Lateral transport of water during drying of alkyd emulsions

Magnetic resonance imaging is used to determine noninvasively the spatial and temporal distribution of water in drying drops of alkyd oil-in-water emulsions. This is the first time that magnetic resonance imaging is used for this purpose. The water loss is monitored under regimes of both free and vapor diffusion-limited evaporation. Evidence is provided for lateral gradients in water concentration across the drop. The results are compared with a model of the drying process that includes lateral water diffusion to reduce water concentration gradients resulting from the evaporation. The model has close similarities to a model recently proposed by Routh and Russel (Routh, A.F.; Russel, W.B. AlChEJ. 1998, 44, 2088). The analysis makes no assumptions about the shape of the drop and includes no fit parameters but only known values. The results are also compared with existing models that maintain features of the drop shape but that do not include lateral water diffusion (Parisse, F.; Allain, C.J. Phys. II 1996, 6, 1111). The data are found to be in better agreement with the diffusion model for reasons that are discussed. However, the diffusion model is less successful when applied to the drying of drops of dispersions of hard colloidal particles.