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The Onsager heat of transport for p-tert-butyltoluene was measured, as part of a series of preliminary experiments towards the determination of the importance of temperature gradients on the air-sea flux of carbon dioxide. The results presented in this thesis imply that the temperature gradient is a major contributor to the magnitude of the air-sea flux. The heat of transport has been measured for the p-tert-butyltoluene system by measuring stationary-state pressure changes for known temperature differences on the vapour side of the interface. At the pressure ranges used the number of mean free paths was always outside the Knudsen zone, but the values of Q* were approximately 100 % of the latent heat of vaporisation. Departures from linearity of plots of P against ΔT are attributed to temperature jumps at the surface of the dry upper plate. Both the results taken for p-tert-butyltoluene and the earlier results for water from this laboratory fit to a Type III BET isotherm, where the c parameter is not constant. They also reveal the importance of the temperature gradient in determining the value of the thermal accommodation coefficient, and provide a new method of measuring thermal accommodation coefficients for a variety of surfaces and vapours

O trabalho apresenta uma investigação sobre a influência da umidade atmosférica no processo de absorção de oxigênio atmosférico por corpo de água através de sua superfície líquida livre. Os experimentos realizados em laboratório consistem na reoxigenação de uma massa de água, contida em um tanque cilíndrico, submetida à agitação mecânica. A atmosfera que reabastece de oxigênio a massa líquida é controlada e os ensaios são conduzidos com vários níveis de umidade do ar, o que provoca diferentes taxas de evaporação. Simultaneamente, determinam-se os coeficientes de reoxigenação, K2 (h-1), em cada experimento. Investiga-se neste trabalho a correlação entre esses dois parâmetros. O processo de reoxigenação é analisado inicialmente com o uso de metodologia clássica baseada em modelos matemáticos tradicionais (possivelmente inadequados). Propõe-se neste trabalho, um modelo original para essa mesma análise. / The influence of the atmospheric humidity on the process of absorption of oxygen by a water body through its free surface is presented herein. Laboratory experiments were run for the reoxygenation of a water mass kept in a cylindrical vessel under mechanical agitation. The quality of the atmospheric air transferring oxygen to water is controlled and the tests are run with several levels of air humidity, what renders different rates of evaporation. The corresponding coefficients of reaeration K2 (h-1) are determined for each experiment. The correlation between these two latter parameters is investigated. The process of reoxygenation is analyzed initially using a classical methodology based on traditional mathematical models (possibly inadequate). An original model for the same analysis is proposed in this work.

A theoretical model describing the motion of a small, fast rare gas atom as it passes over a liquid surface is developed and discussed in detail. A key feature of the model is its reliance on coarse-grained capillary wave and local mode descriptions of the liquid surface. Mathematically, the model is constructed with several concepts from probability and stochastic analysis. The model makes predictions that are quantitative agreement with neon-liquid surface scattering data collected by other research groups. These predictions include the dominance of single, rather than multiple, neon-liquid surface collision dynamics, an average of 60 % energy transfer from a neon atom upon colliding with a non-metallic surface, and an average of 25 % energy transfer upon colliding with a metallic surface. In addition to this work, two other investigations into the gas-liquid interface are discussed. The results of an experimental investigation into the thermodynamics of a gas flux through an aqueous surface are presented, and it is shown that a nitrous oxide flux is mostly due to the presence of a temperature gradient in the gas-liquid interface. Evidence for a reaction between a carbon dioxide flux and an ammonia monolayer on an aqueous surface to produce ammonium carbamate is also found. The second of these is an investigation into the mechanism of bromine production from deliquesced sodium bromide aerosol in the presence of ozone, and involves a sensitivity and uncertainty analysis of the computer aerosol kinetics model MAGIC. It is shown that under dark, non-photolytic conditions, bromine production can be accounted for almost exclusively by a reaction between gas-phase ozone and surface-bound bromide ions. Under photolytic conditions, bromine production instead involves a complicated interplay between various gas-phase and aqueous-phase reactions.

The Onsager heat of transport for p-tert-butyltoluene was measured, as part of a series of preliminary experiments towards the determination of the importance of temperature gradients on the air-sea flux of carbon dioxide. The results presented in this thesis imply that the temperature gradient is a major contributor to the magnitude of the air-sea flux. The heat of transport has been measured for the p-tert-butyltoluene system by measuring stationary-state pressure changes for known temperature differences on the vapour side of the interface. At the pressure ranges used the number of mean free paths was always outside the Knudsen zone, but the values of Q* were approximately 100 % of the latent heat of vaporisation. Departures from linearity of plots of P against ΔT are attributed to temperature jumps at the surface of the dry upper plate. Both the results taken for p-tert-butyltoluene and the earlier results for water from this laboratory fit to a Type III BET isotherm, where the c parameter is not constant. They also reveal the importance of the temperature gradient in determining the value of the thermal accommodation coefficient, and provide a new method of measuring thermal accommodation coefficients for a variety of surfaces and vapours

O trabalho apresenta uma investigação sobre a influência da umidade atmosférica no processo de absorção de oxigênio atmosférico por corpo de água através de sua superfície líquida livre. Os experimentos realizados em laboratório consistem na reoxigenação de uma massa de água, contida em um tanque cilíndrico, submetida à agitação mecânica. A atmosfera que reabastece de oxigênio a massa líquida é controlada e os ensaios são conduzidos com vários níveis de umidade do ar, o que provoca diferentes taxas de evaporação. Simultaneamente, determinam-se os coeficientes de reoxigenação, K2 (h-1), em cada experimento. Investiga-se neste trabalho a correlação entre esses dois parâmetros. O processo de reoxigenação é analisado inicialmente com o uso de metodologia clássica baseada em modelos matemáticos tradicionais (possivelmente inadequados). Propõe-se neste trabalho, um modelo original para essa mesma análise. / The influence of the atmospheric humidity on the process of absorption of oxygen by a water body through its free surface is presented herein. Laboratory experiments were run for the reoxygenation of a water mass kept in a cylindrical vessel under mechanical agitation. The quality of the atmospheric air transferring oxygen to water is controlled and the tests are run with several levels of air humidity, what renders different rates of evaporation. The corresponding coefficients of reaeration K2 (h-1) are determined for each experiment. The correlation between these two latter parameters is investigated. The process of reoxygenation is analyzed initially using a classical methodology based on traditional mathematical models (possibly inadequate). An original model for the same analysis is proposed in this work.

L’évaporation d’une solution solvant/soluté est un processus transitoire qui prend fin lorsque le solvant a totalement disparu. Le refroidissement créé par le changement de phase provoque des gradients à la fois thermiques et de concentration en solvant. Ces homogénéités diffusent ensuite dans l’épaisseur de la solution et sont susceptibles d’engendrer un écoulement fluide. L’origine de cette convection peut être liée à des variations de tension de surface ou de densité. Des travaux expérimentaux ont montré que l’épaisseur des dépôts issus de séchages de solutions solvant/soluté semblait pouvoir être corrélée avec les cellules de convection de la zone fluide. Une compréhension approfondie des phénomènes à l’origine de la convection devrait donc participer à un meilleur contrôle de la qualité des dépôts.Sur la base de travaux numériques et expérimentaux publiés, nous avons étudié l’apparition de la convection pour trois types de modèles représentant le processus d’évaporation d’une solution de Polyisobutylène-Toluène : un modèle purement thermique qui s’applique pour les temps courts, un modèle solutal qui est valable sur les temps longs et enfin un modèle couplé thermique/solutal qui représente les transferts sur l’ensemble de la gamme des temps étudiés. Le caractère transitoire de l’évaporation induit une difficulté pour caractériser la naissance de la convection à partir d’un régime de conduction. En effet, cette convection apparaît à partir d’un germe qui est une petite perturbation de la solution diffusive. Si l’amplitude de cette perturbation est trop faible, son amplification à des intensités suffisantes ne pourra pas avoir lieu avant la fin du régime transitoire et l’écoulement ne deviendra donc jamais convectif. Le rôle de la perturbation est donc primordial. Dans des travaux numériques antérieurs, cette perturbation a été imposée à l’état initial, généralement avec une distribution aléatoire du champ thermique ou de vitesse. Lors de cette thèse, nous avons opté pour un modèle plus physique, basé sur l’introduction d’un transfert thermique sur les parois latérales qui joue le rôle de perturbateur de l’écoulement diffusif transitoire.Dans cette thèse, nous avons établi par voie numérique les seuils de transition entre une solution diffusive et un écoulement convectif pour les modèles thermique, solutal et couplé, dans le cas d’une approximation bidimensionnelle du film liquide et des simulations pleinement tridimensionnelles. Des diagrammes spatio-temporels et l’étude des cellules à la surface libre par des reconstructions de Voronoï nous ont permis de mieux comprendre la naissance et la propagation des instabilités dans la solution fluide / The evaporation of a solvent/solute solution is a transient phenomenon which ends when the whole solvent has disappeared. Phase change generates a cooling of the liquid-gas interface, and consequently, it creates thermal and solutal gradients. These homogeneities spread in the core solution and produce, eventually, a fluid flow. This convection can be due to the surface tension and/or buoyancy variations. Experimental works have shown that some coating thicknesses stemming from drying processes are correlated to the size of the convection cells in the fluid region. A thorough understanding of the physical phenomena responsible to fluid convection should contribute to improve the control of deposit quality.Based on numerical and experimental works, we have studied the onset of convection for three kinds of models for the drying process of a Polyisobutylene-Toluène solution: A pure thermal model which is valid for short times, a solutal model devoted to the simulation of long times, only, and a thermal/solutal coupled model which takes into account the heat and mass transfer over a long time period of the evaporation process. The transient nature of the evaporation problem raises the issue of how to define the onset of the convective flow from a diffusive solution. Indeed, this flow motion occurs from a seed which is a small perturbation of the transient diffusive solution. If the perturbation is too weak, the necessary time interval for a significant growing of its magnitude will be greater than the time scale of the transient regime: thus the solution will never be considered as convective. Consequently, the influence of the perturbation is fundamental. In previous numerical works, this perturbation was imposed at the initial state, often through a random spatial distribution applied to the velocity or temperature field. In the present contribution, we have adopted a physical model where the adiabatic lateral walls have been replaced by diathermal walls: The local thermal inhomogeneities create a very weak flow acting as a small disturbance for the transient diffusive solution.In this thesis, we have developed a numerical model to evaluate the thresholds between the diffusive solutions and the convective flows, for the thermal, solutal and thermal/solutal coupled models, for two- and three-dimensional approximations of the Polyisobutylene-Toluène liquid film. Space-time diagrams and convective cell reconstructions at the liquid-gas interface by a Voronoï algorithm allowed us to get a better understanding of the way the disturbances propagate from the lateral walls for finally giving rise to a convective flow in the core fluid

The thesis involves computer simulation and theoretical studies of phase transition in soft-condensed matter systems and theoretical understanding of enzyme kinetics along with kinetic proofreading of tRNA-aminoacylation in biological systems. Based on the system and phenomena of interest, the work has be classified into the following four major parts: I. Surface phenomena and surface energy of vapor-liquid interface. II. Condensation of vapor in two and three dimensions. III. Liquid-solid phase transition in polydisperse systems. IV. Enzyme catalysis and kinetic proofreading in biosynthesis. Above mentioned four parts have further been divided into thirteen chapters. In the following we provide a brief chapter-wise outline of the thesis. Part I deals with surface tension and interfacial properties of vapor-liquid interface for Lennard-Jones (LJ) fluid in both two and three dimensions. In Chapter 1, we provide a brief overview of vapor-liquid interface and existing theoretical and computer simulation studies of surface/line tension. In this chapter we also discuss about the existing experimental studies. In Chapter 2, we present computer simulation studies of surface tension in two dimensional Lennard-Jones system. The sensitivity of line tension on range (potential cut-off) of interparticle interaction is discussed in this chapter. We present Density Functional Theory (DFT) of line tension of vapor-liquid interface based on Weeks-Chandler-Anderson (WCA) and Barker-Hendersen (BH) perturbation techniques. We compare the DFT prediction with the computer simulation results. In general, WCA approach has been found to be successful for 3D system in predicting the surface tension. In 2D, however, it does not give good agreement either for phase diagram or for the line tension. In fact, BH also does not give accurate values of the coexistence parameters, however, it predicts better line tension compared to WCA. In Chapter 3 we present both theoretical and computer simulation studies of gas-liquid surface tension for three dimensional Lennard-Jones fluid. We perform non-equilibrium computer simulation study following Transition Matrix Monte Carlo (TMMC) method to obtain surface tension for various ranges of potential and introduce a new scaling relation of surface tension in order to capture both the temperature and interparticle interaction range dependence. The scaling shows excellent agreement with the simulation result and it can also predict the critical temperature with sufficient accuracy. The width of the gas-liquid interface is found to be insensitive to the range of the potential, whereas the density separation of the bulk vapor and liquid phases increases with increasing range of potential. Thus, the major contribution comes from the increasing density separation of the bulk vapor and liquid phases. Part II consists of four chapters, where we focus on the age old problem of nucleation, from the perspective of thermodynamics and kinetics. We account for the rich history of the problem in the introductory Chapter 4. In this chapter we describe various types and examples of the nucleation phenomena, and a brief account of the major theoretical approaches used so far. We begin with the most successful Classical Nucleation Theory (CNT), and then move on to more recent applications of Density Functional Theory (DFT) and other mean-field types of models. We present various experimental techniques used in the literature to obtain rate of nucleation. We conclude with a comparison between the experiments, theories and computational studies. In the next chapter (Chapter 5) we attempt to understand the mechanism of the gas-liquid nucleation in three dimension at large metastability from microscopic point of view. Here we study the nature of sequential growth of all liquid-like clusters (not just the largest cluster) at different degrees of metastability. Therefore, we have ordered the clusters according to their decreasing sizes and identified them in terms of kth largest cluster where, k = 1 denotes the largest cluster in the system, k = 2 represents the second largest and k = 3 is the third largest and so on. We have studied both the free energies and the trajectories of the liquid-like clusters in this extended set of order parameters. We further define Fkl(n) as the free energy of the kth largest cluster with size n. Classical nucleation theory provides an expression of unconditional free energy of a single cluster, F (n) (the free energy of formation of a cluster of size n), which is an intensive property of the system. The study of our conditional free energy surfaces, Fkl(n), reveals a more detailed, microscopic picture of the system’s cluster size distribution that is necessary to understand the kinetics of nucleation and growth at large metastability. The rate of nucleation shows a cross over at kinetic spinodal (the limit of metastability, ∆F1 l = 0). Below kinetic spinodal only one (largest) cluster crosses the critical size through activation whereas above this point more than one cluster grow simultaneously through barrierless diffusion. We present a theoretical analysis of the free energy of kth largest cluster based on order statistics. The theoretical predictions are in excellent agreement with computer simulation results for the range of supersaturation we studied. While the previous chapter focuses on relatively well-studied nucleation mechanism in 3dimensional (3D) LJ system at large metastability, in Chapter 6 we present our studies on the characteristics of the nucleation phenomena in two dimensional Lennard-Jones fluid for different ranges of interparticle interaction. Using various Monte Carlo (MC) methods, we calculate the free energy barrier of nucleation and bulk densities of equilibrium liquid and vapor phases, and also investigate the size and shape of the critical nuclei. We find an interesting interplay between the range of interaction potential and the extent of metastability. The free energy barrier of nucleation strongly depends on the range of interaction potential. The study is carried out at an intermediate level of supersaturation (away from the kinetic spinodal limit). A surprisingly large cutoff (rc � 7.0�, where � is the diameter of LJ particles) in the truncation of the LJ potential is required to obtain converged results. A lower cutoff leads to a substantial deviation in the values of the nucleation barrier, and characteristics of the critical cluster (with respect to full range of interaction). We observe that in 2D system CNT fails to provide a reliable estimate of the free energy barrier. While it is known to slightly overestimate the nucleation barrier in 3D, it underestimates the barrier by � 50% at the saturation ratio S =1.1 (defined as S = P/Pc, where Pc is the coexistence pressure) and at the reduced temperature T � =0.427 (defined as T � = kBT/�, where � is the depth of the potential well). The reason for the marked inadequacy of the CNT in 2D can be attributed to the non-circular nature of the critical clusters. Although the shape becomes increasingly circular and the clusters become more compact with increase in cutoff radius, an appreciable non-circular nature remains even for full potential (without truncation) to make the predictions of CNT inaccurate. In Chapter 7 we report the computer simulation study of nucleation in three dimensional LJ system. At a fixed supersaturation the free energy barrier of nucleation increases with increasing range of interparticle interaction. On increasing range of intermolecular interaction, the kinetic spinodal where the mechanism of nucleation changes from activated barrier crossing to barrierless diffusion, shifts towards the deep metastable region. Both the critical cluster size and pre-critical minimum in the free energy surface of kth largest cluster shift towards the smaller size at their respective kinetic spinodal as we increase the range of potential. We find only a weak non-trivial (other than supersaturation and surface tension) contribution to the free energy barrier of nucleation. Part III consists of two chapters and focuses on the liquid-solid phase transition of polydisperse fluid. In Chapter 8 we introduce polydisperse systems and their classification based on different identities. We describe the importance and abundance of polydisperse system in nature. The theoretical modeling of different polydisperse systems and their extent of applicability have also been presented. We have discussed about the various factors which control the phase diagram and various phenomena related to the structure and phase transition. In Chapter 9 we present computer simulation study on freezing/melting of Lennard-Jones (LJ) fluid at different polydispersities. The freezing/melting of polydisperse LJ fluids presents an interesting case study, because, as the polydispersity increases the energy-entropy balance becomes increasingly unfavorable for the solid to exist as a stable phase. The energy of the solid increases due to build up of strain energy because of increasing mismatch in size of the neighbors, while the entropy of the liquid increases. These two factors lead to the existence of a terminal polydispersity. We find beyond the terminal ploydispersity, δ. 0.11system remains in the disorder state even at very high pressure and low temperature. The terminal polydispersity obtained in the present study is close to the experimental value (δt. ≈ 12%). Interestingly, contrary to hard sphere polydisperse fluid, LJ fluid does not exhibit reentrant melting. The last part (Part IV) of the thesis consists of three chapters that deal with the enzyme catalysis and kinetic proofreading of tRNA-aminoacyl synthetases. In Chapter 10 we describe protein synthesis process in biological system and corresponding two processes: aminoacylation of tRNA and translation of amino acid in ribosome. Our interest is to understand the enzyme catalysis involved in aminoacylation of tRNA in the process of protein synthesis. We present the classification of 20 aminoacyl-tRNA synthetases into two classes based on their structure and mode of binding to ATP and tRNA. We discuss all the steps involved in whole tRNA-aminoacylation process. Then we introduce kinetic proofreading during aminoacylation reaction. In Chapter 11 we theoretically analyze the single turn over and steady state reaction mechanism of two classes of aminoacyl-tRNA synthetases. Class I enzymes not only differ in their structure but they also differ with respect to the pre-steady kinetics compared to class II enzymes. We find that the strong binding of product to class I enzymes causes the product release step to be rate limiting step leading to the burst of product formation in pre-steady reaction. On the other hand class II enzymes do not show any burst kinetics. The present study based on time dependent probability statistics is successful in explaining all the experimental results quantitatively. In Chapter 12 we present an augmented kinetic scheme and then employ methods of time dependent probability statistics to understand the mechanism of kinetic proofreading of isoleucyl-tRNA synthetase (IRS) which belongs to class I. We investigate that the enhanced hydrolysis of wrong substrate (Val) enables IRS to discriminate the correct substrate (Ile) and wrong substrate (Val) efficiently. It has been observed that an extra CP1 editing domain serves as an activating domain towards enhanced hydrolysis of Val. The present study is able to explain most of the existing experimental observations. In the concluding note, Chapter 13 lists a few relevant problems that may prove worthwhile to be addressed in future. In the Appendices, we present two of the techniques used in our present computer simulation and theoretical studies. Appendix A describes Grand Canonical Transition Matrix Monte Carlo (GC-TMMC) method which is employed in computer simulation studies of nucleation and surface tension. In Appendix B we present the probabilistic method of waiting time distribution computation used in enzyme catalysis and kinetic proofreading.

Doctor of Philosophy / Department of Grain Science and Industry / Finlay I. MacRitchie / Expansion of dough and hence breadmaking performance is postulated to depend on a dual mechanism for stabilization of inflating gas bubbles. Two flours were used in this study, one from the wheat variety Jagger (Jagger) and the other from a composite of soft wheat varieties (soft). The primary stabilizing mechanism is due to the gluten-starch matrix surrounding the bubble. The secondary mechanism operates when gas bubbles come into close contact during later proofing and early baking. When discontinuities occur in the gluten-starch matrix surrounding gas bubbles, thin liquid lamellae stabilized by adsorbed surface active compounds, provide a secondary stabilization. A key parameter in the primary stabilizing dough film is thought to be the property of strain hardening. Jagger flour gave higher test-bake loaf volume than soft wheat flour and higher strain hardening index for dough. Rheological properties of doughs were varied by addition of protein fractions prepared by pH fractionation. Fractions were characterized by SE-HPLC and MALLS. The molecular weight distribution (MWD) of fractions progressively shifted to higher values as the pH of fractionations decreased. Mixograph peak development time paralleled the MWD. However, the strain hardening index and the test-bake loaf volume increased with increasing MWD up to a point (optimum), after which they declined. At a given strain rate the behavior at the optimum appeared to result from slippage of the maximum number of statistical segments between entanglements, without disrupting the entangled network of polymeric proteins. Shift of MWD to MW higher than the optimum results in a stronger network with reduced slippage through entanglement nodes, whereas a shift to lower MWs will decrease the strength of the network due to less number of entanglements per chain. In order to study the secondary stabilizing mechanism, different lipid fractions were added incrementally to the defatted flours. No effects were observed on the rheological properties of the dough. However, large effects on the loaf volume were measured. The additives used were the total flour lipid and its polar and non polar fractions and the fatty acids palmitic, linoleic and myristic. Polar lipids and palmitic acid had positive or little effect on loaf volume respectively. Non polar lipid, linoleic and myristic acids had negative effects on loaf volume. 1 The different effects of the lipid fractions are thought to be related to the type of monolayer that is formed. Polar lipid and palmitic acid form condensed monolayers at the air/water interface whereas non polar lipid, linoleic and myristic acids form expanded monolayers.

[pt] O deslocamento de um líquido em um tubo capilar pela injeção de um gás ocorre em muitas situações, tais como na recuperação avançada de petróleo, no revestimento de conversores catalíticos e na moldagem assistida por injeção de gás. Geralmente o líquido deslocado é uma solução polimérica ou uma dispersão, que é não Newtoniana. Forças viscoelásticas alteram o balanço de forças em várias partes do escoamento e, conseqüentemente, alteram a eficiência do deslocamento, isto é, mudam a quantidade de líquido deixada na parede do capilar. Modelos de tais escoamentos devem se basear em teorias que levem em consideração o comportamento diferenciado de líquidos com microestrutura complexa, tanto no cisalhamento quanto na extensão. Além do mais, escoamentos de deslocamento envolvem uma superfície livre, e o domínio no qual as equações diferenciais são resolvidas é desconhecido a priori, fazendo parte da solução. Estas duas características tornam o problema extremamente complexo. Este problema foi estudado aqui tanto experimentalmente quanto teoricamente. Os experimentos consistiram da visualização do escoamento e medição da massa deslocada pela passagem de uma bolha de gás através de um tubo capilar preenchido por um líquido viscoelástico. Várias soluções de baixo peso molecular de Polietileno Glicol (PEG) e de alto peso molecular de Óxido de Polietileno (PEO) em água foram usadas a fim de avaliar os efeitos do comportamento viscoelástico no escoamento. As propriedades reológicas das soluções foram avaliadas tanto em cisalhamento quanto em extensão. Na análise teórica, o escoamento com superfície livre bidimensional próximo µa interface gás- líquido foi modelado usando três equações diferenciais constitutivas distintas que aproximam o comportamento viscoelástico de soluções poliméricas diluídas, as quais são os modelos Oldroyd-B, FENE-P e FENE-CR, juntamente com as equações de conservação de massa e de quantidade de movimento linear. O sistema de equações foi resolvido pelo Método dos Elementos Finitos. O sistema de equações algébricas não-lineares resultante foi resolvido pelo método de Newton. Os resultados mostram o efeito do caráter viscoelástico do líquido na forma da superfície livre e a espessura do filme líquido deixado na parede. / [en] Displacement of a liquid in a capillary tube by gas injection occurs in many situations, like enhanced oil recovery, coating of catalytic converters and gas-assisted injection molding. Generally the liquid being displaced is a polymeric solution or dispersion, which is not Newtonian. Viscoelastic forces alter the force balance in various parts of the flow and consequently change the amount of liquid left attached to the capillary wall. Models of such flows must rely on theories that can account for the different behavior of microstructured liquids in simple shear and extensional flow. Moreover, displacement flows involve a free surface, and the domain where the differential equations are posed is unknown a priori being part of the solution. These two characteristics make the problem extremely complex. This problem was analyzed here both by experiments and theory. The experiments consisted of flow visualization and measurement of mass displaced by a gas bubble in a capillary tube filled with a viscoelastic liquid. Various solutions of low molecular weight Polyethylene Glycol (PEG) and high molecular weight Polyethylene Oxide (PEO) in water were used in order to evaluate the effect of viscoelastic behavior on the flow. The rheological properties of the solutions were evaluated both in simple shear and predominantly extensional flows. In the theoretical analysis, the two- dimensional free surface flow near the gas-liquid interface was modelled using three different differential constitutive equations that approximate viscoelastic behavior of dilute polymer solutions, namely Oldroyd-B, FENE-P and FENE-CR, together with momentum and continuity equations. The equation system was solved with the Finite Element Method. The resulting non- linear system of algebraic equations was solved by Newton`s method. The results show the effect of the viscoelastic character of the liquid on the free surface shape and the film thickness attached to the capillary wall.