts and solutions of reversibly crosslinked polymers. In our model, crosslinks are considered as local bonds between two monomers. For a blend of A+B+AB polymers, we assume reversible crosslinks between the copolymers AB with a crosslink strength z and interaction weights ω(A) and ω(B) for monomers of type A and B, respectively. The usual mean field model for polymer blends without reversible crosslinks is recovered if z vanishes. With or without crosslinks, the A+B+AB blend can form a lamellar phase with A and B rich regions. If reversible crosslinks are enabled and ω(A) differs strongly from ω(B), the lamellar nanophase separation of A and B monomers is accompanied by a similar segregation of crosslinked and noncrosslinked polymers. If ω(A) and ω(B) are equal, crosslinked copolymers are well mixed with the homopolymers. For a homopolymer solution with reversible crosslinks between the polymers, our calculations show that polymers and solvent molecules are separated macroscopically if the Flory-Huggins interaction parameter and the crosslink strength are suitably high or if the volume fraction of polymers or the chain length are suitably low.

Quaternary Heusler compounds Co(2-x)Rh(x)MnZ (Z = Ga, Sn, Sb): crystal structure, electronic structure, and magnetic properties.

Within the huge family of Heusler compounds only a few quaternary derivatives are known that crystallize in the F43m space group. In this work, the yet unreported compounds CoRhMnZ (Z = Ga, Sn, Sb) and the alloy Co(0.5)Rh(1.5)MnSb were investigated in detail by experimental techniques and theoretical methods. The ab initio calculations predict the CoRhMnZ compounds to be half-metallic ferromagnets or to be close to the half-metallic ferromagnetic state. Calculations of the elastic constants show that the cubic structure is stable in compounds containing Mn. Both calculations and experiment reveal that Mn cannot be exchanged by Fe (CoRhFeGa). The low temperature magnetization of the compounds is in the range of 3.4-5.5 µ(B) depending on the composition. The best agreement between experiment and calculation has been achieved for CoRhMnSn (5 µ(B)). The other compounds are also cubic but tend to anti-site disorder. Compared to Co(2)MnSn it is interesting to note that the magnetic properties and half-metallicity are preserved when replacing one of the 'magnetic' Co atoms by a 'non-magnetic' Rh atom. This allows us to increase the spin-orbit interaction at one of the lattice sites while keeping the properties as a precondition for applications and physical effects relying on a large spin-orbit interaction. The Curie temperatures were determined from measurements in induction fields of up to 1 T by applying molecular field fits respecting the applied field. The highest Curie temperature was found for CoRhMnSn (620 K) that makes it, together with the other well defined properties, attractive for above room temperature spintronic applications.

Indium-gallium segregation in CuIn(x)Ga(1-x)Se2: an ab initio-based Monte Carlo study.

Thin-film solar cells with CuIn(x)Ga(1-x)Se2 (CIGS) absorber are still far below their efficiency limit, although lab cells already reach 20.1%. One important aspect is the homogeneity of the alloy. Large-scale simulations combining Monte Carlo and density functional calculations show that two phases coexist in thermal equilibrium below room temperature. Only at higher temperatures, CIGS becomes more and more a homogeneous alloy. A larger degree of inhomogeneity for Ga-rich CIGS persists over a wide temperature range, which contributes to the observed low efficiency of Ga-rich CIGS solar cells.

Novel method for measuring the adhesion energy of vesicles.

Adhering vesicles with osmotically stabilized volume are studied with Monte Carlo simulations and optical microscopy. The simulations are used to determine the dependence of the adhesion area on the vesicle volume, the surface area, the bending rigidity, the adhesion energy per membrane area, and the adhesion potential range. The simulation results lead to a simple functional expression that is supplemented by a correction term for gravity effects. The obtained equation provides a new tool to analyze optical microscopy data and, thus, to measure the adhesion energy per area by analyzing the geometry of the adhering vesicle. The method can be applied in the weak and ultra-weak adhesion regime, where the adhesion energy per area is below 10(-6) J/m(2). By comparing the shapes of adhering vesicles with different reduced volumes, the bending rigidity can be estimated as well. The new approach is applied to experimental data for lipid vesicles on (i) an untreated and (ii) a monolayer-coated glass surface, providing ultra-weak and weak adhesion strength, respectively.

Structure formation and fractionation in systems of polydisperse attractive rods.

Fractionation effects and the formation of structured domains are investigated in polydisperse systems of attractive spherocylinders with the help of Monte Carlo simulations. For sufficiently high attractive interaction strength and pressure, the large rods in the system aggregate and form a highly ordered hexatic monolayer that coexists with an isotropic fluid of smaller rods. Fractionation diminishes with decreasing interaction strength but is still observed for hard rod systems, in which the large rods form a nematic droplet rather than a monolayer. Results for polydisperse systems are accompanied by phase diagrams for monodisperse systems of attractive spherocylinders. Here, the phase behavior is shown as a function of rod length and pressure.

Free fluid vesicles are not exactly spherical.

At finite temperature, vesicles perform small fluctuations around an average shape. In the limit of low temperature or high bending rigidity, the fluctuations vanish and the vesicle approaches the energetically favored configuration. In the absence of a volume constraint the configuration of lowest energy is a perfect sphere. It is often assumed that the spherical shape is also the most probable shape for finite temperatures. Consequently, a force would have to be applied to make the average shape of the vesicle anisotropic. In this article it is shown that these assumptions are incorrect. At finite temperature, the most probable shapes of a vesicle without volume constraint are prolate or oblate, where the probability for prolate shapes is slightly larger. For larger deviations from the sphere the vesicle behaves as expected. The behavior at small deformations that is found for vesicles without volume constraint as well as in the presence of a finite osmotic pressure is basically an entropic effect. It already occurs in a three-dimensional crossed dumbbells model system. In two dimensions the same model favors the isotropic state.

Temperature dependence of vesicle adhesion.

The influence of thermal fluctuations on the adhesion behavior of fluid vesicles is investigated with the help of Monte Carlo simulations. The adhesion area A(ad) of a fluid vesicle adhering to a smooth attractive substrate is studied systematically for different values of temperature, adhesion strength, and potential range. For low temperatures T , the ratio A(ad) /A between the adhesion area and the total area A of the vesicle is a linear function of T/kappa , where kappa is the bending rigidity. Linear fits of the simulation data allow an extrapolation to T=0 which corresponds well with data obtained from a simplified analytic model. A new ansatz for A(ad) (T) which is based on the eigenmodes of the adhering vesicle explains the linear behavior of A(ad) (T) for low T and helps to define a fit function which reproduces the linear behavior of the obtained simulation data. This fit function may be used in order to determine the bending rigidity and the adhesion strength from the observed adhesion geometry.