There is a Critical Transport Length that Governs Fractal-to-Colloid Transitions in Peptide-Directed Nanoparticle Self-Assembly.

The morphology of a hierarchical nanoparticle assembly is governed by how particles encounter one another ─ by Brownian diffusion or convective transport ─ yet this competition is rarely quantified. Here we define a particle-scale Péclet number (Pe) and derive a critical transport length (L*) to predict when shear disrupts assembly. Using a cationic tripeptide to direct silver nanoparticles, we sweep Pe from 0 to ∼1.1 by varying the shaking rate and observe a morphological transition: Fractal structures at Pe < 1 give way to partially aggregated intermediates near Pe ≈ 1 and nearly dispersed colloids at Pe ≥ 1. X-ray diffraction shows that crystallite size is conserved across all conditions while the (111) texture diminishes with increasing shear. The results suggest that convection suppresses the contact-dependent assembly pathway because L* depends on particle diffusivity, shear rate, and the relevant transport distance, which provides a transport-based guide for nanoparticle assemblies.