U. Tesei

Spectral data of surface viscoelastic modulus acquired via digital fourier transformation

Abstract The time domain behavior of dynamic surface tension was experimentally observed for submicellar aqueous solutions (2 × 10 −3 and 4 × 10 −3 mole m −3 ) of the surfactant dimethyldodecylphosphine oxide. The frequency domain behavior was calculated. by mapping one domain into the other with the Fourier transform operator. Two consecutive numerical processing runs were performed with different discretizations of the same input signal (0.1− and 3.0-sec time intervals between readings, respectively). By choosing an appropriate part of the output array from each run, satisfactorily accurate spectra of surface viscoelastic modulus were obtained. Values of parameters, determined from the reported spectra on the basis of the Lucassen theory, compare favorably with those given in the literature (obtained from equilibrium data).

Dilational viscoelasticity of fluid interfaces: The diffusion model for transient processes

Abstract The functional form of the surface dilational modulus is determined by considering the subsurface concentration changes produced by the diffusion process consequent to a non-periodic (rectangular step) perturbation of surface area (local equilibrium between surface and subsurface is assumed). The mathematical treatment is intended to present a straightforward demonstration that the same (unique) physical quantity, i.e. the dilational modulus, governs both harmonic and transient surface processes. The results definitely show that this modulus is an intrinsic physical property of a diffusive system, independent of the type of surface disturbance (within the linearity approximation).

Measurement of interfacial dilational properties: A software‐driven apparatus

The construction of a time‐resolved surface viscoelastometer for the study of fluid/fluid interfaces is described in detail. Instrument design and operation are based on the formalism of systems theory. An interfacial‐area change (i.e., an external stimulus) is generated according to a given time function; the ensuing dynamic surface tension response is continuously recorded in synchronism with the stimulus. The principal characteristic of the equipment is a computer‐controlled mechanical motion of the area‐confining device (i.e., of a particular ring barrier, in place of the traditional linear barriers). The software allows a wide variety of oscillatory or transient experiments to be conducted. Instrument performance is illustrated by a typical experimental response, obtained under conditions of small‐amplitude area perturbation, for a submicellar aqueous solution of surfactant. The recorded trace is fitted to a theoretical two‐parameter model. Results show the effectiveness of the controlled‐surface‐def...

Non-equilibrium properties of fluid interfaces - Aperiodic diffusion-controlled regime. 1. Theory

Abstract 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.

Viscoelastic dilatation processes of fluid/fluid interfaces: time-domain representation

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.

Dilational properties of monolayers at the oil-air interface

Abstract Time-domain representations of linear surface viscoelasticity are shown to be advantageous in the characterization of dynamic interface properties exhibited by petroleum and derivatives. General relationships are reported which allow time-representations to be analytically or numerically evaluated for any surface relaxation mechanism and for whatever form of the input surface area perturbation. Adopting the translational-diffusion model, an analytical expression is developed for describing the time evolution of the amplitude of surface tension rise and decay pertaining to a forcing tilted-step perturbation. By fitting this expression to experimental values, intrinsic surface properties can be determined. In addition to the technical interest, the theoretical treatment is also discussed in terms of its fundamental importance for the study of transport mechanisms at fluid-fluid interfaces as well as for the determination of thermodynamic equilibrium quantities.

Non-equilibrium surface thermodynamics. Measurement of transient dynamic surface tension for fluid—fluid interfaces by the trapezoidal pulse technique

Abstract The trapezoidal pulse perturbation is shown to be an attractive procedure for the measurement of dynamic properties of fluid—fluid interfaces. The surface response characterizes the surface relaxation behavior well. Theoretically, this procedure has foundations within the framework of linear non-equilibrium thermodynamics. From the experimental point of view, the procedure offers the possibility of checking the experimental reliability and exhibits practical aspects due to the short duration of the surface response. Benefits are also evident in data processing.

Air-Sea Exchange: Sea Salt and Organic Microcomponents in Antarctic Snow

Abstract A characterization of surface active fluorescent organic matter (SAFOM) in Antarctic snow is carried out. Its Fulvic Acids (FA) nature is confirmed. Its enrichment in the smallest aerosol particles is shown. A tentative explanation of the presence of both natural and man-made organic microcomponents (SAFOM-interacting) is given in terms of marine aerosol transport. Their enrichment ratio appears of the same order as that of SAFOM, and their presence in the atmospheric particulate of marine origin supports the hypothesis on the transport of microcomponents in Antarctica “via marine aerosol”.

Nonequilibrium properties of fluid interfaces: aperiodic diffusion-controlled regime 2. Experiments

Abstract 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.

Spectrofluorimetric Evidence of the Transport of Marine Organic Matter in Antarctic Snow Via Air-Sea Interaction

Abstract This paper considers the importance of marine aerosol in the atmosphere. As a consequence of its peculiar generation mechanism, it is also considered as a possible contributor to the transport of man-made microcomponents via air-sea interaction, especially in remote sites. In view of future marine aerosol studies, the dominant presence of marine aerosol components in coastal Antarctic snow is discussed the presence of fluorescent marine organic matter in Antarctic snow is shown. Its “marum” nature is evidenced. We give a tentative interpretation of the experimental data, and make a hypothesis concerning the variations in marine aerosol composition in correspondence to various altitudes.