| AVS 56th International Symposium & Exhibition | |
| Surface Science | Thursday Sessions |
| Session SS-ThP |
| Session: | Surface Science Poster Session |
| Presenter: | T. Furuta, Tokyo Institute of Technology, Japan |
| Authors: | T. Furuta, Tokyo Institute of Technology, Japan A. Nakajima, Tokyo Institute of Technology, Japan M. Sakai, KAST, Japan T. Isobe, Tokyo Institute of Technology, Japan Y. Kameshima, Okayama University, Japan K. Okada, Tokyo Institute of Technology, Japan |
| Correspondent: | Click to Email |
Hydrophobic coatings have been used widely to apply for various engineering products. For assessing these properties, static wettability such as the contact angle or sliding angle is not always a useful criterion for materials design. Recently, the recognition of the importance of dynamic wettability such as droplet’s sliding velocity or acceleration on a tilted surface has been growing gradually.
Water droplets are known to slide down on the hydrophobic solid surface by a caterpillar-like rolling motion with or without slippage at the solid-liquid boundary; a direct observation method for internal fluidity of sliding water droplets was established recently using particle image velocimetry (PIV). The contribution ratio of rolling and slipping motions to overall sliding acceleration reportedly depends on nanoscale roughness and chemical heterogeneity of the hydrophobic coatings.
On the other hand, evaporation is a fundamental phenomenon for liquids. Various studies have been conducted in relation to this phenomenon for liquid droplets on solid surfaces. Ultra-small, e.g. nanoliter-scale, droplets are evapolated and disappeared in a short time, which corresponds to a situation of large volume change ratio per unit period. It might possess similar dependence against nanoscale surface heterogeneity as well as dynamic wettability.
We prepared fluoroalkylsilane coatings with different nanoscale roughness. This study examined evaporation (including nanoliter-scale droplets) and sliding behaviors of water droplets on these coatings using an automatic microscopic contact angle meter and PIV method.
Evaporation and sliding behaviors of water droplets were investigated on smooth and rough coatings. Evaporation behaviors for these two coatings differed when nanoliter-scale droplets were used, although they were nearly identical for microliter-scale droplets. The droplets on the smooth coating exhibit greater sliding acceleration and a larger slipping velocity ratio than those on the rough coating. Both the evaporation behavior of nanoliter-scale droplets and sliding velocity of microliter-scale droplets were affected by nanoscale surface heterogeneity.
Moreover, considering power balance around the three phase contact area, line tension was measured for microliter-scale and sub-nanoliter-scale droplets of an ionic liquid on a highly smooth and homogeneous fluoroalkylsilane coating. Values for microliter-scale droplets were two orders larger than those for sub-nanoliter-scale droplets despite their identical combinations of solid surfaces and liquid. Scale factors related to this measurement and from the liquid play an important role in the discrepancy.