# 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
Figure 5: **(a)** Schematic representation (i) and cross-sectional SEM image (ii) of a silk fibroin inverse opal with an engineered photonic bandgap realized by introducing a planar defect layer of smaller cavities in the middle of six layers of bigger pores (scale bar, 1 \(\upmu\)m). The introduction of this defect modifies the photonic band gap. Reproduced from Ref. [84] with permission from the 2018 Royal Society of Chemistry. **(b)** Optical image of polystyrene colloidal crystals before (i) and after (ii and iii) the gas-flow-assisted melting and reorganization treatment, showing the increase of crystal domain sizes up to several millimeters. The enhancement of periodic order is confirmed by the corresponding laser diffraction patterns. An example of defect-free colloidal crystal obtained by this process is also shown (iv). Reproduced from Ref. [92] with permission from the 2014 American Chemical Society. **(c)** Closely packed particle monolayers prepared by rubbing dry microspheres on elastomeric surfaces. Schematic showing the rubbing process (i) and the resulting monolayer assembly of polystyrene (1 \(\upmu\)m in diameter) beads on the entire surface (20 cm \(\times\) 20 cm) of a PDMS substrate (ii); variation of degree of orientation (\(\upphi\)) of the colloidal crystal according to the rubbing speed and pressure determination of the optimal conditions of the pressure (iii); in-plane grain growth of the monolayer particle assembly produced by unidirectional rubbing at increasing rubbing speed from left to right, and corresponding laser diffraction patterns (inset) (iv). Reproduced from Ref. [91] with permission from 2019 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim, Germany. **(d)** Accelerated annealing process by self-propelled microswimmers. Calculated individual trajectories of self-propelled particles in an ordered monolayer (i); modeled displacement of colloids induced by a swimmer (red dot, the arrow indicates the direction of motion) (ii); optical microscopy of a monolayer of passive silica beads containing a small fraction of active intruders (not visible at this magnification) before (left) and after (right) the accelerated annealing process, showing the elimination of most of defects (the color-code indicates the local orientational field of the hexagonal structure) (scale bars, 50 \(\upmu\)m) (iii). Reproduced from Ref. [93]. An innovative way to achieve a local reorganization of the colloidal crystal was recently proposed by [PERSON] and his co-workers [93]. The idea under this study is to exploit the motion of self-propelled particles to move some of the microspheres packed in a colloidal crystal from their position, so that the particles in the wrong position can rearrange, removing the defect (Figure 5d). As a result, the degree of order of the 2 DPC highly increases. In this system, which represents the proof-of-concept of controlling a self-assembled structure by active particles, polycrystalline monolayers of 5 \(\upmu\)m silica spheres are assembled by sedimentation on the bottom of a microfluidic apparatus consisting of hexagonal cells filled with solvent (Figure 5(d-iii)). The propellers are light-activated microswimmers of 2 \(\upmu\)m-diameter consisting of a hematite (Fe\({}_{2}\)O\({}_{3}\)) portion protruding out of a polymer particle. Under UV light, the photocatalytic hematite triggers the local decomposition of the hydrogen peroxide contained in the solution, creating a gradient that sets the swimmer into motion (Figure 5(d-ii)). Since the speed of the microswimmers is tuned by light intensity, it is possible to operate on the system different reorganization cycles (as in thermal annealing) and even local reorganization by exposing it to light patterns, obtaining a spatial control of the annealing.