Water-perfluorodecalin interface

This chapter describes experimental and conceptual issues in mesoscale self-assembly (MESA), using examples from our work in the assembly of millimeter- and micron(micrometer)-sized polyhedral objects using capillary forces. In MESA, objects (from nm to mm in size) self-assemble into ordered arrays through noncovalent forces. Three systems that use capillary forces in MESA are described these involve the assembly of objects into two-dimensional arrays at the perfluorodecalin/H20 interface, into three-dimensional arrays at curved liquid/liquid interfaces, and into three-dimensional arrays from a suspension in water. The capillary interactions between objects can be viewed as a type of bond that is analogous to chemical bonds that act between atoms and molecules. 

We will describe two mesoscale, self-assembling systems in which the interactions between objects are based on capillary forces. The first is based on polyhedral polydimethylsiloxane (PDMS) objects at a perfluorodecalin (PFD)/H20 interface. These objects have their faces patterned to be either hydrophobic or hydrophilic, and they assemble via lateral capillary forces that originate from interactions between these faces (Fig. 4. la). The second system uses polyhedral objects that are suspended in water and have selected faces covered with a water-insoluble liquid - either a hydrophobic organic liquid or a liquid metal solder these objects assemble via capillary forces into three-dimensional (3D) structures (Fig. 4.1b). 

Figure 6.2 Examples of static self-assembly, (a) Crystal structure of a ribosome, (b) Self-assembled peptide-amphiphile nanofibers, (c) An array of millimeter-sized polymeric plates assembled at a water/perfluorodecalin interface by capillary interactions, (d) Thin film of a nematic liquid crystal on an isotropic substrate, (e) Micrometer-sized metallic polyhedral folded from planar substrates, (f) A 3-D aggregate of micrometer plates assembled by capillary forces.

Recently, capillary action has been used to self-assemble macroscopic objects. Objects of various shapes were cut from polydimethylsiloxane, a polymer that is not wettable by water but is wetted by fluorinated hydrocarbons. Designated surfaces were then made wettable by water by using controlled oxidation. These objects were then floated at an interface between perfluorodecalin (CioFig) and water. When two non-oxidized surfaces (wettable by CioFis) approached each other within a distance of approximately 5 mm, they moved into contact, which with time created an ordered, self-assembled pattern of the objects. The movement and self-assembly was driven by the solvent adhesive forces that produce the capillary action, thereby leading to an elimination of the curved menisci between non-oxidized surfaces. One such pattern is shown to the right. 

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