# Self-organization of a mesoscale bristle into ordered hierarchical helical assemblies

edited by Qichao Hu

A well known phenomenon is the clumping of wet hair.

This can be explained by studying the elasticity of each hair strand and the adhesion among the neighboring hair strands. When the hair strands are immersed in water, and the water is allowed to evaporate, capillary forces between the strands cause them to deform, adhere to each other, and form clumps.

This paper demonstrates that this elastocapillary force can be used to self-assemble stiff and upright nano-pillars to helical clusters and even coiled large-area structures. The fundamental theory involves the following parameters:

<math>r</math>: circular radius of the nano-pillars

<math>L</math>: length of nano-pillars

<math>d</math>: distance between adjacent nano-pillars

<math>E</math>: Yong's modulus

<math>B\simeq Er^4</math>: bending stiffness of the nano-pillars

<math>J</math>: adhesive energy per unit area

<math>\gamma</math>: interfacial tension of the evaporating liquid

There is a competition between the capillary force due to the menisci (formed when the liquid evaporates to the level of the free ends) that wants to pull the free ends of adjacent nano-pillars together, and the force due to the stiffness of the nano-pillars that wants to resist the bending and return to the straight position. These forces are related to the above parameters through the following:

Bending force: <math>F_B\simeq Bd/L^3</math>

Capillary force: <math>F_C\simeq \gamma r</math>

when the two forces are equal to each other, we arrive at the critical length of the nano-pillars, given by

critical nano-pillar length: <math>L_C \simeq (Bd/\gamma r)^{1/3} \simeq (Ed/\gamma)^{1/3}r</math>

At this critical length, when the adhesive energy per unit area is equal to the interfacial tension <math>J=\gamma</math>, we arrive at another important length

<math>L_a\simeq(Ed/J)^{1/3}r</math>

These critical lengths can be used to guide the design of nano-pillars where such bending is possible and maintained even after the liquid evaporates. The following image shows both the schematic and SEM images for nano-pillars of different lengths.

Regime I: <math>L_a>L>L_C</math> the nano-pillars will come together as long as there is capillary force between them, but move apart after the liquid evaporates.

Regime IIa: <math>L\geq L_a>L_C</math> the free ends of the nano-pillars will touch and remain together after the liquid evaporates.

Regime IIb and IIc: <math>L\gg L_a</math> the bending stiffness of the nano-pillars succumb to the adhesive force and twist around each other.

Regime III and IV: These chiral clusters can interact with each other and form large-area chiral structures.

Since the nano-pillars used are assumed to be circularly symmetric, the chirality of the clusters is random. Occasionally, any impurities in the nano-pillars such as asymmetry in the nano-pillar cross section or preferential direction in evaporation will lead to a particular orientation or handedness in the chirality.

As Regime IV continues, the size of each cluster grows, and the number of clusters decreases. This hierarchical assembly represents the self-assembly process of small clusters grow into larger and higher-order structures.

The randomness in the clusters can be improved or controlled by either introducing nano-pillars with elliptical cross sections, changing the direction of evaporation, or pre-tilting the nano-pillars. This leads to a directional bending of the nano-pillars under capillary force and the formation of large area clusters with controlled chirality.

Such technique can be used to self-assemble nano-pillars to large area, complicated, and helical structures with controlled dimension and orientation. One application is in capture-and-release systems. They demonstrate the stable adhesion and trapping of a particle by the helical clusters.