Feedback mechanisms between precipitation and dissolution reactions across randomly heterogeneous conductivity fields

Our study investigates interplays between dissolution, precipitation, and transport processes taking place across randomly heterogeneous conductivity domains and the ensuing spatial distribution of preferential pathways. We do so by relying on a collection of computational analyses of reactive transport performed in two-dimensional systems where the (natural) logarithm of conductivity is characterized by various degrees of spatial heterogeneity. Our results document that precipitation and dissolution jointly take place in the system, with the latter mainly occurring along preferential flow paths associated with the conductivity field and the former being observed at locations close to and clearly separated from these. High conductivity values associated with the preferential flow paths tend to further increase in time, giving rise to a self-sustained feedback between transport and reaction processes. The clear separation between regions where dissolution or precipitation takes place is imprinted onto the sample distributions of conductivity which tend to become visibly left skewed with time (with the appearance of a bimodal behavior at some times). The link between conductivity changes and reaction-driven processes promotes the emergence of non-Fickian effective transport features. The latter can be captured through a continuous-time random-walk model where solute travel times are approximated with a truncated power law probability distribution. The parameters of such a model shift towards values associated with increasingly high non-Fickian effective transport behavior as time progresses.

Entrainment and deposition of boulders in a gravel bed river

Rivers transports coarse sediment (gravel, cobbles, or boulder) as bedload. During a flood, when the discharge is high enough, the sediment grains move by rolling and bouncing on the river bed. Measuring bedload transport in the field is notoriously difficult. Here, we propose a new method to characterize bedload transport by floods. Using a drone equipped with a high resolution camera, we recorded yearly images of a bar of the Vieux-Habitants river, a gravel-bed river located on Basse-Terre Island (Guadeloupe, French West Indies). These images, combined with high frequency measurements of the river discharge, allow us to monitor the evolution of the population of boulders on the river bed. Based on this dataset, we estimate the smallest discharge that can move the boulders, and calculate the effective transport time of the river. We find that transport occurs about 10 hours per year. When plotted as a function of this effective transport time, likelihood of a given boulder remaining at the same location decreases exponentially, with an effective residence time of 17 hours. We then propose a rough estimate of the average number of boulders that the river carries every year.

Driving a Microswimmer with Wall-Induced Flow

Active walls such as cilia and bacteria carpets generate background flows that can influence the trajectories of microswimmers moving nearby. Recent advances in artificial magnetic cilia carpets offer the potentiality to use a similar wall-generated background flow to steer bio-hybrid microrobots. In this paper, we provide some ground theoretical and numerical work assessing the viability of this novel means of swimmer guidance by setting up a simple model of a spherical swimmer in an oscillatory flow and analysing it from the control theory viewpoint. We show a property of local controllability around the reference free trajectories and investigate the bang–bang structure of the control for time-optimal trajectories, with an estimation of the minimal time for suitable objectives. By direct simulation, we have demonstrated that the wall actuation can improve the wall-following transport by nearly 50%, which can be interpreted by synchronous flow structure. Although an open-loop control with a periodic bang–bang actuation loses some robustness and effectiveness, a feedback control is found to improve its robustness and effective transport, even with hydrodynamic wall-swimmer interactions. The results shed light on the potentialities of flow control and open the way to future experiments on swimmer guidance.

RESEARCH OF EFFECTIVE TRANSPORT AND LOGISTICS LINKS IN THE ORGANIZATION OF INTERNATIONAL MULTIMODAL TRANSPORT MINERAL FERTILIZERS

Modern agriculture cannot do without the use of mineral and organic fertilizers. Mineral fertilizers are produced by large holdings both in Russia and abroad, and Russian enterprises have subsidiaries distributed throughout the world. The latter factor makes mineral fertilizers one of the most export-oriented branches of the chemical industry in Russia. According to official statistics in 2019 Russian agricultural companies supplied more than 21 million tons of mineral fertilizers to foreign markets, which determines the peculiarities of the country's market. The purpose of the study is to determine the optimal assignment of consumers to suppliers of mineral fertilizers in the organization of multimodal transportation. The article considers the period of the beginning of the reorientation of the production of mineral fertilizers for export. The issue of creating domestic ports specialized in the transshipment of fertilizers has been studied. The volumes of production and consumption of specific countries and regions that are included in export-import operations with each other are analyzed. Routes of transportation of fertilizers with determination of the shortest distances are studied. The optimization of the existing route option for the delivery of nitrogen and potash fertilizers to Brazil is proposed.

The structural basis of intraflagellar transport at a glance

ABSTRACT The intraflagellar transport (IFT) system is a remarkable molecular machine used by cells to assemble and maintain the cilium, a long organelle extending from eukaryotic cells that gives rise to motility, sensing and signaling. IFT plays a critical role in building the cilium by shuttling structural components and signaling receptors between the ciliary base and tip. To provide effective transport, IFT-A and IFT-B adaptor protein complexes assemble into highly repetitive polymers, called IFT trains, that are powered by the motors kinesin-2 and IFT-dynein to move bidirectionally along the microtubules. This dynamic system must be precisely regulated to shuttle different cargo proteins between the ciliary tip and base. In this Cell Science at a Glance article and the accompanying poster, we discuss the current structural and mechanistic understanding of IFT trains and how they function as macromolecular machines to assemble the structure of the cilium.

Determination of Intrinsic Drug Dissolution and Solute Effective Transport Rate during Laminar Fluid Flow at Different Velocities

The objective of this study was to determine the intrinsic drug dissolution rate (IDR) and the solute effective transport rate of some drugs, using a single particle dissolution technique, satisfying qualified dissolution conditions. The IDR of three poorly water-soluble compounds was measured in milli-Q water using four different fluid velocities. The enveloped surface area of the particles was calculated from the projected area and the perimeter of the particle observed in the microscope. Furthermore, computational fluid dynamics (CFD) simulations were used to theoretically investigate the flow conditions and dissolution rate, comparing box shaped particles and spherical particles with similar dimensions and surface area as the particles used the experiments. In this study, the IDR measurement of the single particles was determined within 5–60 min using particles with an initial projected area diameter (Dp) between 37.5–104.6 µm. The micropipette-assisted microscopy technique showed a good reproducibility between individual measurements, and the CFD simulations indicated a laminar flow around the particles at all flow velocities, even though there were evident differences in local particle dissolution rates. In conclusion, the IDR and solute effective transport rate were determined under well-defined fluid flow conditions. This type of approach can be used as a complementary approach to traditional dissolution studies to gain in-depth insights into the dissolution process of drug particles.

Feedback mechanisms between precipitation and dissolution reactions across randomly heterogeneous conductivity fields

Our study investigates interplays between dissolution, precipitation, and transport processes taking place across randomly heterogeneous conductivity domains and the ensuing spatial distribution of preferential pathways. We do so by relying on a collection of computational analyses of reactive transport performed in two-dimensional systems where the (natural) logarithm of conductivity is characterized by various degrees of spatial heterogeneity. Our results document that precipitation and dissolution jointly take place in the system, the latter mainly occurring along preferential flowpaths associated with the conductivity field, the former being observed at locations close to and clearly separated from these. High conductivity values associated with the preferential flowpaths tend to further increase in time, giving rise to a self-sustained feedback between transport and reaction processes. The clear separation between regions where dissolution or precipitation takes place is imprinted onto the sample distributions of conductivity which tend to become visibly left skewed with time (with the appearance of a bimodal behavior at some times). The link between conductivity changes and reaction-driven processes promotes the emergence of non-Fickian effective transport features. The latter can be captured through a continuous time random walk model where solute travel times are approximated with a truncated power law probability distribution. The parameters of such a model shift towards values associated with increasingly high non-Fickian effective transport behavior as time progresses.