We investigate the evaporation of liquid droplets on solid substrates. Principally, two different scenarios can be thought of:

1. the droplet evaporates on a solid substrate without dissolution of the substrate,
   
2. the evaporating liquid is a solvent for the substrate material and thus dissolves it during the evaporation process.

In the first case I, different evaporation modes have been observed in the literature. The droplet can evaporate with the contact angle being constant and the contact radius decreasing (constant angle mode). Or the contact radius stays constant and the contact angle decreases: the droplet becomes flatter with time (constant radius mode or pinning). Or both, the contact angle and the contact radius change during evaporation (non-constant mode). Usually, droplets evaporate in different modes. We found that by modifying the surface properties of the underlying solid substrate (hydrophobicity, chemical heterogeneity, roughness), the evaporation process can be tuned in a definite way. Thus, we hope to better understand how certain surface properties couple to droplet evaporation.
The mode, in which a sessile droplet evaporates, can also be affected through solute molecules in the droplet. A frequently studied system is a water droplet containing a fluorescence dye. There, the solute concentration increases at the rim of the droplet during evaporation (see image).

Fluorescence image (top view) of an evaporating sessile water droplet, containing a fluorescent dye, on a solid substrate The dye concentration increases at the rim of the droplet. (The bright spot in the middle and the yellow spot on the left are from the droplet acting as an optical lens.)

While the process principally has been described physically, details are still not well understood. Questions, which arise from our observations, concern the coupling of initial solute concentration and the properties of the solid surface to the evaporation. The answer to these questions might support technical applications as e.g. screening tests, where a non-uniform solute distribution within a liquid droplet, as shown in the photo, is unwished.

We also push our investigations to a more complex system, as introduced above as case II. If a droplet of a solvent is deposited on top of the solid substrate, it starts to dissolve the material of the substrate beneath during the droplet evaporation. After the evaporation a characteristic microwell is left (see image).

Microwell formation after evaporation of a droplet of toluene on polystyrene (PS) The image shows an Atomic Force Micrograph (AFM). Please note that the microwell is rather flat. (Image processing with WSxM© from http://www.nanotec.es)

We are working on the understanding of the different physical processes, which are responsible for the microwell to form. Amongst these are the dissolution from the substrate, the evaporation of the solvent droplet, the diffusion of the dissolved material within the liquid phase, and the precipitation of the dissolved material at the rim of the microwell. The experiments have shown that the droplet deposition on top of the substrate determines the final shape of the microwell. To understand the relevance of the different processes occurring simultaneously in the droplet, the group of Prof. Wolfgang Wiechert at the University of Siegen supports our experiments with computer simulations.

This project is part of the Center for Microchemistry, Nanochemistry and Engineering at the University of Siegen and funded by the Deutsche Forschungsgemeinschaft (DFG) as part of the Research Unit 516 (GR2003/2-1 FOR516).