(135c) Microbubbles That Live Longer

Microbubbles are used in medical diagnostics as a contrast agent for ultrasound imaging.  Commercially available contrast media are gas-filled microbubbles that can be administered intravenously. Microbubbles also find use in drug delivery, wastewater treatment or as texture modifying ingredients for cosmetic creams and food products. Unfortunately their high surface tension renders them thermodynamically unstable and the bubbles tend to quickly dissolve in solution if a resistant encapsulating layer does not protect them.

The gas pressure inside a microbubble follows Laplace’s law and depends directly on the surface tension and is inversely proportional to the bubble radius. Thus a 3 micron bubble excurses a pressure of approximately 1 atmosphere, which drives gas into the surrounding solution and the bubble size shrinks. Subsequently, the pressure in the bubble rises, which accelerates the shrinking process until it completely disappears – microbubbles typically last for only a matter of seconds.  For practical applications microbubbles need to be encapsulated to extend their lifetime. The encapsulating layer needs to trap the system in a mechanical equilibrium to block the release of gas and provide elastic properties to the bubble.

The use of hydrophobically modified nanoparticles to encapsulate bubbles, the so-called Pickering method, is a well-established laboratory experiment, however, carrying out the process on an industrial scale is limited by the need to chemically modify the particle’s surface and to establish a viable protocol for large-scale production and post processing. Our work has focused on creating new microbubble encapsulation techniques that overcome these limitations.  The basis of the method we have developed resides in the use of ionic surfactants that adsorb to the gas-liquid interface of the microbubble. These surfactants play two roles; i) they lower the surface tension, which decreases the energy needed to create microbubbles and diminishes the Laplace pressure, and ii) their ionic nature supplies the bubble surface with a residual charge that will induce electrostatic interactions. Capitalizing on the electrostatic interactions by choosing oppositely charged nanoparticles, leads to a strong attraction of the particles to the bubble surface - the particles “stick” to the bubble surface and form a coherent encapsulating layer. This simple process is completely general and can be applied to a wide range of systems. We have demonstrated the method with particles having different geometrical forms (e.g. spheres, platelets, needles), and show that the microbubbles are able to last for over a year and that their mechanical properties can be modulated based on the particle shape.