3, University of Amsterdam, Amsterdam, , Netherlands
2, PSL Research University, ESPCI Paris, Paris, , France
The fracture of disordered or amorphous materials is not fully understood as it involves structural rearrangements as well as micro-fracturing. In atomic solids, the damage region which precedes the fracture tip is often very small, 10s nanometers, and the crack tip propagates rapidly, meters per second. Therefore, it is exceedingly challenging to image the real crack tip and to follow it dynamically. We have developed a disordered cohesive colloidal material where each colloidal particle of micron diameter plays the role of the atoms. Due to this dramatically increased length scale, the material is very soft with a Young’s modulus of 5Pa and each individual colloid is distinguishable with a confocal microscope. We load a high volume fraction liquid dispersion of fluorescent microgel colloids into a microfluidic device with a flow geometry that permits Mode 1 fracture. A cohesive attraction is then externally triggered using a critical Casimir interaction by gently heating the dispersion within the device. We then hydraulically fracture the colloidal solid by flowing fluid devoid of particles into the device and use confocal microscopy to follow the propagating fracture. After locating the instantaneous centers of each colloidal particle, we calculate the local shear strain matrix; a clear damage zone extends well in front of the crack tip. We utilize this damage zone to predict the motion of crack tip; the magnitude of the accumulation of plastic deformation in front of the crack tip correlates well with the directionality. Lastly, we directly observe that micro-cracking ahead of the tip causes intermittency in the crack tip velocity and position.