Research Fellow LRSM of UPENN Caguas, Puerto Rico, United States
Introduction: Pickering emulsions are systems composed of two immiscible fluids stabilized by solid particles around the fluid-fluid interface, providing high stability against coalescence. However, these systems feature discrete droplets in isolated domains, complicating the continuous supply or retrieval of their contents and making them less than ideal for applications that rely on transport across the fluid-fluid interface. Bicontinuous interfacially jammed emulsion gels (bijels) are novel systems consisting of two immiscible fluids arranged bicontinuously by nanoparticles jammed around the fluid-fluid interfaces. Bijels offer an attractive template for applications involving reaction or transport across the interface, such as catalysts, electrodes for batteries and fuel cells, hierarchically porous materials, scaffolds for tissue engineering, and crossflow microreactors. To produce bijels, the resulting bicontinuous morphology requires spinodal decomposition to induce phase separation, which dictates the composition of the starting homogeneous mixture. Spinodal decomposition involves a single thermodynamic phase spontaneously separates into two phases without nucleation (e.g. bubbles). Despite their potential, research on bijels often does not adequately address the challenges in their fabrication. This project aims to develop a reproducible method for bijel production, ensuring consistent quality and facilitating further advancements in this field. Bijels will be produced under spinodal decomposition via Solvent Transfer Induced Phase Separation (STRIPS) and Vaporization Induced Phase Separation (VIPS). We will use an oil-water-ethanol ternary phase diagram for mixture preparation to determine if a broader area around the binodal line of the diagram can facilitate bijel formation, thereby allowing procedural flexibility and minimizing human error during manufacturing.
Materials and
Methods: Precursor solution preparation based on protocol (control): The two immiscible liquids for the starting precursor were 1–6 hexanediol diacrylate (HDA) and a 3.5 pH distilled water, they were made homogenous by adding 100% Ethanol. Silica particle suspension 50wt% was added and modified with Cetyltrimethylammonium bromide (CTAB) for neutrally wetting properties. At last, the photo initiator 2-Hydroxy-2-methylpropiophenone (HMP) and Nile Red Dye were added to UV cure and fluorescence-stain the film for imaging.
Sample film preparation: Approximately 50-80 microL of the precursor were casted on a corner of a glass slide and blade coated resulting in an evenly distributed from top to bottom. The VIPS method was employed by turning on a vent for 8-10 seconds, depending on the liquidness of the solution, immediately after, the STRIPS method was applied by placing the sample in a pool of 3.5 pH DI water for 2 minutes and cured under a UV lamp for 1 minute.
STRIPS: For continuous fabrication, the process involves adding a cosolvent to an oil-water mixture, producing a homogenous mixture of the three liquids and triggering the phase separation by cosolvent removal (ethanol) after immersion in water.
VIPS: Improves scalability by allowing fabrication under ambient conditions, here, a high vapor pressure cosolvent is removed by evaporation to induce phase separation.
Characterization: Confocal microscopy was employed to observe if the film had a uniform structure throughout the X Y and Z planes.
Results, Conclusions, and Discussions: Result 1: A precursor solution was made based on protocol using 38.23% Ethanol, 30.07% Oil (HDA) and, 31.70% 3.0 pH DI water, and 3.0 pH silica particles. This film did not show bijel structure, suggesting that a 3.0 pH does not provide particles that are neutrally wetting, resulting in improper jamming of the silica particles at the interfaces of the liquids.
Result 2, 3, 4, 5: Bijel structures of varying sizes were successfully observed throughout the Z plane of the film.
Result 6, 7: Nucleation of water was observed throughout the film indicating that when there is a high difference in amounts of oil and water, where oil is predominant, there will not be bijel formation
Result 8: A few points were selected around the area of 45%-58% Ethanol, 60%-40% Oil (HDA), 60%-70% 3.5 pH DI water, when preparing the precursor, solutions appeared cloudy, even when a large volume of ethanol was added. The films were UV cured; however, they did not detach from the glass slide and lost their conformation when detachment was attempted.
Trends were observed based on the ternary phase diagram: If there was less than a certain percentage of oil, the films got stuck to the glass. Choosing points near the critical point successfully produced bijels. When fabricating a bijel, the evaporation of ethanol is induced, causing a slower evaporation of water, which shifts the chosen points on the phase diagram down towards phase separation and to the left by reducing water content. As a result, when choosing points with no more than a certain percentage of water, nucleation was observed. Specifically, bijel production was successful in the range of 40-50% ethanol, 50-60% oil, and 43-52% water. The potential applications of bijels in a biomedical approach could include biomaterials design, where bijels can be used to create scaffolds that mimic the extracellular matrix, and tissue engineering, where bijels can support the growth and organization of cells to form functional tissues.
Acknowledgements (Optional): I would like to express my deepest gratitude to the LRSM Program at University of Pennsylvania for the opportunity of doing research, my research partner Keiralys Soto for her contributions, & Dr. Daeyeon Lee laboratory along with mentors Gabriela Gómez-Dopazo and Philip Iccarino for supporting the project. This research was primarily/partially supported by NSF through the University of Pennsylvania Materials Research Science and Engineering Center (MRSEC) (DMR-2309043).
