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Figure 1. Phase diagram showing the jamming transition as a function of temperature T, density ρ and applied shear stress σ.

Jamming is the physical process by which some materials, such as glasses, foams, granular materials, and other complex fluids, become rigid with increasing density. The concept of a jamming transition has been proposed as a type of phase transition, as illustrated in Figure 1. When the packing fraction of particles ρ is increased above a critical level ρ<math>_c</math>, there is a sudden appearance of a finite shear stiffness. This signals a transition between a flowing liquid and a rigid but disordered solid state.

Interfacial Composite Materials

Armored bubble images.
Figure 2. Stable support of different anisotropic "armored interfaces" with various particle and bubble sizes, as documented by Subramaniam et al.

Armored interfaces, or interfacial composite materials (ICMs), are fluid/fluid interfaces covered with rigid particles typically larger than a micron in diameter. Jamming of the colloidal particles gives ICMs some rather unusual mechanical properties. For instance, because the capillary forces acting on the particles at the interface are balanced by repulsive normal forces between the particles that occur because they are jammed, armored bubbles conserve both volume and surface area.

The consequence of this is that bubbles with strange geometric properties can be created by allowing armored bubbles to coalesce, as shown in Figure 2. In image (a), an ellipsoidal bubble has been obtained by fusing two spherical armored bubbles. Extremely high aspect-ratio gas bubbles such as the mm-length spherocylinders shown in image (b) can be formed by several successive coalescence events. Picture (c) displays an example of a membranelike solid created by partially evacuating an armored bubble that was originally spherical. One can even generate a permanent stable change in topology of an air bubble by introducing a hole to create a toroid, as pictured in (d).