A bike-wheel microcell for measurement of thin-film forces

A thin-film balance employing either a Sheludko capillary or a Mysels-inspired porous-medium film holder provides a direct measurement of disjoining-pressure isotherms in free, liquid thin films. However, each film holder suffers its own distinct disadvantages spanning non-uniform and slow liquid exchange, a limited range of measurable disjoining pressures, an inability for reuse, and a requirement for significant chemical amounts. In an attempt to alleviate these disadvantages, we have designed and constructed a miniaturized and microfabricated ‘bike-wheel’ cell as a replacement film holder. Essentially, an inner hole holding the thin film (the hub) is connected radially by 24 small channels (the spokes) to an outer, larger size annulus (the wheel). This design provides a hybrid of the Sheludko capillary and the Exerowa–Sheludko porous-plate film holders and eliminates the undesirable features of each. Moreover, due to its miniaturized dimensions and concomitant fast drainage rates, the bike-wheel film holder is particularly suited for investigation of polymer and/or protein-based systems where thin-film force laws depend on the degree of aging at the interface. The new bike-wheel microcell is validated quantitatively by reproducing a known disjoining-pressure isotherm for 0.1 M aqueous sodium dodecyl sulfate (SDS) foam films, including dynamic stratification, and reversible and oscillatory isotherm branches. Finally, application of the new bike-wheel film holder is made to thin-film forces in aqueous protein-stabilized foam films of bovine serum albumin (BSA) at the isoelectric point. Here we find a repulsive, steric stabilized disjoining-pressure isotherm for fresh protein films, but surface aggregation and non-equilibrium forces for aged films. The new bike-wheel microcell incorporated into the thin-film balance provides a useful tool for studying thin-film forces, especially for larger molecular weight stabilizing species.