PNAS 2011 ; published ahead of print December 19, 2011, doi:10.1073/pnas.1111164109
Marcello Cavallaro, Jr.1,2, Lorenzo Botto1, Eric P. Lewandowski1, Marisa Wang1, and Kathleen J. Stebe1
Anisotropically shaped microparticles with interesting electrical, optical and magnetic properties are now routinely available. When properly assembled, such particles can potentially be developed into new materials with distinctive, dynamically reconfigurable structures. To create these new materials, reliable means to orient, position and integrate particles into complex structures must be developed. In directed assembly, applied fields are used to achieve these goals. Typically, the effectiveness of these applied fields, and the structures that form within them, depends strongly on the material properties of the particles. For example, dipolar particles orient, migrate and assemble in electric fields, as do ferromagnetic particles in magnetic fields.
Fluid interfaces are versatile sites for directed migration and assembly of particles. When particles are located at an interface, they create distortions in the interface. When distortions induced by neighboring particles overlap, long-range capillary interactions occur which cause particles to attract and assemble. This effect is now an important means for microparticle assembly at otherwise planar fluid interfaces, in particular for anisotropically shaped objects, which assemble with preferred orientations. At planar interfaces, the magnitude of the interaction is determined by the particle shape, size, and surface energies. Therefore, once the particles are placed at the interface, the strength of resulting capillary interactions is fixed. Assembly typically occurs at random locations on the interface determined by sites of initial encounter between the particles.
Here, Kate Stebe and collaborators show for the first time that interface curvature can be used as an applied field to direct the location at which particles assemble. The phenomenon is entirely determined by the interplay between particle shape and capillarity. Therefore, it can be applied to colloids made of any material. When placed on a curved interface, a particle distorts the interface shape, changing the interfacial energy. The interfacial energy depends on the particle’s orientation with respect to the principal axes and its location in a curvature gradient, resulting in torques and forces that drive particle rotation and migration. Since fluid interfaces can be molded and reconfigured, curvature-driven capillary interactions are can be used to amplify the magnitude of interactions and to direct microparticle assembly. By molding the interface to impose different curvature fields, Stebe and collaborators demonstrated the alignment of rod-like particles along principle axes of curvature, and show that particles can be driven along well-defined paths at interfaces that coincide with the curvature gradients, and assemble preferentially at sites of high curvature into complex structures.