Armored droplets from particle rafts and an application to environmental remediation

Applications such as handling hazardous materials or containing chemical reactions involve encapsulating one fluid phase by a shell. In addition to surfactants, a variety of physicochemical approaches have been studied where nano- and/or micron-size particles organize at fluid-fluid interfaces to form some sort of shell. Gravity typically plays an insignificant role compared to surface forces in establishing such ``armored interfaces.'' More generally, however, it is well known that capillary and gravitational forces cause particles trapped at interfaces to self-assemble and organize into raft-like structures. We describe gravity-driven instabilities of particle rafts and show how the ``interfacial granular dynamics'' lead to encapsulation strategies involving stable particle-armored droplets. Experimental results are compared with mathematical models of the composite objects in order to establish a quantitative description of our observations. The application of these ideas to environmental remediation will be described.