Contact Angle Measurements by AFM on Droplets of Intermediate-Length Alkanes Adsorbed on SiO$_{2}$ Surfaces*

We have recently discovered that films of intermediate-length alkanes ($n$-C$_{n}$H$_{2n+2}$; 24 $<$ n $<$ 40) do not completely wet a SiO$_{2}$ surface on a nanometer length scale [2]. In a narrow temperature range near the bulk melting point $T_{b}$, we observe a single layer of molecules oriented with their long axis perpendicular to the surface. On heating just above $T_{b}$, these molecules undergo a delayering transition to three-dimensional droplets that remain present up to their evaporation point. Here we report measurements by noncontact Atomic Force Microscopy of the contact angle of these droplets for a film of hexatriacontane ($n$-C$_{36}$H$_{74}$ or C36). Our preliminary measurements indicate that there is a weak maximum in the contact angle at $\sim \quad T_{b}$ + 3 \r{ }C. Further measurements are planned to investigate whether the weak maximum in the contact angle is consistent with the droplets supporting a surface freezing effect as at the bulk fluid/air interface. $^{2}$M. Bai, K. Knorr, M. J. Simpson, S. Trogisch, H. Taub, S. N. Ehrlich, H. Mo, U. G. Volkmann, F. Y. Hansen, cond-mat/0611497.