Advanced real-time digital microscopy of foaming processes, TAPPI Journal January 2023

Application: This study quantifies the accuracy of digital microscopy in measuring foam bubble size distribution and its standard deviation (i.e., foam homogeneity in bubble scale) and foam density directly from the foam process. It highlights the difference in front light and transillumination approaches and helps the reader in designing the right type of system for the application.

Author: Markus Honkanen and Hannu Eloranta

ABSTRACT: The properties of aqueous foams play a major role in foam forming and foam coating. Inline real-time foam measurements provide highly desired opportunities for optimization and control of foaming processes. This paper presents inline digital microscopy measurements of aqueous foams in foaming processes. It presents methods for providing detailed information on foam quality parameters, such as foam density and foam homogeneity in real time from the process. In addition, this study evaluates the performance of transillumination and front-light illumination in imaging of foams. The tests show very good results for the transillumination approach. Limitations of the image-based optical technique are discussed, and the precision of bubble size distribution measurement is assessed with a certificated reference substance. The measured foam densities are compared against the reference foam densities in the range 100•300 g/L, providing a linear correlation with R2 value of 0.99. In the case of heterogenous foams with a wide bubble size distribution, the bubble size-dependent dimensionless depth of field must be taken into account to obtain accurate estimates of foam density. Bubble-scale foam homogeneity is described by the standard deviation of bubble size distribution in foam.