Abstract
"Hedgehog" particles (HPs) possess a micrometer-sized dielectric spherical core which is densely coated with nanoscale metal oxide spikes. This unique surface topography, resembling the appearance of a hedgehog, provides the particles with the exclusive physiochemical property to stably disperse in both polar and nonpolar solvents without the necessity of changing the surface chemistry. Optical extinction measurements of HP ensembles in aqueous solution indicate a broad spectral response in the visible range. However, there remains a dearth of fundamental knowledge about the cause of the broad optical resonance, as it can be a consequence of shape polydispersity in the many-particle system or intrinsic to each individual HP. In this paper, we present the first experimental study of the dark field scattering of individual hydrophilic and hydrophobic HPs. Our measurements disclose that the expansive optical response in the visible spectral range is truly characteristic for the far-field scattering of a single HP. Our results also uncover how intrinsic particle features, such as spike length, as well as environmental changes affect the scattering of individual HPs. In particular, by changing the atmosphere around a hydrophilic HP from air to nitrogen and by completely immersing in water by employing a 3D optical trap, we discovered that the scattering from a hydrophilic HP is strongly modulated by excess water in its interstitial shell.