Utilizing direct-initiation of thiols for photoinitiator-free stereolithographic 3D printing of mechanically stable scaffolds

The automated production of artificial biological structures for biomedical applications continues to gather interest. Different fields of science are combined to find solutions for the arising multidimensional problems. Additive manufacturing in combination with material science provides one solution for the biological issues around 3D cell culture and construction of living tissues. Here, we present the photoinitiator-free stereolithographic fabrication of thiol-ene polymers with microarchitectures in the range of tens of microns for scaffolds up to the millimeter scale. Scaffolds composed of cubic unit cells were designed using computer-aided design (CAD) and subsequently 3D printed with a custom-made laser stereolithography setup. The process parameters were determined step by step with increasing complexity and number of parameters. Gained insights were applied to the fabrication of 3D printed test specimens. The quality of the 3D printed parts was evaluated by measuring the porosity and optical microscopy images. Furthermore, the mechanical properties of the scaffold structures were characterized using compression testing and compared with the bulk material revealing a lower capacity to bear load but higher flexibility. In this study, we demonstrate the advantages of combining the high-precision, freeform fabrication of stereolithography with a biocompatible material for the fabrication of complex microarchitectures for biomedical applications

Effect of Impurities on the Initiation of the Methanol-to-Olefins Process: Kinetic Modeling Based on Ab Initio Rate Constants

The relevance of a selection of organic impurities for the initiation of the MTO process was quantified in a kinetic model comprising 107 elementary steps with ab initio computed reaction barriers (MP2:DFT). This model includes a representative part of the autocatalytic olefin cycle as well as a direct initiation mechanism starting from methanol through CO-mediated direct C–C bond formation. We find that the effect of different impurities on the olefin evolution varies with the type of impurity and their partial pressures. The reactivity of the considered impurities for initiating the olefin cycle increases in the order formaldehyde < di-methoxy methane < CO < methyl acetate < ethanol < ethene < propene. In our kinetic model, already extremely low quantities of impurities such as ethanol lead to faster initiation than through direct C–C bond formation which only matters in complete absence of impurities. Graphic Abstract

Детонация горючей газовой смеси при взаимодействии ударной волны с эллиптической областью тяжелого инертного газа

On the basis of the Euler equations, the interaction of a shock wave in a combustible gas with an elliptical bubble of an inert gas of increased density is numerically simulated within a plane two-dimensional formulation. The finite-volume Godunov-type method of the second order of approximation is applied. Gas combustion is modeled using the Korobeinikov-Levin two-stage kinetics. Various values of the Mach number of the incident wave and the elongation of the inert bubble are considered, and the refraction and focusing of the incident shock are described. Qualitatively different regimes of gas detonation initiation have been found, including direct initiation by a strong wave, ignition upon reflection of an average-intensity wave from the gas interface, and upon focusing of secondary shock waves at lower shock Mach numbers. The dependence of the ignition mode on the shock intensity and the shape of the bubble is determined.

Детонация горючего газового цилиндра при фокусировке падающей ударной волны

Two-dimensional interaction of a shock in air with elliptic area (two-dimensional gas bubble) filled with propane-oxygen mixture with addition of heavy gas is numerically studied using Euler’s equations. Propane combustion is modeled with one-stage Arrhenius kinetics. Three different ignition regimes are found: direct detonation initiation by sufficiently strong shock, detonation near the triple point formed during weaker shock refraction and detonation at the focusing point of even weaker shock. The latter regime is observed only for significantly elongated bubbles. Detonation initiation regime dependence on shock Mach number and bubble diameter ratio is determined. It is shown that due to bubble elongation, critical Mach number may be significantly lowered in comparison with direct initiation.

The role of multidimensional instabilities in direct initiation of gaseous detonations in free space

We numerically investigate the direct initiation of detonations driven by the propagation of a blast wave into a unconfined gaseous combustible mixture to study the role played by multidimensional instabilities in direct initiation of stable and unstable detonations. To this end, we first model the dynamics of unsteady propagation of detonation using the one-dimensional compressible Euler equations with a one-step chemical reaction model and cylindrical geometrical source terms. Subsequently, we use two-dimensional compressible Euler equations with just the chemical reaction source term to directly model cylindrical detonations. The one-dimensional results suggest that there are three regimes in the direct initiation for stable detonations, that the critical energy for mildly unstable detonations is not unique, and that highly unstable detonations are not self-sustainable. These phenomena agree well with one-dimensional theories and computations available in the literature. However, our two-dimensional results indicate that one-dimensional approaches are valid only for stable detonations. In mildly and highly unstable detonations, one-dimensional approaches break down because they cannot take the effects and interactions of multidimensional instabilities into account. In fact, instabilities generated in multidimensional settings yield the formation of strong transverse waves that, on one hand, increase the risk of failure of the detonation and, on the other hand, lead to the initiation of local over-driven detonations that enhance the overall self-sustainability of the global process. The competition between these two possible outcomes plays an important role in the direct initiation of detonations.

Evolution of detonation formation initiated by a spatially distributed, transient energy source

Detonations usually form through either direct initiation or deflagration-to-detonation transition (DDT). In this work, a detonation initiation process is introduced that shows attributes from each of these two processes. Energy is deposited into a finite volume of fluid in an amount of time that is similar to the acoustic time scale of the heated fluid volume. Two-dimensional simulations of the reactive Euler equations are used to solve for the evolving detonation initiation process. The results show behaviour similar to both direct initiation and DDT. Localized reaction transients are shown to be intimately related to the appearance of a detonation. Thermomechanical concepts are used to provide physical interpretations of the computational results in terms of the interaction between compressibility phenomena on the acoustic time scale and localized, spatially resolved, chemical energy addition on a heat-addition time scale.