Our group has had a long fascination with the idea of mixed phase materials and we have worked to synthesize the first examples of π-conjugated polymers that are only soluble in fluorous solvents. These methods can give rise to materials that have stratified structures in the solids and allow for sequential addition of materials in building up solid state structures. However, we are also interested in pure liquid states as they are dynamic and can change morphology by changing the surfactant concentrations or by doing reactions at the surfaces of the droplets dispersed in water. We began with our fluorous polymers in fluorocarbon solvents more than a decade ago, but have found in the last few years that complex multiphase colloids offer many advantages. In particular a 1/1 volume fluorocarbon/hydrocarbon droplet can be made, in effect, to turn inside out by changing the surfactant concentrations. (Zarzar, L. D.; Sresht, V.; Sletten, E. M.; Kalow, J. A.; Blankschtein, D.; Swager, T. M. “Dynamically Reconfigurable Complex Emulsions via Tunable Interfacial Tensions” Nature, 2015, 518, 520-524).
Dynamic morphologies of a complex fluorocarbon/hydrocarbon droplet as observed by a microscope under a gradient of surfactant concentration.
These processes are fascinating and the interaction must always go through the Janus droplet which will align with gravity as a result of the higher density of the fluorous phase relative to the hydrocarbon. Click on the video to see the change in real-time when we add an excess of the fluorocarbon surfactant to a double emulsion that has the hydrocarbon on the outside and the fluorocarbon on the inside (F/H/W). Note the structures forming in the transition, which show how sensitive the droplets are to changes in the concentrations of surfactants.
This area is rapidly expanding and we have created methods to detect enzyme activity, Zika virus, Salmonella, and E. coli. The refractory optics of these droplets are key in these processes and when the Janus droplets are aligned with gravity, optical images are transmitted, but when they are distorted by chemical reactions at their surfaces or are realigned by binding to proteins or pathogenic organisms, they scatter the light. We have found that you can detect pathogens using these methods with your smartphone (Zhang, Q.; Savagatrup, S.; Kaplonek, P.; Seeberger, P. H.; Swager, T. M. “Janus Emulsions for the Detection of Bacteria” ACS Central Science 2017, 3, 309-313). In this case, we make use of the tilting of droplets that are bound to bacteria and we can create threshold detection using the readability or unreadability of a QR code or use a smart phone with a magnifying lens to count the number of tilted droplets, which scales linearly with the concentration of pathogen.
Tilting of Janus droplets a -> b with binding to a protein or bacterial. Demonstration of the ability to detect this tilting by obscuring a QR code for a binary detection. Quantitative detection using a smart phone and a magnifying lens and computational image analysis.