Submitted to: Carbohydrate Polymers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 26, 2001
Publication Date: N/A
Interpretive Summary: Measurement of viscosity, or resistance to flow, is often used to characterize starch solutions. It is common that starch solutions show a decrease in viscosity with increasing deformation (shear) rate. However, some starch solutions showed an increase in viscosity with increasing shear rate. Furthermore, when the sheared solution was observed with optical microscope, some patterns were observed in the solution. In this report, it is shown that the anomaly in viscosity and pattern formation are two different phenomena. The anomaly in viscosity is an indication of existence of gel formed during sample preparation. On the other hand, pattern formation is a shear-induced aggregation of starch molecules. The results are useful to other researchers and to users of starch.
Technical Abstract: Since Dintzis et al. reported shear-thickening behavior and shear-induced pattern formation in semidilute starch solutions for the first time in 1995, considerable efforts have been made to understand the science behind these observations. Despite these efforts, however, many questions regarding this behavior of starch solutions remain. Using a Brookfield programmable rheometer and a custom-built shear microscope, starch solutions in alkaline solution medium were investigated. In this report, we present data leading to the following conclusions: 1) Gently prepared starch solutions are macroscopically heterogeneous with regions of highly concentrated gel-like structures dispersed in dilute starch solution; 2) Shear breaks up these heterogeneous regions, increasing in viscosity (shear-thickening) which is thus seen to be a result of an increase in the concentration of dissolved starch; 3) Pattern formation, observed when the solution is exposed to higher shear rate, is the result of a separate shear-induced aggregation process; 4) Aggregations are not induced below a certain critical threshold shear rate and time is also a factor in the behavior of the aggregate.