Unraveling dispersion and buoyancy dynamics around radial A + B → C reaction fronts: microgravity experiments and numerical simulations.

Radial Reaction-Diffusion-Advection (RDA) fronts for A + B → C reactions find wide applications in many natural and technological processes. In liquid solutions, their dynamics can be perturbed by buoyancy-driven convection due to concentration gradients across the front. In this context, we conducted microgravity experiments aboard a sounding rocket, in order to disentangle dispersion and buoyancy effects in such fronts. We studied experimentally the dynamics due to the radial injection of A in B at a constant flow rate, in absence of gravity. We compared the obtained results with numerical simulations using either radial one- (1D) or two-dimensional (2D) models. We showed that gravitational acceleration significantly distorts the RDA dynamics on ground, even if the vertical dimension of the reactor and density gradients are small. We further quantified the importance of such buoyant phenomena. Finally, we showed that 1D numerical models with radial symmetry fail to predict the dynamics of RDA fronts in thicker geometries, while 2D radial models are necessary to accurately describe RDA dynamics where Taylor-Aris dispersion is significant.

Population Mass Balance Model for Precipitation with Turbidity Measurements.

The discretized population balance theory has been proven to be a useful method to simulate systems in which solid particles are present. In this work, we introduce a new approach to model precipitation reactions based on the temporal evolution of product concentration, from which particle size distribution, its dynamics, and the specific interfacial energies can be obtained. For a reference study, the previously investigated calcium oxalate precipitation was selected, where the reaction was followed via turbidity measurement. From the obtained particle size distribution, we can show that at low supersaturation, growth is the dominant process, while at higher supersaturation, nucleation is the dominant process. Moreover, the temporal change of the distribution curve has allowed us to split the precipitation into a nucleation, a growth-driven intermediate, and a saturation regime. Furthermore, the comparison between the experimental and calculated results has proved that the method is suitable for predicting particle size distributions and specific interfacial energies.

Rotational Mode Specificity in the F- + CH3I(v = 0, JK) SN2 and Proton-Transfer Reactions.

Quasiclassical trajectory computations are performed for the F- + CH3I(v = 0, JK) → I- + CH3F (SN2) and HF + CH2I- (proton-transfer) reactions considering initial rotational states characterized by J = {0, 2, 4, 6, 8, 12, and 16} and K = {0 and J} in the 1-30 kcal/mol collision energy (Ecoll) range. Tumbling rotation (K = 0) counteracts orientation effects, thereby hindering the SN2 reactivity by about 15% for J = 16 in the 1-15 kcal/mol Ecoll range and has a negligible effect on proton transfer. Spinning about the C-I bond (K = J), which is 21 times faster than tumbling, makes the reactions more direct, inhibiting the SN2 reactivity by 25% in some cases, whereas significantly enhancing the proton-transfer channel by a factor of 2 at Ecoll = 15 kcal/mol due to the fact that the spinning-induced centrifugal force hinders complex formation by breaking H-bonds and activates C-H bond cleavage, thereby promoting proton abstraction on the expense of substitution. At higher Ecoll, as the reactions become more direct, the rotational effects are diminishing.

The impact of reaction rate on the formation of flow-driven confined precipitate patterns.

The production of solid materials via chemical reactions is abundant both in nature and in industrial processes. Precipitation reactions coupled with transport phenomena lead to enhanced product properties not observed in the traditional well-stirred systems. Herein, we present a flow-driven pattern formation upon radial injection in a confined geometry for various chemical systems to show how reaction kinetics modifies the emerging precipitation patterns. It is found that chemically similar elements, such as alkaline earth or transition metals react on very different time scales under the same experimental conditions. The patterns are quantified and compared both with literature results obtained in unconfined solution layers and with hydrodynamic simulations.

Fine tuning of pattern selection in the cadmium-hydroxide-system.

Controlling self-organization in precipitation reactions has received growing attention in the efforts of engineering highly ordered spatial structures. Experiments have been successful in regulating the band patterns of the Liesegang phenomenon on various scales. Herein, we show that by adjusting the composition of the hydrogel medium, we can switch the final pattern between the classical band structure and the rare precipitate spots with hexagonal symmetry. The accompanying modeling study reveals that besides the modification of gel property, tuning of the time scale of diffusional spreading of hydroxide ions with respect to that of the phase separation drives the mode selection between one-dimensional band and two-dimensional spot patterns.