Proc. Natl Acad. Sci. USA 108, 20923–20928 (2011)

Floating objects that deform a liquid surface often clump together as a result of gradients in surface height (this is the 'Cheerios effect', named after the cereal that forms aggregates or clings to the sides of a bowl of milk). However, for micrometre-sized particles sitting at and locally deforming a fluid interface, gravitational forces are irrelevant and instead capillary interactions resulting from the minimization of gradients of curvature (and thus surface area) can drive particle assembly. Marcello Cavallaro and colleagues have now exploited the coupling between curvature-driven capillary forces and particle geometry to tailor the curvature field around vertical microposts that pin the contact line of an oil/water interface, and to direct the location of assembly of curvature-inducing microrods. By varying the cross-section of the microposts, the researchers created attractive and repulsive sites for particle assembly to achieve complex particle configurations, such as end-to-end and side-to-side clusters of microrods at the corners of a square micropost. Such curvature-driven migration and assembly should occur for colloidal particles of any material and shape that deform fluid interfaces.