Interfacial Composite Materials

Particles have long been used as stabilizers for interfaces. Pioneering work by Pickering and Ramsden dates back to slightly more than a 100 years ago, where they report colloidal particles serve as stabilizers for emulsions and foams. Since then much work has been done on bulk systems, such as foam rheology, stability etc. but only recently has attention been devoted to the properties of individual bubbles and drops. Indeed, particle-covered or armored interfaces can be considered composites due to the differing properties of the solid particles and the fluid interface. Our group is interested in exploring new ways of making armored interfaces and to study their physical and chemical properties.

Microfluidic method for producing monodisperse armored bubbles

We have managed to build a flow-focusing microfluidic device that controllably assembles particles onto interfaces. We are currently using the device to study the dynamics of particle capture and assembly. The microfluidic method is also continuous, and provides greater control over the properties of the armored objects.

Hydrodynamic focusing for the continuous formation of bubbles with colloidal armour. a, Picture of the flow-focusing device for flow-driven assembly of colloidal armour. Fluids are driven through the channels by connecting custom syringes to a supply of compressed air. The dispersed phase is injected through the inner channel, while the particle containing continuous phase is injected through the outer channels. A 1mm X 1mm view chamber is constructed at the end of the device.  b,  The direction of dispersed phase flow is indicated by a dashed arrow, while the direction of the colloidal suspension flow is indicated by the solid arrows. Flow of the continuous phase liquid focuses the dispersed phase fluid into a narrow thread and targets the particles onto the interface.  The speed of the particles and the frequency of shedding can be controlled by tuning the difference in driving pressures of the inner and outer channels (typical values are suspension: 1.92 psi, gas: 1.69 psi). The interfacial crystal consisting of 4 µm diameter charge-stabilized fluorescent polystyrene beads grows and subsequently experiences greater shear c, which results in the ejection of a jammed shell.

String of Armored Bubbles: Armored bubbles arrayed on an air/water interface. The fluorescent particles are close-packed on the surface, and the bubble is stable.

Tailored production of Janus armour a, Assembly of particles on an air/water interface to produce Janus crystals. The yellow particles are 4.9 mm diameter polystyrene particles dyed with rhodamine, while the green particles are 4.0 mm particles dyed with fluorescein. b, An example of the Janus shell, with approximately two hemispheres of different size particles and fluorescence.

Non-spherical bubbles : please follow the link to the Harvard University Gazette

Publications:

Bala Subramaniam, A., Abkarian, M. & Stone, H. A. Controlled Assembly of Jammed Colloidal Shells on Fluid Droplets. Nature Materials 4, 553 (2005). 

Bala Subramaniam, A., Abkarian, M. Mahadevan, L. & Stone, H. A. Non-spherical Bubbles. Nature 438, 930 (2005).