In an example embodiment, a wet system delivers a flow of cleaning foam through a channel in a proximity head to a meniscus interfacing with a semiconductor wafer. The wet system diverts a sample of the flow from the channel through a transparent cell that is connected to the channel by an input passage that leads from the channel to the transparent cell and by an output passage that leads from the transparent cell back to the channel. The wet system illuminates the sample in the transparent cell with an LED from the top or the back and captures an image of the illuminated sample with a CCD camera. The image shows a morphological attribute of the cleaning foam such as bubble diameter or spacing. The wet system generates a statistical characterization from the morphological attribute and adjusts other attributes of the cleaning foam based on the statistical characterization.