Induced Temperature Gradient Research Articles

Origin of the light-induced spin currents in heavy metal/magnetic insulator bilayers.

Light-induced spin currents with the faster response is essential for the more efficient information transmission and processing. Herein, we systematically explore the effect of light illumination energy and direction on the light-induced spin currents in the W/Y3Fe5O12 heterojunction. Light-induced spin currents can be clearly categorized into two types. One is excited by the low light intensity, which mainly involves the photo-generated spin current from spin photovoltaic effect. The other is caused by the high light intensity, which is the light-thermally induced spin current and mainly excited by spin Seebeck effect. Under low light-intensity illumination, light-thermally induced temperature gradient is very small so that spin Seebeck effect can be neglected. Furthermore, the mechanism on spin photovoltaic effect is fully elucidated, where the photo-generated spin current in Y3Fe5O12 mainly originates from the process of spin precession induced by photons. These findings provide some deep insights into the origin of light-induced spin current.

Numerical Simulations of Combined Dielectrophoresis and Alternating Current Electrothermal Flow for High-Efficient Separation of (Bio)Microparticles.

High-efficient separation of (bio)microparticles has important applications in chemical analysis, environmental monitoring, drug screening, and disease diagnosis and treatment. As a label-free and high-precision separation scheme, dielectrophoresis (DEP) has become a research hotspot in microparticle separation, especially for biological cells. When processing cells with DEP, relatively high electric conductivities of suspending media are sometimes required to maintain the biological activities of the biosample, which results in high temperature rises within the system caused by Joule heating. The induced temperature gradient generates a localized alternating current electrothermal (ACET) flow disturbance, which seriously impacts the DEP manipulation of cells. Based on this, we propose a novel design of the (bio)microparticle separator by combining DEP with ACET flow to intensify the separation process. A coupling model that incorporates electric, fluid flow, and temperature fields as well as particle tracking is established to predict (bio)microparticle trajectories within the separator. Numerical simulations reveal that both ACET flow and DEP motion act in the same plane but in different directions to achieve high-precision separation between particles. This work provides new design ideas for solving the very tricky Joule heating interference in the DEP separation process, which paves the way for further improving the throughput of the DEP-based (bio)microparticle separation system.

On the use of an induced temperature gradient and full‐field measurements to investigate and model the thermomechanical behaviour of an austenitic stainless steel 316

AbstractA temperature gradient was induced in 316 stable austenitic stainless‐steel tension specimens, and the strain and temperature evolution during tensile deformation was monitored using optical and infrared cameras. The combination of global load with full‐field strain and temperature provided local information on the thermomechanical state of the investigated material. The deformation did not fully concentrate on the hotter portion of the specimen, but instead, the hottest portion strain hardened enough so that the colder portions of the specimen also experienced plastic deformation. Evidently, heat release occurred with plastic deformation and altered the initial temperature gradient as deformation progressed. The Taylor–Quinney coefficient was computed in integral and differential forms, and both are presented as a function of temperature and strain. The Johnson–Cook plasticity model was calibrated through an inverse method procedure in which only five tests were used, and the obtained temperature and strain rate dependencies of the model were comparable to those found in the literature for the same material. A local analysis was done to quantify the impact of adiabatic heating on the mechanical behaviour of the material.

Influence of non-irradiated surface optical absorber on temperature gradient induced by photothermal effect in a thin film

This paper presents the model of surface temperature variations, resulting from the photothermal effect induced in a ?thin film ? highly absorbing surface layer? structure, where the thin film is irradiated. The influence of the optical absorption coefficient and sample thickness on the induced temperature gradient is analyzed. It is shown that, depending on the product of these parameters (optical absorbance) in the described structure, the phenomenon of inverse temperature gradient can occur, further influencing the direction and the magnitude of thermoelastic displacement.

The Dufour Effect in Film Cooling Experiments With Foreign Gases

Abstract In typical film cooling experiments, the adiabatic wall temperature may be determined from surface temperature measurements on a low thermal conductivity model in a low-temperature wind tunnel. In such experiments, it is generally accepted that the adiabatic wall temperature must be bounded between the coolant temperature and the freestream recovery temperature as they represent the lowest and highest temperature introduced into the experiment. Many studies have utilized foreign gas coolants to alter the coolant properties such as density and specific heat to more appropriately simulate engine representative flows. In this paper, we show that the often ignored Dufour effect can alter the thermal physics in such an experiment from those relevant to the engine environment that we generally wish to simulate. The Dufour effect is an off-diagonal coupling of heat and mass transfer that can induce temperature gradients even in what would otherwise be isothermal experiments. These temperature gradients can result in significant errors in the calibration of various experimental techniques, as well as lead to results that at first glance may appear non-physical such as adiabatic effectiveness values not bounded by zero and one. This work explores Dufour effect induced temperature separation on two common cooling flow schemes, a leading edge with compound injection through a cylindrical cooling hole, and a flat plate with axial injection through a 7–7–7-shaped cooling hole. Air, argon, carbon dioxide, helium, and nitrogen coolant were utilized due to their usage in recent film cooling studies.

Femtosecond laser induced thermophoretic writing of waveguides in silicate glass

Here in, the fs-laser induced thermophoretic writing of microstructures in ad-hoc compositionally designed silicate glasses and their application as infrared optical waveguides is reported. The glass modification mechanism mimics the elemental thermal diffusion occurring in basaltic liquids at the Earth’s mantle, but in a much shorter time scale (108 times faster) and over a well-defined micrometric volume. The precise addition of BaO, Na2O and K2O to the silicate glass enables the creation of positive refractive index contrast upon fs-laser irradiation. The influence of the focal volume and the induced temperature gradient is thoroughly analyzed, leading to a variety of structures with refractive index contrasts as high as 2.5 × 10–2. Two independent methods, namely near field measurements and electronic polarizability analysis, confirm the magnitude of the refractive index on the modified regions. Additionally, the functionality of the microstructures as waveguides is further optimized by lowering their propagation losses, enabling their implementation in a wide range of photonic devices.

Combined thermomechanical and optical simulations of planar-optical polymer waveguides

In this work, we describe a theoretical approach for combined thermal, mechanical and optical simulation and analysis of planar polymer waveguides. We consider a finite element approach for thermal and stress/deformation simulation. Also, a Crank-Nicholson finite difference beam propagation method (CN-BPM) is implemented to perform the optical simulation. The results of the finite element (thermo-mechanical) analysis are coupled with the CN-BPM results to carry out the optical simulation of poly(methyl methacrylate) (PMMA) waveguides as function of temperature. For thermal simulation, a model was designed where a polysilicon microheater was added to the upper cladding of the PMMA waveguides to vary the temperature between 20 °C and 200 °C. Thus, the impact of the induced temperature gradients on the refractive index modulation of the PMMA waveguides and the corresponding change in numerical aperture are obtained. In addition, the temperature gradients influence the beam intensity profiles and the movement of the primary eyes within the optical waveguides, thus, impacting the optical properties. Furthermore, the thermally induced mechanical stress and deformation were calculated for transverse and axial directions. In the next step, validation of the model by systematic experimental studies will be performed. In general, our approach provides a toolbox for more comprehensive multi-physics theoretical analysis of polymer-optical waveguides which, in future, can be extended to more complex and functional structures as required for flexible sensor networks, as example.

Bulk Si production from Si–Fe melts under temperature gradients, part I: Growth and characterization

Silicon growth from Si–Fe melts was investigated in view of producing high-quality Si using induced temperature gradients. Scanning electron microscopy coupled with energy-dispersive spectroscopy and micro-computed tomography analyses were used to characterize the quality of bulk Si, in particular its microstructure and purity. The migration of Si and Fe atoms along a temperature gradient in the mushy zone of the Si–Fe alloy was evidenced, which became the source of Si precipitation on the C substrate, and ultimately forming bulk Si. Furthermore, the content of residual Fe–Si can be decreased in the bulk Si by controlling the temperature gradient, the holding time, and the alloy composition.

Numerical analyses of crack path instabilities in quenched plates

Crack path instabilities are observed in rapidly quenched rectangular glass plates whereby wavy crack patterns form as a result of the induced temperature gradients. The peculiar characteristic of these instabilities is that the speed of propagation is several order of magnitudes lower that the Rayleigh wave speed. Experimental studies have shown the dependence of the instabilities on certain geometrical, material, and experimental parameters (e.g. plate width, material toughness, speed of quenching). By perturbing these parameters cracks are observed to propagate along a straight line, oscillate with a periodic sinusoidal or semi-circle like morphology, or propagate in a supercritical manner. Here we formulate the problem of a propagating crack in a brittle thermoelastic material while considering the possibility of the crack undergoing bursts of supercritical crack propagation, by extending the model in Negri and Ortner (2008). We also describe a novel higher order computational framework for its numerical solution centered around Universal Meshes, Mapped Finite Element Methods, and Interaction Integral Functionals. We verify the convergence of the results and compare them against experiments. We reveal crack behaviors not previously observed. Particularly we discuss periods of sudden crack propagation, followed by temporary arrest and crack kinking. We identify various crack morphologies: sinusoidal, asymmetric, semi-circle, kinked and flattened oscillations. We investigate the frequency content of the oscillatory crack paths and study their relation to the dominating problem parameters. Additionally, we identify two new thresholds in phase space corresponding to the transition from oscillatory propagation to rapid propagation and arrest, as well as from permanent crack arrest to temporary crack arrest followed by kinking and branching.

Early-Stage Temperature Gradients in Glazed Spandrels Due to Aesthetical Features to Support Design for Thermal Shock

Investigating thermal breakage of glass panes requires careful analysis of the environmental boundary conditions to determine the expected thermal gradient between the sunlit and shaded parts of the glass. This is particularly critical for glazed spandrels, where an opaque posterior insulation layer normally increases the system’s temperatures. The choice of the spandrel system should also be evaluated against the aesthetical impression that it conveys. The aim of this study is to understand how early design factors, such as aesthetical features like color, are driving temperature gradients in the glazed pane to design for thermal shock. Multiple finite-differences analyses in a quasi-static regime for non-ventilated, single glazed spandrels were conducted in three locations (London, New York and Mumbai). Results were then analyzed via a general linear model in SAS 9.4 and Tuckey post hoc analysis. It was shown that a low absorptance of the back insulation (e.g., light color) can lead to a wide range of possible temperature gradients depending on the glass transparency, with higher values of the thermally induced temperature gradients for more opaque glasses. Conversely, a high absorptance of the insulation layer leads to moderate values of glass temperature gradients, which are not substantially sensitive to the effect of the glass transparency.

高功率板条激光介质的纵向强制对流换热技术

Thermal problem becomes more prominent in the highly-pumped laser gain mediums, for which, the forced convective heat transfer with the advantages of reliability and durability is widely used. However, a flow direction induced temperature gradient always appears within the laser operating substance during the convective heat transfer. Subsequently, it is significantly responsible for the detrimental thermal stress which mainly cause the wave front distortion. Herein, considering the idea of temperature matching between flow field and the operating substance, a cooling configuration for double face pumped slab crystal based on longitudinal forced convective heat transfer was presented, which showed a more efficient cooling and achieved a most homogeneous temperature distribution within the crystal. The influences of flow rate, state of flow field and surface roughness were systematically studied that a fully developed flow state, higher flow rate and rougher surface lead to an improvement in cooling capability. In the simulation with 30 L/min flow rate, the calculated convective heat transfer coefficient was as high as 104 W·m−2·K−1, and even higher when a more coarse surface was implemented. Furthermore, a module based on the configuration was fabricated and the experimental results agree well with the simulation, which shows a good temperature distribution and very weak thermal lensing is achieved.

The effect of solidification direction with respect to gravity on ice-templated TiO2 microstructures

Unidirectional ice-templating produces materials with aligned, elongated pores via: (i) directional solidification of particle suspensions wherein suspended particles are rejected and incorporated between aligned dendrites, (ii) sublimation of the solidified fluid, and (iii) sintering of the particles into elongated walls which are templated by the ice dendrites. Most ice-templating studies utilize upward solidification techniques, where solid ice is located at the bottom of the solidification mold (closest to the cold source), the liquid suspension is above the ice, and the solidification front advances upward, against gravity. Liquid water reaches its maximum density at 4 °C; thus, liquid nearest the solid/liquid interface, at 0ºC, is less dense than warmer liquid above (up to 4 °C, above which, a density inversion occurs, and liquid density decreases with increasing temperature). The lower density liquid nearest the solidification front is thus expected to rise due to buoyancy, promoting convective fluid motion in the liquid during solidification. Here, we investigate the effect of solidification direction with respect to the direction of gravity on ice-templated microstructures to study the role of buoyancy-driven fluid motion during solidification. We hypothesize that, for upward solidification, the convective fluid motion that results from the liquid density gradient occurs near the solidification front. For downward solidification, we expect that this fluid motion occurs farther away from the solidification front. Aqueous suspensions of TiO2 nanoparticles (10–30 nm in size, 10, 15, and 21 vol.%) are solidified upward (against gravity, with ice on bottom and water on top), downward (water on bottom, ice on top), and horizontally (perpendicular to gravity). Microstructural investigation of sintered samples shows evidence of buoyancy-driven, convective fluid flow during solidification for samples solidified upwards (against gravity), including (i) tilting of the wall (and pore) orientation with respect to the induced temperature gradient, (ii) ice lens defects (cracks oriented perpendicular to the freezing direction), and (iii) radial macrosegregation. These features are not observed for downward nor horizontal solidification configurations, consistent with the hypothesis that convective fluid motion does not interact directly with the solidification front for downward solidification.

Magnetic field influence on asphaltene aggregation monitored by diffusion NMR spectroscopy: Is aggregation reversible at high magnetic fields?

We have shown that asphaltene self-association induces temperature gradients and convection currents and it is necessary to suppress these effects to obtain accurate data by DOSY NMR techniques. An interesting finding of this investigation is that aggregation process has been found to be affected by the magnetic field. The obtained results have shown for the first time a significant increase in diffusion coefficients of aggregates upon the increase in the magnetic field strength indicating higher molecular mobility of the present species and possible degradation of aggregates. We have shown that changes in diffusion coefficients are not a consequence of relaxation or viscosity effects. It is not yet clear what the reason for such behavior is and whether this effect is intrinsic. Traces of paramagnetic metals present in asphaltene samples may play a role in this process but this hypothesis is yet to be explored. Furthermore, this study has revealed a similar concentration range in which the Middle East crude oil asphaltenes aggregate being in agreement with previous results. Aggregates form different molecular weight species with different composition and number of monomer units. Ongoing experiments will throw more light on the magnetic field dependent asphaltene aggregation. This may have an impact on developing strategies for resolving asphaltene precipitation issues.

Foaming of polystyrene and poly(methyl methacrylate) multilayered thin films with supercritical carbon dioxide

A multilayer film consisting of 32 alternating layers of poly(methyl methacrylate) (PMMA) and polystyrene (PS) produced by forced assembly multiplying co-extrusion has been foamed using supercritical CO2 as a physical blowing agent. Foamability was first assessed by foaming under an induced temperature gradient. Foams were then generated under isothermal conditions at 60, 70, 80, and 90 ⁰C and 100, 200, and 300 bar using a special mold. PMMA layers were found to foam at 60, 70, 80, and 90 ⁰C while PS layers displayed porous morphology only in foams generated at 80 and 90 ⁰C. The films were observed to display delamination during foaming which became more severe in foams generated from higher temperatures with longer saturation times. Preferential foaming in the PMMA over the PS layers is discussed and is linked to the differences in the CO2 sorption and the depression of Tg in the respective polymers.

A molecular dynamics study about the mechanisms of liquid thermal transpiration flow in nanotubes

The possibility and the mechanisms of liquid transport in nanotubes by the effect of thermal transpiration phenomenon are discussed using molecular dynamics simulation (MD) method. Based on the results, it is demonstrated that the thermal transpiration flow for liquids can be established by imposing a temperature gradient to the wall of a nanoscale tube. It can be deduced from MD results that this flow is mainly due to the asymmetric pressure distribution in a narrow region near the tube wall. Fluid layering phenomenon near the wall together with the induced temperature gradient causes the potential component of the pressure in the narrow region near the cold side of the tube becomes higher than the hot side. This causes the fluid to flow in the direction of the temperature gradient. It is shown that the thermal transpiration flow is a plug-like flow. This phenomenon can be used in designing thermal pumps for fluids. The effect of several effective parameters in fluid pumping is investigated. Results show that increasing the imposed temperature gradient, decreasing the nanotube diameter and increasing the fluid-wall binding energy augments the thermal transpiration effect.

Light-induced self-thermophoresis of Janus spheroidal nanoparticles.

A theoretical framework is provided for determining the self-thermophoretic velocity of a light irradiated spheroidal Janus nanoparticle consisting of symmetric dielectric and perfectly conducting semi-spheroids. The analysis is based on solving the linearized Joule heating problem due to uniform laser irradiance and on explicitly determining the temperature fields inside and outside the particle. We employ the thermoelectric (Peltier- Seebeck) methodology to find the surface self - induced temperature gradient and the related slip velocity which determines the autonomous phoretic (self - propulsion) mobility of the Janus particle. Simplified explicit expressions for the self - thermophoretic velocities of spheroidal (prolate and oblate) Janus particles in terms of their aspect ratios are found and few practical limiting cases (i.e., sphere, disk and needle) are also discussed.

Nonlocal and Nonlinear Surface Plasmon Polaritons and Optical Spatial Solitons Induced by the Thermocapillary Effect.

We study the propagation of surface plasmon polaritons (SPPs) on a metal surface which hosts a thin film of a liquid dielectric. The Ohmic losses that are inherently present due to the coupling of SPPs to conductors' electron plasma, induce temperature gradients and fluid deformation driven by the thermocapillary effect, which lead to a nonlinear and nonlocal change of the effective dielectric constant. The latter extends beyond the regions of highest optical intensity and constitutes a novel thermally self-induced mechanism that affects the propagation of the SPPs. We derive the nonlinear and nonlocal Schrödinger equation that describes propagation of low intensity SPP beams, and show analytically and numerically that it supports a novel optical spatial soliton excitation.

Trapping, sorting and transferring of micro-particles and live cells using electric current-induced thermal tweezers

We demonstrate the manipulation of micro-sized particles in water using Joule heating induced temperature gradient. Using simple sputter deposition and photolithography, we have fabricated disparate gold micro-structures on silica glass. Electric current crowding at narrow channels results in temperature gradients that can be readily controlled by varying the electric power input. This also, in turn, leads to a tunable balance between forces due to convective flow and thermophoresis in the surrounding aqueous medium. The net force can actually be used for trapping target objects. We have used the device to perform sorting, trapping and transferring of polystyrene sphere (PS) and live cells. The reported method offers a new approach for photon-free particle manipulation, while also holds promise for direct integration with lab-on-chip devices.

Thermoelectricity Enhanced Electrocatalysis.

We show that thermoelectric materials can function as electrocatalysts and use thermoelectric voltage generated to initiate and boost electrocatalytic reactions. The electrocatalytic activity is promoted by the use of nanostructured thermoelectric materials in a hydrogen evolution reaction (HER) by the thermoelectricity generated from induced temperature gradients. This phenomenon is demonstrated using two-dimensional layered thermoelectric materials Sb2Te3 and Bi0.5Sb1.5Te3 where a current density approaching ∼50 mA/cm2 is produced at zero potential for Bi0.5Sb1.5Te3 in the presence of a temperature gradient of 90 °C. In addition, the turnover frequency reaches to 2.7 s-1 at 100 mV under this condition which was zero in the absence of temperature gradient. This result adds a new dimension to the properties of thermoelectric materials which has not been explored before and can be applied in the field of electrocatalysis and energy generation.

Electrokinetically driven continuous-flow enrichment of colloidal particles by Joule heating induced temperature gradient focusing in a convergent-divergent microfluidic structure

Enrichment of colloidal particles in continuous flow has not only numerous applications but also poses a great challenge in controlling physical forces that are required for achieving particle enrichment. Here, we for the first time experimentally demonstrate the electrokinetically-driven continuous-flow enrichment of colloidal particles with Joule heating induced temperature gradient focusing (TGF) in a microfluidic convergent-divergent structure. We consider four mechanisms of particle transport, i.e., advection due to electroosmosis, electrophoresis, dielectrophoresis and, and further clarify their roles in the particle enrichment. It is experimentally determined and numerically verified that the particle thermophoresis plays dominant roles in enrichment of all particle sizes considered in this study and the combined effect of electroosmosis-induced advection and electrophoresis is mainly to transport particles to the zone of enrichment. Specifically, the enrichment of particles is achieved with combined DC and AC voltages rather than a sole DC or AC voltage. A numerical model is formulated with consideration of the abovementioned four mechanisms, and the model can rationalize the experimental observations. Particularly, our analysis of numerical and experimental results indicates that thermophoresis which is usually an overlooked mechanism of material transport is crucial for the successful electrokinetic enrichment of particles with Joule heating induced TGF.