Temperature Dependence Of Activation Energy Research Articles

Sub-Arrhenius diffusion in a classical system: Binary colloidal mixture in an external potential

In the transport processes in many physical systems, the temperature dependence of diffusivity is assumed to be exhibit Arrhenius behavior, i.e., a linear relationship log(D)∝1∕T . When extended to lower temperatures, deviations are observed in many physical systems. The phenomenological description of these deviations identifies them as sub-Arrhenius and super-Arrhenius regimes based on the concave or convex nature of log(D)∝1∕T curve and the activation energy becomes temperature dependent. In general, super-Arrhenius behavior is observed in classical systems such as supercooled liquids where correlated motions play an important role in the dynamical behavior of the system, whereas sub-Arrhenius behavior is proposed to be intimately related to quantum tunneling effect of penetration of an energy barrier along the reaction path as in the case of chemical reactions. In this article, we show that a purely classical system, such as binary colloidal mixture under an external repulsive potential, can undergo sub-Arrhenius diffusion. The depletion interactions between the repulsive potential barrier and larger particles in the presence of the smaller particles contribute to the temperature dependence of activation energy which in turn lead to the sub-Arrhenius diffusion of larger particles in the mixture.

Optimization of polymer electrolytes with the effect of concentration of additives in PEO-NH4HF2 polymer electrolytes

In the present work, nanocomposite polymer electrolyte films were prepared by solution casting technique using nanosized fumed silica to polyethylene oxide (PEO)-based polymer electrolytes containing ammonium bifluoride (NH4HF2). The ionic conductivity of 1.19 × 10−5 S cm−1 has been observed at room temperature (25°C) for 3 wt% fumed silica in PEO-10 wt% NH4HF2 polymer electrolytes after which the conductivity was observed to decrease. Furthermore, the addition of high dielectric constant plasticizer propylene carbonate (PC) in the optimized composition of nanocomposite polymer electrolytes has increased the number of charge carriers by the large dissolution of ionic salt, amorphous content, and hence the ionic conductivity. Maximum ionic conductivity obtained at room temperature was found to be 1.55 × 10−4 S cm−1 in the case of PEO-10 wt% NH4HF2-3 wt% fumed silica-0.3 (ml) PC polymer electrolytes which is five orders of magnitude higher than that of the polymer host material. Temperature-dependent ionic conductivity, activation energy, and dielectric constant studies have been described for all the compositions of polymer electrolytes. Ionic conductivity and dielectric constant studies were determined from impedance data. Polymer electrolytes containing both fumed silica and PC highlight that there is no phase transition in the polymer electrolyte and temperature dependence of ionic conductivity in the temperature range is of almost Arrhenius type. The lowest activation energy value for the highest conducting polymer electrolyte was found to be 0.172 eV. Change in melting temperature, % crystallinity ( χc), and mechanical properties have also been observed in polymer electrolytes containing fumed silica as well as PC as studied by Differential Scanning Calorimetry/Thermogravimetric Analysis (DSC/TGA) and universal testing machine, respectively.

Tailoring effect of large polaron hopping in the conduction mechanism of Ca-modified BaTiO3 system

Tailoring effects of Ca-doping concentration on the structural, optical, and electrical properties of Ba1−xCaxTiO3 (x = 0.0 to 0.15) nanomaterials prepared via low-temperature sol–gel auto combustion technique are presented. XRD results reveal that all the prepared nanomaterials are crystallized in tetragonal structure with space group P4mm. FT-IR measurements are carried out in order to reveal the vibrational frequencies of bonds in the crystal lattice. A strong absorption peak due to Ti–O bond (TiO6 octahedron) appears at 538 cm−1 that shifts to a higher frequency with an increase in the doping of Ca ions. It is explained by the Coulomb interaction. Optical band gap of the sample increases with increase Ca doping as 3.44 eV (x = 0.0), 3.48 eV (x = 0.10), and 3.45 eV (x = 0.15). The BCT materials with x = 0.10 apparently show that the particle with short-range order can strongly influence the formation of intermediate states in the gap. Dielectric behavior is found governed by the interfacial polarization, whereas the ac conductivity supports the non-Debye process [n < 1] that is described using Jonscher’s power law. The conduction mechanism is found changing from small polaron to large polaron hopping as the Ca ions increase. Moreover, the electrical conductivity arising due to the movement of charged ions is correlated with the hopping and the lattice defects. The temperature-dependent activation energy is obtained in the range of 0.19–0.12 eV, which evidences the NTCR behavior observed for all studied materials.

Kinetic analysis of particle-size based complex kinetic processes

Complex kinetic data arbitrarily created by measuring (calorimetrically) crystallization of mixed selenium powders with different defined particle sizes were evaluated by the three standard approaches to complex kinetic analysis. Performance of the three tested approaches was tested by comparison with the kinetic results obtained for the separate powder fractions. The additivity of the kinetic signals was verified. Single-process methods of kinetic analysis provided qualitative and quantitative information about the temperature dependence of activation energy E and estimation of the ratio between intensities of the involved sub-processes I1/I2. Mathematic deconvolution approach well reflected the correct model-free and model-based kinetics for the partial overlaps. However, it required iterative processing and additional supplemental information about the nature of the sub-processes in case of the evaluation of fully overlapping data. Kinetic deconvolution based on single-curve non-linear optimization utilizing the fixed E values obtained independently provided very good results for both, partial and full overlaps.

An Analytical Approach to Study Annealing Induced Interdiffusion of In and Ga for Truncated Pyramidal InAs/GaAs Quantum Dots

A strain-assisted model for studying the annealing induced interdiffusion of constituent atoms in truncated pyramidal InAs/GaAs self-assembled quantum dot (QD) is developed. In our model, we have considered the equiprobable diffusion of Ga and In atoms in and out of the QD system, thereby resulting in the formation of In rich In1-xGaxAs as the QD material and a combination of Ga rich InxGa1-xAs and GaAs barrier material. This assumption is supported by the high-resolution cross-sectional transmission electron microscopy (HRTEM) images and high-resolution X-ray diffraction (HRXRD) plots of the as-grown and annealed samples. First principle calculations are utilized to evaluate the strain and annealing temperature dependent activation energy, which further assists to solve the Fickian diffusion model. Our simplistic model provides a detailed insight into the effect of annealing phenomenon on the bimodal Photoluminescence (PL) spectra of molecular beam epitaxy (MBE) grown InAs/GaAs QDs, which have evolved as a potential system for innumerous applications including single photon sources, quantum computers, spin LEDs, LASERs and Infrared Photodetectors.

Stress Relaxation Aging Behavior and Constitutive Modelling of AA7150-T7751 under Different Temperatures, Initial Stress Levels and Pre-Strains

Age forming is an advanced manufacture technology for forming large aluminum panels. Temperature, initial stress level and pre-strains have a great effect on the formability and performance. The stress relaxation aging behavior of AA7150-T7751 under different temperatures, initial stress levels and pre-strains was studied through stress relaxation tests, tensile tests and TEM observations. The results show that the formability can be improved with the increase of temperature, initial stress levels and pre-strains. Deformation mechanisms during stress relaxation of the material were analyzed on the basis of creep stress exponent and apparent activation energy. The aging precipitates of the studied alloy were not sensitive to the age forming conditions, but drastically coarsened at over aging temperature, which decreased the mechanical properties. In addition, the relationship between stress relaxation behavior and aging strengthening is discussed. Based on the dislocation theory and the modified Arrhenius equation, a stress relaxation constitutive equation considering the initial mobile dislocation density and temperature dependent activation energy was established. This model can predict very well the stress relaxation behavior under various temperature, stress level and pre-strain conditions, with an average error of 2%.

Influence of Ce3+ substitution on the structural, electrical and magnetic properties of Zn0.5Mn0.43Cd0.07Fe2O4 spinel ferrites

In this paper, the effect of Ce3+ ions addition on the microstructure, electrical and magnetic behaviour of Cadmium doped Mn–Zn ferrites was investigated. A series having composition Zn0·5Mn0·43Cd0·07Fe2-yCeyO4 (y = 0.0–0.7) was synthesized by wet chemical route. X-Ray Diffraction (XRD) analysis showed that prepared particles having size in nano-regime having cubic spinel structure with preferred (311) plane. Weakening of (311) plane intensity and emergence of secondary phase with Ce3+ ion concentration verified the substitution of Ce3+ ions in the spinel lattice of ferrite. The crystallite size varied between 46.4 and 67 nm as the concentration of Ce3+ ions increased from x = 0.0–0.7. Temperature and composition dependent electrical resistivity and activation energy values were calculated by current-voltage (I–V) characteristics in the temperature range 423–823 K by two probe method. It was observed that DC Resistivity and activation energy of synthesized nanoparticle ranges between109 to ~1010 Ohm cm and 11.78–2.56 eV respectively with increasing the concentration of Ce3+ ions. MH curves of prepared samples showed the paramagnetic nature of spinel feerites. The reported data suggested that these particles promising candidates for high-frequency applications.

Measurement of visco-elastic properties in a binary system of hockey stick-shaped and calamitic mesogens.

Temperature variation of the splay elastic modulus (K11) and rotational viscosity ([Formula: see text]) have been measured throughout the entire nematic (N phase of a binary liquid crystal system comprising of host calamitic compound, 4'-octyloxy-4-cyanobiphenyl (8OCB) and small amount of a hockey stick-shaped mesogen, 4-(3-decyloxyphenyliminomethyl) phenyl 4-decyloxycinnamate (SF7). Two different structural conformations play a significant role in the phase behavior leading to a noticeable shrinkage in the nematic width. During cooling in the nematic phase, both K11 and [Formula: see text] enhance monotonically and divulge an evidence of pretransitional fluctuation near the Smectic-A (Sm-A) transition. With increase in the dopant concentration, the slope of the temperature-dependent K11 and [Formula: see text] becomes uniformly steeper. The observed variations are explained in accordance with the formation of smectic-like clusters in the nematic phase. A representative model of the smectic-like clusters in the nematic phase is presented. However, the obtained values of [Formula: see text] have found to be between that of conventional calamitics and typical bent-core mesogens. Measurement of the temperature-dependent orientational order parameter (〈P2〉 ) and activation energy (Ea) has also been carried out for all the studied mixtures, exhibiting a small augmentation due to the addition of a hockey stick-shaped compound. This indicates an enhancement in intermolecular packing within the mixtures which further influences the molecular motions.

Modeling the effect of thermal combined with high‐pressure treatment on intramuscular lipid oxidation in pork

AbstractThe kinetics of lipid oxidation in pork during high‐pressure combined with thermal (HP‐T) treatment were studied within the condition range of 300–600 MPa and 20–60°C. Results showed that the lipid oxidation rate constants (k values) increased with increasing pressure or temperature. Lipid primary oxidation exhibited higher k values than secondary oxidation at each investigated pressure–temperature. At the studied pressure range, the temperature dependence (activation energy) of both primary lipid oxidation and secondary lipid oxidation rate constants decreased with increasing pressure; while the highest sensitivity of lipid oxidation rate constants to pressure (activation volume) was observed at 30°C for primary oxidation, and 40°C for secondary oxidation. Four kinetic models for lipid oxidation basing on Arrhenius and Eyring equation were established successfully.Practical ApplicationsHigh pressure processing plays a more and more important role in food industry, however, the application of this technology in animal products is limited because of increased lipid oxidation due to high pressure treatment, the reason for which reported by different authors is inconsistent. To better understand the real causes of lipid oxidation induced by high pressure and control it purposefully, the kinetic models for lipid primary and secondary oxidation basing on Arrhenius and Eyring equation were established successfully in this article. In practice, the predetermined combinations of temperature and pressure during HP‐T treatment of pork could be evaluated by predicting the effect of these conditions on lipid oxidation using the models established in this study, and thereafter modifying and selecting the optimal treatment conditions. The models should be validated or modified when applied to other types of meat basing on a larger number of experiments.

(Invited) Activity and Stability of Atomically Dispersed (AD) Fe-C-N ORR Catalyst in Polymer Electrolyte Fuel Cell Environment

Activity and stability of (AD)Fe-N-C catalyst with low Fe content (1.5 at%) have been investigated in a polymer electrolyte fuel cell environment. Previous electron microscopy analyses (SEM, EDS and EELS) have revealed the presence of atomically dispersed Fe atoms coordinated with nitrogen (Fe-Nx) and a partially graphitized microporous carbon structure. In this study, 5-cm2 membrane electrode assemblies (MEAs) were prepared by hot pressing anode gas diffusion electrodes (GDEs, 0.2 mgPt/cm2) to N211 membrane, and brush painting the PGM-free cathode catalyst ink; Sigracet SG29BC GDL was used on cathode side without hot pressing. All performance tests were conducted under differential cell conditions with fixed H2, O2 and air flow rates. Polarization curves obtained in H2/O2 show double Tafel slopes which, in conjunction with the position of the redox potential observed in cyclic voltammetry traces, forms the basis for formulating a distributed ORR (oxygen reduction reaction) kinetic model with potential-dependent available sites and extracting reaction order for P(O2) and relative humidity (RH) dependence and activation energy for temperature dependence. Application of this model to polarization data collected in H2/air provides the basis for formulating the oxygen transport model and leads to 7.7±0.6 mA/cm2 modeled catalyst activity (defined as areal current density at 0.9 V IR-corrected cell voltage in H2/O2,1.5 atm, 80oC, and 100% RH), 31.5-34.3 mA/cm2 cell performance at 1.5 atm, 80oC, 100% RH, and 0.8-2 s/cm O2 transport resistance (Rcf) in the cathode catalyst layer (CCL). The derived value of Rcf for ~100-μm thick PGM-free electrode is 2-3 times higher than the measured Rcf for <10-μm thick low-Pt loaded (<0.1 mgPt/cm2) electrodes. As such, Rcf includes resistance to O2 transport in the CCL secondary pores, and the primary pores and ionomer film surrounding the Fe-Nx active sites within the microporous carbon support. Catalyst durability was characterized in accelerated stress tests (AST) with 0.6-0.95 V trapezoid potential cycles in H2/N2. Although the (AD)Fe-N-C catalyst has low initial activity, it exhibits excellent stability in ASTs considering that the measured polarization curves in H2/air show voltage losses of only 5, 15 and 30 mV after 10k, 20k and 30k cycles, respectively, for an average degradation rate of 3.5 mV/h. The voltage losses are nearly independent of the current density, suggesting that the observed performance degradation is mainly due to loss in catalyst activity and the transport resistance is nearly unaffected by AST. The kinetic model indicates a 50% loss in catalyst activity after 30k AST cycles. We observe similar degradation rate on a load cycle (LC) consisting of H2/air polarization curves under different operating pressures (1-2.5 atm), temperatures (60-95oC), O2 mole fraction (2-21%), and relative humidity (30-100%). The degradation rate is faster under a combined AST and LC, with majority of losses occurring in the load cycles. Figure 1 summarizes the modeled loss in active sites for load cycles in H2/O2 (cell 1), load cycles in H2/air (cell 2), AST cycles in H2/N2 (cell 3), and combined AST and load cycles (cell 4). The model indicates <20% increase in the O2 transport resistance in the (AD)Fe-N-C catalyst layer with >75% loss of inactive sites. We have used the coupled kinetic, O2 transport and stability model to project the improvements needed in catalyst activity and electrode structure to approach the automotive target of 1000 mW/cm2 stack power density while meeting the 1.45 kW/oC heat rejection (Q/ΔT) constraint at 2.5 atm inlet pressure, 95oC coolant exit temperature, and 1.5 cathode stoichiometry. The model indicates that we need twelve-fold higher mass activity for reducing the kinetic losses, doubling of active site density and an engineered electrode structure for 50% lower sheet resistance, as well as a 50% reduction in electrode thickness to limit the O2 transport losses. Such an automotive competitive PGM-free catalyst will exceed 22.6 mA/mg mass activity for >43.3 mA/cm2 specific activity at 1.9 mg/cm2 catalyst loading in an electrode structure that is <50-μm thick and engineered for <50 mΩ.cm2 sheet resistance. It will reach 1150 mA/cm2 current density at 656 mV for 750 mW/cm2 power density at Q/ΔT relevant operating conditions, after incurring voltage losses of 405 mV for ORR kinetics, 70 mV for combined proton transport in membrane and CCL, and 35 mV for O2 transport across the gas channel, GDL and CCL. Figure 1

Kinetic Processes in Amorphous Materials Revealed by Thermal Analysis: Application to Glassy Selenium

It is expected that viscous flow is affecting the kinetic processes in a supercooled liquid, such as the structural relaxation and the crystallization kinetics. These processes significantly influence the behavior of glass being prepared by quenching. In this paper, the activation energy of viscous flow is discussed with respect to the activation energy of crystal growth and the structural relaxation of glassy selenium. Differential scanning calorimetry (DSC), thermomechanical analysis (TMA) and hot-stage infrared microscopy were used. It is shown that the activation energy of structural relaxation corresponds to that of the viscous flow at the lowest value of the glass transition temperature obtained within the commonly achievable time scale. The temperature-dependent activation energy of crystal growth, data obtained by isothermal and non-isothermal DSC and TMA experiments, as well as direct microscopic measurements, follows nearly the same dependence as the activation energy of viscous flow, taking into account viscosity and crystal growth rate decoupling due to the departure from Stokes–Einstein behavior.

Temperature, microwave power and pomace thickness impact on the drying kinetics and quality of carrot pomace

This study investigated the effect of air temperature, microwave power, and pomace thickness on the drying kinetics and quality of dried carrot pomace. The study established that the drying of carrot pomace occurs in the falling rate period, suggesting that drying was driven by molecular diffusion. The microwave-drying moisture diffusivity increased with microwave power and ranged between 1.57×10–8 and 2.61×10–8 m2/s. As regards convective air-drying, the moisture diffusivity values were between 3.38×10–10 and 8.27×10–10 m2/s. The microwave powerto-mass activation energy was 15.079 W/g for 5 mm, 7.599 W/g for 10 mm and 9.542 W/g for 15 mm dried samples. Meanwhile, the temperature-dependent activation energy for carrot pomace was found to be 27.637 kJ/mol for 5 mm, 17.92 kJ/mol for 10 mm and 38.76 kJ/mol for 15 mm thickness pomace. Generally, drying time decreased with increasing microwave power or air temperature. The ascorbic acid content of the fresh carrot pomace reduced after both microwave and convective air-drying. However, microwave power, and sample thickness had significant effect on the β-carotene content of dried products but air temperature did not have a significant effect. The effect of temperature and sample thickness on brown pigment formation was substantial with air temperature compared to microwave. The study has demonstrated that microwave drying, compared to conventional drying, enhances moisture removal, drying time, and preservation of carotenoids and ascorbic acid. Therefore, microwave drying can be considered as an alternative method for obtaining quality dried carrot pomace.

Charge transport in band-tail states of irradiated alkali feldspar I: Super-Arrhenius kinetics

Infra-red stimulated luminescence (IRSL) emission in alkali feldspar comprises two components that arise due to tunneling from the excited state of the primary IRSL trap and the band tail states of the host material. Combination of two Becquerel functions fit well to the IRSL decay curve. The second component which has a longer lifetime was identified with that of the excited state tunneling and found that is dominated at the later part of the decay curve. This part was removed from the IRSL signals to isolate the signals that were predominantly of band tail transport. IRSL decay curves measured at various temperatures from 50 °C to 250 °C after preheated to 280 °C, were fitted to Becquerel function. From the Becquerel function, time-dependent decay rates and the distribution of decay rates were extracted. Ensemble average decay rates were calculated from the distribution of decay rates and found they increase with stimulation temperature. IRSL intensity exhibits a similar behavior. Both were yielding diminishing returns at higher temperatures. Hence, both exhibited a non-linearity in Arrhenius plots. These non-Arrhenius data were analyzed using the deformed exponential function, and that yielded a temperature dependent (decreasing from ∼0.12 eV to ∼ 0.02 eV, with temperature) thermal activation energy. At lower temperature, larger thermal energy is needed to activate the charge transport at lower mobility band tail states, and smaller energy is sufficient at high mobility states. Assuming the processes that are responsible for IRSL intensity change and decay rate change with temperature are different, IRSL intensity change was analyzed using the combination of thermal assistance and thermal quenching. The thermal activation energy of 0.22 eV was obtained for quenching, and it is also observed that the quenching doesn't happen at the recombination center but elsewhere.

Cooperative-VBO model for polymer/graphene nanocomposites

The Cooperative–Viscoplasticity theory based on overstress (VBO) theory which is initially developed for polymers is modified to model the mechanical behavior of graphene-polymer nanocomposites. The modifications are proposed for both the elastic and viscoplastic deformation. For the elastic contribution, an effective modulus definition which covers the agglomeration effects is implemented into the Cooperative-VBO theory. A strain rate, temperature, graphene volume fraction and agglomeration rate dependent elasticity modulus is obtained. In order to model the visco-plastic deformation, two scalar material parameters, activation energy and activation volume are defined as functions of graphene volume ratio using Takayanagi averaging approach. The newly defined graphene volume fraction, agglomeration rate, temperature and strain rate dependent elasticity modulus and activation energy and free volume for nanocomposites are incorporated into the Cooperative -VBO model.In this manner, a constitutive model which covers the total viscoelastic-viscoplastic deformation of graphene-polymer nanocomposites is developed. The capabilities of the introduced model are demonstrated by predicting uniaxial compression behavior of pristine epoxy and different graphene content nanocomposites. In addition, storage modulus obtained from a Dynamic Mechanical Analysis (DMA) are compared with predicted values for neat epoxy, 0.1 wt% and 0.5 wt% graphene-epoxy nanocomposite. The simulation results are compared to experimental data from the literature. A fair agreement with experimental data is obtained.

A characteristic energy scale in glasses.

Intrinsically generated structural disorder endows glassy materials with a broad distribution of various microscopic quantities-such as relaxation times and activation energies-without an obvious characteristic scale. At the same time, macroscopic glassy responses-such as Newtonian (linear) viscosity and nonlinear plastic deformation-are widely interpreted in terms of a characteristic energy scale, e.g., an effective temperature-dependent activation energy in Arrhenius relations. Nevertheless, despite its fundamental importance, such a characteristic energy scale has not been robustly identified. Inspired by the accumulated evidence regarding the crucial role played by disorder- and frustration-induced soft quasilocalized excitations in determining the properties and dynamics of glasses, we propose that the bulk average of the glass response to a localized force dipole defines such a characteristic energy scale. We show that this characteristic glassy energy scale features remarkable properties: (i) It increases dramatically in underlying inherent structures of equilibrium supercooled states approaching the glass transition temperature Tg, significantly surpassing the corresponding increase in the macroscopic shear modulus, dismissing the common view that structural variations in supercooled liquids upon vitrification are minute. (ii) Its variation with annealing and system size is very similar in magnitude and form to that of the energy of the softest non-phononic vibrational mode, thus establishing a nontrivial relation between a rare glassy fluctuation and a bulk average response. (iii) It exhibits striking dependence on spatial dimensionality and system size due to the long-ranged fields associated with quasilocalization, which are speculated to be related to peculiarities of the glass transition in two dimensions. In addition, we identify a truly static growing lengthscale associated with the characteristic glassy energy scale and discuss possible connections between the increase of this energy scale and the slowing down of dynamics near the glass transition temperature. Open questions and future directions are discussed.

Three physiological parameters capture variation in leaf respiration of Eucalyptus grandis, as elicited by short-term changes in ambient temperature, and differing nitrogen supply.

We used instantaneous temperature responses of CO2 -respiration to explore temperature acclimation dynamics for Eucalyptus grandis grown with differing nitrogen supply. A reduction in ambient temperature from 23 to 19°C reduced light-saturated photosynthesis by 25% but increased respiratory capacity by 30%. Changes in respiratory capacity were not reversed after temperatures were subsequently increased to 27°C. Temperature sensitivity of respiration measured at prevalent ambient temperature varied little between temperature treatments but was significantly reduced from ~105kJmol-1 when supply of N was weak, to ~70kJmol-1 when it was strong. Temperature sensitivity of respiration measured across a broader temperature range (20-40°C) could be fully described by 2 exponent parameters of an Arrhenius-type model (i.e., activation energy of respiration at low reference temperature and a parameter describing the temperature dependence of activation energy). These 2 parameters were strongly correlated, statistically explaining 74% of observed variation. Residual variation was linked to treatment-induced changes in respiration at low reference temperature or respiratory capacity. Leaf contents of starch and soluble sugars suggest that respiratory capacity varies with source-sink imbalances in carbohydrate utilization, which in combination with shifts in carbon-flux mode, serve to maintain homeostasis of respiratory temperature sensitivity at prevalent growth temperature.

Nutrient limitation suppresses the temperature dependence of phytoplankton metabolic rates

Climate warming has the potential to alter ecosystem function through temperature-dependent changes in individual metabolic rates. The temperature sensitivity of phytoplankton metabolism is especially relevant, since these microorganisms sustain marine food webs and are major drivers of biogeochemical cycling. Phytoplankton metabolic rates increase with temperature when nutrients are abundant, but it is unknown if the same pattern applies under nutrient-limited growth conditions, which prevail over most of the ocean. Here we use continuous cultures of three cosmopolitan and biogeochemically relevant species (Synechococcus sp., Skeletonema costatum and Emiliania huxleyi) to determine the temperature dependence (activation energy, Ea) of metabolism under different degrees of nitrogen (N) limitation. We show that both CO2 fixation and respiration rates increase with N supply but are largely insensitive to temperature. Ea of photosynthesis (0.11 ± 0.06 eV, mean ± SE) and respiration (0.04 ± 0.17 eV) under N-limited growth is significantly smaller than Ea of growth rate under nutrient-replete conditions (0.77 ± 0.06 eV). The reduced temperature dependence of metabolic rates under nutrient limitation can be explained in terms of enzyme kinetics, because both maximum reaction rates and half-saturation constants increase with temperature. Our results suggest that the direct, stimulating effect of rising temperatures upon phytoplankton metabolic rates will be circumscribed to ecosystems with high-nutrient availability.

Investigation of second grade fluid through temperature dependent thermal conductivity and non-Fourier heat flux

Abstract Here we investigated stagnation point flow of second grade fluid over a stretchable cylinder. Heat transfer is characterized by non-Fourier law of heat flux and thermal stratification. Temperature dependent thermal conductivity and activation energy are also accounted. Transformations procedure is applying to transform the governing PDE’s into ODE’s. Obtained system of ODE’s are solved analytically by HAM. Influence of flow variables on velocity, temperature, concentration, skin friction and Sherwood number are analyzed. Obtained outcome shows that velocity enhanced through curvature parameter, viscoelastic parameter and velocities ratio variable. Temperature decays for larger Prandtl number, thermal stratification, thermal relaxation and curvature parameter. Sherwood number and concentration field show opposite behavior for higher estimation of activation energy, reaction rate, curvature parameter and Schmidt number.

String-like collective motion in the α- and β-relaxation of a coarse-grained polymer melt.

Relaxation in glass-forming liquids occurs as a multi-stage hierarchical process involving cooperative molecular motion. First, there is a "fast" relaxation process dominated by the inertial motion of the molecules whose amplitude grows upon heating, followed by a longer time α-relaxation process involving both large-scale diffusive molecular motion and momentum diffusion. Our molecular dynamics simulations of a coarse-grained glass-forming polymer melt indicate that the fast, collective motion becomes progressively suppressed upon cooling, necessitating large-scale collective motion by molecular diffusion for the material to relax approaching the glass-transition. In each relaxation regime, the decay of the collective intermediate scattering function occurs through collective particle exchange motions having a similar geometrical form, and quantitative relationships are derived relating the fast "stringlet" collective motion to the larger scale string-like collective motion at longer times, which governs the temperature-dependent activation energies associated with both thermally activated molecular diffusion and momentum diffusion.

Photothermal DNA Release from Laser-Tweezed Individual Gold Nanomotors Driven by Photon Angular Momentum

Gold nanoparticles offer a unique possibility for contact-free bioanalysis and actuation with high spatial resolution that increases their potential for bioapplications such as affinity-based biosensing, drug delivery, and cancer treatment. Here we demonstrate an ultrasensitive optomechanical method for probing and releasing DNA cargo from individual gold nanoparticles trapped and manipulated by laser tweezers. Single nanorods are operated as rotational nanomotors, driven and controlled by circularly polarized laser light in aqueous solution. By rotational dynamics analysis, we resolve differences in the thickness of adsorbed ultrathin molecular layers, including different DNA conformations, with nanometer resolution. We then utilize photothermal heating to release DNA from single nanomotors while measuring the temperature-dependent kinetics and activation energy of the DNA melting process. The method opens new possibilities for optomechanical quantification and application of thermally induced molecular transitions in strongly confined geometries, such as inside microfluidic devices and single cells.