Self-Assembly of Hexagonal Rods Based on Capillary Forces

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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

By Alex Epstein

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 matter example

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 self-assembly on the mesoscale (that is, intermediate 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. However, the additional degree of freedom involved here makes tunable self-assembly of space-filling structures more impressive.

Fabrication of the mesostructures

Fig. 1 Schematic diagram of the fabrication of the hexagonal rods.

Self-assembly of unmodified, all-hydrophobic rods

Fig. 2 Unmodified, all-hydrophobic rods formed extended, close-packed arrays. (a) Solid hexagonal rods. (b) Hollow hexagonal rods.

Various self-assembled arrays of rods with specific hydrophilic sides

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.