Thermal Rupture Force Distributions in Colloidal Gels

Colloidal gels yield under imposed or gravitational stresses. Such yielding can be an inherently useful property, for instance in controling the flow of consumer care products, or reflect limits in the stability against sedimentation, which ultimately affects product shelf life. Microscopically, the "bonds" between particles in the gel network are broken during yielding. In thermally fluctuating colloidal systems, there is not a single force required to achieve rupture, but a spectrum, as thermal forces can both augment and inhibit the bond breaking. We demonstrate measurement and interpretation of the distribution of rupture forces between pairs of colloidal particles bonded via the van der Waals attraction. The otherwise irreversible bond is broken by pulling the particles apart with optical tweezers. We show that an ensemble of the particle trajectories before, during and after the rupture event may be used to produce a high fidelity description of the distribution of rupture forces. Bond breakage measurements are then extended to depletion interactions to understand the rheology of depletion gels.