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The properties of adsorbed layers of protein fibrils differ significantly from the properties of fibril spread layers on an aqueous subphase. If the dependencies of the dynamic surface elasticity on surface pressure of Lysozyme (LYS) and β-lactoglobulin (BLG) aqueous dispersions proved to be close to the results for native protein solutions, LYS and BLG spread layers on the surface of 0.1 M NaCl solution exhibited the surface elasticity more than two times higher than the values for protein solutions with the same NaCl concentatration, presumably due to lower surface concentrations of hydrolysed peptides in the latter case. The properties of fibril spread and adsorbed layers and also their morphology, unlike the surface properties of protein solutions, depend noticeably on the ionic strength of the aqueous bulk phase. This dependence is stronger in case of LYS layers, which are also more prone to the formation of macroscopic and mesoscopic surface aggregates as compared with BLG layers.

This work is directed to the elucidation of the peculiarities of fullerene C60 interactions with bovine serum albumin (BSA) in mixed layers at the water-air interface. To this aim, the dilational surface elasticity was measured as a function of surface pressure and surface age and optical methods together with the atomic force microscopy (AFM) were applied. The dependencies of the dilational dynamic surface elasticity of the mixed C60/BSA layers on the surface pressure have two local maxima indicating a conformational transition in the layer. At low surface pressures (region of the first maximum) the surface properties are determined mainly by the protein, while in the second region of relatively high surface pressures the contribution of the fullerene is more important. The AFM images show that both C60 layers and mixed C60/BSA layers contain separate large fullerene aggregates with the length in Z-directions of up to 100 nm but the morphology of the regions between these aggregates differs for the two systems. The mixed layer contains some patches of a network of almost merged fullerene/protein aggregates with a length in Z-direction not longer than about 20 nm. The formation and the subsequent reorganization of the network lead to non-monotonic kinetic dependences of the dynamic surface elasticity in the course of protein penetration into the fullerene layer. The obtained results show strong interactions between the components in the surface layer and thereby can change the toxicity of the fullerene.

The interactions between fibrinogen and polyelectrolytes in the surface layer of their mixed solutions were studied by measurements of the dynamic surface tension and surface elasticity as a function of surface age, polyelectrolyte concentration and solution pH. A flexible and negatively charged polyelectrolyte − sodium polystyrolsulfonate (PSS) – influenced the kinetic dependencies of the surface properties both below and above the isoelectric point, where fibrinogen is positively and negatively charged, respectively, indicating the formation of protein/polyelectrolyte complexes. At the same time, the addition of a more rigid and positively charged polyelectrolyte – polydiallyldimethylammonium chloride (PDADMAC) – to fibrinogen solutions resulted in noticeable changes of the studied dynamic surface properties only at pH values higher than the isoelectric point, while at lower pH values no interactions between the two solutes were observed. The formation of the protein/polyelectrolyte complexes did not lead to noticeable changes of the protein tertiary structure in the surface layer and all the kinetic dependences of the dynamic surface properties were monotonic.

The dynamic surface properties of mixed monolayers of 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC) and negatively charged polystyrene particles (PS) of different size were investigated with the aim to model the interactions of particles with biointerfaces. Special attention was paid to the aggregation of the particles at the water/air interface. The particle aggregates have a weaker influence on the dynamic properties of DPPC monolayers as compared with non-aggregated particles because of a shorter three phase contact line. The incorporation of particles into a DPPC monolayer leads to disordering of the monolayer structure, decreases the changes of surface properties in the course of two-dimensional phase transitions and the dynamic surface elasticity of the condensed lipid film. The influence of particles on the properties of mixed monolayers depends strongly on the ratio of lipid molecules and particles, and decreases with the decrease of particle surface concentration. The Brewster angle microscopy gives direct evidence of the PS particle aggregation in the course of compression of mixed monolayers.

The theoretical description of the adsorption of surfactants at interfaces between aqueous solutions and oil was based over a very long time on models derived for the solution/air interface. Thus, most of the experimentally observed peculiarities could not be specifically considered but were merely interpreted in terms of a penetration of oil molecules into the alkyl chain layer of the adsorbed surfactant molecules. These penetrating oil molecules enhance the surfactant adsorption as compared to the water/air interface. Later on, for the special situations at water/oil interfaces a competitive adsorption of surfactant and oil molecules was postulated, allowing a much better description of experimental data. This picture, however, was unable to explain why the interfacial tension of the water/oil interface decreases very quickly when extremely small amounts of surfactants are added to the water. This effect cannot be of competitive nature, but a cooperativity of surfactant and oil molecules forming a mixed adsorption layer is required instead. This cooperative effect means that already few surfactant molecules adsorbed at the interface can induce a significant ordering of oil molecules in the interfacial layer. This new interfacial structure, in turn, attracts further surfactant molecules to adsorb. Improving the theoretical description of experimental data was finally achieved by applying suitable adsorption models for the two adsorbing compounds, i.e. a Frumkin adsorption model for the oil molecules and a Langmuir, Frumkin, or reorientation model for the adsorbing surfactant molecules. Here, the progress in modelling surfactant adsorption at water/oil interfaces is discussed mainly for the homologous series of the cationic surfactants C(n)TAB, of the anionic surfactant SDS, and members of the homologous series of the non-ionic surfactants CnDMPO at water/alkane interfaces. (C) 2020 Elsevier B.V. All rights reserved.

The dynamic surface properties of solutions of pulmonary surfactant (PS) were investigated at various temperatures and in a broad range of surface tensions. The efficient dynamic surface elasticity was determined for the first time in case when the characteristic adsorption time is comparable with the period of surface area oscillations. The mechanical relaxation in the PS adsorption layer is accelerated strongly not only in the course of compression, but also in the course of expansion due to the formation of a sublayer. The harmonic oscillations of the surface area induce stationary oscillations of the surface tension if the characteristic adsorption time does not exceed the oscillation period and the sublayer formation is sufficiently fast. The information on the dynamic properties of the PS adsorption layer allows estimation of the PS efficiency in medical applications.

Polydopamine (PDA) films at the air - water interface can be easily obtained in slightly alkaline dopamine solutions using 02 dissolved in water as an oxidant but their insufficient stability limits their applications. In this work PDA films at the water surface were obtained by polymerization of dopamine hydrochloride (DA) by the enzyme laccase in a slightly acidic environment at a constant concentration of enzyme and DA concentrations in the range from 0.1 to 10 g/l. The dynamic surface elasticity, effective surface tension, film thickness and refractive index were measured as function of time to characterize the main steps of film formation. The obtained results show that laccase accelerates the polymerization process, decreases the required DA concentration for film formation, and insight from Brewster angle microscopy shows that laccase increases the resistance of the films to mechanical deformation. The dynamic surface elasticity of the obtained films is about seven times higher than the highest values for either pure PDA or pure laccase films. Laccase is inferred not only to provide crosslinking of the polymer but is also incorporated in the film resulting in its higher stability. The kinetic dependences of the surface properties allow the main steps of film formation to be distinguished: nucleation of polymer domains in a rapidly-formed protein film, coexistence of the protein and polymer domains, and their transformation into a highly dense composite film with encapsulated protein. Insight from atomic force microscopy images of films transferred to mica show that PDA fills in the gaps of spherical laccase aggregates when the DA concentration is increased and the film becomes less rough. The inclusion of laccase in PDA films significantly increases their stability.

In this work, dynamic surface properties of spread monolayers of DPPC and adsorbed layers of PS solutions were investigated in a broad range of surface pressure. Application of a modified Langmuir trough and a recently developed approach for the analysis of a non-linear response of the adsorption layer to large surface deformations gives a possibility to determine the dilational surface elasticity in the region of high surface pressures corresponding to the physiological state of the lung alveoli surface. Although DPPC is the main component of the complex pulmonary surfactant (PS) mixture, the dynamic surface properties of the adsorption layers of these two compounds are different. Other components in the PS adsorption layer lead to a decrease of the dynamic surface elasticity due to a looser packing of the layer, as confirmed by ellipsometry and infrared reflection absorption spectroscopy. Moreover, in the region of high surface pressures the main relaxation time of the surface stresses is much lower than that for spread DPPC monolayers. The fast relaxation in the adsorption layer can be connected with the redistribution of molecules between the surface and the subsurface layer in the course of compression. The acceleration of the relaxation processes can lead to a decrease of the number and size of collapsed parts of the layer and thereby to a decrease of the number of molecules with lost functional properties. This effect can be one of the reasons for the lower efficiency of synthetic pharmaceutical drugs, which do not have proteins in their composition, as compared with the natural PS extracted from animal lungs.

The interfacial tension response to dilational deformation of interfacial area exhibits a (more or less) nonlinear behavior, depending on the amplitude of the deformation. Studies of such observable interfacial properties in the nonlinear domain suggest valuable information about the two-dimensional microstructure of the interfacial layer, as well as about the structure time-evolution. In this article, the emphasis is centered on the available mathematical methods for quantitatively analyzing and describing the magnitude and the characteristics of the nonlinear interfacial viscoelastic properties. Specifically, in periodic oscillation experiments the nonlinear behavior can be represented by the combination of a linear part (the surface dilational modulus), with an additional complementary Fourier analysis parameterizing the nonlinearity. Also asymmetric Lissajous plots, of interfacial tension versus deformation, are useful tools for expanding the response nonlinearity into four distinct components relevant to significant points of the cyclic loop. In connection with the mathematical methods, nonequilibrium thermodynamic formulations provide a powerful theoretical framework for investigating the interfacial dynamic properties of multiphase systems. Experimental results for adsorption layers of complex components, available in the literature, show notable nonlinear interfacial viscoelastic behavior. In particular in this review, data are illustrated for solutions of polymers and of polyelectrolyte/surfactant complexes. The observed nonlinear findings reveal formation of complexes, patches, and other different interfacial structures.

A new approach to the analysis of nonlinear surface dilational rheological properties was developed with the aim to enlarge the accessible range of surface pressures. The dilational surface elasticity of spread monolayers of DPPC, cholesterol, DMEA and their mixtures was determined in the region of low surface tensions (less than 10 mN/m) corresponding to the state of pulmonary surfactants at the lung interface. The dilational surface elasticity of pure DPPC monolayer proved to be high (∼200 mN/m) up to the collapse. The addition of cholesterol, DMEA or their mixture to the DPPC monolayer increased the dynamic surface elasticity, but could decrease significantly the surface pressure of the monolayer collapse if the concentration of additions exceeds an optimal value.

The non-monotonic concentration and kinetic dependencies of the dynamic dilational surface elasticity of mixed poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate(SDS)/NaCl solutions indicate the formation of micro-aggregates in the surface layer. The increase of the solution's ionic strength results in strong changes of the surface dilational rheological properties and thereby of the interfacial layer structure at lower SDS concentrations due to the large increase of the adsorption rate. Close to the charge match point of the components, however, the rheological properties are influenced by the presence of micro-aggregates embedded in the adsorption layer. In this case, effects of changing the sample history on the rheological behavior and heterogeneity of the interface were further investigated.

The surface properties of a polydopamine layer at the air-water interface were studied by dilatational surface rheology, ellipsometry and Brewster angle microscopy (BAM). A significant increase of the dynamic surface elasticity was discovered when the concentration changed from 0.75 g/l to 2 g/l with the maximum value of about 60 mN/m at a concentration of 1 g/l. The obtained results indicate that the surface film consists of separate domains and the high surface elasticity is a consequence of the interactions between relatively rigid domains of the polymer film. This conclusion was confirmed by Brewster angle microscopy, which demonstrated different steps of the polydopamine film growth. Separate domains appeared at the first step while one can observe a continuous film close to equilibrium. An increase of the initial concentration led to faster polymerization and to the formation of a thicker film. The dynamic surface elasticity decreased in the concentration range from 2 g/l to 5 g/l when the thickness of the polymer film reached about 80 nm. In this case the film could be destroyed in the course of deformation. The cracks in the film resulted in a decrease of the dynamic surface elasticity.

The presented drop and bubble micro manipulator (DBMM) is a new experimental tool for the quantitative analysis of the interaction between two drops or two bubbles or even between a single drop with a bubble in a liquid medium. For each drop or bubble, a coarse and a very fine dosing system allow controlling their size or changing it in respect to a given protocol. An accurate pressure sensor for each drop/bubble can record changes in the capillary pressure with a selected sampling rate in order to follow critical processes between the two liquid menisci, such as coalescence. The stability of liquid films between drops or bubbles can be studied also under external perturbations, i.e. a harmonic oscillation with a given frequency and amplitude can be generated in order to find critical conditions for the coalescence. The described tool can be used in different experimental protocols. Among them there are options to mimic the situation of multilayers and multiple emulsions. Recording of dynamic processes as video with a standard frame rate is possible as well. Alternatively, a fast video camera can be mounted to monitor fast processes, such as the coalescence of two drops or bubbles.

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/dm3) 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.

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 100 Hz are discussed, and elasticity values at up to 80 Hz were measured for the liquid–liquid interface.

Recent application of the methods of surface dilational rheology to solutions of the complexes between synthetic polyelectrolytes and oppositely charged surfactants (PSC) gave a possibility to determine some steps of the adsorption layer formation and to discover an abrupt transition connected with the formation of microaggregates at the liquid surface. The kinetic dependencies of the dynamic surface elasticity are always monotonous at low surfactant concentrations but can have one or two local maxima in the range beyond the critical aggregation concentration. The first maximum is accompanied by the generation of higher harmonics of induced surface tension oscillations and caused by heterogeneities in the adsorption layer. The formation of a multilayered structure at the surface for some systems leads to the second maximum in the dynamic surface elasticity. The hydrophobicity and charge density of a polymer chain influence strongly the surface structure, resulting in a variety of dynamic surface properties of PSC solutions. Optical methods and atomic force microscopy give additional information for the systems under consideration. Experimental results and existing theoretical frameworks are reviewed with emphasis on the general features of all studied PSC systems.

Scanning probe microscopy is used to study adsorption films of sodium polystyrenesulfonate/dodecyltrimethylammonium bromide complexes, which are transferred from aqueous solution surfaces to the mica surface by the Langmuir-Blodgett method. It is established that the results obtained at surfactant concentrations below critical micelle concentration agree with the conclusions on the structure of the films dased on the data of the dilatational surface rheology (Noskov, B.A., Loglio, G., Miller, R., J. Phys. Chem. B, 2004, vol. 108, p. 18615). At low concentrations of the low-molecular-mass surfactant, a thin adsorption film containing a large number of holes is formed on the surface of an aqueous solution. As the surfactant concentration is increased to reach the region of a dramatic reduction in the dynamic surface elasticity, the morphology of the film drastically changes; i.e., dense three-dimensional aggregates are formed on the solution surface.

The dilational dynamic surface elasticity of mixed aqueous polyvinylpyridinium chloride/sodium dodecyl sulfate solutions (PVPmCl/SDS) was measured as a function of time and surfactant concentration. This system represents a typical example of polyelectrolyte/surfactant complex formation and subsequent aggregation on the nano-scale. The concentration dependence of the real part of the dynamic surface elasticity for the mixed PVPmCl/SDS solutions in 0.1 M NaCl and without added NaCl qualitatively agreed with the corresponding results for some previously studied polyelectrolyte/surfactant solutions. This similarity indicates that strong changes of the surface visco-elasticity in a narrow concentration range can be explained by the formation of microparticles in the surface layer. The addition of 0.1 M NaCl to PVPmCl/SDS solutions led to an almost two times decrease of the surface elasticity at low concentrations and to the acceleration of the adsorption layer formation. At the same time the imaginary part of surface elasticity of PVPmCl/SDS solutions was significantly higher than the corresponding results for the earlier studied systems and the adsorption layers proved to be visco-elastic in a broad concentration range. The application of the Maxwell rheological model in this case allowed the calculation of the main relaxation time. The main relaxation process is the surfactant exchange between the surface layer and the bulk phase or in the surface layer between free surfactant molecules and those bound in aggregates. The distinction of the surface visco-elasticity of PVPmCl/SDS solutions from the previously studied systems can be explained by the lower stability of polyelectrolyte/surfactant complexes in the adsorption layer.

Measurements of the dilational dynamic surface elasticity allow determination of some distinctions between the properties of monolayers of charged polystyrene nano- and microparticles at the water/air and water/oil interfaces. For example, the surface elasticity at the water/oil interface is lower and does not depend on the solution's ionic strength unlike the case of water/air interface. The obtained results at relatively low surface pressures (

This short review is devoted to recent achievements in studies of the dilational surface rheological properties of the systems containing complexes of silica nanoparticles with conventional surfactants. It is shown that there is a surfactant concentration range where the dynamic surface elasticity reaches extremely high values up to 1000 mN/m. This result can explain the formation of very stable foams and emulsions containing nanoparticles. In some surfactant concentration ranges the adsorption layer is characterized by a non-linear response to small compressions or expansions of the liquid surface. Possible causes of this behavior and the mechanism of main relaxation processes are briefly discussed. © 2014 Pleiades Publishing, Ltd.

A new methodology, based on fundamental optical principles relevant to the evanescent wave properties, is developed to evaluate the thinning behavior and the local thickness in different positions for curved aqueous emulsion films, generated in a hydrocarbon matrix. This methodology is applied to test data obtained by a specially-designed bread-board cell. The generated films, constituted by an aqueous sodium dodecylsulfate solution at concentration c = 2.5 times the critical micellar concentration, remain stable during several hours. The results show a slow evolution of the film thickness as a function of time starting from micrometric dimension, at the initial film formation, to a few nanometres prior to film breakage. The action of the gravitational drainage is furthermore assessed by the local thickness values in different positions of the film profile. The illustrated examples demonstrate that the evanescent wave effect can be advantageously adopted as a complementary measurement technique, additional to the usual interferometric technique.

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.

Theoretical models of the exchange of matter at interfaces are necessary to interpret relaxation phenomena in surfactant adsorption layers. For liquid-liquid systems, the diffusion-controlled exchanges of surfactant molecules at the interfaces can be influenced by a simultaneous transfer of molecules from one bulk phase to the other. The diffusional mass exchange function is derived taking into account the transfer across the interface. The resulting mass exchange function is used to calculate the interfacial tension response of a liquid-liquid system. As an example, the interfacial response after a ramp-type area perturbation is calculated.

The frequency and concentration dependencies of the dilational elasticity and viscosity of adsorbed surfactant layers is yet not quantitatively understood. Especially at a surface coverage above 50% the measured high frequency limits of the elasticity pass typically a maximum or reach a plateau while classical models expect an exponential increase to very high values. The consideration of an intrinsic two-dimensional compressibility allows a much better description of experimental data. There are various surfactant systems discussed in literature following this new model. Although the impact of these interfacial characteristics on the dilational rheology is significant, the adsorption isotherm and equation of state change only slightly.

The experimental data on the surface rheological characteristics of dodecyl dimethyl phosphine oxide solutions obtained in a fully automatic oscillating bubble device under microgravity conditions in the frequency range 0.01-100 Hz are presented. The complex surface elasticity modulus is obtained form the amplitude- and phase-frequency characteristics of established pressure oscillations in a closed cell without calibration experiments by direct calculation of the necessary coefficients. The characteristics of the adsorption layers obtained from the elasticity modulus are in good agreement with adsorption isotherms and equations of state accounting for the intrinsic (2D) monolayer compressibility.

We have investigated the surface dilational properties of a submicellar aqueous solution of n -dodecyldimethyl-phosphine oxide. We compared (a) the steady-state responses of dynamic surface tension, induced by small-amplitude harmonic disturbances of surface equilibrium (at different frequencies), and (b) the transient response induced on the same sample by a single trapezoidal-pulse disturbance. Using the equations for a diffusive model, we determined the fit parameters from the observed response values both in the frequency and in the time domains. The results enabled us to predict satisfactorily the harmonic dynamic behaviour of the surface, from the relaxation data. The prediction-bservation agreement supports the theoretical treatment and, in particular, the assertion that the viscoelastic link, between surface excitation and response, is a constitutive property of a liquid solution. The experiments also show the existence of a linearity range and the repeatability of measurements.

Samples of fulvic acids, extracted from the Tyrrhenian Sea (Leghorn, Italy), are studied by dynamic surface tension and surface shear rheology measurements. The initial adsorption process follows a diffusion-controlled mechanism, while at longer times it is several orders of magnitude slower than expected from diffusion. The fulvic acids form adsorption layers which exhibit viscoelastic behaviour. Some samples show a continuous increase in shear viscosity and elasticity until the value exceeds the limits of the shear rheometer. For other samples, an increase in these values was only observed during the first hour, after which the values levelled off.

Surface relaxation processes of liquid solutions, under not-far-from-equilibrium conditions, are interpreted from a phenomenological point of view by application of the theory of distributed systems. In the case of diffusion-controlled adsorption, exact analytical expressions describe the transient responses of bulk concentration and dynamic surface tension, consequent to trapezoidal pulses of relative surface area. The mathematical treatment shows that surface responses have a definite physical significance, as they manifest a constitutive property of the system, i.e. the surface dilational modulus.

Dynamic interfacial phenomena are shown to be most satisfactorily interpreted within the general framework of the theory for system analysis. All time-dependent dilational behaviours of interfaces can be quantitatively predicted from knowledge of the dilational modulus, that is, of the transfer function of the interfacial system. The unifying mathematical formalism allows traditional experiments, known in the literature, to be connected one to another. The presented relationships are also the foundations of the transient techniques for the measurement of interfacial quantities. Problems involving diffusive surface relaxation in quiescent hydrodynamic conditions are easily analytically or numerically solved. Other complex relaxation mechanisms introduce no additional conceptual complication.

Computational procedures are devised which allow accurate magnitude spectra of the modulus ɛ(v) to be evaluated from the observed tansient surface tension decay. This is achieved by proper data manipulation and by adaptation of FT algorithms, taking into account the particular characteristics of the time — and frequency — domain functions relevant to the examined dynamic properties of fluid interfaces. Numerical examples, given for 1) exponential-type and 2) diffusion-controlled relaxation mechanisms, show satisfactory accuracy.

Analyses, reported in the literature on the capillary force resulting from the formation of a liquid meniscus, are examined in the light of a new rigorous mathematical treatment. Formal aspects are pointed out, emphasizing the consideration that surface mechanical properties should preferably be conceived as originating by virtue of the existence of a surface free energy rather than of a contractile skin.