From surfactant adsorption kinetics to asymmetric nanomembrane mechanics: pendant drop experiments with subphase exchange.

Adsorption equilibrium is the state in which the chemical potential of each species in the interface and bulk is the same. Dynamic phenomena at fluid-fluid interfaces in the presence of surface active species are often probed by perturbing an interface or adjoining bulk phase from the equilibrium state. Many methods designed for studying kinetics at fluid-fluid interfaces focus on removing the system from equilibrium through dilation or compression of the interface. This modifies the surface excess concentration Γ(i) and allows the species distribution in the bulk C(i) to respond. There are only a few methods available for studying fluid-fluid interfaces which seek to control C(i) and allow the interface to respond with changes to Γ(i). Subphase exchange in pendant drops can be achieved by the injection and withdrawal of liquid into a drop at constant volumetric flow rate R(E) during which the interfacial area and drop volume V(D) are controlled to be approximately constant. This can be accomplished by forming a pendant drop at the tip of two coaxial capillary tubes. Although evolution of the subphase concentration C(i)(t) is dictated by extrinsic factors such as R(E) and V(D), complete subphase exchange can always be attained when a sufficient amount of liquid is used. This provides a means to tailor driving forces for adsorption and desorption in fluid-fluid systems and in some cases, fabricate interfacial materials of well-defined composition templated at these interfaces. The coaxial capillary pendant drop (CCPD) method opens a wide variety of experimental possibilities. Experiments and theoretical frameworks are reviewed for the study of surfactant exchange kinetics, macromolecular adsorption equilibrium and dynamics, as well as the fabrication of a wide range of soft surface materials and the characterization of their mechanics. Future directions for new experiments are also discussed.