On the gap error in parallel plate rheometry that arises from the presence of air when zeroing the gap

When zeroing the gap between two parallel plates, the squeeze flow of air results in a detectable normal force at a finite height that is misinterpreted in rheometry as an indicator for the zero gap position. This leads to a substantial gap error using well-aligned parallel plates that is accurately predicted using Stefan's equation for squeeze flow. Minimizing or accounting for this error enables accurate rheological measurements to be performed in torsional flow using the parallel plate geometry at narrow gap heights, thus allowing shear rates of over 10(5) s(-1) to be achieved. (C) 2005 The Society of Rheology.