From smooth to spiky Pickering emulsions by controlling the interfacial configuration of silica rods

Pickering emulsions stabilized by anisotropic particles exhibit enhanced stability and tuneable interfacial structures, yet controlling the orientation of adsorbed particles remains challenging. While thermodynamic models predict that rod-shaped anisotropic particles typically favor a parallel orientation at the oil-water interface to minimize interfacial energy, a perpendicular orientation is highly desirable for achieving spiky morphologies with unique functionalities. In this study, we demonstrate that the interfacial orientation of end-functionalized silica nanorods can be systematically controlled through post-synthesis silanization. By tuning the silanization time, the rods exhibit a reentrant transition from surface-parallel to surface-normal and back to predominantly parallel configurations when adsorbed on oil-water emulsion droplets. Surface characterization shows that silanization progressively modifies both the hydrophobicity and the charge of the rods while amplifying the intrinsic chemical asymmetry arising from the PVP residues localized at one rod end. To rationalize the observed behavior, we develop a semi-quantitative theoretical model that incorporates wetting energies and electrostatic interactions, thereby capturing the experimentally observed orientation transitions. These results demonstrate how subtle changes in surface chemistry can regulate the configuration of anisotropic colloids at fluid interfaces and provide a framework for designing Pickering emulsions with programmable interfacial morphology.