Wetting Kinetics of Polymer Solutions. Experimental Observations

Abstract

It is known that the spreading rates of small liquid drops over a solid surface exhibit a strong dependence on the viscosity of the liquid. These observations have led us to study the spreading behavior of polymer solutions on high- and low-energy substrates to determine what role complex rheology plays in wetting kinetics. We did not observe any effects of non-Newtonian (nonlinear) behavior, nor were any obvious signs of viscoelasticity (memory) visible. Instead, we observed a most intriguing phenomenon that polymer solutions, despite having low surface energies, do not wet high-energy substrates. In addition, nonwetting drops spread to an equilibrium configuration by either one of two distinct mechanisms. In the first case, they spread as a wetting Newtonian liquid and then stopped, that is, equilibrated, abruptly. In the second, they equilibrated continuously. We offer some possible molecular and continuum arguments to explain the differences between these two mechanisms.

Wetting kinetics of polymer solutions and force‐based contact angles

“…The polymer PEO was dissolved in distilled water at concentrations of 1 -7 g/dl for PEO of molecular weight 3 x10 5 , as well at 0.5-5 g/dl for PEO of molecular weight 4 3 10 6 same as in de Ryck and Qu er e, 4 both molecular weight polymers were used here in the rheological and contact angles experiments. These are polymers of very large molecular weights and can tear on too much stirring.…”

“…Nieh et al 5 measured the spreading rates of a small drop of polystyrene in dibutylphthalate spreading on a glass surface, but no shear thinning character was seen. Instead of fluid mechanical solution to quantify the dynamic contact angle, de Gennes 6 started with an energy balance where the viscous dissipation was balanced by surface work.…”

“…Finally in this section, we note that some effect of heterogeneity in the polymer solution near that contact line is expected. Nieh et al 5 observed that even though the solvent was wetting, the polymer solution was nonwetting. Ybarra et al 16 suggested (as had some before them) that polymer molecules showing random coils cannot reach the contact line corner as shown schematically in Figure 3.…”

Dynamic contact angles of shear thinning fluids

“…One feature of polymer solutions is that they tend to be nonwetting as a rule even when the pure solvent is wetting. 2,15 This happens due to the fact that the region of the contact line is inhomogeneous in the polymer chain segment density that changes with polymer concentration and molecular weight. This change has been used to explain the differences in the equilibrium contact angles and may also be used to explain the differences in the cutoff length which leads to differences in slopes in Figure 1. )…”

“…The measured equilibrium contact angle was k ¼ 20. 2 , but the one used for fitting to Eq. 1 is k ¼ 25.…”