Insoluble layer deposition and dilatational rheology at a microscale spherical cap interface

The dilatational properties of insoluble monolayers are important for understanding the mechanics of biological systems and consumer products, but isolating the dilatational response of an interface is challenging due to the difficulties in separating dilatation from shear and other deformation modes. Oscillations of a microscale bubble radius are useful for generating purely dilatational flows, but the current deposition methods for insoluble layers onto fluid interfaces are not easily scaled down. In this paper, we describe a miscible solvent exchange procedure for generating insoluble layers at an air-water interface pinned at the tip of a capillary tens of micrometers in diameter. We show that the amount of surfactant adsorbed at the interface can be controlled by the initial concentration dissolved in isopropanol (the starting solvent) and the volumetric flow rate of solvent exchange. Surface pressure-area isotherms and dilatational moduli are measured concurrently for three insoluble surfactants: palmitic acid (PA), dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC). The isotherms measured on the microscale interface compare well with previous experiments performed on a Langmuir trough. However, the magnitudes of the dilatational moduli differ from those measured on either Langmuir trough or pendant drop apparatuses. Several possible reasons for the observed differences are discussed. A comparison of the dilatational modulus with the Gibbs elasticity is used to determine the presence of dilatational extra stresses at the interface. The isotherm and dilatational modulus of the insoluble component of the industrial surfactant Tween 80 are measured using this approach. The methods developed here also open the possibility for future study of the important role of finite size effects on microstructure formation and the resulting interfacial mechanics.