A new method to study evaporation of sessile drop from permeable surfaces

Real time monitoring of the mass of an evaporating liquid drop from a real surface by optical methods cannot account for the volume of the drop that gets trapped between asperities. Mass measurement methods do not have high resolution, necessary to account for the small volume of the drop trapped in the pores. This work uses a non resonant mass sensor with a goniometer to simultaneously measure the mass and contact angle of the evaporating drop. Evaporative flux is used as the quantity of comparison across all surfaces. For a pinned sessile drop, the evaporative flux remains almost a constant for any kind of surface. For inkjet printing paper, the negative slope of the flux rate means evaporation is dominant. The drop breaches the hydrophobic coating on the paper to indicate a positive slope of this flux rate. In short, evaporation is strictly controlled by relative humidity and not the nature of the substrate.