# Nano- and Micropatterning on Optical Fibers by Bottom-Up Approach: The Importance of Being Ordered
## 3 Bottom-Up Fabrication of Periodic Structures on OFs
### Other Self-Assembly Methods
#### 3.2.1 Breath Figures  
The formation of breath figures is a natural phenomenon observable anytime water vapors condensate on a cold surface. The fog formed on a glass window while breathing on it, when viewed under a microscope appears as an assembly of closely packed micrometric water droplets [61]. The most typical application of this simple phenomenon in nanotechnology consists in generating the fog on a polymeric solution by blowing some humid gas toward it, so that at the end of the process the water droplets leave their imprint in the polymer film, generating a microporous film with the characteristic honeycomb structure, known as breath figure array [62]. By controlling the main parameters of the process (type of polymer and solvent employed, gas flow rate, humidity content) highly ordered patterns can be easily obtained, even on a large scale [63]. By analogy, breath figures can be assimilated to the assembly of 2 DPCs at the air/water interface, where the interface considered here is that between polymer solution and air, and the packing objects are not solid particles but water droplets. In breath figures, in fact,  
Figure 2: (**a**) Fabrication procedure for creating four different fiber tip nanoprobes and SEM images (scale bar = 3 μm) of real samples obtained using 1-μm nanospheres, and photograph of a fiber tip showing an iridescent nanopattern. Reproduced from Ref. [52]. **(b**) Fiber probe with hybrid graphene/Au nano-triangle nanostructuration: schematic diagram of the experiment setup (i), SEM image of Au nano-triangle array (ii), R6G Raman spectrum obtained by fiber probe (iii), electric field magnitude mapping along x-y plan for z = 0 nm (iv) and schematic model of Au triangle array (v). Reproduced from Ref. [57] with permission from 2017 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim, Germany. **(c)** Fabrication procedure for the nanocavities-patterned fiber probe obtained by combining colloidal assembly and UV nanoimprinting, which includes the coating, patterning, and metallization of the cavities on the fiber end face. Reproduced from Ref. [59] with permission from 2019 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim, Germany. water droplets are stabilized by the polymer molecules preventing their coalescence, and therefore they can move and pack themselves as long as the polymer solution is still in liquid form. When the solvent evaporates leaving a solid polymer film, water droplets are frozen in their positions; as soon as also water evaporates, its imprint remains on the film. Typically, a volatile and non-water-mixable solvent such as chloroform or carbon disulfide is employed to dissolve the polymer. The water droplets naturally form on the cast polymer solution by water condensation, due to a local temperature drop triggered by the solvent evaporation. It is also possible to manage this phenomenon more accurately by regulating the substrate temperature using a Peltier controller. The potentiality of this technology is extensive, especially in terms of production speed: fabricating a colloidal 2 DPC takes tenths of minutes to hours; making a pattern by breath figures takes a few seconds to 2-3 min. Thanks to its extreme simplicity and fast realization, the breath figure process has been applied in materials science in a great number of studies and for different applications [64; 65; 66; 67; 68; 69; 70; 71]. For nanofabrication purposes, breath figure arrays can also represent the starting structure from which different kinds of patterns can be developed by further fabrication steps such as electrodeposition [72], pyrolysis [73], or dry etching [74]. [PERSON] et al. recently used the breath figures to decorate the OF tip with photonic-plasmonic patterns, and their work remains the only example so far [75]. The nanofabrication approach is depicted in Figure 3. This work demonstrates that breath figure formation can be adapted to work on the OF.