Capillary breakup of emulsion filaments tuned by in situ gelation of dispersed droplets.

HYPOTHESIS: The breakup dynamics of fluid filaments are key to many technologies, including liquid-in-liquid printing, bioprinting, and drop-on-demand deposition. While Newtonian fluid threads typically follow self-similar power-law pinch-off, non-Newtonian and multiphase fluids such as particulate suspensions and emulsions can exhibit a variety of thinning behaviors depending on their rheological properties and composition. We hypothesize that phase changes within the dispersed phase of emulsion systems can alter the filament thinning dynamics and droplet formation.

EXPERIMENTS: Water-in-oil and sol-gel-in-oil emulsions are formulated using mineral oil and Span 80 as the continuous phase, and either deionized water or sodium silicate-ammonium bicarbonate mixtures as the dispersed phase. Emulsions are aged to allow in situ gelation of the internal droplets. High-speed imaging is employed to capture the neck thinning dynamics. Rheological properties are measured using a rotational rheometer, and interfacial tension and droplet morphology are tracked over time.

FINDINGS: We find that emulsions with a liquid internal phase exhibit viscous-dominated thinning, whereas gelation leads to elongated filaments that thin significantly more slowly. This transition appears to correlate with a shift in the ratio of viscous to elastic modulus, from greater than one to less than one, driven by gelation of the dispersed droplets and network percolation among the gel particles. These findings provide new insight into how internal-phase transitions in colloidal systems such as emulsions influence filament stability and offer design strategies for applications that require tunable breakup behavior. Moreover, the formation of colloidal gel droplets in the continuous liquid phase enables encapsulation of molecules and nanomaterials as they are deposited or printed.