Formation of crystalline ring patterns on extremely hydrophobic supersmooth substrates: Extension of ring formation paradigms

The mechanism by which nanoparticles suspended in liquids self-organize on substrates to ring patterns has attracted much attention, because of a large body of possible practicable applications. Recent work demonstrated that, for example, gravity affects ring geometry, showing that the mechanism of ring formation is indeed not fully understood. Current models suppose that the pinning of the contact line between drops and substrates is a prerequisite for ring formation and that the process is induced either by surface irregularities of the substrate itself, or by self-pinning, triggered by the attachment of suspended material to the substrate. The latter mode was illustrated for drops of aqueous suspensions, evaporating on atomically flat hydrophilic substrates, e.g., freshly cleaved mica. Conversely, the crucial role of pinning on ring formation was derived from the observation that no rings were formed on smooth Teflon. Here WE! provide the first experimental evidence of the formation of rings on supersmooth hydrophobic surfaces lacking the known conditions required for particle immobilization. Results are suitable to extend existing models and are expected to be instrumental in the predictable design of cell-integrative crystalline ring patterns on biomaterial surfaces and the production of symmetrical ring patterns in proteomics, where the rings are particularly useful to study interaction scenarios between competing proteins.