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We report on a new class of nonionic, photosensitive surfactants consisting of a polar di(ethylene oxide) head group attached to an alkyl spacer of between two and eight methylene groups, coupled through an ether linkage to an azobenzene moiety. Structural changes associated with the interconversion of the azobenzene group between its cis and trans forms as mediated by the wavelength of an irradiating light source cause changes in the surface tension and self-assembly properties. Differences in saturated surface tensions (surface tension at concentrations above the CMC) were as high as 14.4 mN/m under radiation of different wavelengths. The qualitative behavior of the surfactants changed as the spacer length changed, attributed to the different orientations adopted by the different surfactants depending on their isomerization states, as revealed by neutron reflection studies. The self-assembly of these photosensitive surfactants has been investigated by light scattering, small angle neutron scattering, and cryo-TEM under different illuminations. The significant change in the self-assembly in response to different illumination conditions was attributed to the sign change in Gaussian rigidity, which originated from the azobenzene photoisomerization. / Singapore-MIT Alliance (SMA)

Recent studies showed that evaporation of water can induce surface tension gradients along the water surface and ultimately lead to convection, known as Marangoni convection. This study was devoted to visualization and characterization of the evaporation-induced, surface-tension-driven convection in water using laser-based measurement techniques. The evaporation of water at various low vapor-phase pressures in the absence of buoyancy driven flow was investigated. Strong symmetric convection was observed and its velocity field was measured using stereo particle image velocimetry. The temperature field obtained from using both a thermocouple and planar laser induced fluorescence indicated that no buoyancy driven flow was generated. The strength of the convection was found to be correlated with the evaporation rate of water. In addition, the estimated Marangoni number exceeded the critical value for onset of Marangoni convection. It can be concluded that the observed evaporative convection of water can only be Marangoni convection.

Interfacial tension provides valuable information about polymer processes such as foaming, particle (pigment) suspension, wetting, and blending. Among the methods commonly used to measure surface tension, drop shape methods entail several advantages such as simplicity and versatility. The profile of the drop, which is determined by the balance between gravity and surface forces, is easily defined. The surface tension is obtained from the profile of the drop. Recent progress in image analysis and data acquisition systems makes it possible to digitalize drop images directly using a video frame grabber with a digital camera. The digital signals are easily analyzed using different algorithms to determine the surface/interfacial tension from the drop profile. This study concentrates on one of the drop methods, the pendant drop method, which involves the determination of a drop profile of one dense liquid suspended in another liquid at mechanical equilibrium. Despite theoretical simplicity of using sessile and pendant drops for determining the surface tension of polymer melts, research in this area is limited because of the experimental difficulty associated with maintaining equilibrium of highly viscous melts. This paper examines the surface tension of polystyrene melts using Axisymmetric Drop Shape Analysis (ADSA) at high temperatures. This thesis focuses on attaining a stable pendant drop during experiments and modifying experimental designs. The method is verified by experiments in the air and nitrogen, where reproducibility tests and statistical analyses are performed. The surface tension of polystyrene (PS) that melts in supercritical carbon dioxide is obtained while the gas solubility is correlated with the surface tension value determined under various conditions. The Sanchez-Lacombe (S-L) equation of state (EOS) is applied to estimate the Pressure-Volume-Temperature (PVT) data of the PS/supercritical-carbon-dioxide mixtures, which gives density data. The relationship between surface tension and density is described by the empirical Macleod equation. To characterize the stability of pendant drops formed by the polymer melt, the Bond number is found to be useful; in particular, a stable pendant drop is obtained when the Bond number is between 0.4 and 0.8. This thesis presents experimental results of the surface tension of polystyrene in supercritical carbon dioxide, together with theoretical calculations for a corresponding system. The surface tension is determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P), where a high pressure and temperature cell is designed and constructed to facilitate the formation of a pendant drop of polystyrene melt. Self-consistent field theory (SCFT) calculations are applied to simulate the surface tension of a corresponding system, and a good agreement with the experiment is obtained. The physical mechanisms for three main experimental trends are explained using SCFT, and none of the explanations depend on the configurational entropy of the polymer constituents. These calculations therefore rationalize the use of simple liquid models for the quantitative prediction of surface tensions of polymers. As pressure and temperature increase, the surface tension of polystyrene decreases. A linear relationship is found between surface tension and temperature, and between surface tension and pressure; the rate of surface tension change with temperature is dependent on pressure. A linear relationship is found between surface tension and temperature, and between surface tension and pressure within a temperature range of 170-210C and a pressure range of 500-2,000 psi. Two monodisperse polystyrenes of Mw ~ 100,000 and Mw ~400,000 and a polydisperse polystyrene were investigated to show the temperature and pressure effect on the surface tension in supercritical nitrogen. Regardless of the molecular weight and polydispersity, the surface tension of polystyrenes decreases as the pressure and temperature increase. Monodisperse polystyrene of a higher molecular weight has a higher surface tension by 6-9 mJ/m2 at each experimental condition. The surface tension dependence on temperature and on pressure is more significant for the higher molecular weight polystyrene; the surface tension has been varied more in the higher molecular weight polystyrene than in the lower molecular weigh polystyrene. For a polydisperse polystyrene, high surface tension values seem to be determined predominantly by its high molecular weight portion of polystyrene molecules. An empirical equation was generated to relate surface tension to the density difference between the polymer and supercritical nitrogen. This research should have implications in understanding polymer foaming processes and have application in various polymer engineering fields including polymer characterizations, polymer synthesis, and surface modifications.

The surface tension of an interface separating two bulk phases is one of the most widely studied properties in surface science research. The importance of surface or interfacial tension is reflected in the diverse number of applications which are influenced by surface tension related effects. This thesis represents a comprehensive experimental and theoretical investigation on molecular adsorption and surface tension from a class of organic compounds in aqueous solutions. The research illustrates the effect of both liquid and vapor phase adsorption on the interfacial properties. Adsorption from both sides of the vapor/liquid interface is considered simultaneously rather than exclusive of one another, which has been the conventional practice. In the experimental study, the surface tension of a number of different volatile organic compounds is measured using the Axisymmetric Drop Shape Analysis-Profile (ADSA-P) method. The experiments were performed in a controlled environment under conditions where the surface tension can be affected by both vapor and liquid phase adsorption. The vapor phase was exerted by the presence of an environment solution containing the same organic component as in the drop solution. The results show that initially the surface tension is influenced by the organic concentration in both the liquid and the vapor phase. At the final steady-state the liquid phase becomes less important and the primary factor influencing the surface tension is the vapor phase concentration. The ADSA-P technique is verified by reproducing a select number of cases using the Wilhelmy plate method. A possible consequence of the surface tension phenomenon is illustrated through time-dependent contact angle experiments. The behavior of the interface at steady-state conditions is investigated by measuring the surface tension response to a change in drop volume. It is concluded that the organic compounds considered in the current study may represent a rather general group of molecules whose surface behavior is unique to that of many conventional surfactants. In the theoretical study an empirical model is proposed to describe the relation between the steady-state surface tension and the concentration of the environment and drop solutions. The results confirm the experimental observation that the final steady-state surface tension is determined primarily by the organic concentration in the vapor phase. In addition, a modified adsorption isotherm is developed to account for simultaneous adsorption from both sides of the vapor/liquid interface at steady-state conditions. The derivation is based upon the classic Langmuir analysis, and the new equation is consistent with the Langmuir isotherm under traditional conditions where adsorption occurs from one side of the interface. The modified isotherm is shown to be consistent with the experimental data and is used to generate the equilibrium parameters for three of the systems studied in this research. The adsorption isotherm is then extended to model the dynamic adsorption process through the creation of a new kinetic transfer equation. As with the adsorption isotherm, the transfer equation is based on Langmuir kinetics and is capable of simulating adsorption from both sides of the interface during surface equilibration. The kinetic transfer equation is validated against experimental data from two systems which exhibit a transfer-controlled adsorption mechanism. The theoretical predictions from the transfer equation fit well with the experimental data for both systems. However, significant variability is observed in the least squares estimates of the kinetic rate constants. The variability is attributed to the limitations of empirical models that utilize adjustable fitting parameters to optimize the model predictions, and the wide range of surfactant concentrations studied. Specific concentration regions are identified where the variability in the rate constants is minimal and thus, where the model is most appropriate.

Interfacial tension provides valuable information about polymer processes such as foaming, particle (pigment) suspension, wetting, and blending. Among the methods commonly used to measure surface tension, drop shape methods entail several advantages such as simplicity and versatility. The profile of the drop, which is determined by the balance between gravity and surface forces, is easily defined. The surface tension is obtained from the profile of the drop. Recent progress in image analysis and data acquisition systems makes it possible to digitalize drop images directly using a video frame grabber with a digital camera. The digital signals are easily analyzed using different algorithms to determine the surface/interfacial tension from the drop profile. This study concentrates on one of the drop methods, the pendant drop method, which involves the determination of a drop profile of one dense liquid suspended in another liquid at mechanical equilibrium. Despite theoretical simplicity of using sessile and pendant drops for determining the surface tension of polymer melts, research in this area is limited because of the experimental difficulty associated with maintaining equilibrium of highly viscous melts. This paper examines the surface tension of polystyrene melts using Axisymmetric Drop Shape Analysis (ADSA) at high temperatures. This thesis focuses on attaining a stable pendant drop during experiments and modifying experimental designs. The method is verified by experiments in the air and nitrogen, where reproducibility tests and statistical analyses are performed. The surface tension of polystyrene (PS) that melts in supercritical carbon dioxide is obtained while the gas solubility is correlated with the surface tension value determined under various conditions. The Sanchez-Lacombe (S-L) equation of state (EOS) is applied to estimate the Pressure-Volume-Temperature (PVT) data of the PS/supercritical-carbon-dioxide mixtures, which gives density data. The relationship between surface tension and density is described by the empirical Macleod equation. To characterize the stability of pendant drops formed by the polymer melt, the Bond number is found to be useful; in particular, a stable pendant drop is obtained when the Bond number is between 0.4 and 0.8. This thesis presents experimental results of the surface tension of polystyrene in supercritical carbon dioxide, together with theoretical calculations for a corresponding system. The surface tension is determined by Axisymmetric Drop Shape Analysis-Profile (ADSA-P), where a high pressure and temperature cell is designed and constructed to facilitate the formation of a pendant drop of polystyrene melt. Self-consistent field theory (SCFT) calculations are applied to simulate the surface tension of a corresponding system, and a good agreement with the experiment is obtained. The physical mechanisms for three main experimental trends are explained using SCFT, and none of the explanations depend on the configurational entropy of the polymer constituents. These calculations therefore rationalize the use of simple liquid models for the quantitative prediction of surface tensions of polymers. As pressure and temperature increase, the surface tension of polystyrene decreases. A linear relationship is found between surface tension and temperature, and between surface tension and pressure; the rate of surface tension change with temperature is dependent on pressure. A linear relationship is found between surface tension and temperature, and between surface tension and pressure within a temperature range of 170-210C and a pressure range of 500-2,000 psi. Two monodisperse polystyrenes of Mw ~ 100,000 and Mw ~400,000 and a polydisperse polystyrene were investigated to show the temperature and pressure effect on the surface tension in supercritical nitrogen. Regardless of the molecular weight and polydispersity, the surface tension of polystyrenes decreases as the pressure and temperature increase. Monodisperse polystyrene of a higher molecular weight has a higher surface tension by 6-9 mJ/m2 at each experimental condition. The surface tension dependence on temperature and on pressure is more significant for the higher molecular weight polystyrene; the surface tension has been varied more in the higher molecular weight polystyrene than in the lower molecular weigh polystyrene. For a polydisperse polystyrene, high surface tension values seem to be determined predominantly by its high molecular weight portion of polystyrene molecules. An empirical equation was generated to relate surface tension to the density difference between the polymer and supercritical nitrogen. This research should have implications in understanding polymer foaming processes and have application in various polymer engineering fields including polymer characterizations, polymer synthesis, and surface modifications.

No description available.

No description available.

No description available.