Production of electricity by friction is well known but poorly understood and it 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 10 65 mechanisms leading to solids contact electrification and/or triboelectric charging? The explanation of the production of electrostatic charge comes from a transfer of electrons, ions or both, which are presented in the literature? 16 Poor knowledge on charge accumulation and dissipation 70 mechanisms 17-19 is a root of large-scale personal and property losses, including serious industrial accidents and explosions that are described further in the next section. This is ultimately due to the lack of scientific understanding of the basic phenomena.On the other hand, fundamental electrostatic concepts are well 75 established for semiconductors and metals. 20 When two metals with different work functions are brought into contact, electrons migrate across the interface creating a potential difference between them.

Friction between dielectric surfaces produces patterns of fixed, stable electric charges that in turn contribute electrostatic components to surface interactions between the contacting solids. The literature presents a wealth of information on the electronic contributions to friction in metals and semiconductors but the effect of triboelectricity on friction coefficients of dielectrics is as yet poorly defined and understood. In this work, friction coefficients were measured on tribocharged polytetrafluoroethylene (PTFE), using three different techniques. As a result, friction coefficients at the macro- and nanoscales increase many-fold when PTFE surfaces are tribocharged, but this effect is eliminated by silanization of glass spheres rolling on PTFE. In conclusion, tribocharging may supersede all other contributions to macro- and nanoscale friction coefficients in PTFE and probably in other insulating polymers.

The electroneutrality principle 1 expresses the fact that all pure substances carry a net charge of zero. However, real substances in the environment are usually under significant static potential gradients and thus under nonzero electric potential. 2 The electrochemical potential (μ) (eq 1) of ions under a given electric potential (V)

An attempt to understand the microscopic origin of the high viscosity of Brazilian heavy crude oils was made combining macroscopic (rheological measurements) and microscopic [small-angle X-ray scattering (SAXS) measurements] techniques. A clear relationship between the asphaltene content and viscosity was found, while the removal of asphaltene via flocculation led to a large viscosity drop, confirming them as the origin of high viscosity. The SAXS analyses of crude oils confirmed the presence of asphaltene aggregates as fractal-like particles of colloidal dimensions. Afterward, a systematic investigation was performed on the effects of a series of additives and physical treatments on the crude oil viscosity. Physical methods did not cause any significant viscosity drop as well as more than 80 additives tested. SAXS measurements on oil samples containing toluene and heptane indicated little effect on the asphaltene nanoaggregates within the dimensions probed by SAXS, confirming a general mode of action based on aggregate dilution instead of disruption.

Asphaltene precipitation is a key problem in the petroleum industry and has been the focus of many studies on their aggregates present in crude oils and on the effects of additives to inhibit their formation and/or deposition. However, most of these studies were performed using model systems, such as asphaltene solutions in organic solvents, making the comparison to real systems more difficult. Herein, we combine different modes (height and phase modes) of atomic force microscopy to identify colloidal particles associated with asphaltene aggregates present in crude oils. Following this methodology, a mica plate is inserted into oil and washed with toluene to remove excess oil. In addition, nanoparticles with dimensions ranging from a few to hundreds of nanometers were observed. Overall, more particles are observed when flocculants, such as heptane, are added, whereas their size decreases when a good solvent for asphaltenes (toluene) is added. Similar colloidal particles are also observed repeating this methodology with asphaltene solutions in toluene, confirming that these somewhat reproduce the asphaltene association observed in crude oils. The addition of an inhibitor, such as dodecylbenzenesulfonic acid, led to observation of more and smaller nanoparticles. The present experimental approach not only confirms the existence of asphaltene colloidal particles in crude oils but also provides an accessible methodology to directly assess how these particles are affected by changes in oil composition or inhibitors.

The study of the asphaltene deposition mechanism is critical to understand and solve important problems in the petroleum industry. The same is valid for the selection of inhibitors to control or prevent asphaltene flocculation and/or deposition. However, most of the current information on these processes is obtained by experiments performed using model solvent systems. In the present study, we used quartz crystal microbalance (QCM) measurements as well as laser scanning confocal microscopy to characterize the asphaltene deposition directly measured in a Brazilian crude oil at different conditions of flocculant concentration and using inhibitors with different chemical features. Measurements under accelerated sedimentation (LUMiSizer) were also employed to evaluate inhibitor capacity in crude oil systems, in this case using a large excess of n-heptane. Overall, QCM results suggest that the diffusion-limited aggregation (DLA) model can be used to describe systems close to or above the concentration of the onset of asphaltene precipitation. The transition to a behavior that follows the reaction-limited aggregation (RLA) model occurs when an inhibitor added or the flocculant concentration is reduced farther from the onset. Moreover, accelerated sedimentation shows that the inhibitors tested act by preventing aggregate growth. Therefore, these results highlight the importance of performing time-dependent experiments directly in crude oils and support the use of these methodologies to optimize inhibitor selection for different crude oils.

Transfer of reaction products formed on the surfaces of two mutually rubbed dielectric solids makes an important if not dominating contribution to triboelectricity. New evidence in support of this statement is presented in this report, based on analytical electron microscopy coupled to electrostatic potential mapping techniques. Mechanical action on contacting surface asperities transforms them into hot-spots for free-radical formation, followed by electron transfer producing cationic and anionic polymer fragments, according to their electronegativity. Polymer ions accumulate creating domains with excess charge because they are formed at fracture surfaces of pulled-out asperities. Another factor for charge segregation is the low polymer mixing entropy, following Flory and Huggins. The formation of fractal charge patterns that was previously described is thus the result of polymer fragment fractal scatter on both contacting surfaces. The present results contribute to the explanation of the centuries-old difficulties for understanding the "triboelectric series" and triboelectricity in general, as well as the dissipative nature of friction, and they may lead to better control of friction and its consequences.

Asphaltenes constitute the heavy petroleum fraction responsible for deposition events that may lead to reduced oil production, therefore of great interest for flow assurance. These molecules selfassemble in solutions leading to formation of aggregates that eventually grow towards precipitation and blockages in reservoirs and pipelines. Based on the Yen-Mullins aggregation model, two complementary scenarios are involved in asphaltenes phase behavior: one called thermodynamic, in which interacting molecules and other species can be assumed to be in equilibrium, and a second one, involving interacting colloidal particles, both being described by different theoretical frameworks. For the first, molecular interactions can explain the experimental observations or theoretical models. For the second stage, colloidal forces such as those described by Derjaguin, Landau, Verwey and Overbeek (DLVO) theory, steric particle stabilization and diffusion or reaction limited aggregation processes might control the process. Our evaluation is that this second approach is underrepresented in the current literature. For this reason, this review focuses on describing evidences for the presence of colloidal particles in crude oils obtained with different experimental techniques, drawing attention to this important attribute and we raise a few questions that we believe must be addressed in order to better understand the contributions from colloidal aspects.