Solvent-free confinement of ordered microparticle monolayers: effect of host substrate and pattern symmetry.

The self-organisation of individual suspended colloids into ordered structures that can be mediated by confinement has garnered interest recently. Despite the push for solvent reduction for sustainability reasons, the comprehension and development of solvent-free assembly methods remain largely unaddressed. In this study, we explore the effect of confinement without rigid geometrical constraints, i.e., wall-less confinement on the assembly of monodisperse PMMA powder microspheres (diameters of 3 μm and 10 μm) on fluorocarbon-patterned heterogeneous substrates using a solvent-free rubbing assembly approach. Our findings reveal that the PMMA microspheres self-align on the fluorocarbon patterns, adapting to various geometrical shapes of these patterns through symmetry matching. The assembly process is driven by triboelectric charging and elastic properties of the microspheres and substrates. Moreover, we observe that the host substrate and the particle and pattern size ratio significantly influence the ordering of the microparticles on the fluorocarbon patterns. Ultimately, we demonstrate the successful use of fluorocarbon patterns to assemble tunable crystal patterns on rigid substrates, which typically do not exhibit any ordering.