Printing spongy liquid-based structures: Spontaneous emulsification at micellar solution-nanoparticle dispersion interfaces

All-in-liquid three-dimensional (3D) printed devices have broad potential applicability in processes ranging from energy storage to drug delivery and tissue engineering. Conventionally, they are produced by the jamming of nanoparticle-polymer at the oil-water interface, where one liquid is arrested in a desired non-equilibrium shape in the second liquid phase. Such structures lack the multiscale porosity that presents in equivalent solid hierarchies. Remarkably, we report on printing spongy all-in-liquid materials utilizing direct ink writing techniques, similar to those of solid architectures. Stable liquid columns of nanoparticle dispersions are formed inside micellar solution at Weber numbers three orders of magnitude smaller than that previously reported in the liquid-liquid system, featuring the printing capability at significantly low injection rates. Liquid columns are stabilized due to the rapid formation of a highly viscoelastic microemulsion phase at the nanoparticle dispersion-micellar solution interfaces. The printed aqueous phase turns into an emulsion zone, creating a porous texture in the oil phase. Consequently, a 3D structure with flexible walls consists of layered microemulsions is achieved, counterintuitive with the current liquid-based printed structures. Spongy liquids can revolutionize the current state of art liquid-in-liquid printing techniques and open novel routes to design liquid lab-on-chip devices.