ABSTRACT
Because of the variability in the measured data and because of the limited number of force curves analysed, one can not yet claim to have confirmed the drag law (27). Nevertheless, it is of interest to compare the fitted drag coefficient, a = 3.2 x 103 N m - 1 s - 1 with the value of 6 x 10~2 N m - 1 s - 1 estimated by de Ruijter et al [137] from molecular dynamics simulations of a spreading hexadecane droplet. The large discrepancy between the two may be due, in part, to the low viscosity of the simulated liquid (two orders of magnitude less than that of water), to the low surface tension (about one-fifth that of water), and to a low level of coupling between the substrate and the liquid in the simulations. The average speed of the contact line in the simulations is about 1 m/s [137], whereas in the experiments [116] and in the theory [127] the bubble spreads at about 10 /zm/s. In both simulations and theory, the product of drag coefficient and velocity is 3-6 x 10"2 N/m, which is of the same order of magnitude as the surface tension.
