# Nano- and Micropatterning on Optical Fibers by Bottom-Up Approach: The Importance of Being Ordered
## 3 Bottom-Up Fabrication of Periodic Structures on OFs
### Post-Processing Methods to Increase the Order
determined the optimal tetrahydrofuran (THF) concentration and vapor annealing time to obtain films with hexagonally close-packed perpendicularly aligned cylinders traversing the entire thickness of the film, low defect densities, and extended order [86]. Another reported method to increase the order of a periodic pattern is mechanical rubbing. This post-processing treatment is widely used in liquid crystal orientational epitaxy, for the alignment of polymeric layers which guide the orientation of liquid crystal molecules [87]. Besides liquid crystals, mechanical rubbing can also be applied to the alignment of different materials such as anisotropic nanocrystals [88], nanowires [89], and conjugated polymers [90], resulting in improved material properties like polarized photoluminescence and electric conductivity. However, mechanical rubbing is not limited to the alignment of anisotropic particles or molecules within a direction. Interestingly, [PERSON]'s group showed that it is possible to obtain highly ordered 2 DPC by rubbing dry PS spheres [91]. The assembly process consists in rubbing the nanospheres between two pieces of PDMS elastomer. This randomly directed friction enables the particles to roll one on the other until they pack themselves in a monolayer (Figure 5c). The process bases its success on the fact that the particle-substrate adhesion energy must be larger than the particle-particle attraction energy and the external shear force applied to the aggregate must be transferred to all particles in the aggregate. In other words, the particles must stick preferentially to the substrate than to themselves. At the above condition, once the particles stick with the substrate, it is difficult to be separated out again. In this way, it is feasible to break the multilayered agglomerates and form two monolayers of small and randomly oriented crystals, remaining on each elastomer surface. Then, in order to increase the size of the colloidal crystals, a second rubbing process, this time following a precise direction, must be applied repeatedly back and forth. The result of this post-processing treatment is the formation of large-scale and highly ordered 2 DPCs. By testing different rubbing conditions, the authors could determine the optimal parameters in terms of rubbing speed and force (Figure 5(c-iii)). The possibility of assembling spheres of different materials (PS, silica, PMMA) as well as of using different kinds of rubbing elastomers are also demonstrated. Given the ability to increase the long-range order of the assembly and minimizing the crystal defects, this process is very appealing with respect to the classical assembly of colloidal dispersions used in NSL. The only limitation is that the dry process works on elastomeric surfaces only; however, coating any kind of substrate (glass, silicon) with a thin layer of PDMS is sufficient to perform the assembly. A different approach to increase the order of 2 DPCs has been proposed by [PERSON] and [PERSON] [92]. In this process, the monolayer colloidal crystal is formed by assembling the PS spheres at the air/water interface as described by many previously reported papers. The only difference is that a small amount of surfactant is introduced into the water phase to reduce the interaction energy density between water and PS particles, thereby the lateral attraction between PS particles. When the water surface is full of colloidal particles and the monolayer becomes \"frozen\" on the water surface, a compressed nitrogen flow is directed towards it, so that the \"crystallites\" are \"melted\" and then recrystallized. This \"melting\"--recrystallization process is carried on like the annealing treatment in molecular crystallization to eliminate defects (Figure 5b). A growth front of ordered 2 DPC is generated and moved forward with the nitrogen flow by carefully moving the nitrogen nozzle along the surface, until 2D hexagonal colloidal single crystals of centimeter size are obtained.