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Single drops or bubbles are frequently used for the characterization of liquid− fluid interfaces. Their advantage is the small volume and the various protocols of their formation. Thus, several important methods are based on single drops and bubbles, such as capillary pressure and profile analysis tensiometry. However, these methods are often applied under dynamic conditions, although their principles are defined under equilibrium conditions. Thus, specific attention has to be paid when these methods are used beyond certain limits. In many cases, computational fluid dynamics (CFD) simulations have allowed researchers, to extend these limits and to gain important information on the interfacial dynamics. Examples discussed here are the capillary pressure tensiometry used for short time and profile analysis tensiometry for long time dynamic interfacial tension measurements, the oscillating drop methods for measuring dilational visco-elasticity. For measuring the coalescence of two drops the liquid dynamics of the subsequently formed liquid bridges have to be considered. In this paper, a thorough review of important experimental and computational findings, related to the dynamics of drops, including its formation, growth, oscillation, detachment, and coalescence is presented. Emphasis is however on some selected important developments. In addition, the paper tries to predict the main directions of advancement in interfacial research for the near future. [ABSTRACT FROM AUTHOR]

Dilational and shear viscoelasticities are important properties of interfacial layers. These quantities are particularly relevant in all systems which contain a huge internal interfacial area such as foams and emulsions. Therefore, also the 3D rheological behavior of foams or emulsions studied by respective methods is superimposed by the 2D interfacial rheology. We report on recent developments in dilational and shear rheology from an experimental point of view as well as discuss the state of the art of the underlying theories. Examples of most relevant experiments are also presented and discussed. Although not yet extensively investigated, the links between bulk rheology of foams and emulsions and the rheology of the corresponding interfacial layers are discussed. [ABSTRACT FROM AUTHOR]

This paper deals with computational and experimental studies on the oscillatory flow at high frequencies up to 100 Hz performed with the Oscillating Drop and Bubble Analyzer (ODBA) setup based on the capillary pressure technique. The CFD results are validated considering pressure amplitude experimental data. The simulated results of phase shift between the generated oscillatory flow and the consequent pressure amplitudes show also good agreement with the experimental data. In absence of any compressibility and viscoelasticity effects and assumptions, a complex velocity field during oscillation is the main reason for the observation of a phase shift. The results of velocity profiles at the moment of maximum instant flow rate illustrate a transient of the regular parabolic laminar flow inside the tip at low frequencies to a complex flow profile at intermediate and high frequencies. For the moment of maximum pressure amplitude a complex shape with triple maximum/minimum velocity regions is observed. The evolution of the velocity profile shape depends significantly on the frequency and capillary tip size, however, not by the volume amplitude. The results are in good correlation with the concept of the hydrodynamic relaxation time, however, the presented approach reveals more details. The creation of a double parabolic-like flow inside the tip, which can be defined as fluid flow through much smaller tubes or channels is the main reason for observing a maximum pressure loss with a certain phase shift to the maximum instant liquid flow rate. [ABSTRACT FROM AUTHOR]

The kinetics of coalescence is studied experimentally using a new technique for tracking the process in the bulk phase. For this aim, aqueous solutions of KSCN (colorless) and FeCl 3 (light yellow) are used to make individual W/O emulsions. Any coalescence occurred between drops containing KSCN solution and those containing FeCl 3 solution would combine these solutions. This leads to a reddish brown solution due to the formation of iron(III)thiocyanate. The intensity change of this red color with time represents the dynamics of coalescence occurring between drops of different emulsified aqueous droplets. The detector response to any changes in the system is recorded as a function of time. In this study, the effects of different parameters such as the concentration of Span 80 used as emulsifier in individual emulsions, the mixing rate of the emulsion, and the kinetics of coalescence are investigated semi-quantitatively. [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 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]

Abstract: Profile analysis tensiometry (PAT) is presently the most frequently used technique for measuring surface tensions of liquids. The basis of this methodology is however an equilibrium force balance as given by the Gauss–Laplace equation. Therefore, its application under dynamic conditions, i.e. for growing drops or bubbles, is questionable. We discuss the limits of the applicability of PAT under dynamic conditions by using a growing drop configuration equipped with a high speed video camera. The systems studied are the water/air and water/hexane interface. The obtained “dynamic” drop profiles are analyzed by fitting the classical Gauss–Laplace equation. The results are additionally compared with experimental data obtained from capillary pressure tensiometry. The analysis allows defining three different regions related to respective drop expansion rates. [Copyright &y& Elsevier]

By means of post-tensioning technique, planar space trusses, particularly the single chord space trusses can be formed into curved forms. This novel process of the shape formation results in the architecturally interesting forms with the sufficient structural stability and stiffness, and with the additional advantage of being economical due to eliminating the need to use scaffolding or heavy cranes for their construction.This paper deals with the shape formation and loading tests as well as analyses of the single chord space trusses shaped by post-tensioning technique. To validate analyses and investigate effect of curvature of the shaped models on the results, two models are included; a gently curved dome-shape and a sharply curved barrel-vault. First, shape formation analysis is carried out and next, buckling load of shaped models in loading analyses are determined and findings are compared with the experimental results. Also, effect of the residual stresses of shape formation step on the results of loading analysis is investigated. [ABSTRACT FROM AUTHOR]

This paper presents the results of nonlinear analysis to predict the ultimate load of a barrel-vault space truss shaped by post-tensioning. Experimental study on shape formation, load tests to determine the ultimate load of the post-tensioned barrel-vault space truss and individual compression tests on members to determine their buckling load have been carried out in previous research. The nonlinear analysis carried out herein uses the reduced stiffness of various members according to member test results to determine the ultimate load behavior of the model barrel-vault space truss. The paper also presents the ultimate load behavior of large-span barrel-vault space trusses, taking into account the buckling of their members. As a result, it is shown that these structures can be used to cover large spans with sufficient structural stiffness. Lastly, some design aid diagrams are proposed. [ABSTRACT FROM AUTHOR]

Measurements by capillary pressure tensiometry, under microgravity conditions aboard the International Space Station, supplied a consistent set of reliable results for the dynamic interfacial tension and for the interfacial dilational viscoelastic modulus, quantitatively characterizing the dynamics of Span-80 adsorbed layers at the paraffin-oil/water interface. The experiments were executed at three different temperatures, i.e., 20, 30 and 40 °C, according to a pre-established built-in time-line in the orbiting facility. The interfacial area was subjected to perturbations with various functional forms (square pulses, ramps and harmonic oscillations), at three consecutive amplitudes (5%, 10% and 20%). Each experiment was performed in three successive repetitions, in view of an advantageous telemetered data redundancy. The interfacial responses to imposed perturbations, for the studied minimal surfactant concentration of Span-80 in paraffin-oil (that is (2 ÷ 3) × 10 −5 mol/dm 3 ) revealed a diffusion-controlled adsorption mechanism, definitely matching the Lucassen & Van den Tempel model in the frequency-domain representation. The interfacial responses also showed a linearity range up to the 20% amplitude. Interfacial relaxation responses to transient interfacial perturbations substantially validated the diffusion-controlled model for the adsorption mechanism, in the time-domain representation. [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: We investigated the feasibility of using magneto-optical sensors for measuring the magnetic leakage fields in non-destructive evaluation. We used magneto-optical garnet thin films as a sensor and the effect of crack dimension on the sensor''s response were investigated via a simple Faraday rotation technique. Our results show that the sensor signal displays a linear increase as the length and depth of the cracks increase however; the change in crack width does not have a significant effect on the magnetic leakage field and the sensor''s signal. This technique is capable of high resolution magnetic imaging of small surface cracks and thus has great potential for application in the magnetic flux leakage test industry. [Copyright &y& Elsevier]