Microfluidic fabrication of endoskeletal droplets
Tamas A. Prileszky and Eric M. Furst
Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, DE 19716, United States
Microfluidic devices can produce emulsion droplets rapidly and uniformly. The characteristic low flow rates and pressures intrinsic to the small length scales that microfluidics utilize lend the devices to producing Newtonian emulsions. However, there is interest in developing oil-in-water emulsions where the dispersed oil phase has a significant yield stress, termed endoskeletal droplets.1
Endoskeletal droplets draw their name from an internal scaffold comprising a percolating network of petrolatum crystallites with a sufficiently high yield stress to resist the surface tension force that drives Newtonian emulsions to the spherical morphology. Resisting surface tension allows the droplets to maintain anisotropic shapes, enhancing their efficiency as delivery vehicles because of the corresponding decrease in transport length scales and increase in surface-area-to-volume ratios. Moreover, the yield stress of the internal network can be changed in situ, permitting dynamic shape changes in response to changes in environmental conditions.
We investigate a microfluidic system for continuously generating endoskeletal droplets by incorporating droplet generation, heating, and cooling regions into a single device. The devices are tuned to ensure that droplets are generated at high temperatures as Newtonian fluids, then cooled until the internal network crystallizes, resulting in the yield stress droplets of interest. In addition, the geometry of the microfluidic channels imparts the droplets with three dimensions of anisotropy after ejection from the device, greatly enhancing surface-area-to-volume ratio.
Figure 1: Endoskeletal droplets ejected from a microfluidic device.
1. Caggioni, M., Bayles, A. V, Lenis, J., Furst, E. M. & Spicer, P. T. Interfacial stability and shape change of anisotropic endoskeleton droplets. Soft Matter 10, 7647–52 (2014).