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Spontaneous emulsification of 3-(trimethoxysilyl) propyl methacrylate (TPM) can produce complex and active colloids, nanoparticles, or monodisperse Pickering emulsions. Despite the applicability of TPM in particle synthesis, the nucleation and growth mechanisms of TPM emulsions are still poorly understood. We investigate droplet formation and growth of TPM in aqueous solutions under quiescent conditions. Our results show that in the absence of stirring the mechanisms of diffusion and stranding likely drive the spontaneous emulsification of TPM through the formation of co-soluble species during hydrolysis. In addition, turbidity and dynamic light scattering experiments show that the pH modulates the growth mechanism. At pH 10.1, the droplets grow via Ostwald ripening, while at pH 11.5, the droplets grow via monomer addition. Adding surfactants [Tween, sodium dodecyl sulfate (SDS), or cetyltrimethylammonium bromide] leads to <100 nm droplets that are kinetically stable. The growth of Tween droplets occurs through addition of TPM species while the number density of droplets is kept constant. In addition, in the presence of the ionic surfactant SDS, electrostatic repulsion between the solubilized TPM species and SDS leads to a significant increase in the number density of droplets as well as additional nucleation events. Finally, imaging of the solubilization of TPM in capillaries shows that in the absence of a surfactant, TPM hydrolysis is likely the rate-limiting step for emulsification, whereas the presence of silica particles in the aqueous phase likely acts as a catalyst of TPM hydrolysis. Our experiments highlight the importance of diffusion and solubilization of TPM species in the aqueous phase in the nucleation and growth of droplets.

Nanoemulsions are a versatile means to create a variety of consumer products and complex materials. Producing nanoemulsions with a high volume fraction of the dispersed phase is generally limited to mechanically intensive processes, such as high-pressure homogenization, and often results in polydisperse droplet size distributions. Low-energy methods, such as spontaneous emulsification, can produce monodispersed droplets, but the volume fraction of the dispersed phase is usually much lower. Here, we report on the spontaneous emulsification of 3-(trimethoxysilyl) propyl methacrylate (TPM) into an alkaline aqueous phase (pH > 10.0) that contains surfactants (Tween 20, sodium dodecyl sulfate (SDS), or cetyltrimethylammonium bromide (CTAB)). The nanoemulsions are monodisperse with droplet diameters that range between 15 and 500 nm. The small droplet size is due to the presence of surfactants that stabilize the droplets against coalescence. The spontaneous emulsion process can produce emulsions with a dispersed volume fraction of up to 10% in CTAB solutions and up to 30% using Tween 20 and SDS. After the emulsification process, the TPM droplets can be polymerized to produce nanoparticles. Using dynamic light scattering and scanning electron microscopy, we characterize the relationship between the surfactant concentration and the size of the droplets in the nanoemulsions. We find that the droplet diameter is primarily determined by the molar ratio of oil to surfactant. We also find that the pH in the aqueous phase also modulates the droplet diameter when using an ionic surfactant. This work expands the spontaneous emulsification of TPM in the absence of stabilizing particles to the nanoscale while producing one of the highest volume fractions of nanoemulsion droplets obtained via a low-energy mechanism.

Mosquito compound eyes are elaborate multifunctional hierarchical structures. The presence of ordered curved features spanning length scales of nanometers to millimeters provides the mosquito eye with a wide field of view, an infinite depth of field, and antifogging properties. Developing bio-inspired compound lenses is challenging because of the need to mimic all characteristic curvatures along with their functionalities. Herein, we show how the curvature inherent to nanoparticles, emulsion droplets, and liquid marbles can be employed to mimic the hierarchical structure and functionality of mosquito compound eyes. At the nanometer to micrometer length scale we employ nanoparticle-stabilized emulsion droplets of photocurable oil to form microlenses with nanoscale surface features. After polymerization, the microlenses form a monolayer on an oil droplet to create an optically clear, millimeter scale, liquid marble that functions as a compound lens. We characterize the optical and surface properties of the compound lenses and find that they reproduce the functionality of the mosquito eye. Additionally, we exploit the mobility and reconfigurability of liquid marbles to create arrays (centimeter scale) of compound lenses and other types of functional lenses such as the Janus lens that magnifies the image acquired by the compound lens. Simple and scalable methods to create compound lenses could aid in the development of miniaturized advanced vision systems.