A novel technique for in situ measurements of stress development within a drying film

A novel technique capable of monitoring the stress evolving process within a drying film has been developed in this work. The device uses a thin cantilever beam made of stainless steel to hold a thin layer of fluid material. The (bending) force exerted on the beam as a result of the lateral stress developed within the film is measured via a pendant-balance transmitting mechanism. The reliability of the device was tested with two different biopolymer fluids (18 wt% sodium caseinate and 30 wt% waxy maize starch) dried at different temperatures and relative humidity (RH). The results revealed three stages of stress development for these biopolymer films: an initial delay, a sharp stress increase, followed by a steady stress plateau. Further analysis of experimental data showed that stress development and moisture loss can be normalised to form a master curve. However, it appeared that the magnitude of stress increase has no direct link with the rate of moisture loss, indicating possibly different mechanisms of the stress increase and the loss of moisture. Results also showed that the total stress of a dried starch film was much higher than that of a dried caseinate film.