Stabilization Mechanisms, Rheological and Tribological Characterization of Water-in-Water Emulsions: Role of Waxy Corn Starch Nanocrystals and Basil Seed Gum

Water-in-water (W/W) emulsions were made using waxy corn starch nanocrystal (WCSN) and basil seed gum (BSG) at various concentrations, and their stability, rheological, tribological characteristics, and microstructure were investigated in detail at pH ≈ 7. Increasing the WCSN concentration (0.5%–2% w/w) at a constant level of BSG (0.2% w/w) resulted in a decrease in the Z-average particle size from 376.38 nm to 212.31 nm, while significantly increasing the absolute value of the zeta potential from -49.09 mV to -55.0 mV, which indicates improved electrostatic stability. Rheological analysis revealed a significant increase (p < 0.05) in elastic modulus (G′) from 0.325 Pa to 1.355 Pa, accompanied by a decrease in tanδ_LVE from 0.775 to 0.335, indicating a more elastic response. These changes, observed in stress sweep tests at 25 °C, were further supported by an increase in critical stress (τ_c) from 0.020 Pa to 0.060 Pa, demonstrating greater resistance to external mechanical forces. The tribological experiment also showed that with increasing the concentration of WCSN, the friction coefficient decreases due to smaller droplet size and ball-bearing effect. Confocal laser scanning microscopy (CLSM) imaging confirmed the formation of a denser network at higher WCSN concentrations, leading to increased viscosity and steric stabilization, effectively preventing coalescence. After 28 days of storage, emulsions with 0.2% BSG and 0.5%–2% WCSN exhibited no visible phase separation. The findings suggested the formation of a nanocrystal-mediated gel-like network that entrapped BSG molecules. These results demonstrate the potential of WCSN-BSG systems for stable W/W emulsions in food and pharmaceutical applications.