Monitoring the Evaporation Process of Liquid Samples in Sub-Nanoliter Vials

One would expect that the evaporation process of liquid droplets was described many years ago. On the contrary, just recently some underlying principles of the dynamic process of evaporation have been published. It seems that during the evaporation of a liquid droplet, the liquid at the edge of the droplet is ”pinned” to the underlying surface. This pinning prevents the droplet from shrinking. This implies that the footprint of the droplet remains constant. The evaporated liquid at the edge is replenished by liquid from the bulk of the droplet. This means that there is a flow of liquid moving towards the edge of the droplet. If the liquid droplet contains particles, like in a droplet of coffee, these particles will be transported outwards by the flow. Finally, when all the liquid has evaporated, the (coffee) particles will form a ring of stain. The phenomena described above have also been monitored in the sub-nanoliter vials of a microarray. The behavior of the liquid inside the vials during evaporation differs a lot from the behavior of an evaporating bulk solution. We have monitored this small scale evaporation process with a wide-field epiillumination microscope system and with a confocal scanning laser microscope system. With a conventional wide-field microscope it is only possible to monitor the projection of the liquid in the vial during evaporation, i.e. it is an indirect measurement of the shape of the meniscus, which is the interface between the air and the solution. On the other hand, with a confocal microscope system it is possible to follow the evaporation as a function of time: this is a direct measurement of the liquid behavior. The goal of these ASCI’99, Proc. 5th Annual Conference of the Advanced School for Computing and Imaging (Heijen, NL, June 15-17), ASCI, Delft, 1999, 58-64. experiments was to monitor the shape of the meniscus during evaporation. With these measurements it is possible to validate numerical simulations of the evaporation process. When experimental results and simulation results agree, it is interesting to investigate which geometries of the vial extends the evaporation process as long as possible.