Hollow capsules formed by the self-assembly of caseins on sessile air bubbles

This paper introduces an easy method to produce size-controllable hollow capsules by incubating air bubbles in high concentrated protein solutions. We use the Axis-symmetric drop shape analysis (ADSA) system to keep the bubble size constant as well as to set the initial sizes of objective air bubbles. Dynamic surface tension results indicate that protein molecules start to assemble on those air bubbles surfaces as soon as the air bubbles have been injected into the protein solutions; while “empty bag” like structures come forth when air bubbles begin to shrink after long time incubation. Upon abruptly volume vibration, aged air bubbles exhibit the propensity of resisting the forced dilatation/shrinkage while the fresh air bubbles in the same solution tend to collapse immediately. Parallel ellipsometry experiments confirm that protein molecules are able to form multilayer films at the planer air/water interfaces after such a long time adsorption. Brewster Angel Microscopy results reveal that the aged interfacial protein films are much more rigid than the fresh films, indicating that protein molecules may slowly form cross-link structures over time. We believe that protein molecules assemble a well-organized multilayer shell on air bubble surface after long time incubation; such kind of shells yield the mechanic stability of the air bubbles and inhibit the material exchange across the interfaces of the air bubbles. We hope this report can be a positive inspiration to excite the potential for future development in drug encapsulation techniques.