Generation of Amphiphilic Janus Bubbles and Their Behavior at an Air-Water Interface

This paper presents the generation of amphiphilic Janus bubbles and their behavior at an air-water interface. Janus bubbles are generated by selectively depositing gold onto one side of dried nanoparticle-shelled bubbles. To generate nanoparticle-shelled bubbles that can withstand drying without significant changes in their structure, it is critical to control the ratio of bubble radius to shell thickness using a microfluidic technique. It is observed that the behavior of Janus bubbles at an air-water interface is very different from that of unmodified nanoparticle-shelled bubbles. Interfacial assembly of amphiphilic Janus bubbles shows that they interact with one another via long-ranged attractions. The origin of this long-ranged attraction is quadrupolar capillary interactions due to the undulation of the three-phase contact line around the Janus boundary. The interparticle forces between interface-trapped Janus bubbles are determined using a particle tracking method. The shape of the deformed air-water interface around Janus bubbles is directly observed as well as the orientation of Janus bubbles using a gel-trapping technique. These observations verify that the air-water interface is pinned around the boundary between the two hemispheres and that the chemical heterogeneity of this boundary leads to irregular contact line around Janus bubbles.