This dataset contains the instantaneous velocity vector fields from PIV data taken over large acrylic plates fouled with diatomaceous biofilm of varying patchiness. See associated article, Boundary layer hydrodynamics of patchy biofilms, for methods description. Each zip folder contains data for one of the 4 non-uniform biofilms examined, PB-1 (patchy biofilm 1); PB-2 (patchy biofilm 2); SB-1 (sparse biofilm 1); SB-2 (sparse biofilm 2). For each biofilm, the corresponding folder contains 4000 statistically independent instantaneous velocity vector fields. Each vector field is saved in a .mat file, and the workspace variable that contains the data is called ‘vecfield’. Size calibration and water temperature are provided in the spreadsheet ‘experiment_metadata.xlsx’ Column 1: X (streamwise distance [pixels]) Column 2: Y (wall-normal distance from bottom of frame [pixels]) Column 3: U (streamwise velocity vector [pixels / 250 microseconds]) Column 4: V (vertical velocity vector [pixels / 250 microseconds]) Column 5: CHC (number of tracked particles. A value < 1 gives the location of the biofilm, which was masked out)