# Advances in Riblets Design
## 4 Manufacturing and Fabrication of Riblets
3D printing methods are another approach enabling the manufacture of riblet plates of different shapes. In a similar investigation to [PERSON] et al. [65], [PERSON] et al. [51] used a Stereo Lithography Apparatus (SLA) 3D printing technique to manufacture the riblet test plates, with the possibility of mounting pin-hole probes. Likewise, [PERSON] et al. [33] produced riblets by using multi-materials 3D printing. They designed and fabricated a sheet of flexible shark skin by use of micro-CT imaging from the shark, in which thousands of 3D-printed synthetic scales were mounted in a linear array pattern on the flexible membrane. Mold-making and casting is another approach applied to replicate shark skin geometries [89; 115; 116]. A method of vacuum casting was used for bio-replicating different scales of shark skin. [PERSON] et al. [89] implemented unsaturated polyester resin under vacuum conditions to produce a mold, in which the cavity inside the mold was then poured with silicone rubber in a vacuum and using an oven to produce the film of shark skin. [PERSON] et al. [116] conducted a synthetic bio-replication from pre-treated shark skin to produce flexible female silicon rubber by soft die formation. They applied a nano-long-chain drag reduction interface by use of a water-borne resin epoxy. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Method** & **Advantages** & **Disadvantages** \\ \hline Casting and mold making [89; 115; 89] & Effective replication & Time-consuming mold creation \\  & Versatile material compatibility & Possibility of misalignment \\ \hline Chemical Etching and mold making & High-precision micro-riblets & Limited to specific materials \\ mold making & Repeatability & Chemical handling \\  & Moderate implementation time & Shape limitations \\ \hline \multirow{6}{*}{Adhesive films [85; 118]} & \multirow{3}{*}{
\begin{tabular}{} \end{tabular} } & Difficulties during the application on curved surfaces \\  & & Time of application & Possibility of detachment during the flight \\ \cline{1-1}  & Uniform thickness & Limited in riblet configurations \\ \cline{1-1}  & Suitable for aviation & Application challenges \\ \cline{1-1}  & & Environmental impact \\ \hline \multirow{6}{*}{ Painting [66; 67; 84; 119]} & Quick application & \\ \cline{1-1}  & Cost-effective & \\ \cline{1-1}  & Suitable for complex shapes & Limited precision \\ \cline{1-1}  & In some cases possible to have different height & Environmental impact \\ \cline{1-1}  & Durability & \\ \cline{1-1}  & Suitable for aviation & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._Etching techniques have also been explored for the manufacture of riblet geometries. Generally, the etching process has been used for mold making due to the possibility of making negative volumes [83; 99; 117]. The fabrication of microstructure triangular riblets by use of anisotropic etching, performed by [PERSON] et al. [83] and [PERSON] et al. [99], made a silicon wafer with tetramethylammonium hydroxide (TMAH) as etchant. The authors produced micro-riblets with a polydimethylsiloxane (PDMS), and the micro-modling technique was used by manufacturing silicon wafers with micromachining (photolithography and anisotropic etching). They produced a 180 um height and 300 um spacing of triangular riblets, in which the mold space was filled by PDMS (poly-dimethyl-siloxane). The method was also used over the curved surfaces. The authors successfully tested and fabricated the surface on a NACA 0012 airfoil and cylinder geometries. This method requires more caution during the implementation process to avoid misalignments between the pattern of the UV mask and the silicone mold, which can reduce the quality of fabrication. [PERSON] et al. [117] fabricated microstructures by using both the anodization and the etching methods. The fabrication was iteratively controlled by the use of anodization and pore-widening treatments.