Precise Control of Protrusion Site, Size, and Roughness on Non-Spherical Colloids

Anisotropic colloidal particles serve as valuable models for fundamental research and as versatile building blocks for functional materials. Our previous work (Luo et al. Soft Matter 2021, 17, 10696) demonstrated that delaying the addition of the cross-linker divinylbenzene (DVB) creates seeds whose surfaces bear one large shell buckling-induced indentation (SBI) and several smaller phase-separated indentations (PSIs). When these dented seeds are subjected to seeded emulsion polymerization, monomer selectively nucleates at the dent sites; however, the factors governing this site preference remain unclear. Here, we systematically varied the delayed feeding time t_d of DVB to prepare seeds bearing well-defined dents and subjected them to seeded emulsion polymerization. Particles with coexisting rough and smooth surface domains were obtained. Under identical temperature, amount of monomer and initiator conditions, monomer preferentially nucleates within the SBI of low-t_d seeds (t_d ≤ 3.5 h), yielding one larger protrusion at the SBI site and finer protrusions at the PSIs ones; conversely, for high-t_d seeds (t_d = 4.5 h), nucleation within one of the PSIs of the seeds produces larger protrusion. Raising the temperature or initiator concentration smooths the originally rough seed surface, resulting in Janus particles with one smooth and one rough lobe. As buckling and phase separation impart distinct cross-link densities to SBI and PSIs, we ascribe the observed site selectivity and surface roughness to spatial variations in seed cross-linking. Our approach offers a simple route to Janus particles whose protrusion locations and roughness can be programmed, providing a flexible platform for next-generation functional colloids.