Sporopollenin symphony: Spores and pollen in three phase systems

Three-phase systems, including particle-stabilized emulsions and capillary suspensions, are extensively used in food, cosmetics, and materials science industries. From a soft matter perspective, particle-particle interactions primarily control their stabilization, formation, and rheological properties. While most studies have focused on spherical and smooth particles, particles with diverse characteristics could be used to tune the properties of such systems and could provide further insights into how particle properties influence their behavior. A more comprehensive understanding of different particle properties enables precise control and tailoring systems for specific applications, potentially leading to innovations in product formulation and processing techniques across multiple industries. The aim of this thesis was to gain more insight into how morphology, size, and surface characteristics of particles of biological origin affect three-phase systems. As particles, we used Lycopodium clavatum spores to stabilize all-aqueous emulsions, and Lycopodium clavatum spores and Pine pollen to form capillary suspensions with different rheological properties. In Chapter 2, we used Lycopodium clavatum spores to extract sporopollenin exine capsules (SpECs), which are spores without their inner and outer (fat) components. These particles were able to stabilize a Dex-in-PEG emulsion. Results showed particle porosity plays a major role in emulsion stabilization and particle distribution depends on particle charge. We highlighted the role of particle adsorption at the water-water interface, akin to Pickering stabilization, and that of interparticle electrostatic interactions for emulsion stabilization. In Chapter 3, SpECs were derivatized to obtain non-charged and positively charged particles. Results showed the role of particle-particle interactions in effective adsorption at the interface and the observed enhanced stability. Next, we explored the role of particle shape and surface characteristics in capillary suspensions, where a small amount of an immiscible secondary liquid forms liquid bridges between particles, which created strong attractive capillary interactions, causing the particles to cluster together leading to a system transition from a liquid state to a solid-like gel. In Chapter 4, we used tetrahedral-shaped Lycopodium spores, for which dodecane bridges were formed between their trilete sides that led to a specific particle aggregation behavior, determined the gel strength of the system, and under large amplitude oscillatory shear (LAOS) caused the networks to exhibit complex LAOS responses. Finally, in Chapter 5, we included Pine pollen with a Mickey mouse-shaped morphology, and silica particles with a spherical shape as additional particles. These were used to structure dodecane continuous capillary suspensions with water as secondary liquid. Results showed that the type and volume of bridges were dependent on particle morphology. Tetrahedral spores had directional bridges, while pollen and silica had point-like bridges. The formation of omnidirectional bridges enhanced particle aggregation and gel strength more than directional bridges. At large amplitude deformation, the types of bridges affected the critical strain, where point-like bridges had more particle interactions and decreased critical strain, while directional bridges slightly increased critical strain, due to structural reorganization upon shear. In Chapter 6 the findings in this thesis were discussed, focusing on how key features of spores and pollen influenced systems’ properties.