Electroconvection in a layer of liquid subjected to unipolar injection is characterized by two stability criteria, a linear and a nonlinear one, with an associated hysteresis loop. Experimentally it is found that the velocity field fluctuates around its mean value. A temporal analysis of the measured current, which is directly related to the velocity, revealed the existence of a well-defined frequency correlated to the mean rotation time of a fluid particle in the convective cell, thus indicating that these fluctuations are not stochastic but related to the intrinsic dynamics of the system. Here a method of superparticles is used to solve the problem of the non-stationary electroconvection of the liquid. A good agreement between theoretical and experimental results is obtained.

This paper presents the linear stability analysis of an interface between air and an insulating liquid subjected to a perpendicular electric field, in the presence of unipolar injection of charge. Depending on the characteristics of the liquid and the depth of the liquid layer two different instability thresholds may be found. One of them is characterized by a wavelength of the order of the liquid layer thickness and corresponds to the well-known volume instability of a liquid layer subjected to charge injection. The other one is characterized by a wavelength some ten times the liquid layer thickness and corresponds to the so-called rose-window instability, an instability associated to the balance of surface stresses. C 2014 AIP Publishing LLC.

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, 2378 (1994).The general rigorous theory corresponding to double potential step chronoamperometry for a reversible charge transfer reaction in a spherical electrode whose area increases with t', where z > 0, is presented. The static mercury drop electrode (z = 0) and the dropping mercury electrode ( z = 213) are addressed specifically. The explicit equations for the current versus time response assuming unequal diffusion coefficients for the oxidized and reduced species are valid when the product is soluble in both the electrolytic solution and the electrode. Comparisons with previous general treatments in the literature are made. The theory is applied to the detection of amalgamation in RP and DNP techniques.Moreover, in this work we show the origin of errors appearing in some general treatments in the literature, these errors lead to physically meaningless results when amalgamation takes place. 2378 (1994). On prCsente la thCorie gCnCrale rigoureuse correspondant 2 la chronoampkromCtrie avec une Ctape h double potentiel d'une rCaction rCversible de transfert de charge dans une Clectrode sphCrique dont la superficie augmente comme t Z , oh z > 0. On examine d'une f a~o n spCcifique les cas des Clectrodes avec goutte de mercure statique (z = 0) et avec goutte de rnercure tombante (z = 213). Les Cquations explicites pour le courant en fonction du temps de rCponse, en supposant des coefficients de diffusions diffkrents pour les espbces oxydCes et rkduites, sont valides lorsque le produit est soluble dans le solution Clectrolytique ainsi que dans 1'Clectrode. On prCsente des cornparaisons avec les traitements gCnCraux rapportCs antCrieurement dans la littkrature. On applique la thCorie ii la dCtection d'arnalgamation dans les techniques de RP et de DNP.De plus, dans ce travail, on rnontre l'origine des erreurs qui apparaissent des quelques traftements gCnCraux rapportks dans la littkrature et qui, lorsqu'une amalgamation se produit, rendent les risultats physiquement sans signification.[Traduit par la rCdaction] Introduction In this paper we study the application of a double potential pulse to a reversible charge transfer process at stationary and nonstationary spherical electrodes. We consider the general case when the diffusion coefficients of the oxidized and reduced species are different, and the reduced species can dissolve in the electrolytic solution as well as in the electrode. In order to obtain an exact expression for the response corresponding to the second potential pulse, it is essential to consider the perturbation of the second pulse on the concentration profiles generated by the first potential pulse. Previous investigators (1, 2) have neglected this perturbation.The difficulties in the calculation of the response to the second potential step for a spherical electrode arise from surface concentrations which are dependent on potential, diffusion co-