Steady-state Migration Research Articles

Experimental and numerical studies on the chloride diffusion coefficient of calcium silicate hydrate gel

The chloride diffusivity of calcium silicate hydrate (C-S-H), as the main hydration product of cement paste, plays an important role for the durability of concrete structures, especially for low w/c ratio ones. In this paper, in order to measure the chloride diffusion coefficient of C-S-H, a diffusion path conversion method is proposed to convert the primary diffusion path of ions from capillary pores to gel pores. By combining diffusion path conversion method with steady-state migration test, the diffusion coefficients of C-S-H are obtained. Based on 3D nanostructure, which is generated by a hybrid scheme of Grand Canonical Monte Carlo and Molecular Dynamics, the chloride diffusion coefficient of C-S-H is predicted by a 3D diffusive lattice modelling. Results from the experiments and simulation are compared with the data in the literature. Our work contributes to the understanding of C-S-H diffusivity and is of great importance for predicting the long-term durability of concrete structure.

Revelations of interannual dune evolution from the swiftest aeolian system on Mars by MRO/HiRISE long-term monitoring

The north polar region of Mars, with its greater atmospheric pressure and vast inventory of ever-changing volatiles (e.g., CO2, H2O), hosts arguably the most active and diverse aeolian bedform systems on the planet. Here, we explore how these dune fields evolve spatiotemporally using up to 8 Mars years (16 Earth years) of MRO HiRISE observations to test the impact of various boundary conditions on annual mobility. A high degree of sand flux heterogeneity was observed for some dunes, whereas other sites displayed steady-state migration relative to long-term rates. These large changes in annual migration are attributed to the variable length of the frost-free seasons, sediment availability in relation to the timing of peak katabatic winds, and the influence of global dust storms on seasonal ice thickness. Consistent with our previous work, we continue to observe extremely high transport rates at Olympia Cavi. All stages of aeolian system evolution are observable (sand patch > protodune > dune), along with additional phenomena not previously observed outside of terrestrial settings (e.g., dune calving and collisions, remote transfer). Transitory protodunes may evolve from modest sand mounds to prominent barchans with slipfaces several meters tall within 3–5 Mars years, while adjacent duneforms may suffer slipfaces collapse as they lose sand supply. These Martian protodunes, which appear to be larger than terrestrial equivalents, and mature dunes found downwind are among the swiftest yet reported on Mars. These rapidly evolving cryo-aeolian systems provide a window into longer-term landscape evolution of non-polar dune fields.

Experiment and simulation of chloride ion transport and binding in concrete under the coupling of diffusion and convection

Diffusion and convection are the main factors affecting chloride ion transport in unsaturated cementitious materials. In this study, capillary salt absorption experiment and steady-state migration experiment were used to investigate the chloride ion migration in unsaturated concrete, and a modified chloride ion diffusion-convection model was established for concrete considering curing age, initial water saturation and the combined effect of chloride ion. The results show that the chloride ion migration in unsaturated concrete can be divided into six stages: safety stage, initial stage, convection stage, deceleration stage, diffusion stage and stablilization stage, the chloride ion binding coefficient is approximately linear with the initial saturation, and the concrete with fly ash content of 40% or sand content of 60% has the best blocking effect on chloride ion migration.

Chitosan nanoparticles encapsulating lemongrass (Cymbopogon commutatus) essential oil: Physicochemical, structural, antimicrobial and in-vitro release properties

This study was aimed to encapsulate lemongrass (Cymbopogon commutatus) essential oil (LGEO) into chitosan nanoparticles (CSNPs) and to investigate their physicochemical, morphological, structural, thermal, antimicrobial and in-vitro release properties. CSNPs exhibited spherical morphology with an average hydrodynamic size of 175–235 nm. Increasing EO loading increased the average size of CSNPs from 174 to 293 nm (at CS:EO ratio from 1:0 to 1:1.25). SEM and AFM confirmed the results obtained by hydrodynamic size indicating that EO loading led to formation of large aggregated NPs. The successful physical entrapment of EO within NPs was shown by fourier-transform infrared spectroscopy. X-ray diffractogram of loaded-CSNPs compared to non-loaded CSNPs exhibited a broad high intensity peak at 2θ = 19–25° implying the entrapment of LGEO within CSNPs. Thermogravimetric analysis (TGA) showed that encapsulated EO was decomposed at a temperature of 252 °C compared to a degradation temperature of 126 °C for pure LGEO, indicating a two-fold enhancement in thermal stability of encapsulated CSNPs. Differential scanning calorimetry also proved the physical entrapment of EO into polymeric matrix of chitosan. In-vitro release study showed a time- and pH-dependent release of EO into release media demonstrating a three-stage release behavior with a rapid initial release of EO, followed by a steady state migration of EO from its surrounding envelope at the later stages. Antimicrobial assay showed strong antimicrobial properties of free form of LGEO against the bacteria (both gram positive and gram negative) and fungi species tested. Moreover, loaded-CSNPs exhibited stronger antibacterial and anti-fungal activities than non-loaded CSNPs.

Autogenous Healing of Cracked Mortar Using Modified Steady-State Migration Test against Chloride Penetration

Concrete is a popular building material all over the world, but because of different physiochemical processes, it is susceptible to crack development. One of the primary deterioration processes of reinforced concrete buildings is corrosion of steel bars within the concrete through these cracks. In this regard, a self-healing technique for crack repair would be the best solution to reduce the penetration of chloride ions inside concrete mass. In this study, a rapid chloride migration (RCM) test was conducted to determine the self-healing capacity of cracked mortar. With the help of the RCM test, the steady-state migration coefficient of cracked and uncracked specimens incorporating expansive and crystalline admixtures was calculated. Based on the rate of change of the chloride ion concentrations in the steady-state condition, the migration coefficient was calculated. Furthermore, bulk electrical conductivity tests were also conducted before and after the migration test to understand the self-healing behavior. It was evident from the test results that the self-healing of cracks was helpful to reduce the penetration of chloride ions and that it enhanced the ability of cracked mortar to restrict the chloride ingress. Using this test method, the self-healing capacity of the new self-healing technologies can be evaluated. The RCM test can be an acceptable technique to assess the self-healing ability of cement-based materials in a very short period, and the self-healing capacity can be characterized in terms of the decrease of chloride migration coefficients.

Release Kinetic Models of Vanillin and Physicomechanical Properties of Thermoplastic Starch and Chitosan Nanocomposite Films: Effects of Mixing Order

The effects of mixing sequence of starch–chitosan nanocomposite films on antimicrobial properties of vanillin, the release kinetics of vanillin, and physicomechanical changes of films have been reported. Four types of films were prepared based on the order of mixing. SC1: starch–glycerol–MMT–vanillin; SC2: starch–glycerol–chitosan–MMT–vanillin; SC3: starch–MMT–glycerol–vanillin–chitosan and SC4: starch–chitosan–glycerol–vanillin–MMT. All formulation exhibited high antimicrobial activity against E. coli, S. enterica, and Z. bailii, except P. aeruginosa, which showed lower sensitivity. Migration dynamics of vanillin from films into simulants showed high vanillin migrated into water and 10% ethanol at 25 and 40 °C as analyzed by high-performance liquid chromatography (HPLC). The diffusion coefficients of vanillin in water ranged between 0.38 × 10−13m2 s−1 and 4.30 × 10−13m2 s−1 and in 10% ethanol between 1.38 × 10−13m2 s−1 and 5.16 × 10−13m2 s−1 following the Fickian diffusion mechanism and first-order kinetics. The diffusion was temperature-dependent following the Arrhenius equation with high activation energies of 15.00–52.80 kJmol−1 in water and 35.80–56.50 kJ mol−1 in 10% ethanol. The plot of the mass fraction of $${{m_{t} } \mathord{\left/ {\vphantom {{m_{t} } {m_{\infty } }}} \right. \kern-\nulldelimiterspace} {m_{\infty } }}$$ against time for each sample shows the burst release of vanillin in between 30 min and 1 h and then attained the steady-state migration over 48 h. The plasticization effect of vanillin reduced the tensile strength and elastic modulus of films, while it increased the elongation at the break by 154% that was reversed after the addition of chitosan. These nanostructured starch films showed promising applications in the antimicrobial packaging industry.

Diffusion in barriers: Insights from flux tests

The performance of barriers — widely used in packaging materials to prevent undesired environmental substances from contaminating the product — can be evaluated using, for example, steady-state permeation fluxes. While this method is simpler than the standard lag-time method it does, in contrast to the latter, not provide diffusion coefficients. Furthermore, permeation flux measurements depend on test conditions (vapor pressure, barrier thickness); this limits the usefulness of the flux method because comparison between films of different type and/or thickness becomes difficult. This work attempts to overcome such limitations for the case of a single barrier layer. “Apparent” diffusion coefficients are extracted from (transient) fluxes; the time of initial exposure need not be known. Additionally, the applied approximations allow steady-state migration for different conditions to be captured by a “master line”.

Chemokines and other mediators in the development and functional organization of lymph nodes.

Secondary lymphoid organs like lymph nodes (LNs) are the main inductive sites for adaptive immune responses. Lymphocytes are constantly entering LNs, scanning the environment for their cognate antigen and get replenished by incoming cells after a certain period of time. As only a minor percentage of lymphocytes recognizes cognate antigen, this mechanism of permanent recirculation ensures fast and effective immune responses when necessary. Thus, homing, positioning, and activation as well as egress require precise regulation within LNs. In this review we discuss the mediators, including chemokines, cytokines, growth factors, and others that are involved in the formation of the LN anlage and subsequent functional organization of LNs. We highlight very recent findings in the fields of LN development, steady-state migration in LNs, and the intranodal processes during an adaptive immune response.

Applicability of Diffuse Ultrasound to Evaluation of the Water Permeability and Chloride Ion Penetrability of Cracked Concrete.

This study aims to explore the applicability of diffuse ultrasound to the evaluation of water permeability and chloride ion penetrability of cracked concrete. Lab-scale experiments were conducted on disk-shaped concrete specimens, each having a different width of a penetrating crack that was generated by splitting tension along the centerline. The average crack width of each specimen was determined using an image binarization technique. The diffuse ultrasound test employed signals in the frequency range of 200 to 440 kHz. The water flow rate was measured using a constant water-head permeability method, and the chloride diffusion coefficient was determined using a modified steady-state migration method. Then, the effects of crack width on the diffusion characteristics of ultrasound (i.e., diffusivity, dissipation), water flow rate, and chloride diffusion coefficient are investigated. The correlations between the water flow rate and diffuse ultrasound parameters, and between the chloride diffusion coefficient and diffuse ultrasound parameters, are examined. The results suggest that diffuse ultrasound is a promising method for assessing the water permeability and chloride ion penetrability of cracked concrete.

Influence of complex interfacial rheology on the thermocapillary migration of a surfactant-laden droplet in Poiseuille flow

The effect of surface viscosity on the motion of a surfactant-laden droplet in the presence of a non-isothermal Poiseuille flow is studied, both analytically and numerically. The presence of bulk-insoluble surfactants along the droplet surface results in interfacial shear and dilatational viscosities. This, in turn, is responsible for the generation of surface-excess viscous stresses that obey the Boussinesq-Scriven constitutive law for constant values of surface shear and dilatational viscosities. The present study is primarily focused on finding out how this confluence can be used to modulate droplet dynamics in the presence of Marangoni stress induced by nonuniform distribution of surfactants and temperature along the droplet surface, by exploiting an intricate interplay of the respective forcing parameters influencing the interfacial stresses. Under the assumption of negligible fluid inertia and thermal convection, the steady-state migration velocity of a non-deformable spherical droplet, placed at the centerline of an imposed unbounded Poiseuille flow, is obtained for the limiting case when the surfactant transport along the interface is dominated by surface diffusion. Our analysis proves that the droplet migration velocity is unaffected by the shear viscosity whereas the dilatational viscosity has a significant effect on the same. The surface viscous effects always retard the migration of a surfactant-laden droplet when the temperature in the far-field increases in the direction of the imposed flow although the droplet always migrates towards the hotter region. On the contrary, if a large temperature gradient is applied in a direction opposite to that of the imposed flow, the direction of droplet migration gets reversed. However, for a sufficiently high value of dilatational surface viscosity, the direction of droplet migration reverses. For the limiting case in which the surfactant transport along the droplet surface is dominated by surface convection, on the other hand, surface viscosities do not have any effect on the motion of the droplet. These results are likely to have far-reaching consequences in designing an optimal migration path in droplet-based microfluidic technology.

Triple junction drag effects during topological changes in the evolution of polycrystalline microstructures

Experiments, theory and atomistic simulations show that finite triple junction mobility results in non-equilibrium triple junction angles in evolving polycrystalline systems. These angles have been predicted and verified for cases where grain boundary migration is steady-state. Yet, steady-state never occurs during the evolution of polycrystalline microstructures as a result of changing grain size and topological events (e.g., grain face/edge switching - “T1” process, or grain disappearance “T2” or “T3” processes). We examine the non-steady evolution of the triple junction angle in the vicinity of topological events and show that large deviations from equilibrium and/or steady-state angles occur. We analyze $∖tau$ the characteristic relaxation time of triple junction angles τ by consideration of a pair of topological events, beginning from steady-state migration. Using numerical results and theoretical analysis we predict how the triple junction angle varies with time and how τ varies with triple junction mobility. We argue that it is precisely those cases where grain boundaries are moving quickly (e.g., topological process in nanocrystalline materials), that the classical steady-state prediction of the triple junction angle about finite triple junction mobility is inapplicable and may only be applied qualitatively.

MyD88 Signaling Regulates Steady-State Migration of Intestinal CD103+ Dendritic Cells Independently of TNF-α and the Gut Microbiota.

Intestinal homeostasis and induction of systemic tolerance to fed Ags (i.e., oral tolerance) rely on the steady-state migration of small intestinal lamina propria dendritic cells (DCs) into draining mesenteric lymph nodes (MLN). The majority of these migratory DCs express the α integrin chain CD103, and in this study we demonstrate that the steady-state mobilization of CD103(+) DCs into the MLN is in part governed by the IL-1R family/TLR signaling adaptor molecule MyD88. Similar to mice with complete MyD88 deficiency, specific deletion of MyD88 in DCs resulted in a 50-60% reduction in short-term accumulation of both CD103(+)CD11b(+) and CD103(+)CD11b(-) DCs in the MLN. DC migration was independent of caspase-1, which is responsible for the inflammasome-dependent proteolytic activation of IL-1 cytokine family members, and was not affected by treatment with broad-spectrum antibiotics. Consistent with the latter finding, the proportion and phenotypic composition of DCs were similar in mesenteric lymph from germ-free and conventionally housed mice. Although TNF-α was required for CD103(+) DC migration to the MLN after oral administration of the TLR7 agonist R848, it was not required for the steady-state migration of these cells. Similarly, TLR signaling through the adaptor molecule Toll/IL-1R domain-containing adapter inducing IFN-β and downstream production of type I IFN were not required for steady-state CD103(+) DC migration. Taken together, our results demonstrate that MyD88 signaling in DCs, independently of the microbiota and TNF-α, is required for optimal steady-state migration of small intestinal lamina propria CD103(+) DCs into the MLN.

Benchmarks for multicomponent diffusion and electrochemical migration

In multicomponent electrolyte solutions, the tendency of ions to diffuse at different rates results in a charge imbalance that is counteracted by the electrostatic coupling between charged species leading to a process called “electrochemical migration” or “electromigration.” Although not commonly considered in solute transport problems, electromigration can strongly affect mass transport processes. The number of reactive transport models that consider electromigration has been growing in recent years, but a direct model intercomparison that specifically focuses on the role of electromigration has not been published to date. This contribution provides a set of three benchmark problems that demonstrate the effect of electric coupling during multicomponent diffusion and electrochemical migration and at the same time facilitate the intercomparison of solutions from existing reactive transport codes. The first benchmark focuses on the 1D transient diffusion of HNO3 (pH = 4) in a NaCl solution into a fixed concentration reservoir, also containing NaCl—but with lower HNO3 concentrations (pH = 6). The second benchmark describes the 1D steady-state migration of the sodium isotope 22Na triggered by sodium chloride diffusion in neutral pH water. The third benchmark presents a flow-through problem in which transverse dispersion is significantly affected by electromigration. The system is described by 1D transient and 2D steady-state models. Very good agreement on all of the benchmarks was obtained with the three reactive transport codes used: CrunchFlow, MIN3P, and PHREEQC.

Balance between cell−substrate adhesion and myosin contraction determines the frequency of motility initiation in fish keratocytes

Cells are dynamic systems capable of spontaneously switching among stable states. One striking example of this is spontaneous symmetry breaking and motility initiation in fish epithelial keratocytes. Although the biochemical and mechanical mechanisms that control steady-state migration in these cells have been well characterized, the mechanisms underlying symmetry breaking are less well understood. In this work, we have combined experimental manipulations of cell-substrate adhesion strength and myosin activity, traction force measurements, and mathematical modeling to develop a comprehensive mechanical model for symmetry breaking and motility initiation in fish epithelial keratocytes. Our results suggest that stochastic fluctuations in adhesion strength and myosin localization drive actin network flow rates in the prospective cell rear above a critical threshold. Above this threshold, high actin flow rates induce a nonlinear switch in adhesion strength, locally switching adhesions from gripping to slipping and further accelerating actin flow in the prospective cell rear, resulting in rear retraction and motility initiation. We further show, both experimentally and with model simulations, that the global levels of adhesion strength and myosin activity control the stability of the stationary state: The frequency of symmetry breaking decreases with increasing adhesion strength and increases with increasing myosin contraction. Thus, the relative strengths of two opposing mechanical forces--contractility and cell-substrate adhesion--determine the likelihood of spontaneous symmetry breaking and motility initiation.

Cutaneous B-1 B cells can secrete IL-10 and require integrin α4β1 (VLA-4) for migration into skin (CAM3P.201)

Abstract B1 B cells are innate-like B cells that provide key effector functions during infection and inflammation. Despite their importance, the target organs, function and trafficking of this B cell subset are largely unknown. Here we show that, unexpectedly, B cells, including B1(-like) cells with the potential to secrete IL-10, were part of the normal cutaneous immune system of mice and humans. In the mouse, we found steady-state migration of peritoneal B1 B cells into skin. In addition, B1, but not follicular B2, B cells efficiently migrated into inflamed skin by virtue of α4β1 integrin (VLA-4) expression. Importantly, B1 B cells expressed high affinity VLA-4 at steady state, indicating that B1 cells were poised to migrate into inflamed sites. Our data suggest that IL-10+ innate-like B cells are recruited into skin inflammation to dampen cutaneous inflammation, and we propose that dysregulation of the recruitment or function of B1(-like) B cells contributes to inflammatory skin diseases, such as psoriasis.

The influence of alkali content on the electrical resistivity and transport properties of cementitious materials

This paper examines the role of alkali content on the transport properties of cementitious materials. While cement with high alkali content may be a concern due to the potential for alkali silica reaction (ASR), the alkali content also influences the electrical properties of pore solution and the bulk cementitious system. Higher concentrations of alkalis decrease the resistivity of the pore solution. However, the alkalis may also impact the microstructure formed during hydration. This paper examines the role of alkali content on both the pore solution and on the microstructure that forms. Water vapor desorption measurements indicate that systems with a higher alkali content have a lower overall porosity. This dense microstructure is not caused simply by a higher degree of hydration (DOH) (i.e., an increase in reacted products), as suggested by loss on ignition (LOI), but rather possibly from the improved morphology and distribution of calcium hydroxide, as observed with scanning electron microscope (SEM).These improvements in the microstructure alter the electrical resistivity and the results of steady and non-steady state migration tests. This indicates that with higher alkali content, cement may have a beneficial influence on reducing chloride ion transport. Furthermore, alkali leaching during saturated curing is observed, quantified, and discussed as it relates to the interpretation of electrical measurements and microstructure formed.

Determining the steady-state chloride migration coefficient of ITZ in mortar by using the accelerated chloride migration test

In this study, an electrochemical technique is applied to accelerate chloride ion migration in mortar to estimate its transport properties. The steady-state chloride migration coefficient of mortar was determined experimentally as a function of the aggregate volume fraction to investigate the effect of aggregate content. The influence of the aggregate–cement paste interfacial transition zone (ITZ) on the steady-state chloride migration behavior in mortar was investigated using a three-phase model. The three-phase material was composed of cement paste matrix, aggregate, and ITZ. The chloride diffusion is considered to take place in the cement paste and the ITZ phases. The chloride flow is controlled by dilution in the cement paste, and the chloride flow is controlled by the combining effects of tortuosity and dilution in the ITZ. The experimental results are fitted with the three-phase model. Based on the experimental and regression analytical results, the approximate chloride migration coefficient of ITZ with different assumed thicknesses of ITZ is determined.

Research on the performance of concrete materials under the condition of freeze-thaw cycles

Durability of concrete has been a social concern due to the poor performance of concrete caused by various environmental conditions. This paper aims to study the effects of damage caused by freeze-thaw cycles which often appeared in cold areas on the performance of concrete with polypropylene fibres, such as strength, carbonation depth and chloride ion diffusion. Concrete samples with different degrees of damage were produced by different freeze-thaw cycles. The concrete sample shows great water absorption after freeze-thaw cycles. Due to damage by freeze-thaw cycles, the total pore volume, porosity and average pore diameter of concrete samples damaged by freeze-thaw cycles were generally larger than that of normal concrete samples. Compressive strength and splitting strength of concrete exposed to freeze-thaw cycles decreased with the number of freeze-thaw cycles and greater damage caused higher loss of strength, especially for the splitting tensile strength. The carbonation depth of the concrete after freeze-thaw cycles was also greater than that of the concrete before freeze-thaw cycles. Chloride ion diffusion rates were evaluated using the free chloride ion concentration titration method and the steady state migration testing method. Results indicated that freeze-thaw cycles could accelerate chloride ion diffusion rates. More severe concrete damage results in greater chloride diffusion.

Evaluation of the sulfate resistance of fly ash and slag concrete by using modified ACMT

Pozzolanic concrete has been widely used in various projects. In this study, fly ash and slag concretes were placed in the solution of 5% sodium sulfate under drying–wetting and temperature change cycles and long-term immersion, to simulate the effect of structures in tidal zones and seawater, respectively. The durability of concrete is assessing by using modified Accelerated Chloride Migration Test (ACMT). The results indicate that adding 20% fly ash and 40% slag concrete curing at 23°C and 100% R.H. for 4months, the chloride ion non-steady state migration coefficient (Mn) were 69% and 28% that of ordinary concrete, respectively. Ordinary and fly ash concrete were placed in the solution of 5% sodium sulfate under drying–wetting and temperature change cycles for 12weeks, Mn were reduced to 82% and 33% of the initial value, respectively. This result represented that after the cycle procedure the ability of resisting chloride ion penetration increased. However, Mn of slag concrete became 2times of the initial value, it represented that slag concrete was deteriorated under drying–wetting and temperature change cycles. In addition, results showed that Mn was approximately 3–10times against steady state migration coefficient (Ms), and it is higher than the predicted value of the NIST program. This result indicating the modified ACMT will underestimate the durability of concrete, but it still can be a method for comparing durability of different kinds of concrete.

Theoretical analysis of chloride ion migration tests in cementitious material research

Chloride ion migration tests, or so called electrical accelerated tests, for testing the chloride diffusion coefficient of cementitious materials are reviewed. The ion migration theory of electrical accelerated tests is presented. Four main testing methods that include chloride penetration test, steady state migration test, unsteady state migration test and salt saturated conductivity test (SSCT) are introduced, and their theoretical calculations of chloride diffusion coefficient are described. Through analysis of the theoretical calculations, it is found that all electrical accelerated tests are based on ion migration theory, although different suppositions are adopted. Among the migration tests, steady state migration test is the most suitable to determine the chloride diffusion coefficient. DCIPT, DRCM and DSSCT calculated by chloride penetration test, unsteady state migration test and SSCT can be used to identify the resistance to chloride ion penetration by cementitious materials, but not the real diffusion coefficient values.