Comments on “Role of bubble size for the performance of continuous foam fractionation in stripping mode” by Hofmann et al.

A recent publication in this journal reported the effect of bubble size on foam fractionation performance. Whilst the bubble size is indeed an important parameter, bubble size measurement in the said article is likely affected by a well-known source of error, namely bubble distortion by the column wall which leads to an erroneous interpretation of the experimental phenomenon. Consequently, the authors resorted to empirical equations with little physical basis to correlate their experimental data, while established two-phase flow theories have been largely overlooked. This letter attempts to clarify such issues.