Spreading of “heavy” droplets: II. Experiments

Abstract

We present systematic measurements of the spreading of PDMS oils on flat oxidized silicon wafers. We have determined the complete drop profile for droplet radii larger than the capillary length, κ −1, defined by κ −1 = ( γ ρg ) 1 2 , where γ is the surface tension of the liquid, ρ its density, and g the gravitational acceleration; The droplet shape is found to be quasi-static, up to a critical radius R c, which is found to be one order of magnitude higher than the capillary length. The droplet is essentially flat except on the edges and its thickness at center, h 0, is related to the dynamical contact angle, θ d, by h 0 ≈ κ −1 θ d. This measured profile is in perfect agreement with the numerical profile which has been calculated in the first paper of this series ( J. Colloid Interface Sci. 142, 518, 1991). The time dependence of the drop radius and thickness obeys apparent power laws, with the exponents 0.126 ± 0.002 and −0.27 ± 0.01, respectively. In addition, we have determined the logarithmic prefactor L which characterizes the liquid-solid system in the calculation of the viscous dissipation, L = 2.6 ± 0.1. Thickness measurements are based on two new methods, one mechanical and one optical, which are described in detail.