Former principal researchers at MPI-P:
Guangfen Li, Georgiana Micu, Norbert Höhn, Svetlana Karabacheva
Former cooperation partners:
Prof. Dr. Wolfgang Wiechert (Forschungszentrum Jülich, formerly Universität Siegen)
Prof. Dr. Steffen Hardt (CSI, TU Darmstadt)
Prof. Dr. Friedhelm Schönfeld (Hochschule Rhein-Main)
Prof. Dr. Rainer Jordan (TU Dresden)
We investigate the evaporation of liquid droplets on solid substrates. Principally, two different scenarios can be thought of:
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
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,
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)
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
Forschungszentrum Jülich supported our experiments
with computer simulations.
This project was 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).