Elimination of Coffee-Ring Formation by Humidity Cycling: A Numerical Study.

Periodically switching between evaporation and condensation, or "humidity cycling", has potential for controlling the film shape that results from volatile droplets containing a nonvolatile material. It does not require adaptation of material properties nor the introduction of an external field to achieve a change in film shape. It was shown experimentally by Doi and coworkers [Kajiya et al. Langmuir 2010, 26, pp 10429-10432] that ring-shaped deposits can be removed through careful selection of the atmospheric conditions. We present a model, based on lubrication theory, that can predict the final film shape resulting from the humidity cycling process. We confirm that the refluidization of gelled regions during condensation and the subsequent inward flow is the mechanism responsible for the improved profiles. Furthermore, we find that an increase in the time spent condensing to that spent evaporating results in flatter films and that an optimal humidity cycling frequency exists.