Assembly using capillary faces

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). 

The PDMS objects assembled at the PFD/H20 interface upon agitation using an orbital shaker (Fig. 4.4). We chose this interface for four reasons, (i) PDMS does not swell in contact with either liquid, (ii) The interface has a high surface tension (y = 0.05 J m 2) for comparison, the H20/diethyl ether interface has a surface tension of y = 0.01 J m-2 [ref. 59]. Capillary forces are, thus, strong, (iii) PDMS (p = 1.05 g cirr3) has a density between those of PFD (p = 1.91 gcirr3) and water (p = 1.00 g cm-3) the objects floated at the interface, (iv) A thin film of PFD remained between the faces of objects when they assembled. This film acted as a lubricant and allowed the objects to move laterally relative to each other, and thus to minimize the free energy of the system. 

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