2, CEA-Saclay, Gif-sur-Yvette, , France
Colloidal particles at low volume fractions can gelate into disordered solids by their aggregation into a space-filling network. The mechanical properties of the resulting colloidal gel depend on the connectivity of the final network as well as on the architecture of its constituent particle strands. Rheological and mechanical tests over colloidal gels have shown interesting results such as yield localization and delayed yielding. However, direct observation the flow and fracture behavior of colloidal gels at mesoscopic and microscopic scales is hindered by difficulties in gripping and imposing a controlled load on a soft material and by the combination of large deformations and viscoelastic processes.
In this work, we use two experimental setups fabricated with microfluidic technologies to analyze the mechanical response of aqueous colloidal systems made with silica nanoparticles and with the synthetic hectorite clay Laponite RD (both with diameters around 25 nm). The first one is a Hele-Shaw-like cell with a built-in notch. By injecting water over the notch, a flow rate controlled crack is initiated. The second one consists of a chamber where a rectangular gel sample is surrounded by an immiscible oil. The aspiration of the oil at a controlled flow rate imposes a proportional rate of displacement to the oil-gel interfaces, resulting in the nucleation of a single crack at a notch.
For given particle volume fraction and gel time, a wide range of behaviors was observed as a function of ionic strength for both materials, from a liquid flowing under load to an elastic solid which breaks. The measurement of the displacement fields in the vicinity of the crack tip by Digital Image Correlation and of the Crack Tip Opening Displacement enable the determination of the stress intensity factor and the energy release rate during fracture. The comparison of both methods allows us to estimate the size of the non-linear fracture zone, which tends to increase for decreasing ionic strengths. We also estimate the characteristic time by considering the influence of the crack speeds.