A versatile subphase exchange cell for interfacial shear rheology

Interfacial rheological properties play a key role in the stability of a wide range of high-interface materials and thin films. For many systems, it is desirable to understand the response of the interface to a change of composition in the surrounding bulk phases. Stimuli, such as changes in pH or electrolyte concentration, can have a major effect on the structure and properties of the interfacial layer, or induce adsorption and desorption phenomena. Shear rheology is a particularly sensitive measure of such changes, as it only probes the extra stresses in the interface, regardless of possible variations in interfacial tension. In the present work, a widely used geometry for interfacial rheometry, the double-wall ring, is modified to enable subphase exchanges. The trade-off between the speed of exchange and the stress exerted by the flow in the subphase onto the interface is carefully considered. The optimal geometrical design is found by employing Computational Fluid Dynamics (CFD). A geometry with inlets positioned at the bottom and outlets near the interface minimizes the mixing time. Experiments on interfaces with colloidal particles and proteins, subjected to changes in electrolyte concentration and pH, respectively, are used to evaluate the performance of the setup.