Dynamic wetting of silica by aqueous triblock copolymer solutions at low concentrations

We report on the wetting of silica by aqueous solutions of triblock copoly( ethylene oxide-tetrahydrofuran-ethylene oxide), PEO-PTHF-PEO. The wetting b ehavior was studied by:means of a Wilhelmy force balance and direct images of the contact angle: The results. show that the three-phase contact Line ( tcl) advances in jumps over the surface when this Is immersed with constant rate into copolymer solutions. This stick-slip behavior was studied with r espect to both the concentration of copolymers in the solution and the imme rsion rate of the silica plate into the solution. The wetting results are c ompared with adsorption data obtained by ellipsometry and dynamic Surface t ension measurements. It is clear that the stick-slip spreading behavior res ults from the same basic mechanisms as have previously been proposed for sh ort chain cationic surfactants. The pinning of the contact line is due to t he formation (through adsorption) of an autophobic copolymer layer at the s olid-vapor interface. At low copolymer concentrations, transport of copolym ers to the solid-vapor interface seems to occur mainly through surface diff usion from the liquid-vapor interface to the tcl. Depinning and slip occurs when the dynamic contact angle has increased above a critical value which exceeds that stipulated by the Young's equation in the presence of the auto phobic layer at the solid-vapor interface.