Measurement of volume fraction distribution in a drying film by imaging with a digital camera

We propose a method to measure the evolution of the volume fraction distribution in colloidal films during their drying process, using only the transmitted light intensity recorded with a commercial digital camera. From the Lambert-Beer law, the volume fraction of a dispersion film with a certain film thickness can be measured from the transmitted light intensity. A pseudo-absorption capacity was imparted to the transparent dispersion by adding a red dye, and the ratio of green to blue light absorbance was measured for the liquid film. For any film, the volume fraction increased spatially towards a compaction front, and the volume fraction in this region reached approximately 63.4%. The volume fraction inside the liquid region of the film also increased during drying. These trends are similar to previous results obtained using other methods. The magnification in this method can be easily changed by replacing the imaging equipment, and changes in volume fraction distribution near the drying front were observed at high magnification using different lenses. This method, based on captured photographic images, measure the two-dimensional distribution of the volume fraction, making it easy to observe changes in the volume fraction distribution caused by drying at both macroscopic and microscopic levels. It is likely that the proposed method will be difficult to apply to anisotropic particles or to larger particles which cause significant scattering.