Dynamic Concentration Gradients Research Articles

Investigation of interfacial mass transfer phenomena applying non-invasive Raman imaging and density gradient theory

This study presents a novel non-invasive Raman measurement setup for investigating diffusion and mass transfer processes at liquid/liquid interfaces, based on a photometric setup utilizing customized photon multiplier (CPMs). The results highlight the effectiveness, reliability and applicability of this setup in visualizing and analyzing acetone mass transfer across the aqueous-organic interface. The study focuses on the liquid/liquid extraction of acetone from the aqueous phase into the organic phase, using toluene as the organic solvent. The study's key findings indicate that – in the absence of forced convection – mass transfer across the interface is not the limiting factor in the extraction process; instead, diffusion in the bulk phase takes precedence. Acetone's rapid local transport from the aqueous to the organic phase is observed, with a temporary concentration minimum on the aqueous side of the interface. In the organic phase, local equilibrium is reached near the interface first, only after which acetone disperses into the bulk phase. This dual mechanism—fast interfacial mass transfer and slower bulk-phase diffusion—marks a significant feature of the considered extraction process, which were further substantiated by dynamic concentration gradient theory simulations. The research further demonstrates the feasibility of upscaling interfacial mass transfer simulations using adaptive meshing. However, macroscopic effects like induced convection challenge purely diffusive models. The simulations confirm rapid enrichment of acetone at the interface, followed by slower mass transfer through the interface and bulk phase diffusion, consistent with the experimental data. Experimental and modeled results align well with literature data for liquid/liquid extraction, thereby validating the accuracy of the presented novel diffusion measurement setup and demonstrating its potential to investigate mass transfer phenomena near liquid/liquid interfaces.

Study on mass transfer in reactive liquid-liquid systems

This work examines the mass transfer in reactive liquid-liquid systems applying the dynamic concentration gradient theory (CGT). The CGT is based on a square gradient approach delivering an expression for the Gibbs energy of an inhomogeneous system as basis for the mass transfer in a two-phase system. For this work, the CGT was combined for the first time with a reaction kinetics model to model the reactive mass transfer. The model was validated by experiments. As model reaction the reversible diacetone alcohol formation in a water-toluene system was chosen. To parameterize the Gibbs excess energy model, phase equilibria were measured and furthermore, the predictive power of the model to calculate interfacial tension in equilibrium was also checked by own experiments. To analyze the mass transfer in liquid-liquid systems the model prediction was compared to experimental data from Nitsch-cell experiments. It was found that the model predictions are in excellent agreement to the forward reaction predictions as well as to the backward reaction.

Effective kinetics driven by dynamic concentration gradients under coupled transport and reaction

Biogeochemical reaction kinetics are generally established from batch reactors where concentrations are uniform. In natural systems, many biogeochemical processes are characterized by spatially and temporally variable concentration gradients that often occur at scales which are not resolved by field measurements or biogeochemical and reactive transport models. Yet, it is not clear how these sub-scale chemical gradients affect reaction kinetics compared to batch kinetics. Here we investigate this question by studying the paradigmatic case of localized pulses of solute reacting with a solid or a dissolved species in excess. We consider non-linear biogeochemical reactions, representative of mineral dissolution, adsorption and redox reactions, which we quantify using simplified power-law kinetics. The combined effect of diffusion and reaction leads to effective kinetics that differ quantitatively and qualitatively from the batch kinetics. Depending on the nonlinearity (reaction order) of the local kinetics, these effects lead to either enhancement or decrease of the overall reaction rate, and result in a rich variety of reaction dynamics. We derive analytical results for the effective kinetics, which are validated by comparison to direct numerical simulations for a broad range of Damköhler numbers and reaction order. Our findings provide new insights into the interpretation of imperfectly mixed lab experiments, the effective kinetics of field systems characterized by intermittent reactant release and the integration of sub-scale concentration gradients in reactive transport models.

The Dynamic Concentration Gradient of the Min System Coordinates with Cell Growth and Contributes to Cell Size

Pole-to-pole oscillation of the Min system mediates the cell division site placement that underlies the spatial and temporal organization of the Min proteins. This study reports dynamic modulation of the concentration gradient as the cell elongates, which is coupled with a near constant oscillation period under a given growth condition. The changing concentration gradient leads to gradual clearance of MinD at midcell, causing progressive relief from the inhibitory effect of the Min system on the assembly of the FtsZ ring. This concentration gradient differs among cells growing with different carbon sources or concentrations, which is in synchrony with the daughter cell sizes after division. The carbon stress responses of the MinD concentration gradient involve regulation by the sigma factor RpoS. Taken together, the results showed the plasticity of Min oscillation over the cell cycle that gradually sets up the assembly of the FtsZ ring.

Navigation of Silver/Carbon Nanoantennas in Organic Fluids Explored by a Two-Wave Mixing.

Within this work are analyzed third-order nonlinear optical properties with a potential influence on the dynamic mechanics exhibited by metal/carbon nanofluids. The nanofluids were integrated by multiwall carbon nanotubes decorated with Ag nanoparticles suspended in ethanol or in acetone. Optical third-order nonlinearities were experimentally explored by vectorial two-wave mixing experiments with a Nd-YAG laser system emitting nanosecond pulses at a 532 nm wavelength. An optically induced birefringence in the metal/organic samples seems to be responsible for a significant modification in density and compressibility modulus in the nanosystems. The measured nonlinear refractive index was associated with a thermal process together with changes in density, compressibility modulus and speed of sound in the samples. Nanofluid diffusivity was studied to characterize the dynamic concentration gradients related to the precipitation of nanostructures in the liquid solutions. The evolution of the nanoparticle density suspended in the nanofluids was considered as a temporal-resolved probabilistic system. It is stated that the incorporation of Ag nanoparticles in carbon nanotubes produces strong mechanical changes in carbon-based nanofluids. According to numerical simulations and optical evaluations, immediate applications for developing dynamic nanoantennas optical logic gates and quantum-controlled metal/carbon systems can be contemplated.

Stepwise construction of dynamic microscale concentration gradients around hydrogel-encapsulated cells in a microfluidic perfusion culture device.

Inside living organisms, concentration gradients dynamically change over time as biological processes progress. Therefore, methods to construct dynamic microscale concentration gradients in a spatially controlled manner are needed to provide more realistic research environments. Here, we report a novel method for the construction of dynamic microscale concentration gradients in a stepwise manner around cells in micropatterned hydrogel. In our method, cells are encapsulated in a photodegradable hydrogel formed inside a microfluidic perfusion culture device, and perfusion microchannels are then fabricated in the hydrogel by micropatterned photodegradation. The cells in the micropatterned hydrogel can then be cultured by perfusing culture medium through the fabricated microchannels. By using this method, we demonstrate the simultaneous construction of two dynamic concentration gradients, which allowed us to expose the cells encapsulated in the hydrogel to a dynamic microenvironment.

Study of Dynamic Concentration Gradient on Mass Transfer Coefficient: New Approach to Mobile–Immobile Modeling

The theory of mobile–immobile partitioning to capture a medium’s heterogeneity for simulating the interaction of contaminant mass between these two regions is still limited to the lump value of mass transfer coefficient (MTC) that fails to capture the long tails of breakthrough curves (BTCs). For a time-dependent solute source, BTCs consists of two parts, for example, rising and falling limbs. During the rising part, the concentration in the mobile region is higher and mass transfer occurs from the mobile to immobile region. However, during falling limb concentration in the immobile region have higher values, resulting in the reverse diffusive mass transfer process. This study focuses on overcoming the reported limitations of the mobile–immobile model (MIM) in the prediction of long tails of BTC during the falling limb. To achieve this objective, we propose an approach that is based on the dynamics of time resident concentration and its gradient between hydraulically coupled mobile and immobile regions. In this modified MIM, we estimated two distinct diffusive MTCs for rising and falling limbs (RFMT) of BTCs using a nonlinear least square optimization algorithm. Two experimental data sets available in the literature were simulated using a numerical solution of the proposed model and asymptotic time-dependent dispersion function. The estimated parameters supported the hypothesis that for pulse type input, liquid phase transport during the rising limb of BTCs is governed by advection and dispersion, whereas during the falling limb it is majorly diffusive dominated that can be represented by the new MTCs. Simulated results of RFMT are then compared with continuous-time random walk (CTRW) and constant mass transfer (CMT) approaches to compare the quality of the simulation. A better overall simulation of experimental BTCs was obtained using RFMT in comparison with other models. Sensitivity analysis is also carried out to evaluate the capabilities of RFMT over the rising and falling portions of BTCs. This theory finds its application in quantifying persistent chemical residuals in the immobile region that acts as a source when purging and subsequently helps when designing appropriate cleansing operations.

Molecular architecture of the DNA-binding sites of the P-loop ATPases MipZ and ParA from Caulobacter crescentus

The spatiotemporal regulation of chromosome segregation and cell division in Caulobacter crescentus is mediated by two different P-loop ATPases, ParA and MipZ. Both of these proteins form dynamic concentration gradients that control the positioning of regulatory targets within the cell. Their proper localization depends on their nucleotide-dependent cycling between a monomeric and a dimeric state and on the ability of the dimeric species to associate with the nucleoid. In this study, we use a combination of genetic screening, biochemical analysis and hydrogen/deuterium exchange mass spectrometry to comprehensively map the residues mediating the interactions of MipZ and ParA with DNA. We show that MipZ has non-specific DNA-binding activity that relies on an array of positively charged and hydrophobic residues lining both sides of the dimer interface. Extending our analysis to ParA, we find that the MipZ and ParA DNA-binding sites differ markedly in composition, although their relative positions on the dimer surface and their mode of DNA binding are conserved. In line with previous experimental work, bioinformatic analysis suggests that the same principles may apply to other members of the P-loop ATPase family. P-loop ATPases thus share common mechanistic features, although their functions have diverged considerably during the course of evolution.

Towards single-cell real-time imaging of energy metabolism in the brain

Towards single-cell real-time imaging of energy metabolism in the brain

Optical imaging and analytical modelling of the dynamic concentration gradient of viscous inlet solutions in a microfabricated network design

The design and operation of a gradient network is particularly critical when mixing solutions of different viscosities, which is common with real samples, especially in life science applications. The dynamic concentration gradient formed from mixing equal inlet flow rates of (0.01–1.5 mol/kg) sucrose solution with deionized water in a network was analytically modelled and experimentally investigated. The generation of a stable profile was highly dependent on both the inlet flow rate and the inlet viscosity ratio of the mixed solutions for each case study. The optimum flow rate per inlet for a stable profile was also determined. The experimental investigations show that optimizing the diffusion rate and providing sufficient mixing are key parameters to validate the analytical model. An inlet viscosity ratio of >1.009:1 (with respect to water inlet) was sufficient to perturb the outlet concentration profile. The linearity of the outlet concentration profile decreased and tended towards a sigmoidal profile with increasing sucrose concentration. Enhanced linearity of the outlet concentration profile, through optimizing the inlet flow rate ratio for each case study was also performed.