Control of Shape and Size of Nanopillar Assembly by Adhesion-Mediated Elastocapillary Interaction
Traditionally, chemistry involves the synthesis of new molecules: precursor molecules are mixed together in the right conditions (provided by the chemists) and the desired molecules will be produced without further, direct human intervention. This molecular synthesis is so accomplished as an art and technology that it can make virtually any target molecules.  The interesting and logical question to ask if such a self-assembly is possible for structures in the micro- and nano-scale, and what the constraints of such self assembly is. A major difference between classical molecular synthesis and self-assembly in the nano-/micro- scale is that while the former involves covalent bonds, the latter are often driven by surface forces whose strengths are of the order of KBT.
The work by Kang et al shows just how such the self-assembly of nanopillars are influenced by a combination of such surface forces. Previous studies have assumed that the self-assembly of micro- and nano-pillars are the results of competition between elasticity and capillarity, but Kung et al shows that adhesion has to be taken into account as well.
Figure 1. Left, Effect of the pillar modulus on the size and pattern of assembled clusters. (a-c) SEM images of assembly of fiber array with different moduli (E): (a) E ~ 400 MPa, (b) E ~ 1 GPa, (c) E ~ 2.4 GPa. The diameter and the length of the fibers were fixed to 250 nm and 8 μm, respectively. The scale bars are 20 μm. The insets show high-magnification images of the assemblies, which were used to check the onset of the chirality for each condition. "Right\", Effect of the plasma treatment on the size and pattern of assembled clusters. (a-c) SEM images of assembly of post arrays with different plasma treatment time. The plasma treatment increases adhesion as well as decreases the diameter of the nanopillars. (a) No plasma treatment (control); (b) 1 min plasma treatment; (c) 2 min plasma treatment. The modulus and the height of the pillars were fixed at 1 GPa and 8 μm, respectively. The scale bars are 20 μm. The insets show the high-magnification images used for determining the shape of individual clusters.
1. G.M. Whitesides and M. Boncheva, "Beyond molecules: Self-assembly of mesoscopic and macroscopic components", PNAS (2002) 99 8 4768-4774
2. S.H. Kang, B. Pokroy, L. Mahadevan, and J. Aizenberg, "Control of Shape and Size of Nanopillar Assembly by Adhesion-Mediated Elastocapillary Interaction", ACS Nano 4 11 6323-6331