File Descriptions: X-ray diffraction patterns .hdf, .h5, and .edf files contain X-ray diffraction (XRD) patterns collected from Diamond I11 (.hdf) and droplet microfluidics-coupled XRD (DMC-XRD) patterns from ESRF ID13 (.h5 and .edf). XRD.hdf files are saved in a folder identifying the experiment to which they belong, where "TJ", "Pos1", and "Pos2" refer to the channel position at which data were collected (the T-junction, Position 1, and Position 2, respectively) and "XXmin" refers to the experimental time point in minutes. .h5 files can be found in the open access repository available at https://doi.org/10.5518/472. Files are named "Porous_BG_posXX_DMC_XRD.h5", where XX corresponds to the channel position from which data were acquired: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, or 34. Each file contains all of the frames belonging to a single scan. .edf files are saved in a folder identifying the experiment and channel position to which they belong. Each file contains only a single frame of the 500 belonging to each scan, for example, "raw_K300_CaCO3_2_51_0010.edf", where 51 is the scan number and 0010 is the individial frame number. Matlab scripts to process these different types of XRD files are included. Continuous_Flow_XRD_processing_HDF.m is used to process .hdf files of continuous flow data Segmented_Flow_XRD_processing_HDF.m is used to process .h5 files of segmented flow data Segmented_Flow_XRD_processing_EDF.m is used to process .edf files of segmented flow data The correct sample-to-detector distance for the various experiments are already saved within the respective scripts, but calibrant patterns are also included for the continuous flow experiments (Silicon_calibrant.hdf) if you wish to further refine wavelength and distance. Sample-to-detector distance was pre-determined for segmented flow experiments and provided by beamline scientists. To read HDF5 (.h5) files in Matlab, you must follow the instructions found here: http://hasyweb.desy.de/services/computing/nexus/hdf5-external-filters/install_on_windows.html Briefly, this instructs you to install the needed C++ runtime environent on your PC (with the file vc_redist.x64.exe which is included here). Then you must install the plugin which enables Matlab to read Lz4 compressed data (using the included file: h5lz4-1.0-win64.msi). To read EDF (.edf) files in Matlab, you must download the required pmedf functions included herein and add their file path to the script. Optical micrographs OM.tif files contain in situ optical microscopy data from each flow condition. Files are saved in a folder identifying the experiment to which they belong, where "TJ", "Pos1", and "Pos2" refer to the channel position at which data were collected (the T-junction, Position 1, and Position 2, respectively) and "XXmin" refers to the experimental time point in minutes. Scanning electron micrographs SEM.tif contain ex situ scanning electron microscopy data of the final device state produced by each flow condition. Files are saved in a folder identifying the experiment to which they belong, where "TJ", "Pos1", and "Pos2" refer to the channel position at which data were collected (the T-junction, Position 1, and Position 2, respectively) and "XXmin" refers to the experimental time point in minutes. Technique Descriptions: Droplet Microfluidics Coupled X-Ray Diffraction (DMC-XRD) DMC-XRD analysis of segmented flow experiments at ESRF beamline ID13 (Microfocus) was performed with an X-ray beam of 13 keV and 12 x 15 mum^2 spot size using an EigerX 4M detector. Microfluidic devices were mounted on a computer-controlled XYZ stage, where alignment and positioning were facilitated with an inline optical microscope. After the coordinates of each analysis position were determined, the source flows were switched on and allowed to equilibrate. Then 10-20 second exposures were collected at each position at 50 frames-per-second (fps). DMC-XRD Data Processing A Matlab algorithm was developed to cycle through the frames of a particular time-resolved exposure, where frames containing oil scattering are discarded and frames containing water scattering are background subtracted. The background subtraction routine consists of subtracting a frame from the same exposure, but one not containing any crystals, from the target frames. It is not possible to use a single background reference for all channels for all experiments as small differences in sample-to-detector distance, texturing/imperfections in the windows, and possible beam clipping of channel walls, make each exposure too unique for application of a universal background reference. Any remaining background noise is removed with a threshold identified for each experiment. These frames are summed together to form a composite 2D pattern incorporating all the diffraction observed during that exposure. This pattern is then integrated, and the detector parameters (pixel size, aspect ratio) and the sample-to-detector distance are taken into account to produce a 1D pattern displaying intensity as a function of 2theta. Reference data for particular crystal polymorphs are then plotted against these 1D patterns to identify particular peaks, where errors in peak position are typically < 0.05 deg. XRD of Crystallization in Continuous Flow and Data Processing XRD analysis of continuous flow experiments at Diamond beamline I11 (High-Resolution Powder Diffraction) was performed with an X-ray beam of 15 keV and 200 x 200 mum^2 spot size using a Pixium RF4343 detector. Microfluidic devices were mounted on a computer-controlled XYZ stage, where alignment and positioning were facilitated by scanning across the device to find specific channel positions. After the coordinates of each analysis position were determined, the source flows were switched on and allowed to equilibrate. Then 60 s exposures were collected at each indicated position throughout the experiment. Data were processed similarly to DMC-XRD data with an adapted script provided herein ("Continuous_Flow_XRD_processing_HDF.m"). Scanning electron microscopy (SEM) SEM was conducted with an FEI NanoSEM Nova 450 from bottom Kapton windows after 40 min of experiment time. The windows were removed from the device, rinsed with water to remove dissolved salts, rinsed with ethanol to terminate the reaction, cut down to a smaller size, and allowed to dry in air. Samples were mounted on aluminium SEM stubs with double sided Cu tape. All samples were coated with 2 nm Ir conductive layer prior to analysis. Optical microscopy Optical microscopy was conducted with a Leica M165 FC stereo microscope in bright field transmission mode. Images and videos were recorded using a USB 3.0 Leica DMC2900 color camera with a 3.1 Megapixel CMOS sensor using the Leica Application Suite (LAS) software installed with Single and MultiTime modules.