Spreading of silicone oils on glass in two geometries.

A basic problem in liquid spreading is the hydrodynamic description of the viscous breaking force near the moving contact line. A solution to the problem of divergence at the triple line has been illustrated with two experimental configurations. It consists of observing that the rheological behavior of a Newtonian liquid is modified near the triple line due to high shear rates. Above a critical value of the shear rate, near the triple line and near the solid surface, the liquid becomes shear-thinning, meaning that the apparent viscosity of the liquid decreases as the shear rate increases. As a result, there is no divergence of the viscous energy dissipation and of the braking force as observed for a purely Newtonian behavior. This description of the viscous braking phenomenon in liquid spreading is well supported by spreading experiments of silicone oils on glass substrates in two different wetting configurations. The liquids used are two silicone oils (10,000 and 100,000 cSt). These liquids are Newtonian below a critical value of the shear rate. Above this critical value, the liquid viscosity decreases according to a power law of the shear rate. One series of experiments consider the spreading of silicone oil droplets on treated and untreated glass substrates. The other configuration consists of using the glass substrates as Wilhelmy plates and to determine the advancing dynamic contact angle as a function of the imposed speed of sinking of the plate into oil reservoirs. The two series of experiments satisfy the same dynamic wetting laws. The overall experimental results are compatible with the hypothesis of a Newtonian/non Newtonian transition of the rheological properties of liquids near the wetting front although the main origin of dissipation appears to result from Newtonian viscous braking. The same dynamic law applies for the drop and Wilhelmy plate geometries.