Effect of cellulose nanocrystals on the emulsion stability and rheological properties of microalgal Pickering emulsions

This study investigates the effect of cellulose nanocrystals (CNCs) to Pickering emulsions prepared with microalgal particles (Spirulina sp. (SPI), Chlorella sp. HS2 (CLO)). The microalgae particles show a weak interfacial localization and Pickering behavior on the O/W emulsion depending on the size (avg. drop size ∼5.39 μm with SPI and 22.15 μm with CLO), resulting in a different stabilization effect. When CNC is additionally mixed with the Pickering emulsions including large microalgae particles (CLO), CNC replaces microalgae particles and localizes at the interface, enhancing strong emulsion stabilization. For the Pickering emulsions including small microalgae (SPI), CNC localizes at the continuous phase, forming a network structure regardless of the concentration. This interfacial localization behavior of CNC against microalgae particles is reflected in the rheological behavior of the Pickering emulsion. Depending on the location of CNC, the emulsions exhibit the two-step yielding behavior, mainly attributed to the CNC network in the continuous phase. The complex role of particles in the emulsion system is more sensitively reflected in the large amplitude oscillatory shear (LAOS) region, characterized using the sequence of physical process (SPP) rheological analysis. The maximum elasticity (E_max) in SPP analysis, which indicates the recovery of the deformed structure, exhibits a significant difference, discriminating structural characteristics of CNC dispersion incorporated with microalgae particles. Emulsion with CLO-CNC has lower E_max than the SPI-CNC case because CNC particles disperse at the interface and the continuous phase. Then the distance between CNC particles is longer, resulting in a weak network structure throughout the emulsion. Due to a weak network of CNC, the emulsion is more vulnerable to coalescence compared to the SPI-CNC system. Therefore, this study suggests that CNC particles added to the Pickering emulsion with microalgae compete to localize at the interface and give coalescence suppression effects to the emulsion. Also, for the Pickering emulsion system composed of multi-particles, rheological analysis including SPP analysis successfully indicates structural characteristics and flow-induced stabilization of Pickering emulsions with multi-particles that microscopic characterization could not detect.