Mário Alberto Tenan

Physical Properties of Water Near a Gold Surface: A Nanorheological Analysis

Water at room temperature is not simply a medium for which uniform properties can always be assumed. Water close to solid hydrophobic or hydrophilic surfaces has elasticity, which is measured by monitoring the quartz crystal microbalance (QCM) resonant frequency and resistance. Small additions of salt are shown to modify this elasticity. Furthermore, near the hydrophobic QCM gold electrode, undersaturated aqueous NaCl solutions present a high concentration of ion pairs, which is confirmed by atomic force microscopy through force versus distance measurements.

Determination of the electromechanical parameters of the electrochemical quartz crystal microbalance

Abstract In this work we present a simple mechanical/electrical model for the electrochemical quartz crystal microbalance (EQCM). It consists of a harmonic oscillator (effective mass equal to one half of the crystal mass) coupled to an elastic element and in contact with a viscous fluid. As its electrical analog we consider a RLC series circuit. Simple arguments based on energy/power relationships lead to an easy identification of the mechanical parameters with the electrical ones. That identification together with considerations of the parallel capacitance effects of the electrodes allow us to test/calibrate an EQCM and also to interpret quartz crystal impedance–analyzer results. Model predictions are consistent with the experimental values. This is a subject of considerable relevance for all EQCM experiments.

Measurements of spatial-temporal structures in precipitated iron sulfate

Abstract Current pulses may cause an unusual spatial-temporal structure in precipitated films on iron electrode surfaces produced in sulfuric acid solutions. The precipitated films show an arrangement of cells resembling those found in convection at fluid-fluid interfaces. This phenomenon is the result of macroscopic fluid flow induced by interfacial tension gradients generated by the chemical kinetics of metal dissolution. The fluid motion induced by surface tension variation accelerates the aggregation of particles into a preferential square cell geometry which was observed on the samples prepared using a freeze-drying technique. Both the square pattern cell sizes and the interfacial tension variation have an identical functional dependence on sulfuric acid concentration.

Surface tension-induced convection as a particle aggregation mechanism

Abstract In the present work it is shown that convection—diffusion processes cause a peculiar morphology of precipitated iron sulfate on the metal surface. The calculated solution velocity distribution at the electrode surface shows a pattern of square cells. This fluid motion accelerates the aggregation of particles into a preferential square-cell geometry, which was observed on the samples. The samples, which were prepared using a freeze-drying technique, show that the square pattern cell sizes have the same functional dependence on sulfuric acid concentration as on the interfacial tension variation. The stability condition for cell pattern formation and also turbulence threshold are discussed as are the wall effects on the cell nonuniformity distribution.

Wetting effect on the hydrodynamics of thin-film formation

Abstract In this paper we show that ascending sulfuric acid solution films are formed on vertical partially immersed iron electrodes polarized at the passive potential region. A model is proposed for the film formation mechanism, according to which the driving force for film spreading is the change in solid-liquid contact angles, arising from chemical reactions at the electrode-solution interface.