The aging behaviour of dense suspensions or pastes at rest is almost exclusively attributed to structural dynamics. Here, we identify another aging process, contactcontrolled aging, consisting in the progressive stiffening of solid-solid contacts, after their initial formation rapidly arrests the microstructure. By combining rheometry, confocal microscopy, and particle-scale mechanical tests using laser tweezers, we demonstrate that this process governs the shear modulus aging of dense aqueous silica and polymer latex suspensions at moderate ionic strengths. We further show that contact-controlled aging becomes relevant as soon as Coulombic interactions are sufficiently screened out that the formation of solid-solid contacts is not limited by activation barriers. Given that this condition only requires moderate ion concentrations, contact-controlled aging should be generic in a wide class of materials, such as cements, soils, or 3D inks, thus questioning our understanding of aging dynamics in these systems.

Electret based smart materials have been attracting increasing attention for their versatility combined with easy fabrication. In particular, electret microparticles can be embedded in micro- and nano-electronic devices, enabling applications such as sensing, actuating, biological transducers and energy harvesting. In this work, silica micro-electrets are charged by electron injection in a SEM environment. The particle charge distribution is precisely controlled adjusting the energy of the primary beam. The surface potential, measured in the SEM chamber by the shift of the Duane–Hunt limit and by secondary electron spectroscopy reaches up to 200 V for 1 micron particles. The increase of the particle surface potential with the electron penetration depth is explained by a theoretical model, which also provides the value of about 0.1 C cm−3 for the charge concentration. The charge decay is studied in time monitoring the secondary electron emission by an in–lens SEM detector, showing that most of the charge injected deeper than 200 nm is retained in the particles for several months after the charging process. The capability to reach high values of surface potential stable over time on micrometric scale makes these materials as ideal candidates for applicative purposes and strategic elements in nanotechnology.

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