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.