Self-Assembly of Hexagonal Rods Based on Capillary Forces

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Original entry: Alexander Epstein, APPHY 226, Spring 2009

Self-Assembly of Hexagonal Rods Based on Capillary Forces

Authors: Scott R. J. Oliver, Ned Bowden, and George M. Whitesides.

Journal of Colloid and Interface Science 224, 425–428 (2000)

Soft matter keywords

self-assembly, capillarity, mesostructures., hydrophobicity, hydrophilicity


Abstract from the original paper

A series of well-ordered, extended mesostructures has been generated from hexagonal polyurethane rods (15 x 3.2 mm) by self-assembly using capillary forces. The surface of one or more sides of the rods was rendered hydrophilic by exposure to an oxygen plasma. This modification determined the pattern of hydrophobic and hydrophilic faces; the hydrophobic sides were coated with a thin film of a hydrophobic lubricant. Agitation of the rods in an approximately isodense aqueous environment resulted in their self-assembly, in a process reflecting the action of capillary forces, into an array whose structure depends on the pattern of hydrophobic sides; capillarity also aligned the ends of the rods. We also carried out experiments in reaction chambers that restricted the motion of the rods; this restriction served to increase the size and regularity of the assemblies.

Soft matters

They may curiously resemble stacked pencils in water. But these hexagonal rods are no pencils, and their various modes of stacking reflect notable advances in the control of 3-dimensional Mesoscale Self-Assembly, or MESA (that is, intermediate in size between micro and macro). The system of molded hydrophobic hexagonal rods with some of the sides treated for hydrophilicity is both experimentally practical and elegant. Previous work was conducted by this group in two dimensions using hexagonal plates that assembled at a liquid-liquid or liquid-air interface through capillarity. However, the additional degree of freedom involved here makes tunable self-assembly of space-filling structures more impressive.

Fig. 1 Schematic diagram of the fabrication of the hexagonal rods.
Fig. 2 Unmodified, all-hydrophobic rods formed extended, close-packed arrays. (a) Solid hexagonal rods. (b) Hollow hexagonal rods.
Fig. 3 Various oligomeric arrays are formed by pieces with specific sides patterned hydrophilic. Hydrophilic sides, nonshaded (white) and thin lines; hydrophobic sides, shaded (grey) sides and thick lines. (a) [1]-rods in a spherical flask resulted exclusively in dimers. (b) [1,2]-rods formed a mixture of trimers and bilayers, also using a spherical flask. (c) A rectangular chamber was used for [1,4]-rods; this more restricted container yielded an extended flat array. (d) [1,3,5]-rods in a cylindrical flask induced the formation of an open array.

Fabrication

Fig. 1 illustrates how the authors fabricated the unit rods, which were width 3.2 mm by length 15 mm. To produce rods with patterns of hydrophobic sides, those sides were masked with Scotch tape and the pieces were exposed to oxygen plasma, rendering unmasked sides hydrophilic. Since the hydrophilic sides are wetting, they will remain in contact with water; hydrophobic, non-wetting sides will "bond" to other rods as the water seeks to minimize its surface area. Thus, capillarity drives self-assembly. The rods were also coated with a hydrophobic coating that doubled as a UV-curable adhesive (for fixing the assembled formations later). The choice of the rod material, polyurethane, was very important. The authors found that unlike PDMS, PU pieces do not connect irreversibly by van der Waals bonding in the water and form random assemblies; rather the PU pieces can slide laterally.


Self-assembly

When all rod sides were hydrophobic and the rods were tumbled in a rotating round flask for 30 minutes, the authors observed extended, close-packed arrays as in Fig. 2. By repeating this experiment with hollow hexagonal rods, the authors then demonstrated a method to create large hexagonal arrays of channels--potentially useful for microfluidic applications.

The most interesting results are the various structures formed from rods with specific hydrophilic (bonding) sides. As shown in Fig. 3, rods were patterned as either [1], [1,2] [1,4], or [1,3,5], each corresponding to a predictable structure. The work has implications for directing 3-D self-assembly of geometric objects in suspension simply by modifying their surface properties, controlling capillary forces between these objects.


Hydrophilic sides Self-assembled structure
[1] Dimers
[1,2] Trimers and bilayers
[1,4] Extended flat array
[1,3,5] Open hexagonal array