David Mendez Soares

Floating liquid bridge tensile behavior: Electric-field-induced Young's modulus measurements

A floating bridge is formed spontaneously when high voltage is applied to polar fluids in two capillary tubes that were in contact and then separated. This bridge bends under its own weight, and its bending profile was used to calculate its Youngs modulus. For electric field intensities of ∼106 V/m, water bridges exhibit viscoelastic behavior, with Youngs moduli of ∼24 MPa; dimethylsulfoxide (DMSO) bridges exhibited Youngs moduli of ∼60 kPa. The scheme devised to measure the voltage drop across the water bridge for high voltages applied between the electrodes shows that the bulk water resistance decreases with increasing voltage.

The formation of nanostructures on silicon surfaces in the presence of hydrogen

The presence of hydrogen in HF solutions at a silicon substrate surface is shown to be sufficient to produce a photoluminescent porous silicon layer. The photoluminescence measurements of bubbled and anodized samples show similar spectra when illuminated with UV radiation. This is strong evidence that the hydrogen produced by the anodic silicon dissolution reaction is also responsible for the formation of nanostructures.

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.

Deactivation mechanism of nickel cathodes in alkaline media

Abstract Time-dependent hydrogen evolution reaction (HER) on Ni electrodes shows a large increase in electrode over potential with time. This is ascribed to hydride formation at active Ni cathode surfaces. Hydride formation was detected by X-ray diffraction. By taking into account the nickel electronic density of states variation following hydrogen sorption, we are able to satisfactorily explain the increase in nickel over potential after a few hours of HER.

Floating liquid bridge charge dynamics

The interaction of liquid with electric fields is investigated in a configuration where up to 13 kV are applied between electrodes resulting in a 106 V/m electric field in the capillaries and where there is the formation of a free-standing fluid bridge in the interelectrode gap. The Mott-Gurney equation was fitted to the measured ionization current vs applied voltage curve which indicates that the ionization rate at the high-voltage anode electrode dimethylsulfoxide (DMSO) interface and space charging in the interelectrode gap determine the floating liquid bridge current for a given cathode-to-anode voltage. Space charge effects were measured in the cathode becker and also at the liquid bridge since the ionized charges at the anode migrate to the bridge outer surface and decrease the interfacial tension from 43 mJ/m2 to 29 mJ/m2. Two distinct structural regions then form the bridge, a charged plastic (bulk modulus ∼100 MPa) conducting outer layer with a surface conductivity of ∼10−9 Ω−1, which shapes and supports the floating fluid structure, and an inner liquid cylinder, where DMSO molecules flow.

Magnetic force images of nanomagnetic domains taken with platinum-coated tips

This article deals with magnetic force microscope images of nanosized domains in Co-coated films made by Pt-coated tips as well as micromagnetic images of data tracks written in recording media. Pt-coated tips have improved image delineation of the magnetic field distribution compared to images obtained by Co-coated hard magnetic tips. The force acting on Pt-coated tips in the magnetic field of the substrate was modeled assuming a paramagnetic tip. Due to the ferromagnetic nature of the interaction between the tip and substrate the spatial resolution of hard magnetic tips was shown to be inadequate to measure details of the features of nanosized domains. A comparison of the magnetic images made by Pt-coated tips with topographic images shows that magnetic domains resist thermal erasure at ambient temperature when they are formed of eight metallic grains.

Hydrated Excess Proton Raman Spectral Densities Probed in Floating Water Bridges

Excess proton structures in water remain unclear. The motion and nature of excess protons in water were probed using a supported water bridge structure in electric field (E) with an intensity of ∼106 V/m. The experimental setup generated protons that exhibit a long lifetime. The effect of excess protons in water induced a ∼3% variation in the pH for a 300 V overvoltage at the cathode. The current versus voltage curves show a current space-charge-limited operation. By measuring the space-charge distribution in both the cathode and anode and by adjusting the Mott–Gurney law to the measured excess hydrated proton current and the voltage drop in the cationic space-charge region, the protonic mobility was determined to be ∼200 × 10–8 m2/(V·s) (E ≈ 4 × 106 V/m). This measured mobility, which is typically five times larger than the reported mobility for protons in water, is in agreement with the mechanism outlined by Grotthuss in 1805. The measured mid-Raman spectrum covering 1000–3800 cm–1 range indicates the species character. The hydrated excess proton spectral response through the mid-Raman at 1760 and 3200 cm–1 was attributed to the Zundel complex and the region at ∼2000 to ∼2600 cm–1 response is attributed to the Eigen complex, indicating a core structure simultaneously with a Eigen-like and Zundel-like character, suggesting a rapid fluctuation between these two structures or a new specie.

Atomic scale patterns formed during surface scanning by atomic force microscopy tips

In this work, tip sliding at the water/substrate interfacial region was used to investigate the pattern observed during image acquisition with atomic resolution in atomic force microscopy. The process responsible for the pattern formation is the oscillatory movement of the tip in the direction that is normal to scanning induced by a change in the water interfacial dielectric permittivity from e≈4 at the interface to e≈80 (bulk value) that results in a variation of the measured force acting on the tip of ≈30pN.

Large area industrial electrochemical cells tester

Abstract A method has been developed for characterizing electrodes of full size electrolytic cells. It consists of interrupting the current on only a small fraction of the working electrode, called probe electrode, and measuring the potential difference between probe electrode and reference electrode. The working and probe electrodes are kept at the same potential for most of the operation by an external connection.