Nonaffine deformation and tunable yielding of colloidal assemblies at the air-water interface

Silica nanoparticles trapped at the air-water interface form a 2D solid state with amorphous order. We propose a theoretical model to describe how this solid-like state deforms under a shear strain ramp up to and beyond a yielding point which leads to plastic flow. The model accounts for all the particle-level and many-body physics of the system: nonaffine displacements, local connectivity and its evolution in terms of cage-breaking, and interparticle interactions mediated by the particle chemistry and colloidal forces. The model is able to reproduce experimental data with only two non-trivial fitting parameters: the relaxation time of the cage and the viscous relaxation time. The interparticle spring constant contains information about the strength of interparticle bonding which is tuned by the amount of surfactant that renders the particles hydrophobic and mutually attractive. This framework opens up the possibility of quantitatively tuning and rationally designing the mechanical response of colloidal assemblies at the air-water interface. Also, it provides a mechanistic explanation for the observed non-monotonic dependence of yield strain on surfactant concentration.