Quantification of fluid flow direction

Injected fluorescent particles were tracked using the trackmate plugin in Fiji/ImageJ (Tinevez et al., 2017) using the LoG detector for 8-10 pixel blobs and a threshold of 10-50 depending on the recording. Other parameters were initial search radius: 15,0, max search radius: 15,0, max frame gap: 2. The tracks were then imported in MATLAB. Only tracks containing more than 20 data points and covering a distance longer than 10 pixels were included in the further analysis. The direction between two consecutive data point was then plotted using the surf function in MATLAB.