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An increase in temperature typically leads to a decrease in the interfacial tension of a water/oil interface. The addition of surfactants to the system can complicate the situation significantly, i.e., the interfacial tension can increase or decrease with an increasing temperature. For most concentrations of the two studied surfactants, the cationic tetradecyl trimethyl ammonium bromide (TTAB) and the nonionic tridecyl dimethyl phosphine oxide (C13DMPO), the measured interfacial tension of the aqueous mixed surfactant solutions against hexane increases when the temperature decreases between 30 °C and 20 °C. However, with a further temperature decrease between 20 °C and 15 °C, the reverse effect has also been observed at some concentrations, i.e., a decrease of interfacial tension. Additionally, the corresponding dilational interfacial visco-elasticity shows some discrepant temperature effects, depending on the bulk concentration and oscillation frequency. The experiments have been performed with a capillary pressure tensiometer under the conditions of micro-gravity. The reason for the positive and negative interfacial tension and visco-elasticity gradients, respectively, within certain ranges of the temperature, concentration and mixing ratios, are discussed on the basis of all available parameters, such as the solubility and partitioning of the surfactants in the two liquid phases and the oscillation frequency.

Capillary pressure experiments are performed in microgravity conditions on board the International Space Station to quantify the dynamic interfacial behavior of mixed adsorption layers of TTAB and C13DMPO at the water/hexane interface. While the non-ionic surfactant C13DMPO is soluble in both bulk phases, water and hexane, the cationic surfactant TTAB is only soluble in the aqueous phase. The interfacial layer is thus formed by TTAB molecules adsorbing from the aqueous phase while the C13DMPO molecules adsorb from the aqueous phase, and transfer partially into the hexane phase until both the equilibrium of adsorption and the distribution between the two adjacent liquid phases is established. The experimental constrains as well as all possible influencing parameters, such as interfacial and bulk phase compressibility, interfacial curvature, calibration of pressure and absolute geometry size, are discussed in detail. The experimental results in terms of the dilational interfacial viscoelasticity of the mixed adsorption layers in a wide range of oscillation frequencies show that the existing theoretical background had to be extended in order to consider the effect of transfer of the non-ionic surfactant across the interface, and the curvature of the water/hexane interface. A good qualitative agreement between theory and experiment was obtained, however, for a quantitative comparison, additional accurate information on the adsorption isotherms and diffusion coefficients of the two studied surfactants in water and hexane, alone and in a mixed system, are required.

The adsorption of proteins at liquid interfaces happens at rather low bulk concentrations due to their rather high surface activity. In contrast typical surfactants start to decrease the surface tension at bulk concentration in the range of mmol/l and reach a minimum value at about two or three orders of magnitude higher concentration. The two proteins studied here, β-lactoglobulin and β-casein, adsorb already remarkably at much lower concentrations, i.e. less than 1 μmol/l. When smallest amounts of a non-ionic surfactant are added to low concentrated protein solutions, changes in the surface tension are observed which cannot be explained by the existing theoretical models. An agreement with the experimental data can be achieved only when an increased surface activity of the protein due to the interaction with the surfactants is assumed. [ABSTRACT FROM AUTHOR]

Dynamic interfacial parameters are the key properties of interfaces in many modern technologies and can be studied in various ways. For applications like foams and emulsions, the dynamics of adsorption and the dilational and shear rheology of liquid–fluid interfaces are investigated most frequently. This work gives an insight into recently developed new experimental approaches, such as fast capillary pressure tensiometry for growing and oscillating drops. These experiments are presented in comparison to more classical techniques like drop profile tensiometry and capillary wave damping. Progress in these experiments based on generated interfacial perturbations can be expected only by a close link to respective CFD simulations. We also present the state of the art of CFD simulations, which have reached a high level during the last decade and provide a substantial basis for dynamic interfacial experiments. [ABSTRACT FROM AUTHOR]

Abstract: The dynamic interfacial tension and dilational rheology of alkyltrimethylammonium bromides (C n TAB) solutions are studied for n =10, 12, 14 and 16 using drop profile analysis and capillary pressure tensiometry. The experimental data are analyzed on the basis of two theoretical models, the Frumkin Ionic Compressibility and the Langmuir Compressibility models [1]. We show that even the simplified Langmuir Compressibility model is able to adequately describe the dynamic properties of the investigated ionic surfactants. Dynamic experiments point out the difficulties related to short chain surfactants due to their very fast adsorption kinetics which cannot be studied with conventional tensiometer setups. [Copyright &y& Elsevier]

Abstract: The inflow pattern of liquid into a droplet is studied experimentally using a surface active dye and compared with results of CFD simulations. The results show visual agreement between experiments and simulations. The CFD simulations show also good agreement with the surface tension measured by drop profile analysis tensiometry (PAT). The inflow of the surfactant induces a Marangoni instability caused by the local arrival of the surfactant at the drop surface. The onset of this Marangoni instability observed experimentally has a delay of about 10s when compared with the simulation results. Different scenarios are discussed, including a boundary layer barrier, a kinetic-controlled adsorption mechanism, the way of renewing a mobile interface, and a critical Marangoni number required for the onset of such instability. It turns out that besides the commonly defined critical Marangoni number, the main reason for delay of Marangoni instability is possibly the mobility of the drop surface which can prevent the establishment of a quasi static adsorbed layer required for the appearance of such surface effect. [Copyright &y& Elsevier]

Highlights: [•] Description of a novel measuring technique for coalescence studies. [•] Coalescence behaviour of two single drops or bubbles as model for foams and emulsions. [•] Coalescence studies of concentric drops as model for multiple emulsions. [•] Application of described technique to surfactant stabilised liquid/liquid interfaces. [•] Application of technique to protein stabilised liquid/air interfaces. [ABSTRACT FROM AUTHOR]

Abstract: The oscillating drop and bubble analyzer (ODBA) is an experimental set-up based on the measurement of capillary pressure under static and dynamic conditions. It allows studies of slow and fast dynamic surface and interfacial tensions, following different growing and oscillating drop or bubble protocols, as well as determination of the dilational interfacial visco-elasticity of liquid interfacial layers. For the visco-elasticity studies, drops or bubbles are subjected to harmonic oscillations of area or volume in a broad frequency range, and the resulting harmonic capillary pressure response is analyzed by Fourier analysis. Also, transient relaxations can be easily performed, which are of particular importance for isodense systems. The limits of applicability for highly dynamic conditions are given by the hydrodynamics of the fluid flow inside the capillary tip and the pressure cell which depend on fluid properties, capillary tip size, and geometry. For the growing drop protocol, additional phenomena during drop formation and detachment play a significant role. For oscillating drops or bubbles, the highest accessible frequencies also depend on the absolute drop size and applied oscillation amplitude. In this work, the capability of the technique for measurements in the frequency range between 1 and 100Hz are discussed, and elasticity values at up to 80Hz were measured for the liquid–liquid interface. [Copyright &y& Elsevier]

Abstract: Each experimental method has a certain range of application, and so do the instruments for measuring dynamic interfacial tension and dilational rheology. While the capillary pressure tensiometry provides data for the shortest adsorption times starting from milliseconds at liquid/gas and tens of milliseconds at liquid/liquid interfaces, the drop profile tensiometry allows measurements in a time window from seconds to many hours. Although both methods together cover a time range of about eight orders of magnitude (10−3 s to 105 s), not all surfactants can be investigated with these techniques in the required concentration range. The same is true for studies of the dilational rheology. While drop profile tensiometry allows oscillations between 10−3 Hz and 0.2Hz, which can be complemented by measurements with capillary pressure oscillating drops and the capillary wave damping method (up to 103 Hz) these six orders of magnitude in frequency are often insufficient for a complete characterization of interfacial dilational relaxations of surfactant adsorption layers. The presented analysis provides a guide to select the most suitable experimental method for a given surfactant to be studied. The analysis is based on a diffusion controlled adsorption kinetics and a Langmuir adsorption model. [Copyright &y& Elsevier]

Abstract: For the understanding of short-time adsorption phenomena and high-frequency relaxations at liquid interfaces particular experimental techniques are needed. The most suitable method for respective studies is the capillary pressure tensiometry. However, under gravity conditions there are rather strong limitations, in particular due to convections and interfacial deformations. This manuscript provides an overview of the state of the art of experimental tools developed for short-time and high-frequency investigations of liquid drops and bubbles under microgravity. Besides the brief description of instruments, the underlying theoretical basis will be presented and limits of the applied methods under ground and microgravity conditions will be discussed. The results on the role of surfactants under highly dynamic conditions will be demonstrated by some selected examples studied in two space shuttle missions on Discovery in 1998 and Columbia in 2003. [ABSTRACT FROM AUTHOR]

Abstract: The Capillary Pressure Tensiometry is an effective technique for dilational rheological investigations when it is used according to the oscillating drop/bubble method. This method allows for an indirect determination of the dilational viscoelasticity versus frequency through the acquisition of the pressure signal obtained as response to harmonic perturbations of the drop/bubble interfacial area. A critical aspect of this technique concerns the calculation of the real and imaginary part of the dilational viscoelasticity from the directly acquired experimental data. Such calculation requires a specific experiment theory accounting also for the compressibility of the closed part of the experimental cell and for the fact that the acquired pressure is a superposition of different fluid dynamic contributions. Here we present an extension of previously proposed approaches, considering two different CPT configurations which refer to some already in use. The pressure inside the measurement cell is theoretically calculated considering the contributions, not only of the capillary pressure, but also of the Poiseuille pressure due to the fluid motion through the capillary and of hydrodynamic inertial terms. Simulations of experimental pressure signal are calculated as a function of specific experimental parameters in order to evaluate their influence on the measurement and to improve the effectiveness of the apparatus and the experimental methodology. Moreover in order to validate the assumptions of this approach, specifically acquired experimental data are analysed. [Copyright &y& Elsevier]

Abstract: Dilational rheology represents a powerful tool to investigate equilibrium and dynamic properties of simple and more complex interfacial layers containing surfactants, proteins, polymers or micro–nano sized particles. Concerning the experimental techniques for dilational rheology, drop/bubble tensiometers based on the acquisition of the drop/bubble profile and capillary pressure tensiometers are especially effective. This article focuses on oscillating drop/bubble methodologies where harmonic variations of the interfacial area are utilized for the measurement of the dilational viscoelasticity in the frequency domain. The increasing efficiency of these techniques of the last ten years is due, from one side, to the implementation of advanced instrumentations which make faster the drop/bubble control and the data acquisition and, on the other side, to the application of new theoretical approaches for data acquisition and interpretation. A critical analysis of such drop/bubble instruments is presented where their potentialities and limitations are underlined. Moreover, recent improvements in the definition of calculation methods based on the modelling of the experimental set up are reviewed together with some examples of experimental studies based on the utilisation of such methodologies. [Copyright &y& Elsevier]

Abstract: The mechanical properties of liquid–fluid systems, like the dynamic interfacial tension and interfacial rheology are closely related to the kinetic processes involved and to the behaviour of the adsorbed molecules. Therefore, provided suitable models and experimental methods are set, investigating these properties allows qualitative and quantitative information on these processes to be drawn. This paper presents recent developments in dilational rheology of liquid–fluid adsorption layers, including experimental methods, models and experimental data concerned with surfactants undergoing transformations in the adsorption layer. Models account both for relaxation due to surfactant diffusion and to processes internal to the adsorption layer. In particular surfactant reorientation, aggregation phase transitions and interfacial chemical reactions have been considered as possible reorganisation processes. The presented approach, allows the dilational viscoelasticity to be derived as a function of the perturbation frequency and of the equilibrium and kinetic parameters of the system. The results can also be easily specified for insoluble monolayer. The principal experimental techniques are reviewed and the recent progresses in the implementation of an Oscillation Bubble/Drop method for Capillary Pressure Tensiometer are discussed in detail. Two experimental studies of surfactants characterised by re-orientation and aggregation phase transition are presented. Beside providing a wider comprehension of these mechanisms, the interpretation of the dilational visco-elasticity data, according to the developed models, allows the effective estimation of the equilibrium and kinetic parameters. [Copyright &y& Elsevier]