How can I trace boundaries for multiple images?

Hi- I have images of a fluid being injected between two glass plates over time (hundreds of images). I need to find a way to trace the boundary of the fluid in the images. I have used bwtraceboundaries function and this has worked well for a single image. I'm not sure how to use this function for multiple images since I need to select a location of a border pixel on the fluid boundary as the starting point for the trace. Any ideas on what I could possibly use instead? (Ultimately, I need to calculate the velocity of the fluid front over time)