Coating A Surface with Polymer-Filled Droplets

The illustration above shows two polymer-filled drops that have interacted (but not coalesced) as they dried on a silicon substrate. We are interested in the polymer distribution after all the solvent has evaporated as well as interactions between drops during the drying process.

Device Setup

Our overall goal is to determine the origin of the structure formed when multiple drops containing a polymer are sprayed onto a surface and dry. By better understanding the underlying physics, it may be possible to control the surface morphology of an entire coating simply by modifying drop parameters such as polymer concentration, evaporation rate, and surface tension. Such control would have a broad impact on coating technologies such as pesticide spraying and ink jet printing.

In our experiments, drops with a dilute concentration of polymer are deposited on to a silicon wafer using a drop-on-demand device (see picture on left). The drops are around 50 microns and are deposited at a frequency of about 40 Hz.

Effect of Drying Time on Multi-Drop Residue
The residue morphology depends whether the drop lands on a dry or wet surface. If previous drops have not dried before a new drop lands, the drops coalesce. However if the previous drop has dried, the new drop will spread over the dried surface. This phenomena is not surprising, however it does provide a simple method to vary the surface morphology as shown in the videos on the right.

Wet Surface - Drop coalesces with previous drop. Click on the movie.

Dried Surface - Drop dries before next drop hits. The resulting residue consists of concentric rings. Click on the movie.

Collaborators:
Labcoat Technologies
Howard Stone, Division of Engineering and Applied Sciences, Harvard University

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last updated: Nov. 2005