Wetting Transparency of Ultrathin Polymer Films

Fundamental science question:

Scientists can predict wetting, or how a liquid drop will act on a material: how it bubbles up or spreads out, very well on a large scale. However, when things get extremely small, on the order of nanometers (10-9), the math that previously could predict wetting behavior no longer applies. My project on wetting transparency studies how wetting behavior changes as we make thinner and thinner polymer films. In the ultrathin regime (<100 nm), it has been observed that materials become “wetting transparent,” or partially invisible to the liquid droplet! Our goal is to add knowledge to the gap in understanding fundamental wetting theory at this ultrathin regime.

What does that mean for everyday life?

Think about how everything is becoming smaller and smaller: for example, our phones are computers that fit into a pocket, and we even have ‘smart’ watches that we wear around our wrists. A phone screen that is a sticker on your arm seems like a not-so-distant future. Wearable electronics are a new and exciting scientific application that brings biology, adhesion, electronics, and polymer science together. My project can help in the design of wearable electronics so that researchers can understand how sweat or water might impact an extremely thin polymer film adhered to the human body.

Scientific approach:

We want to systematically study how wetting behavior changes as we make thinner polymer films, so we need to make polymer films and then study wetting! We create bilayer polymer films via spincoating, which are crosslinked with UV light to prevent interlayer diffusion. After characterizing the film thickness with ellipsometry and roughness with Atomic Force Microscopy (AFM), we then perform contact angle measurements with water and diidomethane.

Progress so far:

We have made and characterized our polymer bilayer films, and now are in the process of conducting contact angle measurements!

Favorite moment:

I really love experimental test development. I spent many hours on how to get the best contact angle data! There were many variables to change: drop volume, drop rate, recording lighting, frame speed, etc. When I went from chicken scratch-looking data to clean trends (see graphs below), I was very excited! I also spent time learning and creating a computer program that automatically sorts and analyzes my contact angle data for me – an extremely valuable time saver and helpful skill!

Lead Student:

Evon Petek

Related publication (s):

R. Katsumata, M. N. Yogeesh, H. Wong, S.X. Zhou, S.M. Sirard, T. Huang, R.D. Piner, Z. Wu, W. Li, A.L. Lee, M.C. Carlson, M.J. Maher, D. Akinwande, and C.J. Ellison*, “Large area fabrication of graphene nanoribbons by wetting transparency-assisted block copolymer lithography,” Polymer, 110, 131-138 (2016). [DOI]