## 3 Experimental Setup for Measurement of Microflows Hydrodynamics  
To study the microflows hydrodynamics experimentally, we used the micro Particle Image Velocimetry (\(\upmu\)PIV) technique. The \(\upmu\)PIV method is a quantitative method that can be used to characterize the performance of microfluidic systems with spatial resolutions better than one micron [46]. We can distinguish between two types of \(\upmu\)PIV systems: those capable of measuring two-dimensional, two-component velocity fields (2D2C) and more sophisticated systems that are able to measure volumetric three-component (3D3C) fields [47]. The planar 2D2C configuration consists of an inverted microscope, a single or double CCD/CMOS camera, and a pulsed laser or continuous LED light source [48]. It is also possible to use configurations with the stereoscopic microscope. In contrast with the standard PIV method, the light source illuminates the whole volume of fluid with suspended tracer fluorescent particles in it. The major difficulty arises from the fact that, in contrast with standard PIV, where the measurement plane is defined by a laser light sheet, the measurement plane in \(\upmu\)PIV is determined by the depth of focus of the optical system. Further, the amount of light scattered by the particles reaching the camera sensor must be sufficient to register their displacements. At the same time, the use of high-speed cameras with short exposure times, tracer particles of a small diameter, and a large magnification of microscope lenses drastically reduce its amount. The main steps of the PIV measurement technique involve (1) twice imaging of flowing in a channel suspension of tracer particles, (2) the evaluation of the particle image displacement with digital image processing algorithms, and (3) the conversion of the particle image displacement into physical space by a proper calibration. We have used in our investigations a slightly different configuration of the 2D2C measurement system. As the surface area of microfluidic DSSC is relatively large and the channels are wide, we cannot use the inverted microscope equipped with large magnification lenses. The schematic view of our \(\upmu\)PIV system configuration is shown in Figure 9. Our system consisted of a standard stereoscopic microscope Motic K-400L (1) equipped with a Common Main Objective (CMO) (2) and Moticam 2300 CMOS camera (3). The specially designed microfluidic systems (4) with optically transparent top and bottom surfaces, made from poly(methyl methacrylate) (PMMA) with the same channel's configuration as used in our DSSC chips were used in \(\upmu\)PIV experiments. The microsystem placed on the transparent microscope stage (5) was illuminated from the bottom by the LED illuminator consisting of 60 diodes placed on the ring shape (6). This configuration formed dark-field-like lighting conditions with bright, well-lit tracer particles on the dark background. The depth of field of the lens covered the entire depth of the channel. To find the mean fluid velocity in the XY plane, neglecting the fluid velocity gradient along the \(Z\)-axis, we have used polyamide round particles with a large diameter of 50 \(\upmu\)m PSP-50 (Dantec Dynamics, Denmark), which was about half of the microchannel height (120 \(\upmu\)m). The microsystem was connected with a syringe pump (7) that delivers the suspension of tracer particles to the microsystem at a constant flow rate. The suspension flowing out of the microsystem was collected in the reservoir (8). During the experiments, we recorded short films that were captured and saved on the computer (9) as AVI files. The movies were split into frames, and the pairs of images were analyzed in the PIVlab software [49]. In Figure 10, the microsystems used in the study of flow are presented. The height of each microchannel was 0.12 mm and the straight-line length was 41 mm (distance between the liquid supply points).