On the fatigue fracture at adsorption/desorption of water in/from liquid-repellent nanoporous silica

In this work, fatigue fracture tests on liquid-repellent nanoporous silica micro-particles dispersed in water are reported; then, models of the grain cracking and fragmentation are proposed. Such tests can be regarded, from an external standpoint, as conducted under temporally variable but spatially uniform pressure distribution in the liquid surrounding the silica grains, or from an internal standpoint, as surface fatigue that occurs at the cyclical adsorption/desorption of water in/from the nanoporous particles. The test rig represents a compression-decompression cylinder divided into two chambers, one of constant volume and the other of variable volume. Silica is introduced inside the cavity of fixed volume, and a micro-filter is used to separate it from the chamber of variable volume, in which only water is supplied. Experimental results suggest that the fatigue fracture of silica particles occurs from the inside, explosion-like, oppositely to the previously reported implosion-like collapse of silica under wet pressurization. This is accompanied by enhancement of the hydrophilic silanol groups on the silica surface and by redistribution of the size of particles and pores. Critical numbers of cycles to achieve fracture of the silica particles obtained experimentally, and from the models of grain cracking and fragmentation, under cyclical pressurization, are in good agreement.