## 1 Introduction  
Micro-particle image velocimetry (\(\upmu\)-PIV) has been broadly accepted as an optical method to measure flow fields at microscopic scales. The \(\upmu\)-PIV technique was first introduced by [PERSON] et al. in 1998 [1] and is now widely used in the fields of life science, lab-on-a-chip, micro electro mechanical systems (MEMS) and chemistry. Extensive reviews of \(\upmu\)-PIV techniques and applications have been published by [PERSON] [2], [PERSON] [3] and [PERSON] [4]. \(\upmu\)-PIV is a variation of the well-known particle image velocimetry (PIV) technique that is used to measure macroscopic flows [5]. In both \(\upmu\)-PIV and PIV, the velocity is derived from the measured displacement of tracer particles that are suspended in the fluid. The displacement is calculated by cross-correlation of two subsequent images of the particles in the flow. \(\upmu\)-PIV uses dedicated microscopic imaging components in order to investigate regions from a few micrometers up to several millimeters by choosing an appropriate magnification. The light sheet that is applied in PIV is replaced by a volume illumination of the flow in \(\upmu\)-PIV, and the depth of field is determined by the numerical aperture of the objective lens of the microscope [6]. Fluorescent tracer particles are used to overcome the reflection from the nearby surfaces. In a typical setup, a microfluidic device is placed on the stage of the microscope. The device has to allow optical access to the fluid flow. Many microfluidic MEMS devices made from silicon typically feature an optical access since the silicon substrates are often sealed with glass by anodic bonding [7; 8]. PDMS (Polydimethylsiloxane) is also a very popular material for microfluidics, especially in the research community, because it is inexpensive and easy to fabricate by replication of molds made using rapid prototyping or other techniques [8; 9]. Microfluidic devices made from PDMS are also optically transparent and can be investigated in a \(\upmu\)-PIV setup. Although most popular manufacturing techniques offer the possibility to realize transparent windows or sealings, many fluidic devices or components made from silicon or metal do not feature optical access to the fluid flow. In the case of silicon, its transparency in the infrared region can be capitalized upon, so infrared micro-particle image velocimetry can be used to investigate the fluid flow [10; 11; 12]. A further possibility to achieve optical access was investigated in studies utilizing transparent models of the object to be investigated: coronary stents [13] and a stenosed artery [14], respectively. In the first study, an original stent was mounted on a balloon catheter and inserted into a polyurethane hose. Silicone was poured into the hose and cured. In the second study, a stenosis artery model was manufactured from silicone by casting around a pair of rods whose ends are shaped and slotted together. The silicone replicates were then used for \(\upmu\)-PIV investigations. Although the studies utilized molding techniques, they do not describe a method to replicate complex or irregular inner surface structures of fluidic components such as metallic tubes with weld seams or manifolds. The aim of the current study is to present a new method to perform optical flow analysis in transparent models of fluidic components. In order to realize an optical access, a new silicone-based molding technique is used to create a transparent replication of the original component. Silicones are very well suited for the molding step because they are dimensionally stable and have a high elongation at break (several 100%), so they can be removed without any damage or permanent deformation. Silicone molding is a widely used technology because the flexible mold can produce parts with extremely complex geometry, intricate detail and tight tolerances [15; 16].