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

A thin-film balance employing either a Sheludko capillary or a Mysels-inspi red porous-medium film holder provides a direct measurement of disjoining-p ressure isotherms in free, liquid thin films. However, each film holder suf fers its own distinct disadvantages spanning non-uniform and slow liquid er ,change, a limited range of measurable disjoining pressures, an inability f or reuse, and a requirement for significant chemical amounts. In an attempt to alleviate these disadvantages, we have designed and constructed a minia turized and microfabricated 'bike-wheel' cell as a replacement film holder. Essentially, an inner hole holding the thin film (the hub) is connected ra dially 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 concomit ant fast drainage rates, the bike-wheel film holder is particularly suited for investigation of polymer and/or protein-based systems where thin-film f orce laws depend on the degree of aging at the interface. The new bike-whee l microcell is validated quantitatively by reproducing a known disjoining-p ressure 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 st abilized disjoining-pressure isotherm for fresh protein films, but surface aggregation and non-equilibrium forces for aged films. The new bike-wheel m icrocell incorporated into the thin-film balance provides a useful tool for studying thin-film forces, especially for]larger molecular weight stabiliz ing species. (C) 2001 Elsevier Science B.V. All rights: reserved.