MRI investigation of the evaporation of embedded liquid droplets from porous surfaces under different drying regimes

A combination of in situ one-dimensional 1H magnetic resonance profiling and two-dimensional imaging has been applied to study the shape and subsequent dynamic evaporation behaviour of a single liquid droplet after impact onto a porous surface. Diethyl-malonate (DEM) droplets are initially embedded in the porous substrate by impingement, and are then evaporated over a period of several hours; the surface of the substrate being ventilated by a controlled airflow. The configuration is intended to mimic the behaviour of droplets evaporating into atmospheric flows. In order to evaluate the influence of the airflow at the surface of the porous medium, different experimental configurations were tested by varying the speed of the airflow stream above the porous surface. The method produces several types of data, including images of impinged droplets inside the porous substrate and their development with time during the evaporation episode, one-dimensional concentration profiles through the substrates, and corresponding estimates of the mass fraction of liquid remaining, evaporation rate and mass flux per unit area. The results obtained show that although liquid droplets tend to evaporate faster and present larger evaporation rates when exposed to a more efficient removal of vapour from the surface, the limiting effects of the porous medium are even more evident.