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. Evaporation-induced self-assembly has been studied by various groups and 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. For example, the strength of adhesion forces affects the shapes that the array of nanopillars will take, even when the modulus of the material remains constant. (Figure 1, right) The effects of modulus of the material on self-assembly is also studied. It was found that chirality can be introduced in the nanostructures by adjusting the modulus of the material. (Figure 1, left)
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.
Different structures can also be obtained by self-assembly by introducing anisotropy, such as by stretching and tilting, during the assembly processes. See figure 2.
Figure 2. Effect of the anisotropy on the size and pattern of assembled clusters. Left column shows schematics (not drawn to scale) of the method used to fabricate various nanopillar arrays, and right column shows SEM images of the assemblies of corresponding nanopillar arrays. The insets show the high-magnification image of the corresponding SEM image. (a) Elliptical cross section nanopillars (h ~ 8 μm) arranged into a rectangular lattice assemble into anisotropic clusters elongated in the direction of the short axis if the ellipse (scale bar: 50 μm). The long axis indicates the direction in which the tensile force was applied. The inset shows the elliptical cross section of the posts (scale bar: 1 μm). (b) Square array of cylindrical nanopillars oriented perpendicular to the surface assemble into a regular array of tetramers. (c) Pillars tilted along a lattice direction form a regular array of achiral dimers instead of tetramers. (d) Pillars tilted slightly off a diagonal lattice direction form chiral dimers with a uniform twisting direction. Scale bars: 10 μm.
Figure 3. Schematic diagrams depicting the mechanisms of the assembly process. (a) Possible routes of the assembly and disassembly processes that determine the final pattern. C and A indicate the capillary force and the adhesion force, respectively. E is the elastic force, and the subscripts I, II, III, and IV indicate the hierarchy of the assembly. Note that while the interplay between the elastic and capillary force determines the maximum size the assembly can reach while wet, the adhesion force determines stability of the formed clusters and the size and shape of the final product. (b) Different shapes of the assembled clusters. The nanopillars first attached at the tips and, depending on the adhesion force between the pillars, can either undergo a slippage and chiral rearrangement leading to twisted clusters for low adhesion or zip down the nanopillars for high adhesion values.
Continuous and instantaneous imaging show that assembly has two distinct phases: 1) posts assemble hierarchically, as previously reported, but 2) can subsequently undergo hierarchical disassembly as outlined in figure 3. Previous studies assume that once the structures have assembled, they remain static, but whether they remain static or disassemble depends on the balance between the adhesion forces and the stiffness of pillars once the liquid dries. While the initial structure of the material is largely driven by capillarity forces, once the liquid has dried, the combination of elastic forces and adhesion forces will determine the final structures of nanopillars.
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