Three-Dimensional Self-Assembly of Complex, Millimeter-Scale Structures Through Capillary Bonding Structures
Entry by Haifei Zhang, AP 225, Fall 2009
Soft matter keywords
Self-Assembly, Capillary Bonding
Self-assembly is a process in which components, either separate or linked, spontaneously form ordered aggregates. Self-assembly can occur with components having sizes from the molecular to the macroscopic, provided that appropriate conditions are met. Although much of the work in self-assembly has focused on molecular components, many of the most interesting applications of self-assembling processes can be found at larger sizes (nanometers to micrometers). These larger systems also offer a level of control over the characteristics of the components and over the interactions among them that makes fundamental investigations especially tractable.
The authors fabricated the polyurethane pieces using a replica molding procedure, patterned them with adhesive-backed copper tape, and selectively coated their exposed copper surfaces by dipping the pieces into molten alloy. For the self-assembly experiments, the authors placed the pieces in an indented 500-mL round-bottomed flask, filled the flask with an approximately isodense KBr solution, and rotated it at 10-20 rpm while heating in a 60°C water bath. The alloy melted within a few minutes, and collisions between regions bearing molten alloy enabled the pieces to assemble. Upon completion, we stopped the agitation and allowed the solution to cool to room temperature, causing the alloy to solidify and furnishing aggregates sturdy enough to be removed and examined.
In the first system studied, we used shape complementarity between indented regions on square rods to direct the formation of an open-square array (Figure 1). The four indentations bearing alloy-coated copper tape forced adjacent rods to lie at right angles with respect to one another (Figure 1a).
Next, the authors designed a system to mimic the ubiquitous helical conformations of linear polymers (Figure 2). Figure 2a depicts rectangular slabs designed to form helices. Self-assembly gener-ated helical arrays (Figure 2c) and (Figure 2d), displaying the expected screw axis symmetry.
In the final system examined, the authors chose to mimic a process of fundamental importance to molecular recognition and self assembly-discrimination between enantiomeric species. The system consists of chirally striped square rods designed to assemble into open lattices (Figure 3).
The authors believe that the success of the methods for generating well-ordered, topographically complex aggregates renders MESA a promising strategy for fabricating threedimensional structures. In addition, this work provides further support for the notion that concepts abstracted from the molecular sciences can find fruitful application in building structures at larger-size scales. Although the aggregates presented here are large enough to be fabricated using conventional methods such as manual or robotic assembly, miniaturization of the components could lead to structures that would be difficult to prepare using any other method and, ultimately, to assemblies displaying a range of interesting functions. In other work, we have already demonstrated the formation of regular arrays of 10-um-sized polyhedral plates and the formation of electrical connections in three dimensions between mm-sized subunits, using MESA.
Soft matter details
Self-assembly is not limited to molecules
Molecular synthesis is a technology that chemists use to make molecules by forming covalent bonds between atoms. Molecular self-assembly is a process in which molecules (or parts of molecules) spontaneously form ordered aggregates and involves no human intervention; the interactions involved usually are noncovalent. In molecular self-assembly, the molecular structure determines the structure of the assembly (1). Synthesis makes molecules; self-assembly makes ordered ensembles of molecules (or ordered forms of macromolecules). The structures generated in molecular self-assembly are usually in equilibrium states (or at least in metastable states).
Self-assembly is not limited to molecules although the concepts of self-assembly were developed with molecules, and selfassembling processes currently are best understood and most highly developed for molecules, components of any size can selfassemble in a permissive environment. The expanding contact of chemistry with biology and materials science and the direction of technology toward nanometer- and micrometerscale structures, however, has begun to broaden this focus to include matter at scales larger than the molecular. There are now three ranges of sizes of components for which self-assembly is important: molecular, nanoscale (colloids, nanowires and nanospheres, and related structures), and meso- to macroscopic (objects with dimensions from microns to centimeters). The rules for selfassembly in each of these ranges are similar but not identical. Because new types of aggregates, especially those with potential for application in microelectronics, photonics, nearfield optics, and the emerging field of nanoscience, have become increasingly important technologically, interest in self-assembly as a route to aggregates of components larger than molecules has grown. There are many opportunities for fabrication of useful structures of nano- and macroscale components using self-assembly; ultimately, self-assembly may prove to be more important in these areas than in molecular science!
Self assembly used to fabricate 3D structures
For possible application in the fabrication of functional constructs such as densely interconnected 3-D electronic or optical devices, the authors wanted to extend the range of structures accessible through MESA. So they apply concepts derived from the study of molecules6sincluding shape and surface complementarity, helicity, and enantioselective recognition1, to the self-assembly of mesoscale structures that display symmetries more complex than those resulting from simple extended 2- and 3-D crystalline arrays. In this study, the force responsible for aggregation is capillarity, that is, the minimization of interfacial free energy. Capillary interactions may be considered roughly analogous to chemical bonds; this analogy, though convenient, has limitations. Obvious differences exist between these two classes of bonds. Here, capillary bonding occurs between films of a liquid metalsa lowmelting (47 °C) bismuth alloy -- patterned on the surface of mm-sized polyhedral subunits (pieces). When heated above its melting point, the alloy forms capillary bonds that are strong enough to support open lattice structures and, when cooled below its melting point, locks the structures in place. In addition, the resulting metal-metal contacts can serve as a starting point for the design of systems that form electrical connections through self assembly.
Controlling self assembly in soft matter
Controlling self assembly in soft matter enables the creation of novel materials that interact with external fields: photonic materials, electrorheological fluids, liquid crystal elastomers, and etc. There are a variety of self-assembly scenarios in soft matter systems: low-symmetry, non-close-packed ordered lattices, micellar or inverse micellar structures, cluster phases, layered arrangements, or gyroid phases, to name a few of them. The basic laws of statistical mechanics impose that particles have to self-assemble in the energetically most favorable arrangement. The authors believe that the design of systems of components with nano- to macroscale dimensions for self-assembly can be aided enormously by considering analogies with molecular systems. To test this belief, the authors have explored one of many imaginable systems of self-assembling macroscopic components: systems based on capillary interactions. These studies have demonstrated that it is practical to design new systems of self-assembling components essentially de novo and suggest that such systems can find rapid application.
 Peter J. Lu, Jacinta C. Conrad, Hans M. Wyss, Andrew B. Schofield, and David A. Weitz, "Fluids of Clusters in Attractive Colloids." Physical Review Letters 96, 028306 (2006).
 Whitesides, G. M. and B. Grzybowski (2002). "Self-assembly at all scales." Science 295(5564): 2418-2421.
 Whitesides, G. M. and M. Boncheva (2002). "Beyond molecules: Self-assembly of mesoscopic and macroscopic components." Proceedings of the National Academy of Sciences of the United States of America 99(8): 4769-4774.