Geometric Domain Selection in Nanosheet Suspensions: Why Structural Color and X‐Ray Scattering Probe Different Platelet Populations

ABSTRACT Platelet‐based photonic materials derive their optical functionality from nanosheet orientation, yet X‐ray scattering is often assumed to probe the same domain populations that produce structural color. Here we demonstrate that this assumption fails fundamentally in aligned nanosheet suspensions. Using scanning microfocus small‐angle X‐ray scattering (µSAXS) and polarized‐light imaging of confined nematic suspensions of cesium–fluorohectorite double‐bilayer nanosheets, we show that the nanosheets responsible for the uniform blue structural color are not the same nanosheets detected by SAXS. Strong anchoring at the front and back cuvette faces aligns the platelets parallel to those walls, generating a uniform reflection color but orienting their lamellar stacks along the beam direction, which does not meet the x‐ray diffraction conditions. In contrast, µSAXS selectively detects only the edge‐on domains induced by side‐wall anchoring, which rotate the lamellar stacks by ∼90° relative to the face‐anchored population. This geometric selection yields a clear anchoring penetration length of ∼750–1200 µm and reveals uniform d‐spacing within individual interior domains. Our findings establish a general principle for platelet‐based materials: optical and X‐ray measurements interrogate distinct orientation subsets, with direct implications for structural colors, nanoflake coatings, liquid‐crystal nanocomposites, and any system where functionality depends on nanosheet alignment.