Rubia F. Gouveia

Electrostatic charging of hydrophilic particles due to water adsorption.

Kelvin force microscopy measurements on films of noncrystalline silica and aluminum phosphate particles reveal complex electrostatic potential patterns that change irreversibly as the relative humidity changes within an electrically shielded and grounded environment. Potential adjacent to the particle surfaces is always negative and potential gradients in excess of +/-10 MV/m are found parallel to the film surface. These results verify the following hypothesis: the atmosphere is a source and sink of electrostatic charges in dielectrics, due to the partition of OH(-) and H(+) ions associated to water adsorption. Neither contact, tribochemical or electrochemical ion or electron injection are needed to change the charge state of the noncrystalline hydrophilic solids used in this work.

Friction, tribochemistry and triboelectricity: recent progress and perspectives

Production of electricity by friction is well known but poorly understood, and is the source of electrostatic discharge causing serious accidents. Recent results are in agreement with one of the conflicting views on this problem, according to which triboelectricity in polymers is triggered by mechanochemical and wear or mass transfer phenomena. These results also challenge the widely accepted paradigm of one-way charge transfer that is the basis of the triboelectric series. Experimental results from powerful analytical techniques coupled to surface charge mapping support the following hypothesis: charge-bearing species are ionic polymer fragments formed through mechanical action. Beyond this, the atmosphere participates through tribocharge build-up and dissipation due to reactive plasma formation and charge exchange at the gas–solid interface, mediated by adsorption of non-neutral water, or ion partition during water adsorption, as in hygroelectricity phenomena.

Acid-base site detection and mapping on solid surfaces by Kelvin force microscopy (KFM).

Electrostatic potential at the surface of acidic or basic solids changes under higher relative humidity (RH), as determined by using Kelvin force microscopy (KFM). The potential on acid surfaces becomes more negative as the water vapor pressure increases, while it becomes more positive on basic solids. These results verify the following hypothesis: OH(-) or H(+) ions associated with atmospheric water ion clusters are selectively adsorbed on solid surfaces, depending on the respective Brønsted acid or base character. Therefore, Kelvin microscopy, under variable humidity, is a rigorous but convenient alternative to determine the acid-base character of solid surfaces, with a great advantage: it uses only one amphoteric and simple reagent to determine both the acid and base sites. Moreover, this technique provides information on the spatial distribution of acid-base sites, which is currently inaccessible to any other method.

Eletrização de dielétricos: novas propostas para resolver velhos problemas

Electrostatic phenomena were discovered long ago but their interpretation according to well-established atomic-molecular theory is still lacking. As a result, electrostatic phenomena are often irreproducible and uncontrolled, causing serious practical problems. Highly reproducible recent experimental results on electrostatic charging from this and other laboratories are reviewed in this work, together with a description of the relevant but not so usual Kelvin probe and Faraday cup techniques. These results support a new model for electrostatic charging of dielectrics and insulated metals, based on the role of moist atmosphere as a charge reservoir.

A New and Simple Procedure for Electric Potential Mapping by Phase Contrast in AFM

Electrostatic phenomena have been intensively studied after the 17th and 18th centuries with contribution from many scientists like Coulomb, Faraday, Volta, Ampere, Maxwell, DuFay and others. Based on electrostatic principles, many important technologies have been developed, such as electrostatic painting [1], electrospinning [2] and photocopying [3]. However, even considering this long history and the great number of resources available and widely used in the study of electric phenomena in insulators, there are still many open questions, for example, how can charge carriers in insulators be detected, identified and quantified? Electrostatic charging of insulators is poorly known and it often goes out of control because the charge carriers are unknown, in almost every case, resulting in many practical problems, including serious recent industrial accidents. Different authors have put forward new proposals but a persistent problem is the difficulty to produce repeatable and predictable electric potential patterns [4,5]. The study of electrostatic patterns on dielectrics and other solids has been greatly improved by the