Lifetime and fluctuations of specific bonds between anisotropic colloids mediated through depletion interactions.

Abstract

To fabricate large-scale structures using colloidal particle self-assembly, one of the main challenges is to prevent kinetic trapping in metastable states. Therefore, interactions and colloids must be carefully chosen to ensure selectivity to guide the assembly, reversibility to enable large-scale reorganization and flexibility to finely tune colloid positioning. In pursuit of this, we study simple anisotropic colloids in the shape of half-disks fabricated using two-photon lithography and drive their self-assembly through their vertical faces using depletion interactions. Depletion interactions are widely used in the literature to induce colloidal self-assembly and can provide reversible interactions at low depletant concentrations. The specificity is a consequence of the geometry of the colloids, where the attraction between flat faces is favored by depletion interactions. We demonstrate that these interactions are transient, with survival times that depend on the shape of the interacting faces. The bond lifetime as a function of the depletant concentration is correctly predicted by a theoretical model based on the excluded volume. We also show that the flat surfaces can slide relative to each other offering flexibility to the bonds. We quantify this sliding and show that it follows a Boltzmann distribution governed by the depletion energy. Bond breaking between surfaces occurred predominantly when they are offset relative to each other. Incorporating this flexibility on bond lifetime in our model yields better quantitative agreement on the bond lifetimes. This quantification of specific, transient, and flexible bonds between simple anisotropic colloids could pave the way for the self-assembly of larger, defect-free colloidal structures.