This protocol allows for the efficient study of liquid drop impacts on solid surfaces, an everyday phenomenon that's really important in industry. The outcomes can be really complex, and depend on many different experimental parameters. As the data acquisition and analysis are coordinated, many experiments can be conducted over a short period of time, and the quality of data can be assessed promptly.
This method is useful for studying any basic drop impact parameters. Any liquids, drop size, drop impact velocity, any substrate material, and under any environmental conditions. This method is really efficient, but there are a number of important steps required to set it up properly.
Following a demonstration is the best way to avoid errors. To set up the high speed camera, place an alignment marker on the center position of the sample stage, facing the camera, and adjust the magnification of the camera so that the square marker fits within the field of view. When the marker is in focus, capture an image.
Load the graphical user interface for the droplet impact analysis software in MATLAB, click Calibrate Camera, and select the captured image to run the image analysis code. Enter the size of the calibration square in millimeters, and click OK.Move the rectangle until the calibration square is the only object within the rectangle, and click okay. The software will automatically calculate the conversion factor.
To align the experimental system, position the needle mount at eye level for ease of loading, and check that tubing is not twisted. Using a syringe with a secure, clean needle, manually purge the tubing of any residual fluid, while holding the tubing and needle in a vertical position. And fill the syringe with the fluid of interest.
Attach the syringe to the computer-controlled syringe pump, and click and hold the dispense button of the syringe pump to purge the needle until no bubbles are present within the fluid. Set the pump to dispense the appropriate volume for the release of individual fluid droplets, and align the sample under the needle. Then, use the pump to dispense a single droplet on to the sample, and confirm that the droplet lands and spreads across the area of interest on the sample.
When the droplet settings have been confirmed, adjust the vertical position of the sample holder until the surface is level with the center of the field of view of the camera, and adjust the horizontal position of the camera so that the droplet on the sample is aligned on the center of the field of view. Then, adjust the vertical and horizontal positions of the LED to match the position of the camera so that the center of the light appears in the center of the field of view. And adjust the distance of the camera from the droplet so that the droplet comes into focus.
Once the system has been aligned and calibrated, set the frame rate of the camera to an optimal value for the object being recorded. Set the exposure time of the camera to as small of a value as possible while retaining an enough illumination. And adjust the lens aperture to the smallest available setting while retaining enough illumination.
Then, set the trigger for the camera using an end-mode trigger, so that the camera buffers the recording before stopping on the trigger. To conduct an experiment, create a folder to store the movies for the current batch of experiments, and set this folder as the save location for the camera software according to the manufacturer's guide for the camera. Make sure the file format for captured images is set to TIFF.
Click Set Path in the image analysis graphic user interface, and select the save location folder so that the software will monitor this folder for new videos. To create the folder structure for batch experiments, click Make Folders, and enter the minimum droplet release height, the maximum release height, the height step between each experiment, and the number of repeat experiments at each height. And click OK to run the make folders script.
For impact on a dry, solid surface, clean the surface according to a suitable standard protocol, and allow the surface to completely dry. To record a droplet impact event, place the sample on the sample stage so that it is aligned with the camera, and move the needle to the desired droplet release height. Confirm that the view from the camera is unobstructed before capturing and saving an image in the camera software.
Begin the video recording so that the camera is recording and buffering, and use the syringe pump to dispense a single droplet onto the sample. Then trigger the recording to stop once the impact event is complete. Remove the surface from the sample holder, and dry the surface as appropriate.
To prepare the captured video file for analysis, in the high speed camera software, scan the video to find the first frame in which the droplet is completely within the field of view, and crop the start of the video to this frame. Move forward by the number of frames required to capture the phenomena of interest during the impact experiment, and crop the end of the video to this frame. Then, save the video as an AVI file, and the save path to the corresponding folder for the current experimental batch, release height, and repeat number.
In the image analysis interface, click Sort Files, and visually confirm that the obtained background image is now displayed on screen. Then, click run tracing to begin the image processing. The video will be displayed with the resulting image processing overlaid.
Qualitatively check that the image processing is functioning correctly by watching the video. For raw data analysis, in the image analysis graphic user interface, click Process Data to begin calculation of the main variables from the raw process data. Then, enter the frame rate of recording, fluid density, fluid surface tension, and fluid viscosity values, and click OK.The data will be saved in the video folder's MAT file, and export as a CSV file.
The reference square must be unobstructed in the camera field of view, and in focus. An incorrect focus of the reference square will produce a systematic error in the calculated values. The droplet identification software relies on the surface of the sample being presented horizontally to the camera, as observed in this image.
Surfaces that are bent, or poorly resolved, will produce image processing errors. To ensure that the entire droplet spread is tracked by the software, the droplet should land in the center of the sample. If the system is incorrectly aligned, the droplet can drift from the center position, and will be out of focus.
To ensure that the imaged edges of the impacting droplet appear sharp, the shortest exposure time possible with the available light source should be used. Incorrect alignment of the illumination path relative to the camera often affects other settings, such as the camera aperture, and exposure time, resulting in a fuzzy edge to the traveling droplet. The software should be able to trace the entire outline of the droplet in the video images.
If the trace is not completed, the measured values, such as the length of the spreading droplet, will be incorrect. It is essential that the camera is correctly focused, and aligned with the sample, and that the saved video matches the path provided in the software. Otherwise, the analysis will fail.
We've been able to explore variations in impact outcomes over a range of experimental parameters. For example, in determining the velocity of when a droplet begins to splash.
