Negative thermal expansion and its manifestations within confined lamellar-structured concentrated surfactant solutions.

Concentrated surfactant pastes often form lamellar liquid-crystalline phases whose processing depends on alignment, confinement, temperature, and defect topology. Here, we investigate temperature-driven lamellar contraction and parabolic focal conic defect (pFCD) formation in 70 wt% sodium lauryl ether sulfate (SLEnS) lamellar pastes using small-angle X-ray scattering (SAXS), confined cross-polarized microscopy, and confined oscillatory rheometry. SAXS measurements show that the lamellar repeat spacing decreases with increasing temperature in both SLE1S and SLE3S, corresponding to one-dimensional, bilayer-normal negative thermal expansion. This contraction was observed in both bulk-thickness and confined SAXS geometries, indicating that the nanoscale thermal response is not specific to sample thickness, chemistry, or instrument configuration. Under confinement, squeeze-flow alignment appears to produce coherent lamellar stacks that cannot freely accommodate the temperature-dependent spacing decrease. Heating these aligned lamellae produces thermal undulations and optically resolved pFCD fleurettes. SLE3S forms pFCDs at a lower onset temperature and completes defect development over a lower temperature range, whereas SLE1S forms brighter and larger final pFCD textures. Confined rheometry shows non-monotonic changes in storage and loss moduli over the same temperature range as lamellar contraction and pFCD development, suggesting coupling between nanoscale contraction, mesoscale defect formation, and bulk viscoelasticity. Together, these results establish a confinement-mediated mechanism by which lamellar negative thermal expansion generates compressive strain, undulatory defect formation, and altered mechanics in industrially relevant concentrated surfactant pastes.