(A) Aggregation occurs when there is a net attraction and an equilibrium separation between the components. The equilibrium separation normally represents a balance between attraction and repulsion. These two interactions are fixed in molecular self-assembly but can be engineered independently in macroscopic self-assembly. (B and C) Schematic illustration of the essential differences between irreversible aggregation and ordered self-assembly. (B) Components (shown in blue) that interact with one another irreversibly form disordered glasses (shown in green). (C) Components that can equilibrate, or adjust their positions once in contact, can form ordered crystals if the ordered form is the lowest-energy form (shown in red). (D) Biology provides many examples of self-assembly (here, the formation of a protein, an asymmetric, catalytically active nanostructure); these examples will stimulate the design of biomimetic processes.

Examples of two-dimensional (A and B) and 3D (C–F) structures, self-assembled in systems of macroscopic components interacting via capillary interactions. Open hexagonal array (A; reprinted with permission from ref. 33, copyright 1997 American Association for the Advancement of Science) and hexagonal lattice formed around circular templates (B; reprinted with permission from ref. 48, copyright 2000 American Chemical Society) self-assembled from poly(dimethylsiloxane) plates floating at the interface between perfluodecalin and water. (C) Spherical structure formed by self-assembly of hexagonal metal plates on the surface of a drop of perfluodecalin in water (reprinted with permission from ref. 49, copyright 1998 American Chemical Society). (D) Compact 3D structure formed by self-folding of a string of tethered, polymeric polyhedra (reprinted with permission from ref. 54, copyright 2002 American Chemical Society). (E) Large crystal self-assembled from micrometer-sized hexagonal metal plates (reprinted with permission from ref. 36, copyright 2001 American Chemical Society). (F) Aggregate with electrical connectivity self-assembled from polyhedral, polymer components bearing solder patterns of wires and dots (reprinted with permission from ref. 42, copyright 2000 American Association for the Advancement of Science).