Physical change | EPFL Graph Search

Physical changes are changes affecting the form of a chemical substance, but not its chemical composition. Physical changes are used to separate mixtures into their component compounds, but can not usually be used to separate compounds into chemical elements or simpler compounds. Physical changes occur when objects or substances undergo a change that does not change their chemical composition. This contrasts with the concept of chemical change in which the composition of a substance changes or one or more substances combine or break up to form new substances. In general a physical change is reversible using physical means. For example, salt dissolved in water can be recovered by allowing the water to evaporate. A physical change involves a change in physical properties. Examples of physical properties include melting, transition to a gas, change of strength, change of durability, changes to crystal form, textural change, shape, size, color, volume and density. An example of a phy

Modelling vapour expansion of extruded cereals

Laurent Lach

The vapour expansion of extruded cereals is a versatile technique used in the food processing industry to produce a wide variety of light, crisp & crunchy products such as snacks, breakfast cereals and pet foods. The range of textures that can be produced depends on a complex interaction of a many parameters controlling the expansion phase making the development and optimisation of the process a difficult task involving many trials. This thesis is aimed at developing a numerical model of vapour expansion of extruded cereal to improve our understanding of the physical process and to help speed up the development of new products. Rather than try to model the growth of each bubble explicitly a more economical "micro-macro" approach was developed involving the coupling of a 1D model for single bubble growth with a CFD code for modeling the bulk fluid flow. Several bubble growth models have previously been developed and coupled with simplified models for the flow inside the die, but none of them have succeeded in predicting even the right trend in the observed dependence of the expansion with operating conditions. A microscopic model was first developed along similar lines to those described in the literature with some improvements and then coupled to different macroscopic flow models. Although the process requires at least a 2D coupling to properly capture the full behaviour of vapour expansion, considerable insight was gained by coupling with a 1D compressible macroscopic flow model assuming isotropic expansion in order to predict the evolution of the extrudate outside the die. In particular the 1D model predicted the growth and maximum extrudate diameter in qualitative agreement with experimental measurements and showed for the first time why the expansion is observed to be stronger with lower water content or lower temperature. It showed for example that increasing the water content influences the expansion more by changing the partition between lateral and longitudinal expansion than by changing the degree of volume expansion itself. In other words, the increased water content decreases the viscosity and increases the nucleation pressure so the expansion occurs sooner and more rapidly and hence spends more time expanding inside the die where it can only accelerate. There is thus less bubble growth remaining for lateral expansion by the time it reaches the die outlet. For very low moisture content the model predicts a weak reduction in lateral expansion due to the reduced availability of water for vaporization. The resulting peak was also observed experimentally. Two types of 2&3D coupling with the Fluent CFD code were developed and tested: 1) where the equations for the microscopic model are solved in Eulerian form as User Defined Scalar (UDS) equations and 2) where the microscopic model equations are solved in Lagrangian form along stream lines computed using the Discrete Phase Model (DPM). In both cases the free-surface expansion of the extrudate from the die is simulated with the Volume of Fluid (VOF) capability in Fluent. Unfortunately neither approach could be made to converge stably due to numerical problems that could not be rectified due to the closed source nature of Fluent. A similar problem with the 1D model was solved by deriving a more intimate coupling between the micro and macro equations to reduce their order.

EPFL2006

Approches numériques du transport de charges dans les semiconducteurs organiques

Hocine Houili

Using plastics or dyes like molecular semiconductors to make optoelectronic devices at low cost is a project in industry that is of great interest to the "Information Community" of this beginning of the 21st century. Some applications of this technology have already been realized including flexible flat-panel displays made with organic light-emitting diodes, organic transistors for active matrices, organic solar cells and sensors. In all these applications it is essential to deduce and understand the laws that determine the charge transport and the light emission in these materials. The goal of this thesis is to contribute to the theoretical models that describe these transport processes. In order to predict the behaviour of charge carriers in a given device, one can choose two methods. One can either numerically solve the master equations that describe the general behaviour of charges and currents in each part of the device, or one can simulate the detailed behaviour of each charge in the device by fixing the conditions of its movement using a procedure known as "Monte Carlo". In this thesis I addressed the two approaches. I have been interested successively in the prediction of the electric properties of multilayer light-emitting devices, considered as a whole. Then I was able to understand in detail the processes accruing at the organic-organic interfaces by using the Monte Carlo method. My last project involved the study of the channel of an organic transistor. In each one of these cases I took account of the characteristic features of small molecule organic materials in order to develop models for charge transport for the study and optimization of two important types of organic devices : organic light-emitting diodes and field-effect transistors. Amorphous organic semiconductors are mainly used for the fabrication of organic diodes. They are characterized by an energetically disordered density of states that is assumed to be Gaussian. The transport of charges in these materials occurs via hopping from one molecule to another in this density of states. The correlations between the energies of the molecular sites have important effects on transport ; the dependence of mobility on the applied electric field in particular. If the energetic disorder is due to the random orientations of the permanent dipoles of the molecules, then the correlations between the orientations of these dipoles can profoundly change the spatial configuration of the energetic disorder and consequently the charge transport. Due to the fact that organic diodes often comprise of multiple organic layers, understanding device behaviour at the organic-organic interfaces is of critical importance. Electrons and holes accumulate close to these interfaces and they are therefore more likely to recombine and emit light. The Coulomb interactions between the charge carriers and the energy disorder are at the origin of complex processes taking place at these accumulation regions. I studied these processes through a 3D multi-particle Monte Carlo simulation. The short-range Coulomb repulsion forbids the double-occupancy of the sites and shifts the energy level for injection of carriers across the interface upward, thus increasing the electric current of the diode. The long-range Coulomb interactions increase the electric field and the current at the interface although this increase is less important than suggested by 1D models. Effects related to the hopping process such as the effect of the exact form of the hopping law and backward or return hop at the interface are clearly discussed. The Monte Carlo simulations are cross-compared with an analytical model for hopping transport across the interface. This comparison made it possible to understand other aspects of the physical processes at organic-organic interface and supports the Monte Carlo simulations. The study of the electron-hole recombination showed a decrease of the cross-section with an increase of the applied electric field, but this decrease is less than what can be expected by the classical Langevin theory. In spite of their efficiency, Monte Carlo simulations are not attractive when modelling complex multi-layer diodes. The 1D models are better suited in this case. A 1D model called MOLED, which was developed at Laboratoire d'Optoélectronique des Matériaux Moléculaires (LOMM) for the simulation of the organic diodes, is used and described in detail in this thesis. We used MOLED to explore the effect of the energy correlations on the electric field dependence of the mobility. In fact we simulated the time-of-flight experiment, a common method used to determine of the carrier mobility. The crystalline organic semiconductors achieve mobility performances close or even better than amorphous silicon. Their use for the fabrication of field-effect transistors is thus a good choice. The models for charge transport in crystalline organic materials differ much from their counterparts for amorphous organic materials. Indeed band models are more appropriate to describe charge transport in crystalline organic materials. However it is very important to take into account the effects of electronic polarization in these models. I developed a method of calculation of these effects of polarization in the bulk of the organic semiconductor and I extended it to the case of the interface with a dielectric insulator, a situation encountered in the channel of the organic transistor. These calculations showed that when the polarity of the dielectric increases the inter-molecular transfer integral decreases. This is confirmed by experimental measurements showing increasingly low values of mobility when the permittivity of the insulator increases.

EPFL2006

Study of the kinetics of condensation and evaporation of water vapor over atmospherically relevant pure and doped ice films

Christophe Eric Ludovic Delval

This thesis presents the different steps in the development of a multiple diagnostic experimental setup and the results concerning the kinetics of condensation and evaporation of water over pure ice, as well as ice films doped with a known amount of atmospheric trace gases. The goal of this project is to obtain reliable data concerning the evaporative flux, Jev [molec cm-2 s-1], and the concomitant rate of condensation, kcond [s-1], of H2O molecules in the presence of an ice film. A further aim of this work is the detailed investigation of the impact on the aforementioned kinetic processes of the deposition of atmospherically relevant trace gases such as hydrogen chloride (HCl), hydrogen bromide and nitric acid (HNO3) under well defined conditions including those that are representative of the atmosphere. The new experimental apparatus consisting of a stainless steel reactor may be used under three different regimes, namely static, stirred flow and dynamic condition which correspond to equilibrium, low rate of pumping and molecular flow, respectively. The separation of the kinetics into individual processes is principally afforded by the application of these 3 regimes. First, using a combination of the first two regimes, we have measured Jev and γ of H2O in the case of pure as well as HCl- or HBr-doped ice in the range 190-240 K. The resulting Arrhenius expressions for pure ice are: in the presence of an average mole fraction of HCl, ΧHCl, in the range 3.2·10-5 to 6.4·10-3 we have found: and for ΧHBr at a HBr mole fraction smaller than 6.4·10-3 was found, where R=1.987 cal mol-1 K-1. Owing to the rate separation, γ applies to an individual rate process and should be called an accommodation rather than an uptake coefficient. Of utmost importance is the fact that during most of the ice film evaporation the vapor pressure remained that of pure ice even in the presence of small amounts of HCl or HBr on the order of a fraction to several formal monolayers. This is consistent with the enthalpies of sublimation that may be derived following the relation ΔHsubl0 = Econd - Eev. It leads to different thermochemical parameters A (pre-exponential factor) and E values (energy of activation) for the three cases that are identical within experimental uncertainty. In addition, FTIR spectroscopy of the ice films involved in these experiments in transmission have revealed that different species such as amorphous HCl·H2O mixtures or the crystalline HCl·6H2O hexahydrate are formed depending on the doping protocol. These two species significantly affect the temporal change of Jev each in a different manner. For the sake of precision and accuracy of the kinetic results, a quartz crystal microbalance (QCM) was subsequently added to the experimental apparatus and used as a substrate for ice film growth. The development of a QCM-based device for the investigation of the kinetic processes of ice films at atmospherically relevant temperatures was a challenge we have successfully surmounted whereas previously published attempt had given barely convincing results. The Arrhenius representation of the zero order evaporative flux Jev (molec cm-2 s-1) of H2O from pure ice displays a discontinuity at 193±2 K that could be observed owing to the accuracy and precision of the QCM measurements. It leads to: In addition, γ of H2O onto ice was found to be lower than 0.5 for temperatures below 240 K in contrast to the value of 1.0 which is often encountered in the literature. Subsequently, the deposited ice films have been doped with HNO3 using different protocols depending on parameters such as temperature, rate of deposition of HNO3 onto ice as well as number ofHNO3 molecules deposited. The reproducible and quantitative dosing of the doping gas was key to the quality of the results. This study has shed light on a complex interplay between several of these parameters that affect the continuous decrease of Jev during ice evaporation. Using atmospherically relevant conditions in the doping of ice by HNO3 we have observed crystalline α-NAT being most likely the first species that forms in/on ice films. During most of the evaporation of the ice film doped with known amounts of HNO3 we have checked that the vapor pressure over the ice film remains that of pure ice. Finally, HCl-doped ice films have been investigated in experiments that are complementary to the first set in order to verify that HCl doping has a similar impact on the change of Jev of H2O in the presence of small albeit known amounts of HCl. The doping protocol is a fundamental parameter determining the decrease of Jev with H2O evaporation time of doped ice films. In summary, we have developed from scratch a new experimental apparatus that allows the separation of the kinetic processes of condensation and evaporation of pure as well as doped ice films using multiple diagnostics under the same experimental conditions which was heretofore not attainable. The main result for pure ice films is that the uptake coefficient, γ of H2O on ice is always lower than approximately 0.5 for atmospherically relevant temperatures. Its negative temperature dependence has been confirmed. An accurate control of the doping protocol has allowed us to observe that for a fraction to a few monolayers of the doping gases, HCl, HBr and HNO3, the nature of the aforementioned protocol is the predominant factor influencing the continuous temporal decrease of Jev and γ during doped ice film evaporation. Of utmost importance is the fact that the vapor pressure over the doped ice films remains that of pure water ice during almost all of the evaporation process. These results may be of importance in atmospheric modeling for determining the lifetime of ice particles involved in clouds in regions that are undersaturated in the vapor pressure of H2O over ice. In this way the physical process of evaporation affects the importance of heterogeneous chemistry of atmospheric trace gases on ice substrates representative of Cirrus clouds, aviation induced Cirrus clouds and contrails as well as type II PSC's.

EPFL2004