Thermal conductivity vs depth profiling using the hot disk technique-Analysis of anisotropic, inhomogeneous structures

A recently developed method for analyzing the thermal conductivity vs depth variation near a sample surface has been extended to include inhomogeneous samples with anisotropy. If not considered, the anisotropy ratio in the sample structure can distort the depth-position data of the original test method. The anisotropy ratio is introduced in the original computational scheme in order to improve the depth-position estimations for inhomogeneous structures with anisotropy. The proposed approach has been tested in experiments and shown to improve depth position mapping

(Mechanical effects of light on anisotropic micron-sized particles and their wetting dynamics at the water-air interface

Nous présentons une série d’expériences sur des particules micrométriques de polystyrène de formes ellipsoïdales. Les rapports d’aspects (k) des particules sont variables, de 0.2 à 8 environ. Ces ellipsoïdes sont manipulés dans l’eau par faisceau laser modérément focalisé. On observe la lévitation et l’équilibre dynamique de chaque particule, dans le volume et au contact d’une interface, solide-liquide ou liquide-liquide. Dans une première partie, nous montrons que des particules de k modéré sont piégées radialement. Par contre, les ellipsoïdes allongés (k>3) ou aplatis (k3) or flattened (k<0.3) ellipsoids never come to rest, and permanently “dance” around the beam, through coupled translation-rotation motions. The dynamics are periodic or irregular (akin to chaos) depending on the particle type and beam characteristics. We propose a 2d model that indeed predicts the bifurcation between static and oscillating states. In the second part, we apply optical levitation to study the transition from total to partial wetting of the particles at the water-air interface. We show that the dynamics of the transition is about independent of particle shape, and mainly governed by the pinning-depinning mechanism of the contact line

(Mechanical effects of light on anisotropic micron-sized particles and their wetting dynamics at the water-air interface

Nous présentons une série d’expériences sur des particules micrométriques de polystyrène de formes ellipsoïdales. Les rapports d’aspects (k) des particules sont variables, de 0.2 à 8 environ. Ces ellipsoïdes sont manipulés dans l’eau par faisceau laser modérément focalisé. On observe la lévitation et l’équilibre dynamique de chaque particule, dans le volume et au contact d’une interface, solide-liquide ou liquide-liquide. Dans une première partie, nous montrons que des particules de k modéré sont piégées radialement. Par contre, les ellipsoïdes allongés (k>3) ou aplatis (k3) or flattened (k<0.3) ellipsoids never come to rest, and permanently “dance” around the beam, through coupled translation-rotation motions. The dynamics are periodic or irregular (akin to chaos) depending on the particle type and beam characteristics. We propose a 2d model that indeed predicts the bifurcation between static and oscillating states. In the second part, we apply optical levitation to study the transition from total to partial wetting of the particles at the water-air interface. We show that the dynamics of the transition is about independent of particle shape, and mainly governed by the pinning-depinning mechanism of the contact line

Effets mécaniques de la lumière sur des particules anisotropes micrométriques et dynamique du mouillage à l’interface eau-air

We report experiments on ellipsoidal micrometre-sized polystyrene particles. The particle aspect ratio (k) varies between about 0.2 and 8. These particles are manipulated in water by means of a moderately focused laser beam. We observe the levitation and the dynamical state of each particle in the laser beam, in bulk water or in contact to an interface (water-glass, water-air, water-oil). In the first part, we show that moderate-k particles are radially trapped with their long axis lying parallel to the beam. Conversely, elongated (k>3) or flattened (k3) ou aplatis (k<0.3) ne peuvent pas être immobilisés. Ces particules « dansent » autour du faisceau, dans un mouvement permanent associant translation et rotation. Les mouvements sont périodiques, ou irréguliers (chaotiques) selon les caractéristiques de la particule et du faisceau. Un modèle en 2d est proposé qui permet de comprendre l’origine des oscillations. La seconde partie est une application de la lévitation optique pour une étude de la transition mouillage total-mouillage partiel des particules à l’interface eau-air. Nous montrons que la dynamique de la transition ne dépend pratiquement pas de la forme de particule, et qu’elle est déterminée par le mécanisme d’accrochage-décrochage de la ligne de contact

Optical levitation and long-working-distance trapping: From spherical up to high aspect ratio ellipsoidal particles

Radiation pressure forces from a moderately focused vertical laser beam are used to levitate transparent particles, a few micrometers in size. Having recalled basic results about levitation of spheres, and applications to long-working distance trapping, we turn to ellipsoid-shaped particles. Experiments are carried out with polystyrene particles, inside a glass chamber filled with water. The particles are lifted up to contact with the chamber top surface. We examine particle equilibrium in such conditions and show that the system ''bifurcates'' between static on-axis equilibrium with short ellipsoids, to sustained oscillations with longer ones. A similar Hopf bifurcation is found using a simple ray-optics model of the laser-ellipsoid interaction, providing a qualitative account of the observed oscillations

SIMULATION OF THE HOT DISK SENSOR: TEMPERATURE AND ELECTRIC CURRENT DISTRIBUTION

The Hot Disk method, also known as the transient plane source (TPS) technique, is an experimental approach for determining the thermal transport properties of materials. The core of the method is the Hot Disk sensor, an electrically conducting metallic foil (typically nickel), shaped as a double spiral, clad with a protective polymer film or mica. The double spiral serves simultaneously as heat source and temperature probe. The mean temperature increase of the TPS- sensor has been formulated from various analytical approaches such as: the concentric ring sources model, the thermal quadrupoles formalism, and concentric circular strips structure approach. However, full numerical simulation of the sensor has not been addressed so far. Here we develop 3D model of a Hot Disk sensor. The simulation provides information such as temperature and current distribution along each spiral which is not accessible from the experiment. Modeling feature, Joule heating coupled with heat transfer in solids of COMSOL Multiphysics software is used to simulate the sensor. The temperature and current distributions along the nickel wire is obtained. This can potentially be used in further optimizing geometry and estimating better parameters

Finite element modeling of the Hot Disc method

The Hot Disc method, also known as the transient plane source (TPS) technique, is an experimental approach to determine the thermal transport properties of materials. The core of the method is the Hot Disc sensor, an electrically conducting metallic strip, shaped as a double spiral clad with a protective polymer film. The mean temperature increase in the sensor has been approximated from various analytical approaches such as: the concentric ring sources model, the thermal quadrupoles formalism, and concentric circular strips structure approach. However, full numerical simulation of the sensor has not been addressed so far. Here we develop a 3D model of Hot Disc sensors and compare simulated mean temperature increase to experimental recordings. Joule heating coupled with heat transfer of solids (of COMSOL Multiphysics software) is used to simulate the working principle of the sensor. The volume mean temperature increase in the sensor from the simulations proves to be in a good agreement with the corresponding experimental recordings. The temperature distributions of the metallic strip are also evaluated and discussed with respect to the previous experimental findings. Furthermore, the current distribution across the strip is obtained. Such simulation can potentially be used in further optimizing geometry and parameter estimation

Thermal depth profiling of materials for defect detection using hot disk technique

A novel application of the hot disk transient plane source technique is described. The new application yields the thermal conductivity of materials as a function of the thermal penetration depth which opens up opportunities in nondestructive testing of inhomogeneous materials. The system uses the hot disk sensor placed on the material surface to create a time varying temperature field. The thermal conductivity is then deduced from temperature evolution of the sensor, whereas the probing depth (the distance the heat front advanced away from the source) is related to the product of measurement time and thermal diffusivity. The presence of inhomogeneity in the structure is manifested in thermal conductivity versus probing depth plot. Such a plot for homogeneous materials provides fairly constant value. The deviation from the homogeneous curve caused by defects in the structure is used for inhomogeneity detection. The size and location of the defect in the structure determines the sensitivity and possibility of detection. In addition, a complementary finite element numerical simulation through COMSOL Multiphysics is employed to solve the heat transfer equation. Temperature field profile of a model material is obtained from these simulations. The average rise in temperature of the heat source is calculated and used to demonstrate the effect of the presence of inhomogeneity in the system