Renato Cini

Temperature dependence of the surface tension of water by the equilibrium ring method

Abstract An experimental investigation of the temperature coefficient of the surface tension of pure water was carried out by the equilibrium ring method. The method, apparatus, and procedure are described in detail. Data of the maximum pull on the ring at 1°C intervals, or less, in the 0°–50°C temperature range are reported. The precision in the measurements is better than 0.005% of the total pull. The experimental results clearly show that thermal anomalies (“kinks”) are absent at the free surface of pure water. Furthermore, an approximate relationship between maximum pull on the ring and surface tension is discussed. The best-fit equation of the data of the surface tension as a function of temperature is given. Reliable values of the first and second derivatives dγ/dT and d2γ/dT2 are evaluated.

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...

Chemical characterization of cloud episodes at a ridge site in Tuscan Appennines, Italy

Cloudwater samples were collected from November 1992 to March 1995 in Vallombrosa, a mountain site of the Tuscan Apennines (central Italy). Chemical analyses show that all examined inorganic ions contributed significantly to the total ionic content (TIC). The ratio SO42−/NO4− ranged from 0.92 to 3.46 and was >1 for 86% of samples. There is a wide range in the chemical composition of the cloudwater. The total ionic content ranged from 640 to 7476 μeq l−1 and pH from 3.17 to 6.22. The liquid water content (LWC) ranged from 0.06 to 0.94 g m−3 and electrical conductivity from 47 to 485 μΩ−1. The total ionic content decreases while the liquid water content increases. Also analyzed were soluble trace metals (Fe, Pb, Cu, Mn, Cd, Al), synthetic anionic surfactants and the methanesulphonic acid. Chemical analyses evidenced in some cases a high concentration of organic matter. The meteorological analysis for a few samples of individual passages was carried out for the possibility of establishing a correspondence between meteorological events and chemical composition. The sources (marine, crustal and anthropogenic) of chemical components were deduced.

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.

Ripples damping due to monomolecular films

Abstract Elaborating the algebra relevant to ripple waves on the surface of film-covered water, a quartic equation is derived, whose solutions are the Laplace and the Marangoni waves. It is shown that the Laplace wave is the only physically meaningful solution when the film is soluble. When the film is insoluble, the Laplace solution is coupled with the Marangoni solution in the region of maximum damping.

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.

Experimental evidence of a maximum in the frequency domain of the ratio of ripple attenuation in monolayered water to that in pure water

Abstract Two different methods are discussed for measuring the attenuation coefficient of gravity-capillary waves in a tank, which is sufficiently small in size to allow chemical control of the surface properties. Both methods have been used on a 10−6 M solution of Triton X-100. The ratio of the measured attenuation to pure water attenuation plotted as a function of frequency shows a maximum, previously predicted by theory.