Wilhelmy plate artifacts in elastic monolayers

A recent article [L. Pocivavsek et al., Soft Matter4, 2019 (2008)] by some of us pointed out difficulties in interpreting Wilhelmy plate measurements on elastic Langmuir monolayers that support anisotropic stress. Using a simplified geometry it showed conditions in which the Wilhelmy plate measures significantly different stress from the ambient stress. We correct a serious error in this analysis and strengthen its conclusion, showing that the Wilhelmy stress and the ambient stress can have opposite signs.