Surface elasticity and viscosity from oscillating bubbles measured by automatic axisymmetric drop shape analysis

The pendant drop/sessile drop instrument developed by our group and based on video image analysis has been enhanced to measure oscillating drops and bubbles at a rate up to 25 pictures per second. Data analysis has been developed to analyze the results from sinusoidal oscillations in terms of dilatational surface elasticity and viscosity. The polymers ethylhydroxyethyl cellulose (EHEC) and two different poly(oxyethylene)-poly(oxypropylene) ABA block copolymers (PE6200 and PE6800) have been investigated at the air-water interface regarding rate of adsorption and surface dilatational properties. The polymers give surface pressures in the region 20-30 mN m(-1), the surface elastic moduli are between 4 and 30 mN m(-1), and the viscous moduli are generally low, from 0 to 6 mN m(-1). The elastic moduli increase with increasing frequency, but both the slope and the concentration dependency vary. For the most water-insoluble polymer, EHEC, the modulus increases with polymer concentration, the relatively hydrophobic polymer PE6200 shows the opposite behavior, and the most water-soluble polymer, PE6800, shows a maximum in the equilibrium elasticity. These observations can be explained by the changes in the molecular orientation in the surface layer as reflected in pi vs A isotherms. The PE6200 polymer also shows a higher viscous modulus, which may be explained by diffusional transport between surface and bulk. (C) 1998 Academic Press.